arXiv subjects

Rychard J. Bouwens

Publications and source records attributed to Rychard J. Bouwens.

At least 37 records · Page 2Linked to original sources

arXiv2022

High Equivalent Width of Hα+[N II] Emission in z~8 Lyman-break Galaxies from IRAC 5.8μm Observations: Evidence for Efficient Lyman-continuum Photon production in the Epoch of Re-ionization

We measure, for the first time, the median equivalent width (EW) of H$α$+[N II] in star-forming galaxies at $z\sim8$. Our estimate leverages the unique photometric depth of the Spitzer/IRAC $5.8μ$m-band mosaics (probing $\approx 5500 - 7100$ A at $z\sim8$) of the GOODS Reionization Era Wide Area Treasury from Spitzer (GREATS) program. We median stacked the stamps of $102$ Lyman-break galaxies in the $3.6, 4.5, 5.8$ and $8.0μ$m bands, after carefully removing potential contamination from neighbouring sources. We infer an extreme rest-frame EW$_0$(H$α$+[N II])$=2328^{+1326}_{-1127}$ A from the measured red $[3.6]-[5.8]=0.82\pm0.27$ mag, consistent with young ($\lesssim10^7$ yr) average stellar population ages at $z\sim8$. This implies an ionizing photon production efficiency of $\log(ξ_{\mathrm{ion},0}/\mathrm{erg\ Hz}^{-1})=25.97^{+0.18}_{-0.28}$. Such a high value for photo production, similar to the highest values found at $z\lesssim4$, indicates that only modest escape fractions $f_\mathrm{esc}\lesssim0.3$ (at $2σ$) are sufficient for galaxies brighter than $M_\mathrm{UV}<-18$ mag to re-ionize the neutral Hydrogen at $z\sim8$. This requirement is relaxed even more to $f_\mathrm{esc}\le 0.1$ when considering galaxies brighter than $M_\mathrm{UV}\approx -13$ mag, consistent with recent luminosity functions and as typically assumed in studies addressing re-ionization. These exceptional results clearly indicate that galaxies can be the dominant source of reionizing photons, and provide us with an exciting glimpse into what we might soon learn about the early universe, and particularly about the Reionization Epoch, from forthcoming JWST/MIRI and NIRCam programs.

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arXiv2022

The ALMA REBELS Survey: Specific Star-Formation Rates in the Reionization Era

We present specific star-formation rates for 40 UV-bright galaxies at $z\sim7-8$ observed as part of the Reionization Era Bright Emission Line Survey (REBELS) ALMA large program. The sSFRs are derived using improved measures of SFR and stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [CII]-based spectroscopic redshifts. For each source in the sample, we derive stellar masses from SED fitting and total SFRs from calibrations of the UV and FIR emission. The median sSFR is $18_{-5}^{+7}$ Gyr$^{-1}$, significantly larger than literature measurements lacking constraints in the FIR. The increase in sSFR reflects the larger obscured SFRs we derive from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly-dependent on the assumed star formation history (SFH) in reionization-era galaxies. When large sSFR galaxies are modeled with non-parametric SFHs, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The [CII] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses that are easily able to accommodate the dominant old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties in the derived parameters, we find that the sSFR increases rapidly toward higher redshifts for massive galaxies ($9.6<\log(\rm M_*/M_{\odot})<9.8$), with a power law that goes as $(1+z)^{1.7\pm0.3}$, broadly consistent with expectations from the evolving baryon accretion rates.

