Charles Steinhardt - Academia.edu (original) (raw)

Papers by Charles Steinhardt

The Astrophysical Journal

One of the primary goals for the upcoming James Webb Space Telescope is to observe the first gala... more One of the primary goals for the upcoming James Webb Space Telescope is to observe the first galaxies. Predictions for planned and proposed surveys have typically focused on average galaxy counts, assuming a random distribution of galaxies across the observed field. The first and most-massive galaxies, however, are expected to be tightly clustered, an effect known as cosmic variance. We show that cosmic variance is likely to be the dominant contribution to uncertainty for high-redshift mass and luminosity functions, and that median high-redshift and high-mass galaxy counts for planned observations lie significantly below average counts. Several different strategies are considered for improving our understanding of the first galaxies, including adding depth, area, and independent pointings. Adding independent pointings is shown to be the most efficient both for discovering the single highest-redshift galaxy and also for constraining mass and luminosity functions.

The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. Th... more The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. This consists of 2099 square degrees of five-band (u g r i z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 square degrees of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ≈ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 milli-arcsec rms per coordinate, respectively. The spectra cover the range 3800–9200Å, with a resolution of 1800–2100. Further characteristics of the data are described, as are the data products themselves. Subject headings: Atlases—Catalogs—Surveys Enrico Fermi Institute, The University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 Lawrence Berkeley National Laboratory, One Cyclotron Rd., Berkeley CA 94720-8160 Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QJ, United Kingdom Department of Physics, Universit...

The Astrophysical Journal, 2020

Redshifts used in current cosmological supernova samples are measured using two primary technique... more Redshifts used in current cosmological supernova samples are measured using two primary techniques, one based on well-measured host galaxy spectral lines and the other based on supernova-dominated spectra. Here, we construct an updated Pantheon catalog with revised redshifts, redshift sources, and estimated uncertainties for the entire sample to investigate whether these two techniques yield consistent results. The best-fit cosmological parameters using these two measurement techniques disagree, with a supernova-only sample producing Ω m 3.2σ higher and H 0 2.5σ lower than a hostz-only sample, and we explore several possible sources of bias that could result from using the lower-precision supernova-dominated redshifts. In a pilot study, we show that using a host redshift-only subsample will generically produce lower Ω m and matter density Ω m h 2 and slightly higher H 0 than previous analysis which, for the Pantheon data set, could result in supernova and cosmic microwave background...

Monthly Notices of the Royal Astronomical Society, 2021

We use the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) strong lensing ima... more We use the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) strong lensing image catalogue of the merging galaxy cluster Abell 370 to obtain a mass model using free-form lens inversion algorithm grale. The improvement of the strong lensing data quality results in a lens plane rms of only 0.45 arcsec, about a factor of two lower than that of our existing HFF v4 reconstruction. We attribute the improvement to spectroscopic data and use of the full reprocessed HST mosaics. In our reconstructed mass model, we found indications of three distinct mass features in Abell 370: (i) a ∼35 kpc offset between the northern BCG and the nearest mass peak, (ii) a ∼100 kpc mass concentration of roughly critical density ∼250 kpc east of the main cluster, and (iii) a probable filament-like structure passing N-S through the cluster. While (i) is present in some form in most publicly available reconstructions spanning the range of modelling techniques: parametric, hybrid, and free-form...

The Astrophysical Journal Supplement Series, 2020

The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 para... more The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 parallel Cycle 25 Hubble Space Telescope Treasury program taking data from 2018-2020. BUFFALO will expand existing coverage of the Hubble Frontier Fields (HFF) in WFC3/IR F105W, F125W, and F160W and ACS/WFC F606W and F814W around each of the six HFF clusters and flanking fields. This additional area has not been observed by HST but is already covered by deep multi-wavelength datasets, including Spitzer and Chandra. As with the original HFF program, BUFFALO is designed to take advantage of gravitational lensing from massive clusters to simultaneously find high-redshift galaxies which would otherwise lie below HST detection limits and model foreground clusters to study properties of dark matter and galaxy assembly. The expanded area will provide a first opportunity to study both cosmic variance at high redshift and galaxy assembly in the outskirts of the large HFF clusters. Five additional orbits are reserved for transient followup. BUFFALO data including mosaics, value-added catalogs and cluster mass distribution models will be released via MAST on a regular basis, as the observations and analysis are completed for the six individual clusters.

