We present a detailed stellar population analysis of 11 bright (H < 26.6) galaxies at z=9−11 (three spectroscopically confirmed) to constrain the chemical enrichment and growth of stellar mass of ...early galaxies. We use the flexible Bayesian spectral energy distribution (SED) fitting code Prospector with a range of star-formation histories (SFHs), a flexible dust attenuation law and a self-consistent modeling of emission lines. This approach allows us to assess how different priors affect our results, and how well we can break degeneracies between dust attenuation, stellar ages, metallicity and emission lines using data which probe only the rest-frame ultraviolet to optical wavelengths. We measure a median observed ultraviolet spectral slope β= −1.87+0.35−0.43 for relatively massive star-forming galaxies (9<log(M?/M)<10), consistent with no change from z=4 to z=9−10 at these stellar masses, implying rapid enrichment. Our SED-fitting results are consistent with a star-forming main sequence with sub-linear slope (0.7±0.2) and specific star-formation rates of 3−10 Gyr−1. However, the stellar ages and SFHs are less well constrained. Using different SFH priors, we cannot distinguish between median mass-weighted ages of ∼50−150 Myr, which corresponds to 50% formation redshifts of z50∼10−12 atz∼9 and is of the order of the dynamical timescales of these systems. Importantly, the models with different SFH priors are able to fit the data equally well. We conclude that the current observational data cannot tightly constrain the mass-buildup timescales of these z=9−11 galaxies, with our results consistent with SFHs implying both a shallow and steep increase of the cosmic SFR density with time at z >10
Galaxy mergers are expected to have a significant role in the mass assembly of galaxies in the early universe, but there are very few observational constraints on the merger history of galaxies at z ...> 2. We present the first study of galaxy major mergers (mass ratios <1:4) in mass-selected samples out to z 6. Using all five fields of the Hubble Space Telescope/CANDELS survey and a probabilistic pair-count methodology that incorporates the full photometric redshift posteriors and corrections for stellar mass completeness, we measure galaxy pair-counts for projected separations between 5 and 30 kpc in stellar mass selected samples at 9.7 < log10(M /M ) < 10.3 and log10(M /M ) > 10.3. We find that the major merger pair fraction rises with redshift to z 6 proportional to (1 + z)m, with m = 0.8 0.2 (m = 1.8 0.2) for log10(M /M ) > 10.3 (9.7 < log10(M /M ) < 10.3). Investigating the pair fraction as a function of mass ratio between 1:20 and 1:1, we find no evidence for a strong evolution in the relative numbers of minor to major mergers out to z < 3. Using evolving merger timescales, we find that the merger rate per galaxy ( ) rises rapidly from 0.07 0.01 Gyr−1 at z < 1 to 7.6 2.7 Gyr−1 at z = 6 for galaxies at log10(M /M ) > 10.3. The corresponding comoving major merger rate density remains roughly constant during this time, with rates of Γ 10−4 Gyr−1 Mpc−3. Based on the observed merger rates per galaxy, we infer specific mass accretion rates from major mergers that are comparable to the specific star formation rates for the same mass galaxies at z > 3 - observational evidence that mergers are as important a mechanism for building up mass at high redshift as in situ star formation.
We present the physical extent of C ii 158 m line-emitting gas from 46 star-forming galaxies at z = 4-6 from the ALMA Large Program to INvestigate C ii at Early Times (ALPINE). Using exponential ...profile fits, we measure the effective radius of the C ii line ( ) for individual galaxies and compare them with the rest-frame ultraviolet (UV) continuum ( ) from Hubble Space Telescope images. The effective radius exceeds by factors of ∼2-3, and the ratio of increases as a function of Mstar. We do not find strong evidence that the C ii line, rest-frame UV, and far-infrared (FIR) continuum are always displaced over 1 kpc scale from each other. We identify 30% of isolated ALPINE sources as having an extended C ii component over 10 kpc scales detected at 4.1 -10.9 beyond the size of rest-frame UV and FIR continuum. One object has tentative rotating features up to ∼10 kpc, where the 3D model fit shows the rotating C ii-gas disk spread over 4 times larger than the rest-frame UV-emitting region. Galaxies with the extended C ii line structure have high star formation rate, high stellar mass (Mstar), low Ly equivalent width, and more blueshifted (redshifted) rest-frame UV metal absorption (Ly line), as compared to galaxies without such extended C ii structures. Although we cannot rule out the possibility that a selection bias toward luminous objects may be responsible for such trends, the star-formation-driven outflow also explains all these trends. Deeper observations are essential to test whether the extended C ii line structures are ubiquitous to high-z star-forming galaxies.
