Michele Trenti - Academia.edu (original) (raw)

Papers by Michele Trenti

We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-f... more We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-forming galaxies at z~8-8.5, only 600 million years from recombination, using a Y_{105}-dropout selection. The new 4.7 arcmin**2 WFC3/IR observations reach to ~28.8 AB mag (5 sigma) in the Y_{105}J_{125}H_{160} bands. These remarkable data reach ~1.5 AB mag deeper than the previous data over the HUDF, and

A definitive detection of an intermediate-mass black hole (IMBH) would carry significant conseque... more A definitive detection of an intermediate-mass black hole (IMBH) would carry significant consequences for many sub-fields of astrophysics. As a result of the difficulties associated with direct detection of IMBH signatures, we continue the search for indirect evidence of their existence. In previous work, we described our method for suggesting the presence of an IMBH by examining the radial variations in the average mass of main sequence stars. This method can be applied to Galactic globular clusters that are dynamically old enough - those that have experienced at least 5 initial half-mass relaxation times - and that are not too influenced by the Galactic tidal field. Here, we present the results of the application of our method to NGC 2298 using ACS-HST photometry extending from the center to more than twice the half-light radius of the cluster. We find that the profile of mass segregation closely mimics those of our simulations without a large central mass, thus making it very unl...

Science (New York, N.Y.), Jan 6, 2015

In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a stro... more In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.

Recent extremely-deep, near-IR imaging (to 29.3 mag 5sigma) with HST WFC3/IR in the HUDF from our... more Recent extremely-deep, near-IR imaging (to 29.3 mag 5sigma) with HST WFC3/IR in the HUDF from our 192 orbit HUDF09 program have enabled a huge leap in our knowledge of the galaxies populating the early universe at z>7. These early galaxies are compelling targets for many reasons: as beacons of the first sites of star formation, as testbeds to constrain galaxy

We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over ... more We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z~7. We determine the UV-continuum slope beta in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in beta. For luminous L*(z=3) galaxies, we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to the beta~-2 derived at similar luminosities at z~5-6. However, for the lower luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is substantially bluer than is found for similar luminosity galaxies at z~4, just 800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected beta becomes >~-2.7. To produce these very blue beta's (i.e., beta~-3), extremely low metallicities...

We identify 73 z~7 and 59 z~8 candidate galaxies in the reionization epoch, and use this large 26... more We identify 73 z~7 and 59 z~8 candidate galaxies in the reionization epoch, and use this large 26-29.4 AB mag sample of galaxies to derive very deep luminosity functions to <-18 AB mag and the star formation rate density at z~7 and z~8. The galaxy sample is derived using a sophisticated Lyman-Break technique on the full two-year WFC3/IR and ACS data available over the HUDF09 (~29.4 AB mag, 5 sigma), two nearby HUDF09 fields (~29 AB mag, 14 arcmin) and the wider area ERS (~27.5 AB mag) ~40 arcmin**2). The application of strict optical non-detection criteria ensures the contamination fraction is kept low (just ~7% in the HUDF). This very low value includes a full assessment of the contamination from lower redshift sources, photometric scatter, AGN, spurious sources, low mass stars, and transients (e.g., SNe). From careful modelling of the selection volumes for each of our search fields we derive luminosity functions for galaxies at z~7 and z~8 to <-18 AB mag. The faint-end slopes alpha at z~7 and z~8 are uncertain but very steep at alpha = -2.01+/-0.21 and alpha=-1.91+/-0.32, respectively. Such steep slopes contrast to the local alpha<~-1.4 and may even be steeper than that at z~4 where alpha=-1.73+/-0.05. With such steep slopes (alpha<~-1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The star formation rate densities derived from these new z~7 and z~8 luminosity functions are consistent with the trends found at later times (lower redshifts). We find reasonable consistency, with the SFR densities implied from reported stellar mass densities, being only ~40% higher at z<7. This suggests that (1) the stellar mass densities inferred from the Spitzer IRAC photometry are reasonably accurate and (2) that the IMF at very high redshift may not be very different from that at later times.

The Astrophysical Journal, 2007

We present the UDF05 project, a HST Large Program of deep ACS (F606W, F775W, F850LP) and NICMOS (... more We present the UDF05 project, a HST Large Program of deep ACS (F606W, F775W, F850LP) and NICMOS (F110W, F160W) imaging of three fields, two of which coincide with the NICP1-4 NICMOS parallel observations of the Hubble Ultra Deep Field (HUDF). In this first paper we use the ACS data for the NICP12 field, as well as the original HUDF ACS data, to measure the UV Luminosity Function (LF) of z ∼ 5 Lyman Break Galaxies (LBGs) down to very faint levels. Specifically, based on a V − i, i − z selection criterion, we identify a sample of 101 and 133 candidate z ∼ 5 galaxies down to z 850 = 28.5 and 29.25 magnitudes in the NICP12 and in the HUDF fields, respectively. Using an extensive set of Monte Carlo simulations we derive corrections for observational biases and selection effects, and construct the rest-frame 1400Å LBG LF over the range M 1400 = [−21.4, −17.1], i.e. down to ∼ 0.04 L * at z ∼ 5, and complement it with data from the Subaru Deep Field (SDF) from to extend it to the brighter end (M 1400 ≥ −22.2). We show that: (i) Different assumptions regarding the SED distribution of the LBG population, dust properties and intergalactic absorption result in a 25% variation in the number density of LBGs at z ∼ 5; (ii) Under consistent assumptions for dust properties and intergalactic absorption, the HUDF is about 30% under-dense in z ∼ 5 LBGs relative to the NICP12 field, a variation which is well explained by cosmic variance; (iii) The faint-end slope of the LF is independent of the specific assumptions for the input physical parameters, and has a value of α ∼ −1.6, similar to the faint-end slope of the LF that has been measured for LBGs at z ∼ 3 and z ∼ 6. Our study therefore supports no variation in the faint-end of the LBG LF over the whole redshift range z ∼ 3 to z ∼ 6. Based on a comparison with semi-analytical models, we speculate that the z ∼ 5 LBGs might have a top-heavy IMF.

