Annalisa Citro | University of Minnesota - Twin Cities (original) (raw)

Papers by Annalisa Citro

The escape fraction of LyC ionizing radiation fesc_LyC is crucial for understanding reionization... more The escape fraction of LyC ionizing radiation fesc_LyC is crucial for understanding reionization, yet impossible to measure at z ≳ 5.3. Recently, studies have focused on calibrating indirect indicators of fesc_LyC at z ∼ 0.3, finding that Lyα is closely linked to it. What is still unclear is whether the LyC - Lyα relation evolves with redshift, and if Lyα is truly applicable as an fesc_LyC indicator during the reionization epoch. In this study, we investigate seven −21≲MUV≲−19 gravitationally lensed galaxies from the BELLS GALLERY survey at z ∼ 2.3. Our targets have rest-frame Lyα equivalent widths ranging from 10 Å to 100 Å and low dust content ( −2.5≲β≲−1.9), both indicative of high LyC escape. Surprisingly, direct estimates of fesc_LyC using Hubble Space Telescope imaging with F275W and F225W reveal that our targets are not LyC emitters, with absolute fesc_LyC ≲ 6.5% at 3σ significance (with two sources having absolute fesc_LyC (3σ) ≲ 10% and ≲ 16%). The low fesc_LyC, coupled with the high Lyα escape fraction and equivalent width could potentially be attributed to the redshift evolution of the neutral hydrogen column density and dust content. Our results challenge previous studies based on local samples, suggesting that the extrapolation of z ~ 0 Lyα -based LyC indirect estimators into the reionization epoch might not be correct.

We provide one of the most comprehensive metallicity studies at ∼ 4 by analyzing the UV/optical H... more We provide one of the most comprehensive metallicity studies at ∼ 4 by analyzing the UV/optical HST photometry, and rest-frame VLT-FORS2 ultraviolet and VLT-XSHOOTER optical spectra of J0332-3557, a gravitationally lensed galaxy magnified by a factor of 20. With a 5 detection of the auroral O III] 1666 line, we are able to derive a direct gas metallicity estimate for our target. We find Z gas = 12 + log(O/H) = 8.26 ± 0.06, which is compatible with an increasing of both the gas fraction and the outflow metal loading factor from ∼ 0 to ∼ 4. J0332-3557 is the most metal-rich individual galaxy at ∼ 4 for which the C/O ratio has been measured. We derive a low log(C/O) = −1.02 ± 0.2, which suggests that J0332-3557 is in the early stages of ISM carbon enrichment driven mostly by massive stars. The low C/O abundance also indicates that J0332-3557 is characterized by a low star formation efficiency, higher yields of oxygen, and longer burst duration. We find that EW C III]1906,9 is as low as ∼ 3 Å. The main drivers of the low EW C III]1906,9 are the higher gas metallicity and the low C/O abundance. J0332-3557 is characterized by one diffuse and two more compact regions ∼ 1 kpc in size. We find that the carbon emission mostly originates in the compact knots. Our study on J0332-3557 serves as an anchor for studies investigating the evolution of metallicity and C/O abundance across different redshifts.

Holding the major share of stellar mass in galaxies and being also old and passively evolving, ea... more Holding the major share of stellar mass in galaxies and being also old and passively evolving, early-type galaxies (ETGs) are the primary probes in investigating these various evolution scenarios, as well as being useful means to provide insights on cosmological parameters. In this thesis work I focused specifically on ETGs and on their capability in constraining galaxy formation and evolution; in particular, the principal aims were to derive some of the ETGs evolutionary parameters, such as age, metallicity and star formation history (SFH) and to study their age-redshift and mass-age relations. In order to infer galaxy physical parameters, I used the public code STARLIGHT: this program provides a best fit to the observed spectrum from a combination of many theoretical models defined in user-made libraries. the comparison between the output and input light-weighted ages shows a good agreement starting from SNRs of ∼ 10, with a bias of ∼ 2.2% and a dispersion 3%. Furthermore, also me...

We propose a method aimed at identifing galaxies in the short evolutionary phase in which they qu... more We propose a method aimed at identifing galaxies in the short evolutionary phase in which they quench their star-formation (SF). We rely on high- to low-ionization emission line ratios, which rapidly disappear after the SF halt due to the softening of the UV ionizing radiation. In particular, we focus on [O III] 5007/Halpha and [Ne III] 3869/[O II] 3727, simulating their time evolution by means of the CLOUDY photoionization code. We find that these two emission line ratios are able to trace the quenching on very short time-scales (i.e. 10-80 Myr), depending on if a sharp or a smoother SF quenching is assumed. We adopt the [N II] 6584/[O II] 3727 ratio as metallicity diagnostic to mitigate the metallicity degeneracy which affects our method. Using a Sloan Digital Sky Survey galaxy sample, we identify 11 examples of extreme quenching candidates within the [O III] 5007/Halpha vs. [N II] 6584/[O II] 3727 plane, characterized by faint [Ne III] 3869, blue dust-corrected spectra and blue (...