The Astrophysical Journal, 2020

Large photometric surveys provide a rich source of observations of quiescent galaxies, including ... more Large photometric surveys provide a rich source of observations of quiescent galaxies, including a surprisingly large population at z>1. However, identifying large, but clean, samples of quiescent galaxies has proven difficult because of their near-degeneracy with interlopers such as dusty, star-forming galaxies. We describe a new technique for selecting quiescent galaxies based upon t-distributed stochastic neighbor embedding (t-SNE), an unsupervised machine-learning algorithm for dimensionality reduction. This t-SNE selection provides an improvement both over UVJ, removing interlopers that otherwise would pass color selection, and over photometric template fitting, more strongly toward high redshift. Due to the similarity between the colors of high-and lowredshift quiescent galaxies, under our assumptions, t-SNE outperforms template fitting in 63% of trials at redshifts where a large training sample already exists. It also may be able to select quiescent galaxies more efficiently at higher redshifts than the training sample.

The Astrophysical Journal, 2020

We argue that the interplay between cosmic rays, the initial mass function, and star formation pl... more We argue that the interplay between cosmic rays, the initial mass function, and star formation plays a crucial role in regulating the star-forming "main sequence". To explore these phenomena we develop a toy model for galaxy evolution in which star formation is regulated by a combination of a temperature-dependent initial mass function and heating due to starlight, cosmic rays and, at very high redshift, the cosmic microwave background. This produces an attractor, near-equilibrium solution which is consistent with observations of the star-forming main sequence over a broad redshift range. Additional solutions to the same equations may correspond to other observed phases of galaxy evolution including quiescent galaxies. This model makes several falsifiable predictions, including higher metallicities and dust masses than anticipated at high redshift and isotopic abundances in the Milky Way. It also predicts that stellar mass-to-light ratios are lower than produced using a Milky Way-derived IMF, so that inferences of stellar masses and star formation rates for high redshift galaxies are overestimated. In some cases, this may also transform inferred dark matter profiles from core-like to cusp-like.

The Astrophysical Journal, 2020

 Users may download and print one copy of any publication from the public portal for the purpose... more  Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Nature, Jun 21, 2017

At redshift z = 2, when the Universe was just three billion years old, half of the most massive g... more At redshift z = 2, when the Universe was just three billion years old, half of the most massive galaxies were extremely compact and had already exhausted their fuel for star formation. It is believed that they were formed in intense nuclear starbursts and that they ultimately grew into the most massive local elliptical galaxies seen today, through mergers with minor companions, but validating this picture requires higher-resolution observations of their centres than is currently possible. Magnification from gravitational lensing offers an opportunity to resolve the inner regions of galaxies. Here we report an analysis of the stellar populations and kinematics of a lensed z = 2.1478 compact galaxy, which-surprisingly-turns out to be a fast-spinning, rotationally supported disk galaxy. Its stars must have formed in a disk, rather than in a merger-driven nuclear starburst. The galaxy was probably fed by streams of cold gas, which were able to penetrate the hot halo gas until they were ...

The Astrophysical Journal, 2015

We investigate the Eddington ratio distribution of X-ray selected broad-line active galactic nucl... more We investigate the Eddington ratio distribution of X-ray selected broad-line active galactic nuclei (AGN) in the redshift range 1.0 < z < 2.2, where the number density of AGNs peaks. Combining the optical and Subaru/FMOS near-infrared spectroscopy, we estimate black hole masses for broad-line AGNs in the Chandra Deep Field-South (CDF-S), Extended Chandra Deep Field-South (E-CDF-S), and the XMM-Newton Lockman Hole (XMM-LH) surveys. AGNs with similar black hole masses show a broad range of AGN bolometric luminosities, which are calculated from X-ray luminosities, indicating that the accretion rate of black holes is widely distributed. We find that a substantial fraction of massive black holes accreting significantly below the Eddington limit at z 2, in contrast to what is generally found for luminous AGNs at high redshift. Our analysis of observational selection biases indicates that the "AGN cosmic downsizing" phenomenon can be simply explained by the strong evolution of the co-moving number density at the bright end of the AGN luminosity function, together with the corresponding selection effects. However, it might need to consider a correlation between the AGN luminosity and the accretion rate of black holes that luminous AGNs have higher Eddington ratios than low-luminosity AGNs in order to understand the relatively small fraction of low-luminosity AGNs with high accretion rates in this epoch. Therefore, the observed downsizing trend could be interpreted as massive black holes with low accretion rates, which are relatively fainter than less massive black holes with efficient accretion.

