We present results from the first robust investigation of galaxy morphology as a function of environment at z > 1.5. Our study is motivated by the fact that star-forming galaxies contained within a ...protocluster at z = 2.3 in the HS 1700+64 field have significantly older ages and larger stellar masses on average than those at similar redshifts but more typical environmental densities. In the analysis of HST ACS images, we apply nonparametric statistics to characterize the rest-frame UV morphologies of a sample of 85 UV-selected star-forming galaxies at z = 1.7 2.9, 22 of which are contained in the protocluster. The remaining 63 control sample galaxies are not in the protocluster but have a similar mean redshift of z similar to 2.3. We find no environmental dependence for the distributions of morphological properties. Combining the measured morphologies with the results of population synthesis modeling, we find only weak correlations, if any, between morphological and steilar population properties such as stellar mass, age, extinction, and star formation rate. Given the incomplete census of the protocluster galaxy population, and the lack of correlation between rest-frame UV morphology and star formation history at z similar to 2 within our sample, the absence of environmental trends in the distribution of morphological properties is not surprising. In addition, using a larger sample of photometric candidates, we compare morphological distributions for 282 UV-selected and 43 near-IR-selected galaxies. While the difference in the degree of nebulosity between the two samples appears to be a by-product of the fainter average rest-frame UV surface brightness of the near-IR-selected galaxies, we find that, among the lowest surface brightness galaxies, the near-IR-selected objects have significantly smaller angular sizes.
Increasingly large samples of galaxies are now being discovered at redshifts z {approx} 5-6 and higher. Many of these objects are inferred to be young, low in mass, and relatively unreddened, but ...detailed analysis of their high quality spectra will not be possible until the advent of future facilities. In this paper, we shed light on the physical conditions in a plausibly similar low-mass galaxy by presenting the analysis of the rest-frame optical and UV spectra of Q2343-BX418, an L* galaxy at z = 2.3 with a very low mass-to-light ratio and unusual properties: BX418 is young (<100 Myr), low mass (M {sub *} {approx} 10{sup 9} M{sub sun}), low in metallicity (Z {approx} 1/6 Z{sub sun}), and unreddened (E(B - V) {approx_equal} 0.02, UV continuum slope {beta} = -2.1). We infer a metallicity 12 + log(O/H) = 7.9 {+-} 0.2 from the rest-frame optical emission lines. We also determine the metallicity via the direct, electron temperature method, using the ratio O III {lambda}{lambda}1661, 1666/O III {lambda}5007 to determine the electron temperature and finding 12 + log(O/H) = 7.8 {+-} 0.1. These measurements place BX418 among the most metal-poor galaxies observed in emission at high redshift. The rest-frame UV spectrum, which represents {approx}12 hr of integration with the Keck telescope, contains strong emission from Ly{alpha} (with rest-frame equivalent width 54 A), He II {lambda}1640 (both stellar and nebular), C III {lambda}{lambda}1907, 1909 and O III {lambda}{lambda}1661, 1666. The C IV/C III ratio indicates that the source of ionization is unlikely to be an active galactic nucleus. Analysis of the He II, O III, and C III line strengths indicates a very high ionization parameter log U {approx} -1, while Ly{alpha} and the interstellar absorption lines indicate that outflowing gas is highly ionized over a wide range of velocities. It remains to be determined how many of BX418's unique spectral features are due to its global properties, such as low metallicity and dust extinction, and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.
Abstract
We present ultradeep Keck/MOSFIRE rest-optical spectra of two star-forming galaxies at
z
= 2.18 in the COSMOS field with bright emission lines, representing more than 20 hr of total ...integration. The fidelity of these spectra enabled the detection of more than 20 unique emission lines for each galaxy, including the first detection of the auroral O
ii
λλ
7322, 7332 lines at high redshift. We use these measurements to calculate the electron temperature in the low-ionization O
+
zone of the ionized interstellar medium and derive abundance ratios of O/H, N/H, and N/O using the direct method. The N/O and
α
/Fe abundance patterns of these galaxies are consistent with rapid formation timescales and ongoing strong starbursts, in accord with their high specific star formation rates. These results demonstrate the feasibility of using auroral O
ii
measurements for accurate metallicity studies at high redshift in a higher-metallicity and lower-excitation regime previously unexplored with the direct method in distant galaxies. These results also highlight the difficulty in obtaining the measurements required for direct-method metallicities from the ground. We emphasize the advantages that the JWST/NIRSpec instrument will bring to high-redshift metallicity studies, where the combination of increased sensitivity and uninterrupted wavelength coverage will yield more than an order of magnitude increase in efficiency for multiplexed auroral-line surveys relative to current ground-based facilities. Consequently, the advent of JWST promises to be the beginning of a new era of precision chemical abundance studies of the early universe at a level of detail rivaling that of local galaxy studies.