The Astrophysical Journal, 2008

We study cosmic variance in deep high redshift surveys and its influence on the determination of ... more We study cosmic variance in deep high redshift surveys and its influence on the determination of the luminosity function for high redshift galaxies. For several survey geometries relevant for HST and JWST instruments, we characterize the distribution of the galaxy number counts. This is obtained by means of analytic estimates via the two point correlation function in extended Press-Schechter theory as well as by using synthetic catalogs extracted from N-body cosmological simulations of structure formation. We adopt a simple luminosity -dark halo mass relation to investigate the environment effects on the fitting of the luminosity function. We show that in addition to variations of the normalization of the luminosity function, a steepening of its slope is also expected in underdense fields, similarly to what is observed within voids in the local universe. Therefore, to avoid introducing artificial biases, caution must be taken when attempting to correct for field underdensity, such as in the case of HST UDF i-dropout sample, which exhibits a deficit of bright counts with respect to the average counts in GOODS. A public version of the cosmic variance calculator based on the two point correlation function integration is made available on the web.

The Astrophysical Journal, 2008

In many theoretical scenarios it is expected that intermediate-mass black holes (IMBHs, with mass... more In many theoretical scenarios it is expected that intermediate-mass black holes (IMBHs, with masses M ∼ 10 2−4 M ) reside at the centers of some globular clusters. However, observational evidence for their existence is limited. Several previous numerical investigations have focused on the impact of an IMBH on the cluster dynamics or brightness profile. Here we instead present results from a large set of direct N-body simulations including single and binary stars. These show that there is a potentially more detectable IMBH signature, namely on the variation of the average stellar mass between the center and the half-light radius. We find that the existence of an IMBH quenches mass segregation and causes the average mass to exhibit only modest radial variation in collisionally relaxed star clusters. This differs from when there is no IMBH. To measure this observationally requires high resolution imaging at the level of that already available from the Hubble Space Telescope (HST) for the cores of a large sample of galactic globular clusters. With a modest additional investment of HST time to acquire fields around the half-light radius, it will be possible to identify the best candidate clusters to harbor an IMBH. This test can be applied only to globulars with a half-light relaxation time 1 Gyr, which is required to guarantee efficient energy equipartition due to two-body relaxation.

The Astrophysical Journal, 2010

Cosmological simulations of galaxy formation often rely on prescriptions for star formation and f... more Cosmological simulations of galaxy formation often rely on prescriptions for star formation and feedback that depend on halo properties such as halo mass, central over-density, and virial temperature. In this paper we address the convergence of individual halo properties, based on their number of particles N, focusing in particular on the mass of halos near the resolution limit of a simulation. While it has been established that the halo mass function is sampled on average down to N ∼ 20 − 30 particles, we show that individual halo properties exhibit significant scatter, and some systematic biases, as one approaches the resolution limit. We carry out a series of cosmological simulations using the Gadget2 and Enzo codes with N p = 64 3 to N p = 1024 3 total particles, keeping the same large-scale structure in the simulation box. We consider boxes of small (l box = 8 Mpc h −1 ), medium (l box = 64 Mpc h −1 ) and large (l box = 512 Mpc h −1 ) size to probe different halo masses and formation redshifts. We cross-identify dark matter halos in boxes at different resolutions and measure the scatter in their properties. The uncertainty in the mass of single halos depends on the number of particles (scaling approximately as N −1/3 ), but the rarer the density peak, the more robust its identification. The virial radius of halos is very stable and can be measured without bias for halos with N 30. In contrast, the average density within a sphere containing 25% of the total halo mass is severely underestimated (by more than a factor 2) and the halo spin is moderately overestimated for N 100. If sub-grid physics is implemented upon a cosmological simulation, we recommend that rare halos (∼ 3σ peaks) be resolved with N 100 particles and common halos (∼ 1σ peaks) with N 400 particles to avoid excessive numerical noise and possible systematic biases in the results.

The Astrophysical Journal, 2009

Theoretical investigations have suggested the presence of intermediate mass black holes (IMBHs, w... more Theoretical investigations have suggested the presence of intermediate mass black holes (IMBHs, with masses in the 100-10000 M range) in the cores of some globular clusters (GCs). In this paper, we present the first application of a new technique to determine the presence or absence of a central IMBH in globular clusters that have reached energy equipartition via two-body relaxation. The method is based on the measurement of the radial profile for the average mass of stars in the system, using the fact that a quenching of mass segregation is expected when an IMBH is present. Here, we measure the radial profile of mass segregation using main-sequence stars for the globular cluster NGC 2298 from resolved source photometry based on Hubble Space Telescope (HST/ACS) data. NGC 2298 is one of the smallest galactic globular clusters, thus not only it is dynamically relaxed but also a single ACS field of view extends to about twice its half-light radius, providing optimal radial coverage. The observations are compared to expectations from direct N-body simulations of the dynamics of star clusters with and without an IMBH. The mass segregation profile for NGC 2298 is quantitatively matched to that inferred from simulations without a central massive object over all the radial range probed by the observations, that is from the center to about two half-mass radii. Profiles from simulations containing an IMBH more massive than ≈300-500 M (depending on the assumed total mass of NGC 2298) are instead inconsistent with the data at about 3σ confidence, irrespective of the initial mass function and binary fraction chosen for these runs. Our finding is consistent with the currently favored formation scenarios for IMBHs in GCs, which are not likely to apply to NGC 2298 due to its modest total mass. While providing a null result in the quest of detecting a central black hole in globular clusters, the data-model comparison carried out here demonstrates the feasibility of the method which can also be applied to other globular clusters with resolved photometry in their cores.