The expansion history of the Universe can be constrained in a cosmology-independent way by measur... more The expansion history of the Universe can be constrained in a cosmology-independent way by measuring the differential age evolution of cosmic chronometers. This yields a measurement of the Hubble parameter as a function of redshift. The most reliable cosmic chronometers known so far are extremely massive and passively evolving galaxies. Age-dating these galaxies is, however, a difficult task, and even a small contribution of an underlying young stellar population ("frosting") could, in principle, affect the age estimate and its cosmological interpretation. We present several spectral indicators to detect, quantify and constrain such a young component in old galaxies, and study how their combination can be used to maximize the purity of cosmic chronometers selection. In particular, we analyze the CaII H/K ratio, and the presence (or absence) of H$\alpha$ and [OII] emission lines, higher order Balmer absorption lines, and UV flux; each indicator is especially sensitive to a ...

The Astrophysical Journal, 2021

Detailed analyses of high-redshift galaxies are challenging because these galaxies are faint, but... more Detailed analyses of high-redshift galaxies are challenging because these galaxies are faint, but this difficulty can be overcome with gravitational lensing, in which the magnification of the flux enables spectroscopy with a high signal-to-noise ratio (S/N). We present the rest-frame ultraviolet (UV) Keck Echellette Spectrograph and Imager (ESI) spectrum of the newly discovered z = 2.79 lensed galaxy SDSS J1059+4251. With an observed magnitude F814W = 18.8 and a magnification factor μ = 31 ± 3, J1059+4251 is both highly magnified and intrinsically luminous, about two magnitudes brighter than M UV * at z ∼ 2–3. With a stellar mass M * = (3.22 ± 0.20) × 1010 M ⊙, star formation rate SFR = 50 ± 7 M⊙ yr−1, and stellar metallicity Z * ≃ 0.15–0.5 Z ⊙, J1059+4251 is typical of bright star-forming galaxies at similar redshifts. Thanks to the high S/N and the spectral resolution of the ESI spectrum, we are able to separate the interstellar and stellar features and derive properties that woul...

We present a method to select galaxies in the critical phase when the star formation is rapidly s... more We present a method to select galaxies in the critical phase when the star formation is rapidly suppressed ('Quenching'). These objects were spectroscopically searched, in the local Universe (0.04 ≤ z &lt; 0.21), by exploiting the dust-corrected [OIII]λ5007/Hα ratio that is quite sensitive to the ionization parameter. <br> We identified roughly 300 quenching candidates amongst 174000 SDSS star-forming galaxies. We analysed their fundamental properties (such SFR, colours, mass) and the Quenching timescale and we found that they stopped the star formation no more than a few Myrs before. <br> Therefore, they can give precise information about the mechanism driving the Quenching, in particular, the role of galaxy mass and halo mass, the role of environment and the role of stars and AGN feedback.

Despite the progress made towards a more comprehensive knowledge of galaxy evolution, a global pi... more Despite the progress made towards a more comprehensive knowledge of galaxy evolution, a global picture of the mechanisms regulating the formation of stars in galaxies, of how galaxy evolutionary properties correlate with stellar masses and star formation rates (SFRs) and of the processes suppressing the star formation in galaxies and their timescales is still lacking. In this thesis work, we attempt to address some of these open questions, inspecting galaxy evolution back in cosmic time. In particular, we start from the archaeological analysis of passive local galaxies (1), reconstructing their past star formation histories. Then we take a step back towards the phase in which galaxies quench their star formation (2), defining a new methodology able to identify the quenching progenitors of passive galaxies. Finally, we move back to the star-forming phase (3), investigating the properties of high-redshift galaxies which could be the star-forming progenitors of the passive local ones. ...

Monthly Notices of the Royal Astronomical Society, 2018

Detecting galaxies when their star formation is being quenched is crucial to understand the mecha... more Detecting galaxies when their star formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III] λ5007/H α ratio and searching for galaxies with undetected [O III]. Using a sample of ∼174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 ≤ z< 0.21, weidentify the ∼300 quenching galaxy best candidates with low [O III]/H α, out of ∼26000 galaxies without [O III] emission. They have masses between 10 9.7 and 10 10.8 M , consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours, and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments. Most candidates have morphologies similar to star-forming galaxies, suggesting that no morphological transformation has occurred yet. From a survival analysis we find a low fraction of candidates (∼0.58 per cent of the star-forming population), leading to a short quenching timescale of t Q ∼ 50 Myr and an e-folding time for the quenching history of τ Q ∼ 90 Myr, and their upper limits of t Q < 0.76 Gyr and τ Q <1.5 Gyr, assuming as quenching galaxies 50 per cent of objects without [O III] (∼7.5 per cent). Ourresults are compatible with a 'rapid' quenching scenario of satellites galaxies due to the final phase of strangulation or ram-pressure stripping. This approach represents a robust alternative to methods used so far to select quenched galaxies (e.g. colours, specific star-formation rate, or post-starburst spectra).