The Astrophysical Journal, 2015

Calibrating the photometric redshifts of 10 9 galaxies for upcoming weak lensing cosmology experi... more Calibrating the photometric redshifts of 10 9 galaxies for upcoming weak lensing cosmology experiments is a major challenge for the astrophysics community. The path to obtaining the required spectroscopic redshifts for training and calibration is daunting, given the anticipated depths of the surveys and the difficulty in obtaining secure redshifts for some faint galaxy populations. Here we present an analysis of the problem based on the self-organizing map, a method of mapping the distribution of data in a high-dimensional space and projecting it onto a lower-dimensional representation. We apply this method to existing photometric data from the COSMOS survey selected to approximate the anticipated Euclid weak lensing sample, enabling us to robustly map the empirical distribution of galaxies in the multidimensional color space defined by the expected Euclid filters. Mapping this multicolor distribution lets us determine where-in galaxy color space-redshifts from current spectroscopic surveys exist and where they are systematically missing. Crucially, the method lets us determine whether a spectroscopic training sample is representative of the full photometric space occupied by the galaxies in a survey. We explore optimal sampling techniques and estimate the additional spectroscopy needed to map out the color-redshift relation, finding that sampling the galaxy distribution in color space in a systematic way can efficiently meet the calibration requirements. While the analysis presented here focuses on the Euclid survey, similar analysis can be applied to other surveys facing the same calibration challenge, such as DES, LSST, and WFIRST.

The unprecedented large number of quasar candidates included in the latest data release of SDSS (... more The unprecedented large number of quasar candidates included in the latest data release of SDSS (DR5) make it possible to constrain models for quasar evolution by mapping the quasars in a time-luminosity plane. Analyzing the features of the rise and fall of this map, as well as other spectroscopic information from SDSS like the mass obtained from reverberation methods, we can infer typical individual quasar trajectories starting with their formation, growth predominantly by baryonic accretion followed by the luminosity decay. The remarkable smoothness of the luminosity decay for the overall sample of quasars following a long period of our Universe's evolution suggests that there are more similarities than differences in the behavior of quasars at high red-shifts and low red-shifts.

With the emergence of moderately detailed evolutionary models for supermassive black hole growth ... more With the emergence of moderately detailed evolutionary models for supermassive black hole growth in galaxies it is becoming possible to predict evolutionary tracks for the rapid growth phase seen as quasars in, for example, the Mass-Luminosity plane. The Sloan Digital Sky Survey includes 77,429 quasar candidates in their latest data release (DR5). We estimate SMBH masses using the standard method based on reverberation mapping: we use H-beta, MgII, and CIV broad emission line widths and continuum luminosities to construct mass luminosity distributions of SDSS quasars as a function of redshift. This distribution shows several characteristic features. We show typical tracks in this plane for quasars growing predominantly due to luminous accretion. We further use the evolution of the mass-luminosity distribution with redshift to determine how these tracks have changed over time and whether high redshift quasars behave similarly to low-redshift quasars.

This thesis investigates the quasar mass-luminosity plane, as a new tool to explore the relations... more This thesis investigates the quasar mass-luminosity plane, as a new tool to explore the relationship between black hole mass and quasar luminosity over time. Previous techniques used quasar luminosity function and mass functions, which are one-dimensional projections of the mass-luminosity plane. The M --- L plane contains information that cannot be seen in these projections. We use 62,185 quasars from the Sloan Digital Sky Survey DR5 sample to develop several new constraints on quasar accretion. Black hole masses, based on the widths of their Hbeta, Mg II, and C IV lines and adjacent continuum luminosities, were used assuming using standard virial mass estimate scaling laws. In each redshift interval over the range 0.2 < z < 4.0, low-mass quasars reach at their Eddington luminosity, but high-mass quasars fall short, even by a factor of ten or more at 0.2 < z < 0.6. We examine several potential sources of measurement uncertainty or bias and show that none of them can acc...