The Astrophysical Journal, 2009

We detect three (plus one less certain) z 850 -dropout sources in two separate fields (HUDF and N... more We detect three (plus one less certain) z 850 -dropout sources in two separate fields (HUDF and NICP34) of our UDF05 HST NICMOS images. These z ∼ 7 Lyman-Break Galaxy (LBG) candidates allow us to constrain the Luminosity Function (LF) of the star forming galaxy population at those epochs. By assuming a change in only M * and adopting a linear evolution in redshift, anchored to the measured values at z ∼ 6, the best-fit evolution coefficient is found to be 0.43 ± 0.19 mag per unit redshift (0.36 ± 0.18, if including all four candidates), which provides a value of M * (z = 7.2) = −19.7 ± 0.3. This implies a drop of the luminosity density in LBGs by a factor of ∼ 2 − 2.5 over the ∼ 170 Myr that separate z ∼ 6 and z ∼ 7, and a steady evolution for the LBG LF out to z ∼ 7, at the same rate that is observed throughout the z ∼ 3 to 6 period. This puts a strong constraint on the star-formation histories of z ∼ 6 galaxies, whose ensemble star-formation rate density must be lower by a factor 2 at ∼170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z ∼ 6 LBG population must form at redshifts well above z ∼ 7. The rate of ionizing photons produced by the LBG population decreases consistently with the decrease in the cosmic star formation rate density. Extrapolating this steady evolution of the LF out to higher redshifts, we estimate that galaxies would be able to reionize the universe by z ∼ 6, provided that the faint-end slope of the z > 7 LF steepens to α ∼ −1.9, and that faint galaxies, with luminosities below the current detection limits, contribute a substantial fraction of the required ionizing photons. This scenario gives however an integrated optical depth to electron scattering that is ∼ 2σ below the WMAP-5 measurement. Therefore, altogether, our results indicate that, should galaxies be the primary contributors to reionization, either the currently detected evolution of the galaxy population slows down at z 7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.

The Astrophysical Journal, 2006

The Astrophysical Journal, 2013

Abstract: Knowledge of the mass distribution within galaxies is essential for our understanding o... more Abstract: Knowledge of the mass distribution within galaxies is essential for our understanding of their structure, but is challenging to acquire due to the complex spatial distribution of stellar populations. The advent of deep, high-resolution data over a wide range of wavelengths provides an unprecedented opportunity to measure spatial variations in stellar populations and quantify their effects on the mass-to-light distribution. In this Paper we present stellar mass surface density profiles of a mass-selected sample of galaxies at ...

The Astrophysical Journal, 2010

Studying the radial variation of the stellar mass function in globular clusters (GCs) has proved ... more Studying the radial variation of the stellar mass function in globular clusters (GCs) has proved a valuable tool to explore the collisional dynamics leading to mass segregation and core collapse. Recently, Pasquato et al. (2009) used the mass segregation profile to investigate the presence of an intermediate-mass black hole (IMBH) in NGC 2298. As a relaxed cluster with a large core, M 10 (NGC 6254) is suitable for a similar investigation. In order to study the radial dependence of the luminosity and mass function of M 10, we used deep high resolution archival images obtained with the Advanced Camera for Survey (ACS) on board the Hubble Space Telescope (HST), reaching out to approximately the cluster's half-mass radius (r hm ), combined with deep Wide Field and Planetary Camera 2 (WFPC2) images that extend our radial coverage to more than 2 r hm . From our photometry, we derived a radial mass segregation profile and a global mass function that we compared with those of simulated clusters containing different energy sources (namely hard binaries and/or an IMBH) able to halt core collapse and to quench mass segregation. A set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of the total cluster mass, comprising different initial mass functions (IMFs) and primordial binary fractions, was used to predict the observed mass segregation profile and mass function.

The Astrophysical Journal, 2010

We present a unified picture for the evolution of star clusters on the twobody relaxation timesca... more We present a unified picture for the evolution of star clusters on the twobody relaxation timescale. We use direct N-body simulations of star clusters in a galactic tidal field starting from different multi-mass King models, up to 10% of primordial binaries and up to N tot = 65536 particles. An additional run also includes a central Intermediate Mass Black Hole. We find that for the broad range of initial conditions we have studied the stellar mass function of these systems presents a universal evolution which depends only on the fractional mass loss. The structure of the system, as measured by the core to half mass radius ratio, also evolves toward a universal state, which is set by the efficiency of heating on the visible population of stars induced by dynamical interactions in the core of the system. Interactions with dark remnants (white dwarfs, neutron stars and stellar mass black holes) are dominant over the heating induced by a moderate population of primordial binaries (3-5%), especially under the assumption that most of the neutron stars and black holes are retained in the system. All our models without primordial binaries undergo a deep gravothermal collapse in the radial mass profile. However their projected light distribution can be well fitted by medium concentration King models (with parameter W 0 ∼ 8), even though there tends to be an excess over the best fit for the innermost points of the surface brightness. This excess is consistent with a shallow cusp in the surface brightness (µ ∼ R −ν with ν ∼ 0.4−0.7), like it has been observed for many globular clusters from high-resolution HST imaging. Generally fitting a King profile to derive the structural parameters yields to larger fluctuations in the core size than defining the core as the radius where the surface brightness is one half of its central value. Classification of core-collapsed globular clusters based on their surface brightness profile may thus fail in systems that appear to have already bounced back to lower concentrations, particularly if the angular resolution of the observations is limited and the core is not well resolved.