Astronomy & Astrophysics, 2019

We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff–Riley type II (FR... more We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff–Riley type II (FRII) radio galaxy atz = 1.7 in the deep multiband survey around thez = 6.3 quasi-stellar object (QSO) SDSS J1030+0524. Based on a 6 h VLT/MUSE and on a 4 h LBT/LUCI observation, we identify at least eight galaxy members in this structure with spectroscopic redshiftz = 1.687 − 1.699, including the FRII galaxy atz = 1.699. Most members are distributed within 400 kpc from the FRII core. Nonetheless, the whole structure is likely much more extended, as one of the members was serendipitously found at ∼800 kpc projected separation. The classic radio structure of the FRII itself extends for ∼600 kpc across the sky. Most of the identified overdensity members are blue, compact galaxies that are actively forming stars at rates of ∼8–60M⊙yr−1. For the brightest of them, a half-light radius of 2.2 ± 0.8 kpc at 8000 Å rest-frame was determined based on adaptive optics-assisted observations with LBT/SO...

Monthly Notices of the Royal Astronomical Society, 2018

VANDELS is a uniquely deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrog... more VANDELS is a uniquely deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrograph on ESO's Very Large Telescope (VLT). The survey has obtained ultradeep optical (0.48 < λ < 1.0 μm) spectroscopy of 2100 galaxies within the redshift interval 1.0 ≤ z ≤ 7.0, over a total area of 0.2 deg 2 centred on the CANDELS Ultra Deep Survey and Chandra Deep Field South fields. Based on accurate photometric redshift pre-selection, 85 per cent of the galaxies targeted by VANDELS were selected to be at z ≥ 3. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the fundamental aim of the survey is to provide the high-signal-to-noise ratio spectra necessary to measure key physical properties such as stellar population ages, masses, metallicities, and outflow velocities from detailed absorption-line studies. Using integration times calculated to produce an approximately constant signal-tonoise ratio (20 < t int < 80 h), the VANDELS survey targeted: (a) bright star-forming galaxies at 2.4 ≤ z ≤ 5.5, (b) massive quiescent galaxies at 1.0 ≤ z ≤ 2.5, (c) fainter star-forming galaxies at 3.0 ≤ z ≤ 7.0, and (d) X-ray/Spitzer-selected active galactic nuclei and Herschel-detected galaxies. By targeting two extragalactic survey fields with superb multiwavelength imaging data, VANDELS will produce a unique legacy data set for exploring the physics underpinning high-redshift galaxy evolution. In this paper, we provide an overview of the VANDELS survey

The Astrophysical Journal, 2018

The expansion history of the Universe can be constrained in a cosmology-independent way by measur... more The expansion history of the Universe can be constrained in a cosmology-independent way by measuring the differential age evolution of cosmic chronometers. This yields a measurement of the Hubble parameter H(z) as a function of redshift. The most reliable cosmic chronometers known so far are extremely massive and passively evolving galaxies. Age-dating these galaxies is, however, a difficult task, and even a small contribution of an underlying young stellar population could, in principle, affect the age estimate and its cosmological interpretation. We present several spectral indicators to detect, quantify and constrain such contamination in old galaxies, and study how their combination can be used to maximize the purity of cosmic chronometers selection. In particular, we analyze the Ca II H/K ratio, the presence (or absence) of Hα and [OII] emission lines, higher order Balmer absorption lines, and UV flux; each indicator is especially sensitive to a particular age range, allowing us to detect young components ranging between 10 Myr and 1 Gyr. The combination of these indicators minimizes the contamination to a level below 1% in the case of ideal data. More importantly, it offers a way to control the systematic error on H(z) as a function of the contamination by young stellar populations. We show that for our previous measurements of the Hubble parameter, the possible bias induced by the presence of a younger component is well below the current errors. We envision that these indicators will be instrumental in paving the road for a robust and reliable dating of the old population and its cosmological interpretation.

Monthly Notices of the Royal Astronomical Society, 2015

We present the first estimate of age, stellar metallicity and chemical abundance ratios, for an i... more We present the first estimate of age, stellar metallicity and chemical abundance ratios, for an individual early-type galaxy at high-redshift (z = 1.426) in the COSMOS (Cosmological Evolution Survey) field. Our analysis is based on observations obtained with the X-Shooter instrument at the Very Large Telescope (VLT), which cover the visual and near-infrared spectrum at high (R > 5000) spectral resolution. We measure the values of several spectral absorptions tracing chemical species, in particular magnesium and iron, besides determining the age-sensitive D4000 break. We compare the measured indices to stellar population models, finding good agreement. We find that our target is an old (t > 3 Gyr), high-metallicity ([Z/H] > 0.5) galaxy which formed its stars at z form >5 within a short timescale ∼0.1 Gyr, as testified by the strong [α/Fe] ratio (>0.4), and has passively evolved in the first >3-4 Gyr of its life. We have verified that this result is robust against the choice and number of fitted spectral features, and stellar population model. The result of an old age and high-metallicity has important implications for galaxy formation and evolution confirming an early and rapid formation of the most massive galaxies in the Universe.

Astronomy & Astrophysics, 2016

Inferring the star-formation histories of the most massive and passive early-type galaxies at z <... more Inferring the star-formation histories of the most massive and passive early-type galaxies at z < 0.3

Detecting galaxies when their star-formation is being quenched is crucial to understand the mecha... more Detecting galaxies when their star-formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III]5007/Halpha ratio and searching for galaxies with undetected [O III]. Using a sample of ~174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 < z < 0.21,we identify the ~300 quenching galaxy best candidates with low [O III]/Halpha, out of ~26000 galaxies without [O III] emission. They have masses between 10^9.7 and 10^10.8 Mo, consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments.Most candidates have morphologies similar to s...

Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the ... more Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae and Baryon Acoustic Oscillations, providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 to provide new constraints on the Hubble parameter H(z) using the em cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent H(z) m...

Monthly Notices of the Royal Astronomical Society, 2019

Understanding when, how, and where star formation ceased (quenching) within galaxies is still a c... more Understanding when, how, and where star formation ceased (quenching) within galaxies is still a critical subject in galaxy evolution studies. Taking advantage of the new methodology developed by Quai et al. to select recently quenched galaxies, we explored the spatial information provided by the IFU data to get critical insights on this process. In particular, we analyse 10 SDSS-IV MaNGA galaxies that show regions with low [O iii]/H α compatible with a recent quenching of the star formation. We compare the properties of these 10 galaxies with those of a control sample of 8 MaNGA galaxies with ongoing star formation in the same stellar mass, redshift, and gas-phase metallicity range. The quenching regions found are located between 0.5 and 1.1 effective radii from the centre. This result is supported by the analysis of the average radial profile of the ionization parameter, which reaches a minimum at the same radii, while the one of the star-forming sample shows an almost flat trend. ...

Astronomy & Astrophysics, 2018

This paper describes the observations and the first data release (DR1) of the ESO public spectros... more This paper describes the observations and the first data release (DR1) of the ESO public spectroscopic survey “VANDELS, a deep VIMOS survey of the CANDELS CDFS and UDS fields”. The main targets of VANDELS are star-forming galaxies at redshift 2.4 < z < 5.5, an epoch when the Universe had not yet reached 20% of its current age, and massive passive galaxies in the range 1 < z < 2.5. By adopting a strategy of ultra-long exposure times, ranging from a minimum of 20 h to a maximum of 80 h per source, VANDELS is specifically designed to be the deepest-ever spectroscopic survey of the high-redshift Universe. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the survey is obtaining ultra-deep optical spectroscopy covering the wavelength range 4800–10 000 Å with a sufficiently high signal-to-noise ratio to investigate the astrophysics of high-redshift galaxy evolution via detailed absorption line studies of well-defined samples of high-redshift galaxies. VANDE...

Journal of Cosmology and Astroparticle Physics, 2016

Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the ... more Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae (SNe) and Baryon Acoustic Oscillations (BAO), providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS, [1, 2, 3]) Data Release 9 to provide new constraints on the Hubble parameter H(z) using the cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent H(z) measurements in the redshift range 0.3 < z < 0.5, with an accuracy of ∼11-16% incorporating both statistical and systematic errors. Once combined, these measurements yield a 6% accuracy constraint of H(z = 0.4293) = 91.8 ± 5.3 km/s/Mpc. The new data are crucial to provide the first cosmology-independent determination of the transition redshift at high statistical significance, measuring z t = 0.4 ± 0.1, and to significantly disfavor the null hypothesis of no transition between decelerated and accelerated expansion at 99.9% confidence level. This analysis highlights the wide potential of the cosmic chronometers approach: it permits to derive constraints on the expansion history of the Universe with results competitive with standard probes, and most importantly, being the estimates independent of the cosmological model, it can constrain cosmologies beyondand including-the ΛCDM model.

Monthly Notices of the Royal Astronomical Society, 2017

We propose a new methodology aimed at finding star-forming galaxies in the phase which immediatel... more We propose a new methodology aimed at finding star-forming galaxies in the phase which immediately follows the star-formation (SF) quenching, based on the use of high-to lowionization emission line ratios. These ratios rapidly disappear after the SF halt, due to the softening of the UV ionizing radiation. We focus on [O III] λ5007/Hα and [Ne III] λ3869/[O II] λ3727, studying them with simulations obtained with the CLOUDY photoionization code. If a sharp quenching is assumed, we find that the two ratios are very sensitive tracers as they drop by a factor of ∼ 10 within ∼10 Myr from the interruption of the SF; instead, if a smoother and slower SF decline is assumed (i.e. an exponentially declining SF history with e-folding time τ = 200 Myr), they decrease by a factor of ∼2 within ∼80 Myr. We mitigate the ionizationmetallicity degeneracy affecting our methodology using pairs of emission line ratios separately related to metallicity and ionization, adopting the [N II] λ6584/[O II] λ3727 ratio as metallicity diagnostic. Using a Sloan Digital Sky Survey galaxy sample, we identify 10 examples among the most extreme quenching candidates within the [O III] λ5007/Hα versus [N II] λ6584/ [O II] λ3727 plane, characterized by low [O III] λ5007/Hα, faint [Ne III] λ3869, and by blue dust-corrected spectra and (u − r) colours, as expected if the SF quenching has occurred in the very recent past. Our results also suggest that the observed fractions of quenching candidates can be used to constrain the quenching mechanism at work and its timescales .