Comment on "Biases in the Quasar Mass-Luminosity Plane"

We consider the formation of dwarf carbon stars via accretion from a carbon AGB companion in ligh... more We consider the formation of dwarf carbon stars via accretion from a carbon AGB companion in light of the new 107 object sample of Downes et al. (2004). This sample is now large enough to allow good mass determination via comparison of a composite spectrum to theoretical atmospheric models. Carbon dwarfs of spectral type M are indeed main sequence M dwarfs with enhanced metallicity and carbon abundance. We also calculate the predicted abundance of both M and of F/G carbon dwarfs, and show that the latter should be falsifiable in the near future.

Anomalous narrow-line quasars are a new class of Type I quasars with narrow Hβ broader than 1200 ... more Anomalous narrow-line quasars are a new class of Type I quasars with narrow Hβ broader than 1200 km/s, above the velocity believed possible for gas in the quasar narrow-line region. This broadening is tightly correlated with changes in other spectral features, so that the set of ANLs is a distinct population difficult to explain with the current quasar standard model. Further, ANLs comprise over one quarter of all quasars at 0.2 < z < 0.8, so they must be accounted for as part of our understanding of supermassive black hole evolution. Because other quasar narrow lines such as [O II] are diminished or disappear entirely, ANLs raise several fundamental questions about the connection between the black hole and the central region of the galaxy. They also appear inconsistent with a predominantly virial broad-line region, calling into question the technique of virial mass estimation.

We describe a new class of Type I quasars with narrow H\beta\ broader than 1200 km/s, above the v... more We describe a new class of Type I quasars with narrow H\beta\ broader than 1200 km/s, above the velocity believed possible for gas in the quasar narrow-line region. We identify this group of quasars as a distinct population because of a variety of spectral and photometric signatures common to these "anomalous narrow-line quasars" (ANLs) but atypical of other quasars. One prominent signature is suppression of the [OIII]\lambda ?4959, 5007 emission-line, in many cases accompanied by broadening similar to the H\beta\ line. We demonstrate that ANLs comprise at least 11% and most likely approximately one quarter of the SDSS Type I quasar population at 0.2 < z < 0.8. The most striking feature of ANLs is a strong correlation between narrow H\beta\ width and the width of the broad component of the H\beta\ emission line. Another feature of ANLs is a diminished [OII] line, which might indicate a connection between ANLs and the interstellar mediums of their host galaxies, throu...

The Astrophysical Journal, 2015

We use observed optical to near infrared spectral energy distributions (SEDs) of 266 galaxies in ... more We use observed optical to near infrared spectral energy distributions (SEDs) of 266 galaxies in the COSMOS survey to derive the wavelength dependence of the dust attenuation at high redshift. All of the galaxies have spectroscopic redshifts in the range z = 2 to 6.5. The presence of the CIV absorption feature, indicating that the rest-frame UV-optical SED is dominated by OB stars, is used to select objects for which the intrinsic, unattenuated spectrum has a well-established shape. Comparison of this intrinsic spectrum with the observed broadband photometric SED then permits derivation of the wavelength dependence of the dust attenuation. The derived dust attenuation curve is similar in overall shape to the Calzetti curve for local starburst galaxies. We also see the 2175Å bump feature which is present in the Milky Way and LMC extinction curves but not seen in the Calzetti curve. The bump feature is commonly attributed to graphite or PAHs. No significant dependence is seen with redshift between sub-samples at z = 2-4 and z = 4-6.5. The 'extinction' curve obtained here provides a firm basis for color and extinction corrections of high redshift galaxy photometry.