ABSTRACT We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity functions at z = 4-8. We use deep Hubble Space Telescope imaging over the Cosmic Assembly ...Near-infrared Deep Extragalactic Legacy Survey/GOODS fields, the Hubble Ultra Deep Field, and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1-2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2 × 106 Mpc3 over this epoch, allowing us to perform a robust search for faint 18) and bright (M 21) high-redshift galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 candidate galaxies at 3.5 8.5, with >1000 galaxies at 6-8. We measure both a stepwise luminosity function for candidate galaxies in our redshift samples, and a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end, our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright candidate galaxies at 6. Our best-fit value of the characteristic magnitude is consistent with −21 at 5, which is different than that inferred based on previous trends at lower redshift, and brighter at ∼2 significance than previous measures at z = 6 and 7. At z = 8, a single power law provides an equally good fit to the UV luminosity function, while at z = 6 and 7 an exponential cutoff at the bright end is moderately preferred. We compare our luminosity functions to semi-analytical models, and find that the lack of evolution in is consistent with models where the impact of dust attenuation on the bright end of the luminosity function decreases at higher redshift, although a decreasing impact of feedback may also be possible. We measure the evolution of the cosmic star-formation rate (SFR) density by integrating our observed luminosity functions to , correcting for dust attenuation, and find that the SFR density declines proportionally to (1 ) at 4, which is consistent with observations at 9. Our observed luminosity functions are consistent with a reionization history that starts at 10, completes at 6, and reaches a midpoint (x 0.5) at 6.7 9.4. Finally, using a constant cumulative number density selection and an empirically derived rising star-formation history, our observations predict that the abundance of bright z = 9 galaxies is likely higher than previous constraints, although consistent with recent estimates of bright 10 galaxies.
We present results from near-infrared spectroscopy of 26 emission-line galaxies at z ~ 2.2 and z ~ 1.5 obtained with the Folded-port InfraRed Echellette (FIRE) spectrometer on the 6.5 m Magellan ...Baade telescope. Our FIRE follow-up spectroscopy (R ~ 5000) over 1.0-2.5 mu m permits detailed measurements of the physical properties of the z ~ 2 emission-line galaxies. Dust-corrected star formation rates for the sample range from ~5-100 M sub(middot in circle) yr super(-1) with a mean of 29 M sub(middot in circle) yr super(-1). We derive a median metallicity for the sample of 12 + log(O/H) = 8.34 or ~0.45 Z sub(middot in circle). The average ionization parameter measured for the sample, log U approximately -2.5, is significantly higher than what is found for most star-forming galaxies in the local universe, but similar to the values found for other star-forming galaxies at high redshift. We speculate that the elevated nitrogen abundance could result from substantial numbers of Wolf-Rayet stars in starbursting galaxies at z ~ 2.