The escape fraction of LyC ionizing radiation fesc_LyC is crucial for understanding reionization... more The escape fraction of LyC ionizing radiation fesc_LyC is crucial for understanding reionization, yet impossible to measure at z ≳ 5.3. Recently, studies have focused on calibrating indirect indicators of fesc_LyC at z ∼ 0.3, finding that Lyα is closely linked to it. What is still unclear is whether the LyC - Lyα relation evolves with redshift, and if Lyα is truly applicable as an fesc_LyC indicator during the reionization epoch. In this study, we investigate seven −21≲MUV≲−19 gravitationally lensed galaxies from the BELLS GALLERY survey at z ∼ 2.3. Our targets have rest-frame Lyα equivalent widths ranging from 10 Å to 100 Å and low dust content ( −2.5≲β≲−1.9), both indicative of high LyC escape. Surprisingly, direct estimates of fesc_LyC using Hubble Space Telescope imaging with F275W and F225W reveal that our targets are not LyC emitters, with absolute fesc_LyC ≲ 6.5% at 3σ significance (with two sources having absolute fesc_LyC (3σ) ≲ 10% and ≲ 16%). The low fesc_LyC, coupled with the high Lyα escape fraction and equivalent width could potentially be attributed to the redshift evolution of the neutral hydrogen column density and dust content. Our results challenge previous studies based on local samples, suggesting that the extrapolation of z ~ 0 Lyα -based LyC indirect estimators into the reionization epoch might not be correct.

We provide one of the most comprehensive metallicity studies at ∼ 4 by analyzing the UV/optical H... more We provide one of the most comprehensive metallicity studies at ∼ 4 by analyzing the UV/optical HST photometry, and rest-frame VLT-FORS2 ultraviolet and VLT-XSHOOTER optical spectra of J0332-3557, a gravitationally lensed galaxy magnified by a factor of 20. With a 5 detection of the auroral O III] 1666 line, we are able to derive a direct gas metallicity estimate for our target. We find Z gas = 12 + log(O/H) = 8.26 ± 0.06, which is compatible with an increasing of both the gas fraction and the outflow metal loading factor from ∼ 0 to ∼ 4. J0332-3557 is the most metal-rich individual galaxy at ∼ 4 for which the C/O ratio has been measured. We derive a low log(C/O) = −1.02 ± 0.2, which suggests that J0332-3557 is in the early stages of ISM carbon enrichment driven mostly by massive stars. The low C/O abundance also indicates that J0332-3557 is characterized by a low star formation efficiency, higher yields of oxygen, and longer burst duration. We find that EW C III]1906,9 is as low as ∼ 3 Å. The main drivers of the low EW C III]1906,9 are the higher gas metallicity and the low C/O abundance. J0332-3557 is characterized by one diffuse and two more compact regions ∼ 1 kpc in size. We find that the carbon emission mostly originates in the compact knots. Our study on J0332-3557 serves as an anchor for studies investigating the evolution of metallicity and C/O abundance across different redshifts.

Holding the major share of stellar mass in galaxies and being also old and passively evolving, ea... more Holding the major share of stellar mass in galaxies and being also old and passively evolving, early-type galaxies (ETGs) are the primary probes in investigating these various evolution scenarios, as well as being useful means to provide insights on cosmological parameters. In this thesis work I focused specifically on ETGs and on their capability in constraining galaxy formation and evolution; in particular, the principal aims were to derive some of the ETGs evolutionary parameters, such as age, metallicity and star formation history (SFH) and to study their age-redshift and mass-age relations. In order to infer galaxy physical parameters, I used the public code STARLIGHT: this program provides a best fit to the observed spectrum from a combination of many theoretical models defined in user-made libraries. the comparison between the output and input light-weighted ages shows a good agreement starting from SNRs of ∼ 10, with a bias of ∼ 2.2% and a dispersion 3%. Furthermore, also me...

We propose a method aimed at identifing galaxies in the short evolutionary phase in which they qu... more We propose a method aimed at identifing galaxies in the short evolutionary phase in which they quench their star-formation (SF). We rely on high- to low-ionization emission line ratios, which rapidly disappear after the SF halt due to the softening of the UV ionizing radiation. In particular, we focus on [O III] 5007/Halpha and [Ne III] 3869/[O II] 3727, simulating their time evolution by means of the CLOUDY photoionization code. We find that these two emission line ratios are able to trace the quenching on very short time-scales (i.e. 10-80 Myr), depending on if a sharp or a smoother SF quenching is assumed. We adopt the [N II] 6584/[O II] 3727 ratio as metallicity diagnostic to mitigate the metallicity degeneracy which affects our method. Using a Sloan Digital Sky Survey galaxy sample, we identify 11 examples of extreme quenching candidates within the [O III] 5007/Halpha vs. [N II] 6584/[O II] 3727 plane, characterized by faint [Ne III] 3869, blue dust-corrected spectra and blue (...

The expansion history of the Universe can be constrained in a cosmology-independent way by measur... more The expansion history of the Universe can be constrained in a cosmology-independent way by measuring the differential age evolution of cosmic chronometers. This yields a measurement of the Hubble parameter as a function of redshift. The most reliable cosmic chronometers known so far are extremely massive and passively evolving galaxies. Age-dating these galaxies is, however, a difficult task, and even a small contribution of an underlying young stellar population ("frosting") could, in principle, affect the age estimate and its cosmological interpretation. We present several spectral indicators to detect, quantify and constrain such a young component in old galaxies, and study how their combination can be used to maximize the purity of cosmic chronometers selection. In particular, we analyze the CaII H/K ratio, and the presence (or absence) of H$\alpha$ and [OII] emission lines, higher order Balmer absorption lines, and UV flux; each indicator is especially sensitive to a ...