The Astrophysical Journal

One of the primary goals for the upcoming James Webb Space Telescope is to observe the first gala... more One of the primary goals for the upcoming James Webb Space Telescope is to observe the first galaxies. Predictions for planned and proposed surveys have typically focused on average galaxy counts, assuming a random distribution of galaxies across the observed field. The first and most-massive galaxies, however, are expected to be tightly clustered, an effect known as cosmic variance. We show that cosmic variance is likely to be the dominant contribution to uncertainty for high-redshift mass and luminosity functions, and that median high-redshift and high-mass galaxy counts for planned observations lie significantly below average counts. Several different strategies are considered for improving our understanding of the first galaxies, including adding depth, area, and independent pointings. Adding independent pointings is shown to be the most efficient both for discovering the single highest-redshift galaxy and also for constraining mass and luminosity functions.

The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. Th... more The Sloan Digital Sky Survey has validated and made publicly available its First Data Release. This consists of 2099 square degrees of five-band (u g r i z) imaging data, 186,240 spectra of galaxies, quasars, stars and calibrating blank sky patches selected over 1360 square degrees of this area, and tables of measured parameters from these data. The imaging data go to a depth of r ≈ 22.6 and are photometrically and astrometrically calibrated to 2% rms and 100 milli-arcsec rms per coordinate, respectively. The spectra cover the range 3800–9200Å, with a resolution of 1800–2100. Further characteristics of the data are described, as are the data products themselves. Subject headings: Atlases—Catalogs—Surveys Enrico Fermi Institute, The University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 Lawrence Berkeley National Laboratory, One Cyclotron Rd., Berkeley CA 94720-8160 Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QJ, United Kingdom Department of Physics, Universit...

The Astrophysical Journal, 2020

Redshifts used in current cosmological supernova samples are measured using two primary technique... more Redshifts used in current cosmological supernova samples are measured using two primary techniques, one based on well-measured host galaxy spectral lines and the other based on supernova-dominated spectra. Here, we construct an updated Pantheon catalog with revised redshifts, redshift sources, and estimated uncertainties for the entire sample to investigate whether these two techniques yield consistent results. The best-fit cosmological parameters using these two measurement techniques disagree, with a supernova-only sample producing Ω m 3.2σ higher and H 0 2.5σ lower than a hostz-only sample, and we explore several possible sources of bias that could result from using the lower-precision supernova-dominated redshifts. In a pilot study, we show that using a host redshift-only subsample will generically produce lower Ω m and matter density Ω m h 2 and slightly higher H 0 than previous analysis which, for the Pantheon data set, could result in supernova and cosmic microwave background...

Monthly Notices of the Royal Astronomical Society, 2021

We use the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) strong lensing ima... more We use the Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) strong lensing image catalogue of the merging galaxy cluster Abell 370 to obtain a mass model using free-form lens inversion algorithm grale. The improvement of the strong lensing data quality results in a lens plane rms of only 0.45 arcsec, about a factor of two lower than that of our existing HFF v4 reconstruction. We attribute the improvement to spectroscopic data and use of the full reprocessed HST mosaics. In our reconstructed mass model, we found indications of three distinct mass features in Abell 370: (i) a ∼35 kpc offset between the northern BCG and the nearest mass peak, (ii) a ∼100 kpc mass concentration of roughly critical density ∼250 kpc east of the main cluster, and (iii) a probable filament-like structure passing N-S through the cluster. While (i) is present in some form in most publicly available reconstructions spanning the range of modelling techniques: parametric, hybrid, and free-form...

The Astrophysical Journal Supplement Series, 2020

The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 para... more The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 parallel Cycle 25 Hubble Space Telescope Treasury program taking data from 2018-2020. BUFFALO will expand existing coverage of the Hubble Frontier Fields (HFF) in WFC3/IR F105W, F125W, and F160W and ACS/WFC F606W and F814W around each of the six HFF clusters and flanking fields. This additional area has not been observed by HST but is already covered by deep multi-wavelength datasets, including Spitzer and Chandra. As with the original HFF program, BUFFALO is designed to take advantage of gravitational lensing from massive clusters to simultaneously find high-redshift galaxies which would otherwise lie below HST detection limits and model foreground clusters to study properties of dark matter and galaxy assembly. The expanded area will provide a first opportunity to study both cosmic variance at high redshift and galaxy assembly in the outskirts of the large HFF clusters. Five additional orbits are reserved for transient followup. BUFFALO data including mosaics, value-added catalogs and cluster mass distribution models will be released via MAST on a regular basis, as the observations and analysis are completed for the six individual clusters.