Abstract
We present observations of CO(3−2) in 13 main-sequence
z
= 2.0–2.5 star-forming galaxies at
log
(
M
*
/
M
⊙
)
=
10.2
–
10.6
that span a wide range in metallicity (O/H) based on rest-optical ...spectroscopy. We find that
L
CO
(
3
−
2
)
′
/SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at
z
∼ 2. We constrain the CO-to-H
2
conversion factor (
α
CO
) and find that
α
CO
inversely correlates with metallicity at
z
∼ 2. We derive molecular gas masses (
M
mol
) and characterize the relations among
M
*
, SFR,
M
mol
, and metallicity. At
z
∼ 2,
M
mol
increases and the molecular gas fraction (
M
mol
/
M
*
) decreases with increasing
M
*
, with a significant secondary dependence on SFR. Galaxies at
z
∼ 2 lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR−
M
*
, O/H−
M
*
, and
M
mol
−
M
*
relations is correlated such that, at fixed
M
*
,
z
∼ 2 galaxies with larger
M
mol
have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at
z
∼ 2, where O/H is inversely correlated with both SFR and
M
mol
at fixed
M
*
. These results suggest that the scatter of the
z
∼ 2 star-forming main sequence, mass–metallicity relation, and
M
mol
–
M
*
relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive
z
∼ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium.
Abstract We use medium-resolution JWST/NIRSpec observations from the Cosmic Evolution Early Release Science Survey to place the first constraints on dust attenuation and star formation based on ...Paschen lines for a sizable sample of 63 galaxies at redshifts z = 1.0–3.1. Our analysis indicates strong correlations between the Balmer decrement, H α /H β , and line ratios that include Paschen lines (i.e., Pa α /H β , Pa β /H β , and the Paschen decrement, Pa α /Pa β ), suggesting that the former is sensitive to the overall dust obscuration toward H ii regions in high-redshift galaxies. The line ratios are used to derive nebular reddening, E ( B − V ) neb , and star formation rates (SFRs). There is marginal evidence that the SFRs deduced from Paschen lines may exceed by ≈25% those derived from Balmer lines alone, suggesting the presence of star formation that is optically thick in Balmer lines, though deeper observations are needed to confirm this result. Using the Paschen-line constraints on the bolometric SFRs, we reevaluate the relationship between dust obscuration and UV spectral slope, and find a reddening of the UV continuum that, on average, follows the SMC extinction curve. This analysis highlights the need for deeper spectroscopy of more representative samples to evaluate nebular dust attenuation and bolometric SFRs in high-redshift galaxies, and their relationship to the reddening of the UV continuum.
Abstract
We present results from the MOSFIRE Deep Evolution Field survey on broad flux from the nebular emission lines H
α
, N
ii
, O
iii
, H
β
, and S
ii
. The sample consists of 127 star-forming ...galaxies at 1.37 <
z
< 2.61 and 84 galaxies at 2.95 <
z
< 3.80. We decompose the emission lines using narrow and broad Gaussian components that we define as having FWHM < 275 km s
−1
and FWHM > 300 km s
−1
, respectively, for both individual galaxies and stacks. For individual galaxies, broad emission is detected at >3
σ
in <10% of galaxies and the broad flux accounts for 10%–70% of the total flux. In the stacks, we find a slight increase in broad to narrow flux ratio with mass but note that we cannot reliably detect broad emission with FWHM < 275 km s
−1
, which may be significant at low masses. When placed on the N2-BPT diagram (O
iii
/H
β
versus N
ii
/H
α
), the broad components of the stacks are shifted toward higher O
iii
/H
β
and N
ii
/H
α
ratios compared to the narrow component. We compare the location of the broad components to shock models and find that the broad component could be explained as a shocked outflow, but we do not rule out other possibilities, such as the presence of an AGN. We discuss the possible consequences of shocked emission on the galaxy location in emission line diagnostic diagrams and calculation of SFR. We attempt to estimate the mass outflow rate/star formation rate, but caution that our results strongly rely on the assumptions regarding the physical properties of the outflow.