The Astrophysical Journal, 2021

Detailed analyses of high-redshift galaxies are challenging because these galaxies are faint, but... more Detailed analyses of high-redshift galaxies are challenging because these galaxies are faint, but this difficulty can be overcome with gravitational lensing, in which the magnification of the flux enables spectroscopy with a high signal-to-noise ratio (S/N). We present the rest-frame ultraviolet (UV) Keck Echellette Spectrograph and Imager (ESI) spectrum of the newly discovered z = 2.79 lensed galaxy SDSS J1059+4251. With an observed magnitude F814W = 18.8 and a magnification factor μ = 31 ± 3, J1059+4251 is both highly magnified and intrinsically luminous, about two magnitudes brighter than M UV * at z ∼ 2–3. With a stellar mass M * = (3.22 ± 0.20) × 1010 M ⊙, star formation rate SFR = 50 ± 7 M⊙ yr−1, and stellar metallicity Z * ≃ 0.15–0.5 Z ⊙, J1059+4251 is typical of bright star-forming galaxies at similar redshifts. Thanks to the high S/N and the spectral resolution of the ESI spectrum, we are able to separate the interstellar and stellar features and derive properties that woul...

We present a method to select galaxies in the critical phase when the star formation is rapidly s... more We present a method to select galaxies in the critical phase when the star formation is rapidly suppressed ('Quenching'). These objects were spectroscopically searched, in the local Universe (0.04 ≤ z &lt; 0.21), by exploiting the dust-corrected [OIII]λ5007/Hα ratio that is quite sensitive to the ionization parameter. <br> We identified roughly 300 quenching candidates amongst 174000 SDSS star-forming galaxies. We analysed their fundamental properties (such SFR, colours, mass) and the Quenching timescale and we found that they stopped the star formation no more than a few Myrs before. <br> Therefore, they can give precise information about the mechanism driving the Quenching, in particular, the role of galaxy mass and halo mass, the role of environment and the role of stars and AGN feedback.

Despite the progress made towards a more comprehensive knowledge of galaxy evolution, a global pi... more Despite the progress made towards a more comprehensive knowledge of galaxy evolution, a global picture of the mechanisms regulating the formation of stars in galaxies, of how galaxy evolutionary properties correlate with stellar masses and star formation rates (SFRs) and of the processes suppressing the star formation in galaxies and their timescales is still lacking. In this thesis work, we attempt to address some of these open questions, inspecting galaxy evolution back in cosmic time. In particular, we start from the archaeological analysis of passive local galaxies (1), reconstructing their past star formation histories. Then we take a step back towards the phase in which galaxies quench their star formation (2), defining a new methodology able to identify the quenching progenitors of passive galaxies. Finally, we move back to the star-forming phase (3), investigating the properties of high-redshift galaxies which could be the star-forming progenitors of the passive local ones. ...

Monthly Notices of the Royal Astronomical Society, 2018

Detecting galaxies when their star formation is being quenched is crucial to understand the mecha... more Detecting galaxies when their star formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III] λ5007/H α ratio and searching for galaxies with undetected [O III]. Using a sample of ∼174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 ≤ z< 0.21, weidentify the ∼300 quenching galaxy best candidates with low [O III]/H α, out of ∼26000 galaxies without [O III] emission. They have masses between 10 9.7 and 10 10.8 M , consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours, and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments. Most candidates have morphologies similar to star-forming galaxies, suggesting that no morphological transformation has occurred yet. From a survival analysis we find a low fraction of candidates (∼0.58 per cent of the star-forming population), leading to a short quenching timescale of t Q ∼ 50 Myr and an e-folding time for the quenching history of τ Q ∼ 90 Myr, and their upper limits of t Q < 0.76 Gyr and τ Q <1.5 Gyr, assuming as quenching galaxies 50 per cent of objects without [O III] (∼7.5 per cent). Ourresults are compatible with a 'rapid' quenching scenario of satellites galaxies due to the final phase of strangulation or ram-pressure stripping. This approach represents a robust alternative to methods used so far to select quenched galaxies (e.g. colours, specific star-formation rate, or post-starburst spectra).

Astronomy & Astrophysics, 2019

We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff–Riley type II (FR... more We report the discovery of a galaxy overdensity around a Compton-thick Fanaroff–Riley type II (FRII) radio galaxy atz = 1.7 in the deep multiband survey around thez = 6.3 quasi-stellar object (QSO) SDSS J1030+0524. Based on a 6 h VLT/MUSE and on a 4 h LBT/LUCI observation, we identify at least eight galaxy members in this structure with spectroscopic redshiftz = 1.687 − 1.699, including the FRII galaxy atz = 1.699. Most members are distributed within 400 kpc from the FRII core. Nonetheless, the whole structure is likely much more extended, as one of the members was serendipitously found at ∼800 kpc projected separation. The classic radio structure of the FRII itself extends for ∼600 kpc across the sky. Most of the identified overdensity members are blue, compact galaxies that are actively forming stars at rates of ∼8–60M⊙yr−1. For the brightest of them, a half-light radius of 2.2 ± 0.8 kpc at 8000 Å rest-frame was determined based on adaptive optics-assisted observations with LBT/SO...