The Astrophysical Journal, 2020

Large photometric surveys provide a rich source of observations of quiescent galaxies, including ... more Large photometric surveys provide a rich source of observations of quiescent galaxies, including a surprisingly large population at z>1. However, identifying large, but clean, samples of quiescent galaxies has proven difficult because of their near-degeneracy with interlopers such as dusty, star-forming galaxies. We describe a new technique for selecting quiescent galaxies based upon t-distributed stochastic neighbor embedding (t-SNE), an unsupervised machine-learning algorithm for dimensionality reduction. This t-SNE selection provides an improvement both over UVJ, removing interlopers that otherwise would pass color selection, and over photometric template fitting, more strongly toward high redshift. Due to the similarity between the colors of high-and lowredshift quiescent galaxies, under our assumptions, t-SNE outperforms template fitting in 63% of trials at redshifts where a large training sample already exists. It also may be able to select quiescent galaxies more efficiently at higher redshifts than the training sample.

The Astrophysical Journal, 2020

We argue that the interplay between cosmic rays, the initial mass function, and star formation pl... more We argue that the interplay between cosmic rays, the initial mass function, and star formation plays a crucial role in regulating the star-forming "main sequence". To explore these phenomena we develop a toy model for galaxy evolution in which star formation is regulated by a combination of a temperature-dependent initial mass function and heating due to starlight, cosmic rays and, at very high redshift, the cosmic microwave background. This produces an attractor, near-equilibrium solution which is consistent with observations of the star-forming main sequence over a broad redshift range. Additional solutions to the same equations may correspond to other observed phases of galaxy evolution including quiescent galaxies. This model makes several falsifiable predictions, including higher metallicities and dust masses than anticipated at high redshift and isotopic abundances in the Milky Way. It also predicts that stellar mass-to-light ratios are lower than produced using a Milky Way-derived IMF, so that inferences of stellar masses and star formation rates for high redshift galaxies are overestimated. In some cases, this may also transform inferred dark matter profiles from core-like to cusp-like.

The Astrophysical Journal, 2020

 Users may download and print one copy of any publication from the public portal for the purpose... more  Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Nature, Jun 21, 2017

At redshift z = 2, when the Universe was just three billion years old, half of the most massive g... more At redshift z = 2, when the Universe was just three billion years old, half of the most massive galaxies were extremely compact and had already exhausted their fuel for star formation. It is believed that they were formed in intense nuclear starbursts and that they ultimately grew into the most massive local elliptical galaxies seen today, through mergers with minor companions, but validating this picture requires higher-resolution observations of their centres than is currently possible. Magnification from gravitational lensing offers an opportunity to resolve the inner regions of galaxies. Here we report an analysis of the stellar populations and kinematics of a lensed z = 2.1478 compact galaxy, which-surprisingly-turns out to be a fast-spinning, rotationally supported disk galaxy. Its stars must have formed in a disk, rather than in a merger-driven nuclear starburst. The galaxy was probably fed by streams of cold gas, which were able to penetrate the hot halo gas until they were ...