ABSTRACT We present gas kinematics for 178 star-forming galaxies at from the MOSFIRE Deep Evolution Field survey. We have developed models to interpret the kinematic measurements from fixed-angle ...multi-object spectroscopy, using structural parameters derived from Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey Hubble Space Telescope/F160W imaging. For 35 galaxies, we measure resolved rotation with a median of . We derive dynamical masses from the kinematics and sizes and compare them to baryonic masses, with gas masses estimated from dust-corrected star formation rates (SFRs) and the Kennicutt-Schmidt relation. When assuming that galaxies with and without observed rotation have the same median , we find good agreement between the dynamical and baryonic masses, with a scatter of and a median offset of . This comparison implies a low dark matter fraction (8% within an effective radius) for a Chabrier initial mass function (IMF), and disfavors a Salpeter IMF. Moreover, the requirement that / should be independent of inclination yields a median value of for galaxies without observed rotation. If, instead, we treat the galaxies without detected rotation as early-type galaxies, the masses are also in reasonable agreement ( , ). The inclusion of gas masses is critical in this comparison; if gas masses are excluded, there is an increasing trend of / with higher specific SFR (SSFR). Furthermore, we find indications that decreases with increasing SSFR for our full sample, which may reflect disk settling. We also study the Tully-Fisher relation and find that at fixed stellar mass was higher at earlier times. At fixed baryonic mass, we observe the opposite trend. Finally, the baryonic and dynamical masses of the active galactic nuclei in our sample are also in excellent agreement, suggesting that the kinematics trace the host galaxies.
We present results on the dust attenuation curve of z, ~ 2 galaxies using early observations from the MOSFIRE Deep Evolution Field survey. Our sample consists of 224 star-forming galaxies with z ...sub(spec) = 1.36-2.59 and high signal-to-noise ratio measurements of H alpha and H beta obtained with Keck/MOSFIRE. We construct composite spectral energy distributions (SEDs) of galaxies in bins of Balmer decrement to measure the attenuation curve. We find a curve that is similar to the SMC extinction curve at lambda gap 2500 A. At shorter wavelengths, the shape is identical to that of the Calzetti et al. relation, but with a lower normalization. Hence, the new attenuation curve results in star formation rates (SFRs) that are approximately 20% lower, and stellar masses that are Delta log(M super(*)/M sub(middot in circle))Asymptotically = to 0.16 dex lower, than those obtained with the Calzetti relation. We find that the difference in the total attenuation of the ionized gas and stellar continuum correlates strongly with SFR, such that for dust-corrected SFRs gap 20 M sub(middot in circle) yr super(-1), assuming a Chabrier initial mass function, the nebular emission lines suffer an increasing degree of obscuration relative to the continuum. A simple model that can account for these trends is one in which the UV through optical stellar continuum is dominated by a population of less-reddened stars, while the nebular line and bolometric luminosities become increasingly dominated by dustier stellar populations for galaxies with large SFRs, as a result of the increased dust enrichment that accompanies such galaxies. Consequently, UV- and SED-based SFRs may underestimate the total SFR at even modest levels of approximately 20 M sub(middot in circle)yr super(-1).
Abstract
We present rest-optical spectroscopic properties of a sample of four galaxies in the Atacama Large Millimeter/submillimeter Array Hubble Ultra Deep Field (ALMA HUDF). These galaxies span the ...redshift range
and the stellar mass range
. They have existing far-infrared and radio measurements of dust-continuum and molecular gas emission from which bolometric star formation rates (SFRs), dust masses, and molecular gas masses have been estimated. We use new
H
- and
K
-band near-infrared spectra from the Keck/Multi-object Spectrometer for Infrared Exploration (MOSFIRE) to estimate SFRs from dust-corrected H
α
emission (SFR(H
α
)) and gas-phase oxygen abundances from the ratio N
ii
/H
α
. We find that the dust-corrected SFR(H
α
) is systematically lower than the bolometric SFR by a factor of several, and measure gas-phase oxygen abundances in a narrow range,
(
). Relative to a large
z
∼ 2 comparison sample from the MOSFIRE Deep Evolution Field (MOSDEF) survey, the ALMA HUDF galaxies scatter roughly symmetrically around the best-fit linear mass–metallicity relation, providing tentative evidence for a flattening in the SFR dependence of metallicity at high stellar mass. Combining oxygen abundances with estimates of dust and molecular gas masses, we show that there is no significant evolution in the normalization of the dust-to-gas ratio (DGR) versus metallicity relation from
z
∼ 0 to
z
∼ 2. This result is consistent with some semi-analytic models and cosmological simulations describing the evolution of dust in galaxies. Tracing the actual form of the DGR versus metallicity relation at high redshift now requires combined measurements of dust, gas, and metallicity over a significantly wider range in metallicity.
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