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arXiv2022

The ALMA REBELS Survey: Efficient Ly$α$ Transmission of UV-Bright z$\simeq$7 Galaxies from Large Velocity Offsets and Broad Line Widths

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly$α$ transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly$α$ profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at $z\sim7$ remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [CII]-based redshifts from ALMA. Here we report MMT/Binospec Ly$α$ spectroscopy of eight UV-bright ($\mathrm{M_{UV}}^{}\sim-22$) galaxies at $z\simeq7$ selected from the ALMA REBELS survey. We detect Ly$α$ in 4 of 8 galaxies and use the [CII] systemic redshifts to investigate the Ly$α$ velocity profiles. The Ly$α$ lines are significantly redshifted from systemic (average velocity offset=223 km/s) and broad (FWHM$\approx$300$-$650 km/s), with two sources showing emission extending to $\approx$750 km/s. We find that the broadest Ly$α$ profiles are associated with the largest [CII] line widths, suggesting a potential link between the Ly$α$ FWHM and the dynamical mass. Since Ly$α$ photons at high velocities transmit efficiently through the $z=7$ IGM, our data suggest that velocity profiles play a significant role in boosting the Ly$α$ visibility of the most UV-luminous reionization-era galaxies.

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arXiv2021

The Spitzer/IRAC Legacy over the GOODS Fields: Full-Depth 3.6, 4.5, 5.8 and 8.0um Mosaics and Photometry for > 9000 Galaxies at z~3.5-10 from the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS)

We present the deepest Spitzer/IRAC $3.6$, $4.5$, $5.8$ and $8.0μ$m wide-area mosaics yet over the GOODS-N and GOODS-S fields as part of the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) project. We reduced and mosaicked in a self-consistent way observations taken by the 11 different Spitzer/IRAC programs over the two GOODS fields from 12 years of Spitzer cryogenic and warm mission data. The cumulative depth in the $3.6μ$m and $4.5μ$m bands amounts to $\sim 4260$ hr, $\sim 1220$ hr of which are new very deep observations from the GREATS program itself. In the deepest area, the full-depth mosaics reach $\gtrsim200$ hr over an area of $\sim100$ arcmin$^2$, corresponding to a sensitivity of $\sim29$ AB magnitude at $3.6μ$m ($1σ$ for point sources). Archival cryogenic $5.8μ$m and $8.0μ$m band data (a cumulative 976 hr) are also included in the release. The mosaics are projected onto the tangential plane of CANDELS/GOODS at a $0.3''$ pixel$^{-1}$ scale. This paper describes the methodology enabling, and the characteristics of, the public release of the mosaic science images, the corresponding coverage maps in the four IRAC bands, and the empirical Point-Spread Functions (PSFs). These PSFs enable mitigation of the source blending effects by taking into account the complex position-dependent variation in the IRAC images. The GREATS data products are in the Infrared Science Archive (IRSA). We also release the deblended $3.6$-to-$8.0μ$m photometry for $9192$ Lyman-Break galaxies at $z\sim3.5-10$. GREATS will be the deepest mid-infrared imaging until JWST and, as such, constitutes a major resource for characterizing early galaxy assembly.

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arXiv2021

MMT Spectroscopy of Lyman-alpha at z$\simeq$7: Evidence for Accelerated Reionization Around Massive Galaxies