Monthly Notices of the Royal Astronomical Society, 2018

VANDELS is a uniquely deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrog... more VANDELS is a uniquely deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrograph on ESO's Very Large Telescope (VLT). The survey has obtained ultradeep optical (0.48 < λ < 1.0 μm) spectroscopy of 2100 galaxies within the redshift interval 1.0 ≤ z ≤ 7.0, over a total area of 0.2 deg 2 centred on the CANDELS Ultra Deep Survey and Chandra Deep Field South fields. Based on accurate photometric redshift pre-selection, 85 per cent of the galaxies targeted by VANDELS were selected to be at z ≥ 3. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the fundamental aim of the survey is to provide the high-signal-to-noise ratio spectra necessary to measure key physical properties such as stellar population ages, masses, metallicities, and outflow velocities from detailed absorption-line studies. Using integration times calculated to produce an approximately constant signal-tonoise ratio (20 < t int < 80 h), the VANDELS survey targeted: (a) bright star-forming galaxies at 2.4 ≤ z ≤ 5.5, (b) massive quiescent galaxies at 1.0 ≤ z ≤ 2.5, (c) fainter star-forming galaxies at 3.0 ≤ z ≤ 7.0, and (d) X-ray/Spitzer-selected active galactic nuclei and Herschel-detected galaxies. By targeting two extragalactic survey fields with superb multiwavelength imaging data, VANDELS will produce a unique legacy data set for exploring the physics underpinning high-redshift galaxy evolution. In this paper, we provide an overview of the VANDELS survey

The Astrophysical Journal, 2018

The expansion history of the Universe can be constrained in a cosmology-independent way by measur... more The expansion history of the Universe can be constrained in a cosmology-independent way by measuring the differential age evolution of cosmic chronometers. This yields a measurement of the Hubble parameter H(z) as a function of redshift. The most reliable cosmic chronometers known so far are extremely massive and passively evolving galaxies. Age-dating these galaxies is, however, a difficult task, and even a small contribution of an underlying young stellar population could, in principle, affect the age estimate and its cosmological interpretation. We present several spectral indicators to detect, quantify and constrain such contamination in old galaxies, and study how their combination can be used to maximize the purity of cosmic chronometers selection. In particular, we analyze the Ca II H/K ratio, the presence (or absence) of Hα and [OII] emission lines, higher order Balmer absorption lines, and UV flux; each indicator is especially sensitive to a particular age range, allowing us to detect young components ranging between 10 Myr and 1 Gyr. The combination of these indicators minimizes the contamination to a level below 1% in the case of ideal data. More importantly, it offers a way to control the systematic error on H(z) as a function of the contamination by young stellar populations. We show that for our previous measurements of the Hubble parameter, the possible bias induced by the presence of a younger component is well below the current errors. We envision that these indicators will be instrumental in paving the road for a robust and reliable dating of the old population and its cosmological interpretation.

Monthly Notices of the Royal Astronomical Society, 2015

We present the first estimate of age, stellar metallicity and chemical abundance ratios, for an i... more We present the first estimate of age, stellar metallicity and chemical abundance ratios, for an individual early-type galaxy at high-redshift (z = 1.426) in the COSMOS (Cosmological Evolution Survey) field. Our analysis is based on observations obtained with the X-Shooter instrument at the Very Large Telescope (VLT), which cover the visual and near-infrared spectrum at high (R > 5000) spectral resolution. We measure the values of several spectral absorptions tracing chemical species, in particular magnesium and iron, besides determining the age-sensitive D4000 break. We compare the measured indices to stellar population models, finding good agreement. We find that our target is an old (t > 3 Gyr), high-metallicity ([Z/H] > 0.5) galaxy which formed its stars at z form >5 within a short timescale ∼0.1 Gyr, as testified by the strong [α/Fe] ratio (>0.4), and has passively evolved in the first >3-4 Gyr of its life. We have verified that this result is robust against the choice and number of fitted spectral features, and stellar population model. The result of an old age and high-metallicity has important implications for galaxy formation and evolution confirming an early and rapid formation of the most massive galaxies in the Universe.

Astronomy & Astrophysics, 2016

Inferring the star-formation histories of the most massive and passive early-type galaxies at z <... more Inferring the star-formation histories of the most massive and passive early-type galaxies at z < 0.3

Detecting galaxies when their star-formation is being quenched is crucial to understand the mecha... more Detecting galaxies when their star-formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III]5007/Halpha ratio and searching for galaxies with undetected [O III]. Using a sample of ~174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 < z < 0.21,we identify the ~300 quenching galaxy best candidates with low [O III]/Halpha, out of ~26000 galaxies without [O III] emission. They have masses between 10^9.7 and 10^10.8 Mo, consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments.Most candidates have morphologies similar to s...

Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the ... more Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae and Baryon Acoustic Oscillations, providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 to provide new constraints on the Hubble parameter H(z) using the em cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent H(z) m...