The Astrophysical Journal, 2011

Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than ... more Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than 10 12 M ⊙ and are thus expected to be surrounded by overdense regions. In a series of 10 constrained simulations, we have tested the environment of such QSOs. Comparing the computed overdensities with respect to the unconstrained simulations of regions empty of QSOs, assuming there is no bias between the DM and baryon distributions, and invoking an observationally-constrained duty-cycle for Lyman Break Galaxies, we have obtained the galaxy count number for the QSO environment. We find that a clear discrepancy exists between the computed and observed galaxy counts in the Kim et al. samples. Our simulations predict that on average eight z ∼ 6 galaxies per QSO field should have been observed, while Kim et al. detect on average four galaxies per QSO field compared to an average of three galaxies in a control sample (GOODS fields). While we cannot rule out a small number statistics for the observed fields to high confidence, the discrepancy suggests that galaxy formation in the QSO neighborhood proceeds differently than in the field. We also find that QSO halos are the most massive of the simulated volume at z ∼ 6 but this is no longer true at z ∼ 3. This implies that QSO halos, even in the case they are the most massive ones at high redshifts, do not evolve into most massive galaxy clusters at z = 0.

The Astrophysical Journal, 2011

In this paper, we present a derivation of the rest-frame 1400Å luminosity function (LF) at redshi... more In this paper, we present a derivation of the rest-frame 1400Å luminosity function (LF) at redshift six from a new application of the maximum likelihood method by exploring the five deepest HST/ACS fields, i.e., the HUDF, two UDF05 fields, and two GOODS fields. We work on the latest improved data products, which makes our results more robust than those of previous studies. We use un-binned data and thereby make optimal use of the information contained in the dataset. We focus on the analysis to a magnitude limit where the completeness is larger than 50% to avoid possibly large errors in the faint end slope that are difficult to quantify. We also take into account scattering in and out of the dropout sample due to photometric errors by defining for each object a probability that it belongs to the dropout sample. We find the best fit Schechter parameters to the z ∼ 6 LF are: α = 1.87 ± 0.14, M * = −20.25 ± 0.23, and φ * = 1.77 +0.62 −0.49 × 10 −3 Mpc −3 . Such a steep slope suggests that galaxies, especially the faint ones, are possibly the main sources of ionizing photons in the universe at redshift six. We also combine results from all studies at z ∼ 6 to reach an agreement in 95% confidence level that −20.45 < M * < −20.05 and −1.90 < α < −1.55. The luminosity density has been found not to evolve significantly between z ∼ 6 and z ∼ 5, but considerable evolution is detected from z ∼ 6 to z ∼ 3.

The Astrophysical Journal, 2009

The First Stars in the Universe form out of pristine primordial gas clouds that have been radiati... more The First Stars in the Universe form out of pristine primordial gas clouds that have been radiatively cooled to a few hundreds of degrees Kelvin either via molecular or atomic (Lyman-α) hydrogen lines. This primordial mode of star formation is eventually quenched once radiative and/or chemical (metal enrichment) feedbacks mark the transition to Population II stars. In this paper we present a model for the formation rate of Population III stars based on Press-Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model we derive the star formation history of Population III stars, their contribution to the re-ionization of the Universe and the time of the transition from Population III star formation in minihalos (M ≈ 10 6 M ⊙ , cooled via molecular hydrogen) to that in more massive halos (M 2 × 10 7 M ⊙ , where atomic hydrogen cooling is also possible). We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal free stars contribute only to a minor fraction of the total number of photons required to re-ionize the universe. In addition, metal free star formation is primarily located in minihalos and chemically enriched halos become the dominant locus of star formation very early in the life of the Universe -at redshift z ≈ 25 -even assuming a modest fraction (0.5%) of enriched gas converted in stars. If instead

The Astrophysical Journal, 2009

While the average metallicity of the intergalactic medium rises above Z 10 −3 Z ⊙ by the end of t... more While the average metallicity of the intergalactic medium rises above Z 10 −3 Z ⊙ by the end of the reionization, pockets of metal-free gas can still exist at later times. We quantify the presence of a long tail in the formation rate of metalfree halos during late stages of reionization (redshift z ≈ 6), which might offer the best window to detect Population III stars. Using cosmological simulations for the growth of dark matter halos, coupled with analytical recipes for the metal enrichment of their interstellar medium, we show that pockets of metal-free gas exist at z ≈ 6 even under the assumption of high efficiency in metal pollution via winds. A comoving metal-free halo formation rate d 2 n/dt dV 10 −9 Mpc −3 yr −1 is expected at z = 6 for halos with virial temperature T vir ≈ 10 4 K (mass ∼ 10 8 M ⊙ ), sufficient to initiate cooling even with strong negative radiative feedback. These halos will appear as absorption systems with a typical hydrogen -2column density of ∼ 10 20 cm −2 , a sky covering factor 5 × 10 −3 and a number density of 25 arcmin −2 for 5.5 ≤ z ≤ 6.5. Under the assumption of a single Population III supernova formed per metal-free halo, we expect an observed supernova rate of 2.6 × 10 −3 deg −2 yr −1 in the same redshift range. These metalfree stars and their supernovae will be isolated and outside galaxies (at distances 150 h −1 kpc) and thus significantly less biased than the general population of ∼ 10 8 M ⊙ halos at z ≈ 6. Supernova searches for metal-free explosions must thus rely on large area surveys. If metal-free stars produce very luminous supernovae, like SN2006gy, then a multi-epoch survey reaching m AB = 27 at 1 µm is sufficient for detecting them at z = 6. While the Large Synoptic Survey Telescope (LSST) will not reach this depth in the z band, it will be able to detect several tens of Population III supernovae in the i and r bands at z 5.5, when their observed rate is down to 3 − 8 × 10 −4 deg −2 yr −1 .