Studying giant star-forming clumps in distant galaxies is important to understand galaxy formation and evolution. At present, however, observers and theorists have not reached a consensus on whether ...the observed "clumps" in distant galaxies are the same phenomenon that is seen in simulations. In this paper, as a step to establish a benchmark of direct comparisons between observations and theories, we publish a sample of clumps constructed to represent the commonly observed "clumps" in the literature. This sample contains 3193 clumps detected from 1270 galaxies at 0.5 ≤ z < 3.0 . The clumps are detected from rest-frame UV images, as described in our previous paper. Their physical properties (e.g., rest-frame color, stellar mass ( M * ), star formation rate (SFR), age, and dust extinction) are measured by fitting the spectral energy distribution (SED) to synthetic stellar population models. We carefully test the procedures of measuring clump properties, especially the method of subtracting background fluxes from the diffuse component of galaxies. With our fiducial background subtraction, we find a radial clump U − V color variation, where clumps close to galactic centers are redder than those in outskirts. The slope of the color gradient (clump color as a function of their galactocentric distance scaled by the semimajor axis of galaxies) changes with redshift and M * of the host galaxies: at a fixed M * , the slope becomes steeper toward low redshift, and at a fixed redshift, it becomes slightly steeper with M * . Based on our SED fitting, this observed color gradient can be explained by a combination of a negative age gradient, a negative E(B − V) gradient, and a positive specific SFR gradient of the clumps. We also find that the color gradients of clumps are steeper than those of intra-clump regions. Correspondingly, the radial gradients of the derived physical properties of clumps are different from those of the diffuse component or intra-clump regions.
Abstract
We report the discovery of an accreting supermassive black hole at
z
= 8.679. This galaxy, denoted here as CEERS_1019, was previously discovered as a Ly
α
-break galaxy by Hubble with a Ly
α
...redshift from Keck. As part of the Cosmic Evolution Early Release Science (CEERS) survey, we have observed this source with JWST/NIRSpec, MIRI, NIRCam, and NIRCam/WFSS and uncovered a plethora of emission lines. The H
β
line is best fit by a narrow plus a broad component, where the latter is measured at 2.5
σ
with an FWHM ∼1200 km s
−1
. We conclude this originates in the broadline region of an active galactic nucleus (AGN). This is supported by the presence of weak high-ionization lines (N V, N IV, and C III), as well as a spatial point-source component. The implied mass of the black hole (BH) is log (
M
BH
/
M
⊙
) = 6.95 ± 0.37, and we estimate that it is accreting at 1.2 ± 0.5 times the Eddington limit. The 1–8
μ
m photometric spectral energy distribution shows a continuum dominated by starlight and constrains the host galaxy to be massive (log M/M
⊙
∼9.5) and highly star-forming (star formation rate, or SFR ∼ 30 M
⊙
yr
−1
; log sSFR ∼ − 7.9 yr
−1
). The line ratios show that the gas is metal-poor (
Z
/
Z
⊙
∼ 0.1), dense (
n
e
∼ 10
3
cm
−3
), and highly ionized (log
U
∼ − 2.1). We use this present highest-redshift AGN discovery to place constraints on BH seeding models and find that a combination of either super-Eddington accretion from stellar seeds or Eddington accretion from very massive BH seeds is required to form this object.
Abstract
Observations and simulations of interacting galaxies and mergers in the local universe have shown that interactions can significantly enhance the star formation rates (SFRs) and fueling of ...active galactic nuclei (AGN). However, at higher redshift, some simulations suggest that the level of star formation enhancement induced by interactions is lower due to the higher gas fractions and already increased SFRs in these galaxies. To test this, we measure the SFR enhancement in a total of 2351 (1327) massive (
M
*
> 10
10
M
⊙
) major (1 <
M
1
/
M
2
< 4) spectroscopic galaxy pairs at 0.5 <
z
< 3.0 with Δ
V
< 5000 km s
−1
(1000 km s
−1
) and projected separation <150 kpc selected from the extensive spectroscopic coverage in the COSMOS and CANDELS fields. We find that the highest level of SFR enhancement is a factor of
1.23
−
0.09
+
0.08
in the closest projected separation bin (<25 kpc) relative to a stellar mass-, redshift-, and environment-matched control sample of isolated galaxies. We find that the level of SFR enhancement is a factor of ∼1.5 higher at 0.5 <
z
< 1 than at 1 <
z
< 3 in the closest projected separation bin. Among a sample of visually identified mergers, we find an enhancement of a factor of
1.86
−
0.18
+
0.29
(∼3
σ
) for coalesced systems. For this visually identified sample, we see a clear trend of increased SFR enhancement with decreasing projected separation (2.40
−
0.37
+
0.62
versus
1.58
−
0.20
+
0.29
for 0.5 <
z
< 1.6 and 1.6 <
z
< 3.0, respectively). The SFR enhancements seen in our interactions and mergers are all lower than the level seen in local samples at the same separation, suggesting that the level of interaction-induced star formation evolves significantly over this time period.