Reionization-era galaxies tend to exhibit weak Ly$α$ emission, likely reflecting attenuation from an increasingly neutral IGM. Recent observations have begun to reveal exceptions to this picture, with strong Ly$α$ emission now known in four of the most massive z=7$-$9 galaxies in the CANDELS fields, all of which also exhibit intense [OIII]$+$H$β$ emission (EW$>$800 $\mathrm{\mathring{A}}$). To better understand why Ly$α$ is anonymously strong in a subset of massive z$\simeq$7$-$9 galaxies, we have initiated an MMT/Binospec survey targeting a larger sample (N=22) of similarly luminous ($\simeq$1$-$6 L$^{\ast}_{\mathrm{UV}}$) z$\simeq$7 galaxies selected over very wide-area fields ($\sim$3 deg$^2$). We confidently ($>$7$σ$) detect Ly$α$ in 78% (7/9) of galaxies with strong [OIII]$+$H$β$ emission (EW$>$800 $\mathrm{\mathring{A}}$) as opposed to only 8% (1/12) of galaxies with more moderate (EW=200$-$800 $\mathrm{\mathring{A}}$) [OIII]$+$H$β$. We argue that the higher Ly$α$ EWs of the strong [OIII]$+$H$β$ population likely reflect enhanced ionizing photon production efficiency owing to their large sSFRs ($\gtrsim$30 Gyr$^{-1}$). We also find evidence that Ly$α$ transmission from massive galaxies declines less rapidly over $6<z<7$ than in low-mass lensed systems. In particular, our data suggest no strong evolution in Ly$α$ transmission, consistent with a picture wherein massive z$\simeq$7 galaxies often reside in large ionized regions. We detect three closely-separated ($R$ = 1.7 physical Mpc) z$\simeq$7 Ly$α$ emitters in our sample, conceivably tracing a large ionized structure that is consistent with this picture. We detect tentative evidence for an overdensity in this region, implying a large ionizing photon budget in the surrounding volume.

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arXiv2021

Measuring the average molecular gas content of star-forming galaxies at $z=3-4$

We study the molecular gas content of 24 star-forming galaxies at $z=3-4$, with a median stellar mass of $10^{9.1}$ M$_{\odot}$, from the MUSE Hubble Ultra Deep Field (HUDF) Survey. Selected by their Lyman-alpha-emission and H-band magnitude, the galaxies show an average EW $\approx 20$ angstrom, below the typical selection threshold for Lyman Alpha Emitters (EW $> 25$ angstrom), and a rest-frame UV spectrum similar to Lyman Break Galaxies. We use rest-frame optical spectroscopy from KMOS and MOSFIRE, and the UV features observed with MUSE, to determine the systemic redshifts, which are offset from Lyman alpha by 346 km s$^{-1}$, with a 100 to 600 km s$^{-1}$ range. Stacking CO(4-3) and [CI](1-0) (and higher-$J$ CO lines) from the ALMA Spectroscopic Survey of the HUDF (ASPECS), we determine $3σ$ upper limits on the line luminosities of $4.0\times10^{8}$ K km s$^{-1}$pc$^{2}$ and $5.6\times10^{8}$ K km s$^{-1}$pc$^{2}$, respectively (for a 300 km s$^{-1}$ linewidth). Stacking the 1.2 mm and 3 mm dust continuum flux densities, we find a $3σ$ upper limits of 9 $μ$Jy and $1.2$ $μ$Jy, respectively. The inferred gas fractions, under the assumption of a 'Galactic' CO-to-H$_{2}$ conversion factor and gas-to-dust ratio, are in tension with previously determined scaling relations. This implies a substantially higher $α_{\rm CO} \ge 10$ and $δ_{\rm GDR} \ge 1200$, consistent with the sub-solar metallicity estimated for these galaxies ($12 + \log(O/H) \approx 7.8 \pm 0.2$). The low metallicity of $z \ge 3$ star-forming galaxies may thus make it very challenging to unveil their cold gas through CO or dust emission, warranting further exploration of alternative tracers, such as [CII].

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arXiv2021

Significant Dust-Obscured Star Formation in Luminous Lyman-break Galaxies at $z$$\sim$$7$$-$$8$

