arXiv subjects

Guido Roberts-Borsani

Publications and source records attributed to Guido Roberts-Borsani.

69 records · Page 4Linked to original sources

arXiv2022

Early results from GLASS-JWST. III: Galaxy candidates at z$\sim$9-15

We present the results of a first search for galaxy candidates at z$\sim$9--15 on deep seven-bands NIRCam imaging acquired as part of the GLASS-JWST Early Release Science Program on a flanking field of the Frontier Fields cluster A2744. Candidates are selected via two different renditions of the Lyman-break technique, isolating objects at z$\sim$9-11, and z$\sim$9-15, respectively, supplemented by photometric redshifts obtained with two independent codes. We find five color-selected candidates at z$>$9, plus one additional candidate with photometric redshift z$_{phot}\geq$9. In particular, we identify two bright candidates at $M_{UV}\simeq -21$ that are unambiguously placed at $z\simeq 10.6$ and $z\simeq 12.2$, respectively. The total number of galaxies discovered at $z>9$ is in line with the predictions of a non-evolving LF. The two bright ones at $z>10$ are unexpected given the survey volume, although cosmic variance and small number statistics limits general conclusions. This first search demonstrates the unique power of JWST to discover galaxies at the high redshift frontier. The candidates are ideal targets for spectroscopic follow-up in Cycle$-2$.

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arXiv2022

Early results from GLASS-JWST. IX: First spectroscopic confirmation of low-mass quiescent galaxies at $z>2$ with NIRISS

How passive galaxies form, and the physical mechanisms which prevent star formation over long timescales, are some of the most outstanding questions in understanding galaxy evolution. The properties of quiescent galaxies over cosmic time provide crucial information to identify the quenching mechanisms. Passive galaxies have been confirmed and studied out to $z\sim4$, but all of these studies have been limited to massive systems (mostly with $\log{(M_{\rm star}/M_{\odot})}>10.8$). Using James Webb Space Telescope (JWST) NIRISS grism slitless spectroscopic data from the GLASS JWST ERS program, we present spectroscopic confirmation of two quiescent galaxies at $z_{\rm spec}=2.650^{+0.004}_{-0.006}$ and $z_{\rm spec}=2.433^{+0.032}_{-0.016}$ (3$σ$ errors) with stellar masses of $\log{(M_{\rm star}/M_{\odot})}=10.53^{+0.18}_{-0.06}$ and $\log{(M_{\rm star}/M_{\odot})}=9.93^{+0.06}_{-0.07}$ (corrected for magnification factors of $μ=2.0$ and $μ=2.1$, respectively). The latter represents the first spectroscopic confirmation of the existence of low-mass quiescent galaxies at cosmic noon, showcasing the power of JWST to identify and characterize this enigmatic population.

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arXiv2022

Possible Systematic Rotation in the Mature Stellar Population of a $z=9.1$ Galaxy

We present new observations with the Atacama Large Millimeter/submillimeter Array for a gravitationally-lensed galaxy at $z=9.1$, MACS1149-JD1. [O III] 88-$μ$m emission is detected at 10$σ$ with a spatial resolution of $\sim0.3$ kpc in the source plane, enabling the most distant morpho-kinematic study of a galaxy. The [O III] emission is distributed smoothly without any resolved clumps and shows a clear velocity gradient with $ΔV_{\rm obs}/2σ_{\rm tot}=0.84\pm0.23$, where $ΔV_{\rm obs}$ is the observed maximum velocity difference and $σ_{\rm tot}$ is the velocity dispersion measured in the spatially-integrated line profile, suggesting a rotating system. Assuming a geometrically thin self-gravitating rotation disk model, we obtain $V_{\rm rot}/σ_V=0.67_{-0.26}^{+0.73}$, where $V_{\rm rot}$ and $σ_V$ are the rotation velocity and velocity dispersion, respectively, still consistent with rotation. The resulting disk mass of $0.65_{-0.40}^{+1.37}\times10^{9}$ M$_\odot$ is consistent with being associated with the stellar mass identified with a 300 Myr-old stellar population independently indicated by a Balmer break in the spectral energy distribution. We conclude that the most of the dynamical mass is associated with the previously-identified mature stellar population that formed at $z\sim15$.