We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-f... more We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-forming galaxies at z~8-8.5, only 600 million years from recombination, using a Y_{105}-dropout selection. The new 4.7 arcmin**2 WFC3/IR observations reach to ~28.8 AB mag (5 sigma) in the Y_{105}J_{125}H_{160} bands. These remarkable data reach ~1.5 AB mag deeper than the previous data over the HUDF, and

A definitive detection of an intermediate-mass black hole (IMBH) would carry significant conseque... more A definitive detection of an intermediate-mass black hole (IMBH) would carry significant consequences for many sub-fields of astrophysics. As a result of the difficulties associated with direct detection of IMBH signatures, we continue the search for indirect evidence of their existence. In previous work, we described our method for suggesting the presence of an IMBH by examining the radial variations in the average mass of main sequence stars. This method can be applied to Galactic globular clusters that are dynamically old enough - those that have experienced at least 5 initial half-mass relaxation times - and that are not too influenced by the Galactic tidal field. Here, we present the results of the application of our method to NGC 2298 using ACS-HST photometry extending from the center to more than twice the half-light radius of the cluster. We find that the profile of mass segregation closely mimics those of our simulations without a large central mass, thus making it very unl...

Science (New York, N.Y.), Jan 6, 2015

In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a stro... more In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.

Recent extremely-deep, near-IR imaging (to 29.3 mag 5sigma) with HST WFC3/IR in the HUDF from our... more Recent extremely-deep, near-IR imaging (to 29.3 mag 5sigma) with HST WFC3/IR in the HUDF from our 192 orbit HUDF09 program have enabled a huge leap in our knowledge of the galaxies populating the early universe at z>7. These early galaxies are compelling targets for many reasons: as beacons of the first sites of star formation, as testbeds to constrain galaxy

We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over ... more We use the ultra-deep WFC3/IR data over the HUDF and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z~7. We determine the UV-continuum slope beta in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in beta. For luminous L*(z=3) galaxies, we measure a mean UV-continuum slope beta of -2.0+/-0.2, which is comparable to the beta~-2 derived at similar luminosities at z~5-6. However, for the lower luminosity 0.1L*(z=3) galaxies, we measure a mean beta of -3.0+/-0.2. This is substantially bluer than is found for similar luminosity galaxies at z~4, just 800 Myr later, and even at z~5-6. In principle, the observed beta of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected beta becomes >~-2.7. To produce these very blue beta's (i.e., beta~-3), extremely low metallicities...

We identify 73 z~7 and 59 z~8 candidate galaxies in the reionization epoch, and use this large 26... more We identify 73 z~7 and 59 z~8 candidate galaxies in the reionization epoch, and use this large 26-29.4 AB mag sample of galaxies to derive very deep luminosity functions to <-18 AB mag and the star formation rate density at z~7 and z~8. The galaxy sample is derived using a sophisticated Lyman-Break technique on the full two-year WFC3/IR and ACS data available over the HUDF09 (~29.4 AB mag, 5 sigma), two nearby HUDF09 fields (~29 AB mag, 14 arcmin) and the wider area ERS (~27.5 AB mag) ~40 arcmin**2). The application of strict optical non-detection criteria ensures the contamination fraction is kept low (just ~7% in the HUDF). This very low value includes a full assessment of the contamination from lower redshift sources, photometric scatter, AGN, spurious sources, low mass stars, and transients (e.g., SNe). From careful modelling of the selection volumes for each of our search fields we derive luminosity functions for galaxies at z~7 and z~8 to <-18 AB mag. The faint-end slopes alpha at z~7 and z~8 are uncertain but very steep at alpha = -2.01+/-0.21 and alpha=-1.91+/-0.32, respectively. Such steep slopes contrast to the local alpha<~-1.4 and may even be steeper than that at z~4 where alpha=-1.73+/-0.05. With such steep slopes (alpha<~-1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The star formation rate densities derived from these new z~7 and z~8 luminosity functions are consistent with the trends found at later times (lower redshifts). We find reasonable consistency, with the SFR densities implied from reported stellar mass densities, being only ~40% higher at z<7. This suggests that (1) the stellar mass densities inferred from the Spitzer IRAC photometry are reasonably accurate and (2) that the IMF at very high redshift may not be very different from that at later times.

The Astrophysical Journal, 2007

We present the UDF05 project, a HST Large Program of deep ACS (F606W, F775W, F850LP) and NICMOS (... more We present the UDF05 project, a HST Large Program of deep ACS (F606W, F775W, F850LP) and NICMOS (F110W, F160W) imaging of three fields, two of which coincide with the NICP1-4 NICMOS parallel observations of the Hubble Ultra Deep Field (HUDF). In this first paper we use the ACS data for the NICP12 field, as well as the original HUDF ACS data, to measure the UV Luminosity Function (LF) of z ∼ 5 Lyman Break Galaxies (LBGs) down to very faint levels. Specifically, based on a V − i, i − z selection criterion, we identify a sample of 101 and 133 candidate z ∼ 5 galaxies down to z 850 = 28.5 and 29.25 magnitudes in the NICP12 and in the HUDF fields, respectively. Using an extensive set of Monte Carlo simulations we derive corrections for observational biases and selection effects, and construct the rest-frame 1400Å LBG LF over the range M 1400 = [−21.4, −17.1], i.e. down to ∼ 0.04 L * at z ∼ 5, and complement it with data from the Subaru Deep Field (SDF) from to extend it to the brighter end (M 1400 ≥ −22.2). We show that: (i) Different assumptions regarding the SED distribution of the LBG population, dust properties and intergalactic absorption result in a 25% variation in the number density of LBGs at z ∼ 5; (ii) Under consistent assumptions for dust properties and intergalactic absorption, the HUDF is about 30% under-dense in z ∼ 5 LBGs relative to the NICP12 field, a variation which is well explained by cosmic variance; (iii) The faint-end slope of the LF is independent of the specific assumptions for the input physical parameters, and has a value of α ∼ −1.6, similar to the faint-end slope of the LF that has been measured for LBGs at z ∼ 3 and z ∼ 6. Our study therefore supports no variation in the faint-end of the LBG LF over the whole redshift range z ∼ 3 to z ∼ 6. Based on a comparison with semi-analytical models, we speculate that the z ∼ 5 LBGs might have a top-heavy IMF.