We make use of ALMA continuum observations of $15$ luminous Lyman-break galaxies at $z$$\sim$$7$$-$$8$ to probe their dust-obscured star-formation. These observations are sensitive enough to probe to obscured SFRs of $20$ $M_{\odot}$$/$$yr$ ($3σ$). Six of the targeted galaxies show significant ($\geq$$3$$σ$) dust continuum detections, more than doubling the number of known dust-detected galaxies at $z$$>$$6.5$. Their IR luminosities range from $2.7$$\times$$10^{11}$ $L_{\odot}$ to $1.1$$\times$$10^{12}$ $L_{\odot}$, equivalent to obscured SFRs of $20$ to $105$ $M_{\odot}$$/$$yr$. We use our results to quantify the correlation of the infrared excess IRX on the UV-continuum slope $β_{UV}$ and stellar mass. Our results are most consistent with an SMC attenuation curve for intrinsic $UV$-slopes $β_{UV,intr}$ of $-2.63$ and most consistent with an attenuation curve in-between SMC and Calzetti for $β_{UV,intr}$ slopes of $-2.23$, assuming a dust temperature $T_d$ of $50$ K. Our fiducial IRX-stellar mass results at $z$$\sim$$7$$-$$8$ are consistent with marginal evolution from $z$$\sim$$0$. We then show how both results depend on $T_d$. For our six dust-detected sources, we estimate their dust masses and find that they are consistent with dust production from SNe if the dust destruction is low ($<$$90$%). Finally we determine the contribution of dust-obscured star formation to the star formation rate density for $UV$ luminous ($<$$-$$21.5$ mag: $\gtrsim$$1.7$$L_{UV} ^*$) $z$$\sim$$7$$-$$8$ galaxies, finding that the total SFR density at $z$$\sim$$7$ and $z$$\sim$$8$ from bright galaxies is $0.18_{-0.10}^{+0.08}$ dex and $0.20_{-0.09}^{+0.05}$ dex higher, respectively, i.e. $\sim$$\frac{1}{3}$ of the star formation in $\gtrsim$$1.7$$L_{UV} ^*$ galaxies at $z$$\sim$$7$$-$$8$ is obscured by dust.

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arXiv2021

Galaxy Stellar Mass Functions from z~10 to z~6 using the Deepest Spitzer/IRAC Data: No Significant Evolution in the Stellar-to-Halo Mass Ratio of Galaxies in the First Gyr of Cosmic Time

We present new stellar mass functions at $z\sim6$, $z\sim7$, $z\sim8$, $z\sim9$ and, for the first time, $z\sim10$, constructed from $\sim800$ Lyman-Break galaxies previously identified over the XDF/UDF, parallels and the five CANDELS fields. Our study is distinctive due to (1) the much deeper ($\sim200$ hour) wide-area Spitzer/IRAC imaging at $3.6μ$m and $4.5μ$m from the GOODS Re-ionization Era wide Area Treasury from Spitzer (GREATS) program and (2) consideration of $z\sim6-10$ sources over a $3\times$ larger area than previous HST+Spitzer studies. The Spitzer/IRAC data enable $\ge2σ$ rest-frame optical detections for an unprecedented $50\%$ of galaxies down to a stellar mass limit of $\sim10^{8}\mathcal{M}_\odot$ across all redshifts. Schechter fits to our volume densities suggest a combined evolution in characteristic mass $\mathcal{M}^*$ and normalization factor $ϕ^*$ between $z\sim6$ and $z\sim8$. The stellar mass density (SMD) increases by $\sim1000\times$ in the $\sim500$ Myr between $z\sim10$ and $z\sim6$, with indications of a steeper evolution between $z\sim10$ and $z\sim8$, similar to the previously-reported trend of the star-formation rate density. Strikingly, abundance matching to the Bolshoi-Planck simulation indicates halo mass densities evolving at approximately the same rate as the SMD between $z\sim10$ and $z\sim4$. Our results show that the stellar-to-halo mass ratios, a proxy for the star-formation efficiency, do not change significantly over the huge stellar mass build-up occurred from $z\sim10$ to $z\sim6$, indicating that the assembly of stellar mass closely mirrors the build-up in halo mass in the first $\sim1$ Gyr of cosmic history. JWST is poised to extend these results into the "first galaxy" epoch at $z\gtrsim10$.