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arXiv2022

The size-luminosity relation of lensed galaxies at $z=6-9$ in the Hubble Frontier Fields

We measure the size-luminosity relation of photometrically-selected galaxies within the redshift range $z\sim6-9$, using galaxies lensed by six foreground Hubble Frontier Fields (HFF) clusters. The power afforded by strong gravitational lensing allows us to observe fainter and smaller galaxies than in blank fields. We select our sample of galaxies and obtain their properties, e.g., redshift, magnitude, from the photometrically-derived ASTRODEEP catalogues. The intrinsic size is measured with the Lenstruction software, and completeness maps are created as a function of size and luminosity via the GLACiAR2 software. We perform a Bayesian analysis to estimate the intrinsic and incompleteness-corrected size-luminosity distribution, with parameterization $r_e \propto L^β$. We find slopes of $β\sim0.48\pm0.08$ at $z\sim6-7$ and $β\sim0.68\pm0.14$ at $z\sim8.5$, adopting the Bradac lens model. The slope derived by lensed galaxies is steeper than that obtained in blank fields and is consistent with other independent determinations of the size-luminosity relation from the HFF dataset. We also investigate the systematic uncertainties correlated with the choice of lens models, finding that the slopes of size-luminosity relations derived from different models are consistent with each other, i.e. the modeling errors are not a significant source of uncertainty in the size-luminosity relation.

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arXiv2021

The Physical Properties of Luminous $z\gtrsim8$ Galaxies and Implications for the Cosmic Star Formation Rate Density From $\sim$0.35 deg$^{2}$ of (Pure-)Parallel HST Observations

We present the largest systematic, HST-based search to date for luminous $z\gtrsim8$ galaxy candidates using $\sim$1267 arcmin$^{2}$ of (pure-)parallel observations from a compilation of 288 random sightlines with ACS and WFC3 observations, derived from the SuperBoRG data set and together representing a factor $\sim1.12\times$ larger than existing space-based data sets. Using NIR color cuts and careful photo-$z$ analyses, we find 31 $z\gtrsim8$ galaxy candidates over 29 unique sightlines, and derive global galaxy properties such as UV magnitudes and continuum slopes, sizes, and rest-frame optical properties (e.g., SFRs, stellar masses, $A_{\rm v}$). Taking advantage of the (pure-)parallel nature of our data set - making it one of the most representative thus far - and derived SFRs, we evaluate the cosmic star formation rate density for the bright end of the UV luminosity function at $z\sim8-10$ and test the validity of luminosity function-derived results using a conversion factor. We find our method yields comparable results to those derived with luminosity functions. Furthermore, we present follow up observations of 5 (Super)BoRG targets with Keck/MOSFIRE, finding no evidence of Ly$α$ in $>3$ hrs of $Y-$band observations in either, consistent with a largely neutral medium at $z\sim8$. Our results offer a definitive HST legacy on the bright end of the luminosity function and provide a valuable benchmark as well as targets for follow up with JWST.

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arXiv2021

A quantitative assessment of completeness correction methods and public release of a versatile simulation code

Having accurate completeness functions is crucial to the determination of the rest-frame ultraviolet luminosity functions (UVLFs) all the way back to the epoch of reionization. Most studies use injection-recovery simulations to determine completeness functions. Although conceptually similar, published approaches have subtle but important differences in their definition of the completeness function. As a result, they implement different methods to determine the UVLFs. We discuss the advantages and limitations of existing methods using a set of mock observations, and then compare the methods when applied to the same set of Hubble Legacy Field (HLF) images. We find that the most robust method under all our mock observations is the one that defines completeness as a function of both input and output magnitude. Other methods considering completeness only as a function of either input or output magnitude may suffer limitations in a presence of photometric scatter and/or steep luminosity functions. In particular, when the flux scatter is >0.2 mag, the bias in the bright end of the UVLFs is on par with other systematic effects such as the lensing magnification bias. When tested on HLF images, all methods yield UVLFs that are consistent within 2 sigma confidence, suggesting that UVLF uncertainties in the literature are still dominated by small number statistics and cosmic variance. The completeness simulation code used in this study (GLACiaR2) is publicly released with this paper as a tool to analyze future higher precision datasets such as those expected from the James Webb Space Telescope.

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arXiv2021

Improving $z\sim7-11$ Galaxy Property Estimates with JWST/NIRCam Medium-Band Photometry