The Astrophysical Journal, 2008

We study cosmic variance in deep high redshift surveys and its influence on the determination of ... more We study cosmic variance in deep high redshift surveys and its influence on the determination of the luminosity function for high redshift galaxies. For several survey geometries relevant for HST and JWST instruments, we characterize the distribution of the galaxy number counts. This is obtained by means of analytic estimates via the two point correlation function in extended Press-Schechter theory as well as by using synthetic catalogs extracted from N-body cosmological simulations of structure formation. We adopt a simple luminosity -dark halo mass relation to investigate the environment effects on the fitting of the luminosity function. We show that in addition to variations of the normalization of the luminosity function, a steepening of its slope is also expected in underdense fields, similarly to what is observed within voids in the local universe. Therefore, to avoid introducing artificial biases, caution must be taken when attempting to correct for field underdensity, such as in the case of HST UDF i-dropout sample, which exhibits a deficit of bright counts with respect to the average counts in GOODS. A public version of the cosmic variance calculator based on the two point correlation function integration is made available on the web.

The Astrophysical Journal, 2008

In many theoretical scenarios it is expected that intermediate-mass black holes (IMBHs, with mass... more In many theoretical scenarios it is expected that intermediate-mass black holes (IMBHs, with masses M ∼ 10 2−4 M ) reside at the centers of some globular clusters. However, observational evidence for their existence is limited. Several previous numerical investigations have focused on the impact of an IMBH on the cluster dynamics or brightness profile. Here we instead present results from a large set of direct N-body simulations including single and binary stars. These show that there is a potentially more detectable IMBH signature, namely on the variation of the average stellar mass between the center and the half-light radius. We find that the existence of an IMBH quenches mass segregation and causes the average mass to exhibit only modest radial variation in collisionally relaxed star clusters. This differs from when there is no IMBH. To measure this observationally requires high resolution imaging at the level of that already available from the Hubble Space Telescope (HST) for the cores of a large sample of galactic globular clusters. With a modest additional investment of HST time to acquire fields around the half-light radius, it will be possible to identify the best candidate clusters to harbor an IMBH. This test can be applied only to globulars with a half-light relaxation time 1 Gyr, which is required to guarantee efficient energy equipartition due to two-body relaxation.

The Astrophysical Journal, 2010

Cosmological simulations of galaxy formation often rely on prescriptions for star formation and f... more Cosmological simulations of galaxy formation often rely on prescriptions for star formation and feedback that depend on halo properties such as halo mass, central over-density, and virial temperature. In this paper we address the convergence of individual halo properties, based on their number of particles N, focusing in particular on the mass of halos near the resolution limit of a simulation. While it has been established that the halo mass function is sampled on average down to N ∼ 20 − 30 particles, we show that individual halo properties exhibit significant scatter, and some systematic biases, as one approaches the resolution limit. We carry out a series of cosmological simulations using the Gadget2 and Enzo codes with N p = 64 3 to N p = 1024 3 total particles, keeping the same large-scale structure in the simulation box. We consider boxes of small (l box = 8 Mpc h −1 ), medium (l box = 64 Mpc h −1 ) and large (l box = 512 Mpc h −1 ) size to probe different halo masses and formation redshifts. We cross-identify dark matter halos in boxes at different resolutions and measure the scatter in their properties. The uncertainty in the mass of single halos depends on the number of particles (scaling approximately as N −1/3 ), but the rarer the density peak, the more robust its identification. The virial radius of halos is very stable and can be measured without bias for halos with N 30. In contrast, the average density within a sphere containing 25% of the total halo mass is severely underestimated (by more than a factor 2) and the halo spin is moderately overestimated for N 100. If sub-grid physics is implemented upon a cosmological simulation, we recommend that rare halos (∼ 3σ peaks) be resolved with N 100 particles and common halos (∼ 1σ peaks) with N 400 particles to avoid excessive numerical noise and possible systematic biases in the results.

The Astrophysical Journal, 2009

Theoretical investigations have suggested the presence of intermediate mass black holes (IMBHs, w... more Theoretical investigations have suggested the presence of intermediate mass black holes (IMBHs, with masses in the 100-10000 M range) in the cores of some globular clusters (GCs). In this paper, we present the first application of a new technique to determine the presence or absence of a central IMBH in globular clusters that have reached energy equipartition via two-body relaxation. The method is based on the measurement of the radial profile for the average mass of stars in the system, using the fact that a quenching of mass segregation is expected when an IMBH is present. Here, we measure the radial profile of mass segregation using main-sequence stars for the globular cluster NGC 2298 from resolved source photometry based on Hubble Space Telescope (HST/ACS) data. NGC 2298 is one of the smallest galactic globular clusters, thus not only it is dynamically relaxed but also a single ACS field of view extends to about twice its half-light radius, providing optimal radial coverage. The observations are compared to expectations from direct N-body simulations of the dynamics of star clusters with and without an IMBH. The mass segregation profile for NGC 2298 is quantitatively matched to that inferred from simulations without a central massive object over all the radial range probed by the observations, that is from the center to about two half-mass radii. Profiles from simulations containing an IMBH more massive than ≈300-500 M (depending on the assumed total mass of NGC 2298) are instead inconsistent with the data at about 3σ confidence, irrespective of the initial mass function and binary fraction chosen for these runs. Our finding is consistent with the currently favored formation scenarios for IMBHs in GCs, which are not likely to apply to NGC 2298 due to its modest total mass. While providing a null result in the quest of detecting a central black hole in globular clusters, the data-model comparison carried out here demonstrates the feasibility of the method which can also be applied to other globular clusters with resolved photometry in their cores.