We analyze the dependence of galaxy structure (size and Sersic index) and mode of star formation ( Delta *SSFR and SFRIR/SFRUV) on the position of galaxies in the star formation rate (SFR) versus ...mass diagram. Our sample comprises roughly 640,000 galaxies at z ~ 0.1, 130,000 galaxies at z ~ 1, and 36,000 galaxies at z ~ 2. Structural measurements for all but the z ~ 0.1 galaxies are based on Hubble Space Telescope imaging, and SFRs are derived using a Herschel-calibrated ladder of SFR indicators. We find that a correlation between the structure and stellar population of galaxies (i.e., a 'Hubble sequence') is already in place since at least z ~ 2.5. At all epochs, typical star-forming galaxies on the main sequence are well approximated by exponential disks, while the profiles of quiescent galaxies are better described by de Vaucouleurs profiles. In the upper envelope of the main sequence, the relation between the SFR and Sersic index reverses, suggesting a rapid buildup of the central mass concentration in these starbursting outliers. We observe quiescent, moderately and highly star-forming systems to co-exist over an order of magnitude or more in stellar mass. At each mass and redshift, galaxies on the main sequence have the largest size. The rate of size growth correlates with specific SFR, and so does Delta *SSFR at each redshift. A simple model using an empirically determined star formation law and metallicity scaling, in combination with an assumed geometry for dust and stars, is able to relate the observed Delta *SSFR and SFRIR/SFRUV, provided a more patchy dust geometry is assumed for high-redshift galaxies.
Abstract
We present a rest-frame UV–optical (
λ
= 2500–6400 Å) stacked spectrum representative of massive quiescent galaxies at 1.0 <
z
< 1.3 with log(
M
*
/
M
⊙
) > 10.8. The stack is constructed ...using VANDELS survey data, combined with new KMOS observations. We apply two independent full-spectral-fitting approaches, measuring a total metallicity Z/H = −0.13 ± 0.08 with
Bagpipes
and Z/H = 0.04 ± 0.14 with
Alf
, a fall of ∼0.2–0.3 dex compared with the local universe. We also measure an iron abundance Fe/H = −0.18 ± 0.08, a fall of ∼0.15 dex compared with the local universe. We measure the alpha enhancement via the magnesium abundance, obtaining Mg/Fe = 0.23 ± 0.12, consistent with galaxies of similar mass in the local universe, indicating no evolution in the average alpha enhancement of log(
M
*
/
M
⊙
) ∼ 11 quiescent galaxies over the last ∼8 Gyr. This suggests the very high alpha enhancements recently reported for several bright
z
∼ 1–2 quiescent galaxies are due to their extreme masses, log(
M
*
/
M
⊙
) ≳ 11.5, in accordance with the well-known downsizing trend, rather than being typical of the
z
≳ 1 population. The metallicity evolution we observe with redshift (falling Z/H, Fe/H, constant Mg/Fe) is consistent with recent studies. We recover a mean stellar age of
2.5
−
0.4
+
0.6
Gyr, corresponding to a formation redshift
z
form
=
2.4
−
0.3
+
0.6
. Recent studies have obtained varying average formation redshifts for
z
≳ 1 massive quiescent galaxies, and, as these studies report consistent metallicities, we identify models with different star formation histories as the most likely cause. Larger spectroscopic samples from upcoming ground-based instruments will provide precise constraints on ages and metallicities at
z
≳ 1. Combining these with precise stellar mass functions for
z
> 2 quiescent galaxies from the James Webb Space Telescope will provide an independent test of formation redshifts derived from spectral fitting.
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