The Astrophysical Journal, 2015

We investigate the Eddington ratio distribution of X-ray selected broad-line active galactic nucl... more We investigate the Eddington ratio distribution of X-ray selected broad-line active galactic nuclei (AGN) in the redshift range 1.0 < z < 2.2, where the number density of AGNs peaks. Combining the optical and Subaru/FMOS near-infrared spectroscopy, we estimate black hole masses for broad-line AGNs in the Chandra Deep Field-South (CDF-S), Extended Chandra Deep Field-South (E-CDF-S), and the XMM-Newton Lockman Hole (XMM-LH) surveys. AGNs with similar black hole masses show a broad range of AGN bolometric luminosities, which are calculated from X-ray luminosities, indicating that the accretion rate of black holes is widely distributed. We find that a substantial fraction of massive black holes accreting significantly below the Eddington limit at z 2, in contrast to what is generally found for luminous AGNs at high redshift. Our analysis of observational selection biases indicates that the "AGN cosmic downsizing" phenomenon can be simply explained by the strong evolution of the co-moving number density at the bright end of the AGN luminosity function, together with the corresponding selection effects. However, it might need to consider a correlation between the AGN luminosity and the accretion rate of black holes that luminous AGNs have higher Eddington ratios than low-luminosity AGNs in order to understand the relatively small fraction of low-luminosity AGNs with high accretion rates in this epoch. Therefore, the observed downsizing trend could be interpreted as massive black holes with low accretion rates, which are relatively fainter than less massive black holes with efficient accretion.

The Astrophysical Journal, 2015

Calibrating the photometric redshifts of 10 9 galaxies for upcoming weak lensing cosmology experi... more Calibrating the photometric redshifts of 10 9 galaxies for upcoming weak lensing cosmology experiments is a major challenge for the astrophysics community. The path to obtaining the required spectroscopic redshifts for training and calibration is daunting, given the anticipated depths of the surveys and the difficulty in obtaining secure redshifts for some faint galaxy populations. Here we present an analysis of the problem based on the self-organizing map, a method of mapping the distribution of data in a high-dimensional space and projecting it onto a lower-dimensional representation. We apply this method to existing photometric data from the COSMOS survey selected to approximate the anticipated Euclid weak lensing sample, enabling us to robustly map the empirical distribution of galaxies in the multidimensional color space defined by the expected Euclid filters. Mapping this multicolor distribution lets us determine where-in galaxy color space-redshifts from current spectroscopic surveys exist and where they are systematically missing. Crucially, the method lets us determine whether a spectroscopic training sample is representative of the full photometric space occupied by the galaxies in a survey. We explore optimal sampling techniques and estimate the additional spectroscopy needed to map out the color-redshift relation, finding that sampling the galaxy distribution in color space in a systematic way can efficiently meet the calibration requirements. While the analysis presented here focuses on the Euclid survey, similar analysis can be applied to other surveys facing the same calibration challenge, such as DES, LSST, and WFIRST.

The unprecedented large number of quasar candidates included in the latest data release of SDSS (... more The unprecedented large number of quasar candidates included in the latest data release of SDSS (DR5) make it possible to constrain models for quasar evolution by mapping the quasars in a time-luminosity plane. Analyzing the features of the rise and fall of this map, as well as other spectroscopic information from SDSS like the mass obtained from reverberation methods, we can infer typical individual quasar trajectories starting with their formation, growth predominantly by baryonic accretion followed by the luminosity decay. The remarkable smoothness of the luminosity decay for the overall sample of quasars following a long period of our Universe's evolution suggests that there are more similarities than differences in the behavior of quasars at high red-shifts and low red-shifts.

With the emergence of moderately detailed evolutionary models for supermassive black hole growth ... more With the emergence of moderately detailed evolutionary models for supermassive black hole growth in galaxies it is becoming possible to predict evolutionary tracks for the rapid growth phase seen as quasars in, for example, the Mass-Luminosity plane. The Sloan Digital Sky Survey includes 77,429 quasar candidates in their latest data release (DR5). We estimate SMBH masses using the standard method based on reverberation mapping: we use H-beta, MgII, and CIV broad emission line widths and continuum luminosities to construct mass luminosity distributions of SDSS quasars as a function of redshift. This distribution shows several characteristic features. We show typical tracks in this plane for quasars growing predominantly due to luminous accretion. We further use the evolution of the mass-luminosity distribution with redshift to determine how these tracks have changed over time and whether high redshift quasars behave similarly to low-redshift quasars.