The Astrophysical Journal, 2009

We detect three (plus one less certain) z 850 -dropout sources in two separate fields (HUDF and N... more We detect three (plus one less certain) z 850 -dropout sources in two separate fields (HUDF and NICP34) of our UDF05 HST NICMOS images. These z ∼ 7 Lyman-Break Galaxy (LBG) candidates allow us to constrain the Luminosity Function (LF) of the star forming galaxy population at those epochs. By assuming a change in only M * and adopting a linear evolution in redshift, anchored to the measured values at z ∼ 6, the best-fit evolution coefficient is found to be 0.43 ± 0.19 mag per unit redshift (0.36 ± 0.18, if including all four candidates), which provides a value of M * (z = 7.2) = −19.7 ± 0.3. This implies a drop of the luminosity density in LBGs by a factor of ∼ 2 − 2.5 over the ∼ 170 Myr that separate z ∼ 6 and z ∼ 7, and a steady evolution for the LBG LF out to z ∼ 7, at the same rate that is observed throughout the z ∼ 3 to 6 period. This puts a strong constraint on the star-formation histories of z ∼ 6 galaxies, whose ensemble star-formation rate density must be lower by a factor 2 at ∼170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z ∼ 6 LBG population must form at redshifts well above z ∼ 7. The rate of ionizing photons produced by the LBG population decreases consistently with the decrease in the cosmic star formation rate density. Extrapolating this steady evolution of the LF out to higher redshifts, we estimate that galaxies would be able to reionize the universe by z ∼ 6, provided that the faint-end slope of the z > 7 LF steepens to α ∼ −1.9, and that faint galaxies, with luminosities below the current detection limits, contribute a substantial fraction of the required ionizing photons. This scenario gives however an integrated optical depth to electron scattering that is ∼ 2σ below the WMAP-5 measurement. Therefore, altogether, our results indicate that, should galaxies be the primary contributors to reionization, either the currently detected evolution of the galaxy population slows down at z 7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.

The Astrophysical Journal, 2006

The Astrophysical Journal, 2013

Abstract: Knowledge of the mass distribution within galaxies is essential for our understanding o... more Abstract: Knowledge of the mass distribution within galaxies is essential for our understanding of their structure, but is challenging to acquire due to the complex spatial distribution of stellar populations. The advent of deep, high-resolution data over a wide range of wavelengths provides an unprecedented opportunity to measure spatial variations in stellar populations and quantify their effects on the mass-to-light distribution. In this Paper we present stellar mass surface density profiles of a mass-selected sample of galaxies at ...

The Astrophysical Journal, 2010

Studying the radial variation of the stellar mass function in globular clusters (GCs) has proved ... more Studying the radial variation of the stellar mass function in globular clusters (GCs) has proved a valuable tool to explore the collisional dynamics leading to mass segregation and core collapse. Recently, Pasquato et al. (2009) used the mass segregation profile to investigate the presence of an intermediate-mass black hole (IMBH) in NGC 2298. As a relaxed cluster with a large core, M 10 (NGC 6254) is suitable for a similar investigation. In order to study the radial dependence of the luminosity and mass function of M 10, we used deep high resolution archival images obtained with the Advanced Camera for Survey (ACS) on board the Hubble Space Telescope (HST), reaching out to approximately the cluster's half-mass radius (r hm ), combined with deep Wide Field and Planetary Camera 2 (WFPC2) images that extend our radial coverage to more than 2 r hm . From our photometry, we derived a radial mass segregation profile and a global mass function that we compared with those of simulated clusters containing different energy sources (namely hard binaries and/or an IMBH) able to halt core collapse and to quench mass segregation. A set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of the total cluster mass, comprising different initial mass functions (IMFs) and primordial binary fractions, was used to predict the observed mass segregation profile and mass function.

The Astrophysical Journal, 2010

We present a unified picture for the evolution of star clusters on the twobody relaxation timesca... more We present a unified picture for the evolution of star clusters on the twobody relaxation timescale. We use direct N-body simulations of star clusters in a galactic tidal field starting from different multi-mass King models, up to 10% of primordial binaries and up to N tot = 65536 particles. An additional run also includes a central Intermediate Mass Black Hole. We find that for the broad range of initial conditions we have studied the stellar mass function of these systems presents a universal evolution which depends only on the fractional mass loss. The structure of the system, as measured by the core to half mass radius ratio, also evolves toward a universal state, which is set by the efficiency of heating on the visible population of stars induced by dynamical interactions in the core of the system. Interactions with dark remnants (white dwarfs, neutron stars and stellar mass black holes) are dominant over the heating induced by a moderate population of primordial binaries (3-5%), especially under the assumption that most of the neutron stars and black holes are retained in the system. All our models without primordial binaries undergo a deep gravothermal collapse in the radial mass profile. However their projected light distribution can be well fitted by medium concentration King models (with parameter W 0 ∼ 8), even though there tends to be an excess over the best fit for the innermost points of the surface brightness. This excess is consistent with a shallow cusp in the surface brightness (µ ∼ R −ν with ν ∼ 0.4−0.7), like it has been observed for many globular clusters from high-resolution HST imaging. Generally fitting a King profile to derive the structural parameters yields to larger fluctuations in the core size than defining the core as the radius where the surface brightness is one half of its central value. Classification of core-collapsed globular clusters based on their surface brightness profile may thus fail in systems that appear to have already bounced back to lower concentrations, particularly if the angular resolution of the observations is limited and the core is not well resolved.