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arXiv2021

Blue Rest-Frame UV-Optical Colors in z~8 Galaxies from GREATS: Very Young Stellar Populations at ~650 Myr of Cosmic Time

Deep rest-optical observations are required to accurately constrain the stellar populations of $z\sim8$ galaxies. Due to significant limitations in the availability of such data for statistically complete samples, observational results have been limited to modest numbers of bright or lensed sources. To revolutionize the present characterization of $z\sim8$ galaxies, we exploit the ultradeep ($\sim27$ mag, $3σ$) Spitzer/IRAC $3.6μ$m and $4.5μ$m data, probing the rest-frame optical at $z\sim8$, over $\sim200$ arcmin$^2$ of the GOODS fields from the recently completed GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) program, combined with observations in the CANDELS UDS and COSMOS fields. We stacked $\gtrsim100$ $z\sim8$ Lyman-Break galaxies in four bins of UV luminosity ($M_\mathrm{UV}\sim -20.7$ to $-18.4$) and study their $H_\mathrm{160}-[3.6]$ and $[3.6]-[4.5]$ colors. We find young ages ($\lesssim100$ Myr) for the three faintest stacks, inferred from their blue $H_\mathrm{160}-[3.6]\sim 0$ mag colors, consistent with a negative Balmer break. Meanwhile, the redder $H_\mathrm{160}-[3.6]$ color seen in the brightest stack is suggestive of slightly older ages. We explored the existence of a correlation between the UV luminosity and age, and find either no trend or fainter galaxies being younger. The stacked SEDs also exhibit very red $[3.6]-[4.5]\sim0.5$ mag colors, indicative of intense [OIII]+H$β$ nebular emission and SFR. The correspondingly high specific star-formation rates, sSFR$\gtrsim10$Gyr$^{-1}$, are consistent with recent determinations at similar redshifts and higher luminosities, and support the co-evolution between the sSFR and the specific halo mass accretion rate.

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arXiv2021

An Atlas of MUSE Observations towards Twelve Massive Lensing Clusters

Spectroscopic surveys of massive galaxy clusters reveal the properties of faint background galaxies, thanks to the magnification provided by strong gravitational lensing. We present a systematic analysis of integral-field-spectroscopy observations of 12 massive clusters, conducted with the Multi Unit Spectroscopic Explorer (MUSE). All data were taken under very good seeing conditions (0.6") in effective exposure times between two and 15 hrs per pointing, for a total of 125 hrs. Our observations cover a total solid angle of ~23 arcmin$^2$ in the direction of clusters, many of which were previously studied by the MACS, Frontier Fields, GLASS and CLASH programs. The achieved emission line detection limit at 5$σ$ for a point source varies between (0.77--1.5)$\times$10$^{-18}$ erg\,s$^{-1}$\,cm$^{-2}$ at 7000Å. We present our developed strategy to reduce these observational data, detect sources and determine their redshifts. We construct robust mass models for each cluster to further confirm our redshift measurements using strong-lensing constraints, and identify a total of 312 strongly lensed sources producing 939 multiple images. The final redshift catalogs contain more than 3300 robust redshifts, of which 40\% are for cluster members and $\sim$30\% for lensed Lyman-$α$ emitters. 14\% of all sources are line emitters not seen in the available HST images, even at the depth of the FFs ($\sim29$ AB). We find that the magnification distribution of the lensed sources in the high-magnification regime ($μ{=}$ 2--25) follows the theoretical expectation of $N(z)\proptoμ^{-2}$. The quality of this dataset, number of lensed sources, and number of strong-lensing constraints enables detailed studies of the physical properties of both the lensing cluster and the background galaxies. The full data products from this work are made available to the community. [abridged]

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arXiv2020

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The nature of the faintest dusty star-forming galaxies