The past decade has seen impressive progress in the detection of $z>7$ galaxies with the Hubble Space Telescope, however little is known about their properties. The James Webb Space Telescope will revolutionise the high-$z$ field by providing NIR (i.e., rest-frame optical) data of unprecedented depth and spatial resolution. Measuring galaxy quantities such as resolved stellar ages or gas metallicity gradients traditionally requires spectroscopy, as broad-band imaging filters are generally too coarse to fully isolate diagnostics such as the 4000 Å (rest-frame) break, continuum emission from aged stars, and key emission lines (e.g., [OII], [OIII], H$β$). However, in this paper, we show that adding NIRCam images through a strategically chosen medium-band filter to common wide-band filters sets adopted by ERS and GTO programs delivers tighter constraints on these galactic properties. To constrain the choice of filter, we perform a systematic investigation of which combinations of wide-band filters from ERS and GTO programs and single medium-band filters offer the tightest constraints on several galaxy properties at redshifts $z\sim7-11$. We employ the JAGUAR extragalactic catalogs to construct statistical samples of physically-motivated mock photometry and conduct SED-fitting procedures to evaluate the accuracy of galaxy property (and photo-$z$) recovery with a simple star-formation history model. We find that adding $>4.1 μ$m medium filters at comparable depth to the broad-band filters can significantly improve photo-$z$s and yield close to order-of-magnitude improvements in the determination of quantities such as stellar ages, metallicities, SF-related quantities and emission line fluxes at $z\sim8$. For resolved sources, the proposed approach enables spatially-resolved determination of these quantities that would be prohibitive with slit spectroscopy.

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arXiv2020

The evolution of the size-mass relation at $z$=1-3 derived from the complete Hubble Frontier Fields data set

We measure the size-mass relation and its evolution between redshifts 1$ 3\times10^{9}M_{\odot}$ at 1$ 5 \times 10^{9} M_{\odot}$ at 2.5$<z<$3, and at a fixed stellar mass, we find galaxy sizes evolve as $R_{eff}\propto (1+z)^{-1.05\pm0.37}$. The intrinsic scatter is $<0.1$ dex at $z<1.5$ but increases to $\sim0.3$ dex at higher redshift. The results are in good agreement with those obtained in blank fields. We evaluate the uncertainties associated with the choice of lens model by comparing size measurements using five different and publicly available models, finding the choice of lens model leads to a 3.7 % uncertainty of the median value, and $\sim 25$ % scatter for individual galaxies. Our work demonstrates the use of strong lensing magnification to boost resolution does not introduce significant uncertainties in this kind of work, and paves the way for wholesale applications of the sophisticated lens reconstruction technique to higher redshifts and larger samples.

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arXiv2020

The Mass-Metallicity Relation at z=8: Direct-Method Metallicity Constraints and Near-Future Prospects

Physical properties of galaxies at z>7 are of interest for understanding both the early phases of star formation and the process of cosmic reionization. Chemical abundance measurements offer valuable information on the integrated star formation history, and hence ionizing photon production, as well as the rapid gas accretion expected at such high redshifts. We use reported measurements of [O III] 88$μ$m emission and star formation rate to estimate gas-phase oxygen abundances in five galaxies at z=7.1-9.1 using the direct T_e method. We find typical abundances 12+log(O/H) = 7.9 ($\sim$0.2 times the solar value) and an evolution of 0.9$\pm$0.5 dex in oxygen abundance at fixed stellar mass from z$\simeq$8 to 0. These results are compatible with theoretical predictions, albeit with large (conservative) uncertainties in both mass and metallicity. We assess both statistical and systematic uncertainties to identify promising means of improvement with the Atacama Large Millimeter Array (ALMA) and the James Webb Space Telescope (JWST). In particular we highlight [O III] 52$μ$m as a valuable feature for robust metallicity measurements. Precision of 0.1-0.2 dex in T_e-based O/H abundance can be reasonably achieved for galaxies at z$\approx$5-8 by combining [O III] 52$μ$m with rest-frame optical strong lines. It will also be possible to probe gas mixing and mergers via resolved T_e-based abundances on kpc scales. With ALMA and JWST, direct metallicity measurements will thus be remarkably accessible in the reionization epoch.

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arXiv2020

Interpreting the Spitzer/IRAC Colours of 7<z<9 Galaxies: Distinguishing Between Line Emission and Starlight Using ALMA

Prior to the launch of JWST, Spitzer/IRAC photometry offers the only means of studying the rest-frame optical properties of z>7 galaxies. Many such high redshift galaxies display a red [3.6] - [4.5] micron colour, often referred to as the "IRAC excess", which has conventionally been interpreted as arising from intense [OIII]+Hbeta emission within the [4.5] micron bandpass. An appealing aspect of this interpretation is similarly intense line emission seen in star-forming galaxies at lower redshift as well as the redshift-dependent behaviour of the IRAC colours beyond z~7 modelled as the various nebular lines move through the two bandpasses. In this paper we demonstrate that, given the photometric uncertainties, established stellar populations with Balmer (4000 A, rest-frame) breaks, such as those inferred at z>9 where line emission does not contaminate the IRAC bands, can equally well explain the redshift-dependent behaviour of the IRAC colours in 7<z<9 galaxies. We discuss possible ways of distinguishing between the two hypotheses using ALMA measures of [OIII] 88 micron and dust continuum fluxes. Prior to further studies with JWST, we show that the distinction is important in determining the assembly history of galaxies in the first 500 Myr.