The Astrophysical Journal, 2011

Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than ... more Observed high-redshift QSOs, at z ∼ 6, may reside in massive dark matter (DM) halos of more than 10 12 M ⊙ and are thus expected to be surrounded by overdense regions. In a series of 10 constrained simulations, we have tested the environment of such QSOs. Comparing the computed overdensities with respect to the unconstrained simulations of regions empty of QSOs, assuming there is no bias between the DM and baryon distributions, and invoking an observationally-constrained duty-cycle for Lyman Break Galaxies, we have obtained the galaxy count number for the QSO environment. We find that a clear discrepancy exists between the computed and observed galaxy counts in the Kim et al. samples. Our simulations predict that on average eight z ∼ 6 galaxies per QSO field should have been observed, while Kim et al. detect on average four galaxies per QSO field compared to an average of three galaxies in a control sample (GOODS fields). While we cannot rule out a small number statistics for the observed fields to high confidence, the discrepancy suggests that galaxy formation in the QSO neighborhood proceeds differently than in the field. We also find that QSO halos are the most massive of the simulated volume at z ∼ 6 but this is no longer true at z ∼ 3. This implies that QSO halos, even in the case they are the most massive ones at high redshifts, do not evolve into most massive galaxy clusters at z = 0.

The Astrophysical Journal, 2011

In this paper, we present a derivation of the rest-frame 1400Å luminosity function (LF) at redshi... more In this paper, we present a derivation of the rest-frame 1400Å luminosity function (LF) at redshift six from a new application of the maximum likelihood method by exploring the five deepest HST/ACS fields, i.e., the HUDF, two UDF05 fields, and two GOODS fields. We work on the latest improved data products, which makes our results more robust than those of previous studies. We use un-binned data and thereby make optimal use of the information contained in the dataset. We focus on the analysis to a magnitude limit where the completeness is larger than 50% to avoid possibly large errors in the faint end slope that are difficult to quantify. We also take into account scattering in and out of the dropout sample due to photometric errors by defining for each object a probability that it belongs to the dropout sample. We find the best fit Schechter parameters to the z ∼ 6 LF are: α = 1.87 ± 0.14, M * = −20.25 ± 0.23, and φ * = 1.77 +0.62 −0.49 × 10 −3 Mpc −3 . Such a steep slope suggests that galaxies, especially the faint ones, are possibly the main sources of ionizing photons in the universe at redshift six. We also combine results from all studies at z ∼ 6 to reach an agreement in 95% confidence level that −20.45 < M * < −20.05 and −1.90 < α < −1.55. The luminosity density has been found not to evolve significantly between z ∼ 6 and z ∼ 5, but considerable evolution is detected from z ∼ 6 to z ∼ 3.

The Astrophysical Journal, 2009

The First Stars in the Universe form out of pristine primordial gas clouds that have been radiati... more The First Stars in the Universe form out of pristine primordial gas clouds that have been radiatively cooled to a few hundreds of degrees Kelvin either via molecular or atomic (Lyman-α) hydrogen lines. This primordial mode of star formation is eventually quenched once radiative and/or chemical (metal enrichment) feedbacks mark the transition to Population II stars. In this paper we present a model for the formation rate of Population III stars based on Press-Schechter modeling coupled with analytical recipes for gas cooling and radiative feedback. Our model also includes a novel treatment for metal pollution based on self-enrichment due to a previous episode of Population III star formation in progenitor halos. With this model we derive the star formation history of Population III stars, their contribution to the re-ionization of the Universe and the time of the transition from Population III star formation in minihalos (M ≈ 10 6 M ⊙ , cooled via molecular hydrogen) to that in more massive halos (M 2 × 10 7 M ⊙ , where atomic hydrogen cooling is also possible). We consider a grid of models highlighting the impact of varying the values for the free parameters used, such as star formation and feedback efficiency. The most critical factor is the assumption that only one Population III star is formed in a halo. In this scenario, metal free stars contribute only to a minor fraction of the total number of photons required to re-ionize the universe. In addition, metal free star formation is primarily located in minihalos and chemically enriched halos become the dominant locus of star formation very early in the life of the Universe -at redshift z ≈ 25 -even assuming a modest fraction (0.5%) of enriched gas converted in stars. If instead

The Astrophysical Journal, 2009

While the average metallicity of the intergalactic medium rises above Z 10 −3 Z ⊙ by the end of t... more While the average metallicity of the intergalactic medium rises above Z 10 −3 Z ⊙ by the end of the reionization, pockets of metal-free gas can still exist at later times. We quantify the presence of a long tail in the formation rate of metalfree halos during late stages of reionization (redshift z ≈ 6), which might offer the best window to detect Population III stars. Using cosmological simulations for the growth of dark matter halos, coupled with analytical recipes for the metal enrichment of their interstellar medium, we show that pockets of metal-free gas exist at z ≈ 6 even under the assumption of high efficiency in metal pollution via winds. A comoving metal-free halo formation rate d 2 n/dt dV 10 −9 Mpc −3 yr −1 is expected at z = 6 for halos with virial temperature T vir ≈ 10 4 K (mass ∼ 10 8 M ⊙ ), sufficient to initiate cooling even with strong negative radiative feedback. These halos will appear as absorption systems with a typical hydrogen -2column density of ∼ 10 20 cm −2 , a sky covering factor 5 × 10 −3 and a number density of 25 arcmin −2 for 5.5 ≤ z ≤ 6.5. Under the assumption of a single Population III supernova formed per metal-free halo, we expect an observed supernova rate of 2.6 × 10 −3 deg −2 yr −1 in the same redshift range. These metalfree stars and their supernovae will be isolated and outside galaxies (at distances 150 h −1 kpc) and thus significantly less biased than the general population of ∼ 10 8 M ⊙ halos at z ≈ 6. Supernova searches for metal-free explosions must thus rely on large area surveys. If metal-free stars produce very luminous supernovae, like SN2006gy, then a multi-epoch survey reaching m AB = 27 at 1 µm is sufficient for detecting them at z = 6. While the Large Synoptic Survey Telescope (LSST) will not reach this depth in the z band, it will be able to detect several tens of Population III supernovae in the i and r bands at z 5.5, when their observed rate is down to 3 − 8 × 10 −4 deg −2 yr −1 .