We present a characterization of the physical properties of a sample of 35 securely-detected, dusty galaxies in the deep ALMA 1.2-mm image obtained as part of the ALMA Spectroscopic Survey in the {\it Hubble} Ultra Deep Field (ASPECS) Large Program. This sample is complemented by 26 additional sources identified via an optical/infrared source positional prior. Using their well-characterized spectral energy distributions, we derive median stellar masses and star formation rates (SFR) of $4.8\times10^{10}~M_\odot$ and 30 $M_\odot$ yr$^{-1}$, and interquartile ranges of $(2.4-11.7)\times10^{10}~M_\odot$ and $20-50~M_\odot$ yr$^{-1}$, respectively. We derive a median spectroscopic redshift of 1.8 with an interquartile range $1.1-2.6$, significantly lower than submillimeter galaxies detected in shallower, wide-field surveys. We find that 59\%$\pm$13\%, 6\%$\pm$4\%, and 34\%$\pm$9\% of our sources are within, above and below $\pm0.4$ dex from the SFR-stellar mass relation or main-sequence (MS), respectively. The ASPECS galaxies closely follow the SFR-molecular gas mass relation and other previously established scaling relations, confirming a factor of five increase of the gas-to-stellar mass ratio from $z=0.5$ to $z=2.5$ and a mild evolution of the gas depletion timescales with a typical value of 0.7 Gyr at $z=1-3$. ASPECS galaxies located significantly below the MS, a poorly exploited parameter space, have low gas-to-stellar-mass ratios of $\sim0.1-0.2$ and long depletion timescales $>1$ Gyr. Galaxies along the MS dominate the cosmic density of molecular gas at all redshifts. Systems above the main sequence have an increasing contribution to the total gas reservoirs from $z<1$ to $z=2.5$, while the opposite is found for galaxies below the MS.

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arXiv2020

The BUFFALO HST Survey

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.

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arXiv2019

The Brightest $z\gtrsim8$ Galaxies over the COSMOS UltraVISTA Field

We present 16 new ultrabright $H_{AB}\lesssim25$ galaxy candidates at z~8 identified over the COSMOS/UltraVISTA field. The new search takes advantage of the deepest-available ground-based optical and near-infrared observations, including the DR3 release of UltraVISTA and full-depth Spitzer/IRAC observations from the SMUVS and SPLASH programs. Candidates are selected using Lyman-break criteria, combined with strict optical non-detection and SED-fitting criteria, minimizing contamination by low-redshift galaxies and low-mass stars. HST/WFC3 coverage from the DASH program reveals that one source evident in our ground-based near-IR data has significant substructure and may actually correspond to 3 separate z~8 objects, resulting in a sample of 18 galaxies, 10 of which seem to be fairly robust (with a >97% probability of being at z>7). The UV-continuum slope $β$ for the bright z~8 sample is $β=-2.2\pm0.6$, bluer but still consistent with that of similarly bright galaxies at z~6 ($β=-1.55\pm0.17$) and z~7 ($β=-1.75\pm0.18$). Their typical stellar masses are 10$^{9.1^{+0.5}_{-0.4}}M_{\odot}$, with the SFRs of $32^{+44}_{-32}M_{\odot}$/year, specific SFR of $4^{+8}_{-4}$ Gyr$^{-1}$, stellar ages of $\sim22^{+69}_{-22}$\,Myr, and low dust content A$_V=0.15^{+0.30}_{-0.15}$ mag. Using this sample we constrain the bright end of the z~8 UV luminosity function (LF). When combined with recent empty field LF estimates at z~8-9, the resulting z~8 LF can be equally well represented by either a Schechter or a double power-law (DPL) form. Assuming a Schechter parameterization, the best-fit characteristic magnitude is $M^*= -20.95^{+0.30}_{-0.35}$ mag with a very steep faint end slope $α=-2.15^{+0.20}_{-0.19}$. These new candidates include amongst the brightest yet found at these redshifts, 0.5-1.0 mag brighter than found over CANDELS, providing excellent targets for follow-up studies.