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arXiv2020

Observational Constraints on the Multiphase Nature of Outflows Using Large Spectroscopic Surveys at $z\sim$0

Mass outflow rates and loading factors are typically used to infer the quenching potential of galactic-scale outflows. However, these generally rely on observations of a single gas phase which can severely underestimate the total ejected gas mass. To address this, we use observations of high mass ($\geqslant$10$^{10}$ M$_{\odot}$), normal star-forming galaxies at $z\sim$0 from the MaNGA, xCOLD GASS, xGASS and ALFALFA surveys and a stacking of NaD, H$α$, CO(1-0) and HI 21cm tracers with the aim of placing constraints on an average, total mass outflow rate and loading factor. We find detections of outflows in both neutral and ionised gas tracers, with no detections in stacks of molecular or atomic gas emission. Modelling of the outflow components reveals velocities of $|$v$_{\text{NaD}}|$=131 km s$^{-1}$ and $|$v$_{\text{H}α}|$=439 km s$^{-1}$ and outflow rates of $\dot{M}_{\text{NaD}}$=7.55 M$_{\odot}$yr$^{-1}$ and $\dot{M}_{\text{H}α}$=0.10 M$_{\odot}$yr$^{-1}$ for neutral and ionised gas, respectively. Assuming a molecular/atomic outflow velocity of 200 km s$^{-1}$, we derive upper limits of $\dot{M}_{\text{CO}}<$19.43 M$_{\odot}$yr$^{-1}$ and $\dot{M}_{\text{HI}}<$26.72 M$_{\odot}$yr$^{-1}$ for the molecular and atomic gas, respectively. Combining the detections and upper limits, we find average total outflow rates of $\dot{M}_{\text{tot}}\lesssim$27 M$_{\odot}$yr$^{-1}$ and a loading factor of $η_{\text{tot}}\lesssim$6.39, with molecular gas likely contributing $\lesssim$72% of the total mass outflow rate, and neutral and ionised gas contributing $\sim$28% and $<$1%, respectively. Our results suggest that, to first order, a degree of quenching via ejective feedback could occur in normal galaxies when considering all gas phases, even in the absence of an AGN.

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arXiv2020

Outflows in Star-forming Galaxies: Stacking Analyses of Resolved Winds and the Relation to Their Hosts' Properties

Outflows form an integral component in regulating the gas cycling in and out of galaxies, although their impact on the galaxy hosts is still poorly understood. Here we present an analysis of 405 high mass (log M$_{*}$/M$_{\odot}\geqslant10$), star-forming galaxies (excluding AGN) with low inclinations at $z\sim$0, using stacking techniques of the NaD $λλ$5889,5895 A neutral gas tracer in IFU observations from the MaNGA DR15 survey. We detect outflows in the central regions of 78/405 galaxies and determine their extent and power through the construction of stacked annuli. We find outflows are most powerful in central regions and extend out to $\sim$1R$_{e}$, with declining mass outflow rates and loading factors as a function of radius. The stacking of spaxels over key galaxy quantities reveals outflow detections in regions of high $Σ_{\text{SFR}}$ ($\gtrsim$0.01 M$_{\odot}$yr$^{-1}$kpc$^{-2}$) and $Σ_{M_{*}}$ ($\gtrsim$10$^{7}$ M$_{\odot}$kpc$^{-2}$) along the resolved main sequence. Clear correlations with $Σ_{\text{SFR}}$ suggest it is the main regulator of outflows, with a critical threshold of $\sim$0.01 M$_{\odot}$yr$^{-1}$kpc$^{-2}$ needed to escape the weight of the disk and launch them. Furthermore, measurements of the H$δ$ and D$_{n}$4000 indices reveal virtually identical star formation histories between galaxies with outflows and those without. Finally, through stacking of HI 21 cm observations for a subset of our sample, we find outflow galaxies show reduced HI gas fractions at central velocities compared to their non-detection control counterparts, suggestive of some removal of HI gas, likely in the central regions of the galaxies, but not enough to completely quench the host.