Monthly Notices of the Royal Astronomical Society, 2019

Understanding when, how, and where star formation ceased (quenching) within galaxies is still a c... more Understanding when, how, and where star formation ceased (quenching) within galaxies is still a critical subject in galaxy evolution studies. Taking advantage of the new methodology developed by Quai et al. to select recently quenched galaxies, we explored the spatial information provided by the IFU data to get critical insights on this process. In particular, we analyse 10 SDSS-IV MaNGA galaxies that show regions with low [O iii]/H α compatible with a recent quenching of the star formation. We compare the properties of these 10 galaxies with those of a control sample of 8 MaNGA galaxies with ongoing star formation in the same stellar mass, redshift, and gas-phase metallicity range. The quenching regions found are located between 0.5 and 1.1 effective radii from the centre. This result is supported by the analysis of the average radial profile of the ionization parameter, which reaches a minimum at the same radii, while the one of the star-forming sample shows an almost flat trend. ...

Astronomy & Astrophysics, 2018

This paper describes the observations and the first data release (DR1) of the ESO public spectros... more This paper describes the observations and the first data release (DR1) of the ESO public spectroscopic survey “VANDELS, a deep VIMOS survey of the CANDELS CDFS and UDS fields”. The main targets of VANDELS are star-forming galaxies at redshift 2.4 < z < 5.5, an epoch when the Universe had not yet reached 20% of its current age, and massive passive galaxies in the range 1 < z < 2.5. By adopting a strategy of ultra-long exposure times, ranging from a minimum of 20 h to a maximum of 80 h per source, VANDELS is specifically designed to be the deepest-ever spectroscopic survey of the high-redshift Universe. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the survey is obtaining ultra-deep optical spectroscopy covering the wavelength range 4800–10 000 Å with a sufficiently high signal-to-noise ratio to investigate the astrophysics of high-redshift galaxy evolution via detailed absorption line studies of well-defined samples of high-redshift galaxies. VANDE...

Journal of Cosmology and Astroparticle Physics, 2016

Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the ... more Deriving the expansion history of the Universe is a major goal of modern cosmology. To date, the most accurate measurements have been obtained with Type Ia Supernovae (SNe) and Baryon Acoustic Oscillations (BAO), providing evidence for the existence of a transition epoch at which the expansion rate changes from decelerated to accelerated. However, these results have been obtained within the framework of specific cosmological models that must be implicitly or explicitly assumed in the measurement. It is therefore crucial to obtain measurements of the accelerated expansion of the Universe independently of assumptions on cosmological models. Here we exploit the unprecedented statistics provided by the Baryon Oscillation Spectroscopic Survey (BOSS, [1, 2, 3]) Data Release 9 to provide new constraints on the Hubble parameter H(z) using the cosmic chronometers approach. We extract a sample of more than 130000 of the most massive and passively evolving galaxies, obtaining five new cosmology-independent H(z) measurements in the redshift range 0.3 < z < 0.5, with an accuracy of ∼11-16% incorporating both statistical and systematic errors. Once combined, these measurements yield a 6% accuracy constraint of H(z = 0.4293) = 91.8 ± 5.3 km/s/Mpc. The new data are crucial to provide the first cosmology-independent determination of the transition redshift at high statistical significance, measuring z t = 0.4 ± 0.1, and to significantly disfavor the null hypothesis of no transition between decelerated and accelerated expansion at 99.9% confidence level. This analysis highlights the wide potential of the cosmic chronometers approach: it permits to derive constraints on the expansion history of the Universe with results competitive with standard probes, and most importantly, being the estimates independent of the cosmological model, it can constrain cosmologies beyondand including-the ΛCDM model.

Monthly Notices of the Royal Astronomical Society, 2017

We propose a new methodology aimed at finding star-forming galaxies in the phase which immediatel... more We propose a new methodology aimed at finding star-forming galaxies in the phase which immediately follows the star-formation (SF) quenching, based on the use of high-to lowionization emission line ratios. These ratios rapidly disappear after the SF halt, due to the softening of the UV ionizing radiation. We focus on [O III] λ5007/Hα and [Ne III] λ3869/[O II] λ3727, studying them with simulations obtained with the CLOUDY photoionization code. If a sharp quenching is assumed, we find that the two ratios are very sensitive tracers as they drop by a factor of ∼ 10 within ∼10 Myr from the interruption of the SF; instead, if a smoother and slower SF decline is assumed (i.e. an exponentially declining SF history with e-folding time τ = 200 Myr), they decrease by a factor of ∼2 within ∼80 Myr. We mitigate the ionizationmetallicity degeneracy affecting our methodology using pairs of emission line ratios separately related to metallicity and ionization, adopting the [N II] λ6584/[O II] λ3727 ratio as metallicity diagnostic. Using a Sloan Digital Sky Survey galaxy sample, we identify 10 examples among the most extreme quenching candidates within the [O III] λ5007/Hα versus [N II] λ6584/ [O II] λ3727 plane, characterized by low [O III] λ5007/Hα, faint [Ne III] λ3869, and by blue dust-corrected spectra and (u − r) colours, as expected if the SF quenching has occurred in the very recent past. Our results also suggest that the observed fractions of quenching candidates can be used to constrain the quenching mechanism at work and its timescales .