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arXiv2019

The mean H$α$ EW and Lyman-continuum photon production efficiency for faint $z\approx4-5$ galaxies

We present the first measurements of the Lyman-continuum photon production efficiency $ξ_{\textrm{ion,0}}$ at $z\sim4$-5 for galaxies fainter than 0.2 $L^*$ ($-$19 mag). $ξ_{\textrm{ion,0}}$ quantifies the production rate of ionizing photons with respect to the UV luminosity density assuming a fiducial escape fraction of zero. Extending previous measurements of $ξ_{\textrm{ion,0}}$ to the faint population is important, as ultra-faint galaxies are expected to contribute the bulk of the ionizing emissivity. We probe $ξ_{\textrm{ion,0}}$ to such faint magnitudes by taking advantage of 200-hour depth Spitzer/IRAC observations from the GREATS program and $\approx$300 3<$z$<6 galaxies with spectroscopic redshifts from the MUSE GTO Deep + Wide programs. Stacked IRAC [3.6]$-$[4.5] colors are derived and used to infer the H$α$ rest-frame equivalent widths, which range from 403Å to 2818Å. The derived $ξ_{\textrm{ion,0}}$ is $\log_{10}(ξ_{\textrm{ion,0}} / \textrm{Hz erg}^{-1}) = 25.36 \pm 0.08$ over $-$20.5 < M$_{\textrm{UV}}$ < $-$17.5, similar to those derived for brighter galaxy samples at the same redshift and therefore suggesting that $ξ_{\textrm{ion}}$ shows no strong dependence on $M_{UV}$. The $ξ_{\textrm{ion,0}}$ values found in our sample imply that the Lyman-continuum escape fraction for $M_{\textrm{UV}} \approx -19$ star-forming galaxies cannot exceed $\approx$8-20\% in the reionization era.

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arXiv2019

The Super Eight Galaxies: Properties of a Sample of Very Bright Galaxies at $7 < z < 8$

We present the Super Eight galaxies - a set of very luminous, high-redshift ($7.1<z<8.0$) galaxy candidates found in Brightest of Reionizing Galaxies (BoRG) Survey fields. The original sample includes eight galaxies that are $Y$-band dropout objects with $H$-band magnitudes of $m_H<25.5$. Four of these objects were originally reported in Calvi et al. 2016. Combining new Hubble Space Telescope (HST) WFC3/F814W imaging and $Spitzer$ IRAC data with archival imaging from BoRG and other surveys, we explore the properties of these galaxies. Photometric redshift fitting places six of these galaxies in the redshift range of $7.1<z<8.0$, resulting in three new high-redshift galaxies and confirming three of the four high-redshift galaxy candidates from Calvi et al. 2016. We calculate the half-light radii of the Super Eight galaxies using the HST F160W filter and find that the Super Eight sizes are in line with typical evolution of size with redshift. The Super Eights have a mean mass of log(M$_*$/M$_\odot$) $\sim10$, which is typical for sources in this luminosity range. Finally, we place our sample on the UV $z\sim8$ luminosity function and find that the Super Eight number density is consistent with other surveys in this magnitude and redshift range.

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arXiv2019

Detection of a Lensed z$\approx$11 Galaxy in the Rest-Optical with Spitzer/IRAC and the Inferred SFR, Stellar Mass, and Physical Size