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arXiv2019

Near-Infrared Spectroscopy of Galaxies During Reionization: Measuring C$\,$III] in a Galaxy at $\mathbf{z=7.5}$

We present Keck/MOSFIRE $H$-band spectroscopy targeting C$\,$III] $λ$1907,1909 in a $z=7.5056$ galaxy previously identified via Ly$α$ emission. We detect strong line emission at $1.621\pm0.002\,μ$m with a line flux of ($2.63\pm0.52$)$\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$. We tentatively identify this line as [C$\,$III] $λ$1907, but we are unable to detect C$\,$III] $λ$1909 owing to sky emission at the expected location. This gives a galaxy systemic redshift, $z_{sys}=7.5032\pm0.0003$, with a velocity offset to Ly$α$ of $Δ$v$_{Lyα}$ = $88\pm27$ km s$^{-1}$. The ratio of combined C$\,$III]/Ly$α$ is 0.30-0.45, one of the highest values measured for any $z>2$ galaxy. We do not detect Si$\,$III] $λλ$1883, 1892, and place an upper limit on Si$\,$III]/C$\,$III] $<$ 0.35 ($2σ$). Comparing our results to photoionization models, the C$\,$III] equivalent width (W$_{\text{CIII]}} = 16.23\pm2.32\,$Å), low Si$\,$III]/C$\,$III] ratio, and high implied [O$\,$III] equivalent width (from the $Spitzer$/IRAC [3.6]$-$[4.5]$\simeq$0.8 mag color) require sub-Solar metallicities ($Z\simeq0.1-0.2 Z_{\odot}$) and a high ionization parameter, log$\,$U $\gtrsim -1.5$. These results favor models that produce higher ionization, such as the BPASS models for the photospheres of high-mass stars, and that include both binary stellar populations and/or an IMF that extends to 300 M$_{\odot}$. The combined C$\,$III] equivalent width and [3.6]$-$[4.5] color are more consistent with ionization from young stars than AGN, however we cannot rule out ionization from a combination of an AGN and young stars. We make predictions for $James~Webb~Space~Telescope$ spectroscopy using these different models, which will ultimately test the nature of the ionizing radiation in this source.

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arXiv2018

The onset of star formation 250 million years after the Big Bang

A fundamental quest of modern astronomy is to locate the earliest galaxies and study how they influenced the intergalactic medium a few hundred million years after the Big Bang. The abundance of star-forming galaxies is known to decline from redshifts of about 6 to 10, but a key question is the extent of star formation at even earlier times, corresponding to the period when the first galaxies might have emerged. Here we present spectroscopic observations of MACS1149-JD1, a gravitationally lensed galaxy observed when the Universe was less than four per cent of its present age. We detect an emission line of doubly ionized oxygen at a redshift of $9.1096\pm0.0006$, with an uncertainty of one standard deviation. This precisely determined redshift indicates that the red rest-frame optical colour arises from a dominant stellar component that formed about 250 million years after the Big Bang, corresponding to a redshift of about 15. Our results indicate the it may be possible to detect such early episodes of star formation in similar galaxies with future telescopes.

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arXiv2015

Lyman-alpha Emission from a Luminous z=8.68 Galaxy: Implications for Galaxies as Tracers of Cosmic Reionization

We report the discovery of Lyman-alpha emission (Ly$α$) in the bright galaxy EGSY-2008532660 (hereafter EGSY8p7) using the MOSFIRE spectrograph at the Keck Observatory. First reported by Roberts-Borsani et al. (2015), it was selected for spectroscopic observations because of its photometric redshift ($z_{phot}=8.57^{+0.22}_{-0.43}$), apparent brightness (H$_{160}=25.26\pm0.09$) and red Spitzer/IRAC [3.6]-[4.5] color indicative of contamination by strong oxygen emission in the [4.5] band. With a total integration of $\sim$4.3 hours, our data reveal an emission line at $\simeq$11776 Å which we argue is likely Ly$α$ at a redshift $z_{spec}=8.683^{+0.001}_{-0.004}$, in good agreement with the photometric estimate. The line was detected independently on two nights using different slit orientations and its detection significance is $\sim7.5σ$. An overlapping skyline contributes significantly to the uncertainty on the total line flux although the significance of the detected line is robust to a variety of skyline-masking procedures. By direct addition and a Gaussian fit, we estimate a 95\% confidence range of 1.0--2.5$\times10^{-17}$ erg s$^{-1}$ cm$^{-2}$, corresponding to a rest-frame equivalent width of 17--42 Å. EGSY8p7 is the most distant galaxy confirmed spectroscopically to date, and the third luminous source in the EGS field beyond $z_{phot}\gtrsim7.5$ with detectable Ly$α$ emission viewed at a time when the intergalactic medium is believed to be fairly neutral. Although the reionization process was probably patchy, we discuss whether luminous sources with prominent IRAC color excesses may harbor harder ionizing spectra than the dominant fainter population thereby creating earlier ionized bubbles. Further spectroscopic follow-up of such bright sources promises important insight into the early formation of galaxies.

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