This thesis investigates the quasar mass-luminosity plane, as a new tool to explore the relations... more This thesis investigates the quasar mass-luminosity plane, as a new tool to explore the relationship between black hole mass and quasar luminosity over time. Previous techniques used quasar luminosity function and mass functions, which are one-dimensional projections of the mass-luminosity plane. The M --- L plane contains information that cannot be seen in these projections. We use 62,185 quasars from the Sloan Digital Sky Survey DR5 sample to develop several new constraints on quasar accretion. Black hole masses, based on the widths of their Hbeta, Mg II, and C IV lines and adjacent continuum luminosities, were used assuming using standard virial mass estimate scaling laws. In each redshift interval over the range 0.2 < z < 4.0, low-mass quasars reach at their Eddington luminosity, but high-mass quasars fall short, even by a factor of ten or more at 0.2 < z < 0.6. We examine several potential sources of measurement uncertainty or bias and show that none of them can acc...

Comment on "Biases in the Quasar Mass-Luminosity Plane"

We consider the formation of dwarf carbon stars via accretion from a carbon AGB companion in ligh... more We consider the formation of dwarf carbon stars via accretion from a carbon AGB companion in light of the new 107 object sample of Downes et al. (2004). This sample is now large enough to allow good mass determination via comparison of a composite spectrum to theoretical atmospheric models. Carbon dwarfs of spectral type M are indeed main sequence M dwarfs with enhanced metallicity and carbon abundance. We also calculate the predicted abundance of both M and of F/G carbon dwarfs, and show that the latter should be falsifiable in the near future.

Anomalous narrow-line quasars are a new class of Type I quasars with narrow Hβ broader than 1200 ... more Anomalous narrow-line quasars are a new class of Type I quasars with narrow Hβ broader than 1200 km/s, above the velocity believed possible for gas in the quasar narrow-line region. This broadening is tightly correlated with changes in other spectral features, so that the set of ANLs is a distinct population difficult to explain with the current quasar standard model. Further, ANLs comprise over one quarter of all quasars at 0.2 < z < 0.8, so they must be accounted for as part of our understanding of supermassive black hole evolution. Because other quasar narrow lines such as [O II] are diminished or disappear entirely, ANLs raise several fundamental questions about the connection between the black hole and the central region of the galaxy. They also appear inconsistent with a predominantly virial broad-line region, calling into question the technique of virial mass estimation.

We describe a new class of Type I quasars with narrow H\beta\ broader than 1200 km/s, above the v... more We describe a new class of Type I quasars with narrow H\beta\ broader than 1200 km/s, above the velocity believed possible for gas in the quasar narrow-line region. We identify this group of quasars as a distinct population because of a variety of spectral and photometric signatures common to these "anomalous narrow-line quasars" (ANLs) but atypical of other quasars. One prominent signature is suppression of the [OIII]\lambda ?4959, 5007 emission-line, in many cases accompanied by broadening similar to the H\beta\ line. We demonstrate that ANLs comprise at least 11% and most likely approximately one quarter of the SDSS Type I quasar population at 0.2 < z < 0.8. The most striking feature of ANLs is a strong correlation between narrow H\beta\ width and the width of the broad component of the H\beta\ emission line. Another feature of ANLs is a diminished [OII] line, which might indicate a connection between ANLs and the interstellar mediums of their host galaxies, throu...

The Astrophysical Journal, 2015

We use observed optical to near infrared spectral energy distributions (SEDs) of 266 galaxies in ... more We use observed optical to near infrared spectral energy distributions (SEDs) of 266 galaxies in the COSMOS survey to derive the wavelength dependence of the dust attenuation at high redshift. All of the galaxies have spectroscopic redshifts in the range z = 2 to 6.5. The presence of the CIV absorption feature, indicating that the rest-frame UV-optical SED is dominated by OB stars, is used to select objects for which the intrinsic, unattenuated spectrum has a well-established shape. Comparison of this intrinsic spectrum with the observed broadband photometric SED then permits derivation of the wavelength dependence of the dust attenuation. The derived dust attenuation curve is similar in overall shape to the Calzetti curve for local starburst galaxies. We also see the 2175Å bump feature which is present in the Milky Way and LMC extinction curves but not seen in the Calzetti curve. The bump feature is commonly attributed to graphite or PAHs. No significant dependence is seen with redshift between sub-samples at z = 2-4 and z = 4-6.5. The 'extinction' curve obtained here provides a firm basis for color and extinction corrections of high redshift galaxy photometry.