We take advantage of new 100-hour Spitzer/IRAC observations available for MACS0647-JD, a strongly lensed $z\approx11$ galaxy candidate, to provide improved constraints on its physical properties. Probing the physical properties of galaxies at $z>8$ is challenging due to the inherent faintness of such sources and the limited wavelength coverage available. Thanks to the high $\approx$2-6$\times$ lensing magnification of the multiple images of MACS0647-JD, we can use the sensitive Spitzer/IRAC data to probe the rest-frame optical fluxes of MACS0647-JD and investigate its physical properties including the age and the stellar mass. In deriving Spitzer/IRAC fluxes for MACS0647-JD, great care is taken in coping with the impact of three bright ($\approx$8-16 mag) stars in our field to ensure robust results. Assuming a constant star formation rate, the age, stellar mass, and rest-frame UV slope we estimate for MACS0647-JD based on a stack of the photometry are log$_{10}$(age/yr) = 8.6$^{+0.1}_{-2.1}$, log$_{10}$(M$_{*}$/M$_{\odot}$) = 9.1$^{+0.2}_{-1.4}$, and $β= -$1.3$\pm$0.6, respectively. We compare our results with expectations from the EAGLE simulation and find that MACS0647-JD has properties consistent with corresponding to the most massive and rapidly star-forming galaxies in the simulation. We also find that its radius, 105$\pm$28 pc, is a factor of $\approx$2 smaller than the mean size in a separate simulation project DRAGONS. Interestingly enough, the observed size is similar to the small sizes seen in very low-luminosity $z\approx$6-10 galaxies behind lensing clusters.

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arXiv2019

Does radiative feedback make faint z>6 galaxies look small?

Recent observations of lensed sources have shown that the faintest ($M_{\mathrm{UV}} \approx -15\,\mathrm{mag}$) galaxies observed at z=6-8 appear to be extremely compact. Some of them have inferred sizes of less than 40 pc for stellar masses between $10^6$ and $10^7\,\mathrm{M}_{\odot}$, comparable to individual super star clusters or star cluster complexes at low redshift. High-redshift, low-mass galaxies are expected to show a clumpy, irregular morphology and if star clusters form in each of these well-separated clumps, the observed galaxy size would be much larger than the size of an individual star forming region. As supernova explosions impact the galaxy with a minimum delay time that exceeds the time required to form a massive star cluster, other processes are required to explain the absence of additional massive star forming regions. In this work we investigate whether the radiation of a young massive star cluster can suppress the formation of other detectable clusters within the same galaxy already before supernova feedback can affect the galaxy. We find that in low-mass ($M_{200} \lesssim 10^{10}\,\mathrm{M}_{\odot}$) haloes, the radiation from a compact star forming region with an initial mass of $10^{7}\,\mathrm{M}_{\odot}$ can keep gas clumps with Jeans masses larger than $\approx 10^{7}\,\mathrm{M}_{\odot}$ warm and ionized throughout the galaxy. In this picture, the small intrinsic sizes measured in the faintest $z=6-8$ galaxies are a natural consequence of the strong radiation field that stabilises massive gas clumps. A prediction of this mechanism is that the escape fraction for ionizing radiation is high for the extremely compact, high-z sources.

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arXiv2018

Dependence of galaxy clustering on UV-luminosity and stellar mass at $z \sim 4 - 7$

We investigate the dependence of galaxy clustering at $z \sim 4 - 7$ on UV-luminosity and stellar mass. Our sample consists of $\sim$ 10,000 Lyman-break galaxies (LBGs) in the XDF and CANDELS fields. As part of our analysis, the $M_\star - M_{\rm UV}$ relation is estimated for the sample, which is found to have a nearly linear slope of $d\log_{10} M_\star / d M_{\rm UV} \sim 0.44$. We subsequently measure the angular correlation function and bias in different stellar mass and luminosity bins. We focus on comparing the clustering dependence on these two properties. While UV-luminosity is only related to recent starbursts of a galaxy, stellar mass reflects the integrated build-up of the whole star formation history, which should make it more tightly correlated with halo mass. Hence, the clustering segregation with stellar mass is expected to be larger than with luminosity. However, our measurements suggest that the segregation with luminosity is larger with $\simeq 90\%$ confidence (neglecting contributions from systematic errors). We compare this unexpected result with predictions from the \textsc{Meraxes} semi-analytic galaxy formation model. Interestingly, the model reproduces the observed angular correlation functions, and also suggests stronger clustering segregation with luminosity. The comparison between our observations and the model provides evidence of multiple halo occupation in the small scale clustering.

astro-ph.GA↗