We report the first detailed measurement of the shape of the CO luminosity function at high redshift, based on >320 hr of the NSF's Karl G. Jansky Very Large Array (VLA) observations over an area of ...∼60 arcmin2 taken as part of the CO Luminosity Density at High Redshift (COLDz) survey. COLDz "blindly" selects galaxies based on their cold gas content through CO(J = 1 → 0) emission at z ∼ 2-3 and CO(J = 2 → 1) at z ∼ 5-7 down to a CO luminosity limit of log( /K km s−1 pc2) 9.5. We find that the characteristic luminosity and bright end of the CO luminosity function are substantially higher than predicted by semi-analytical models, but consistent with empirical estimates based on the infrared luminosity function at z ∼ 2. We also present the currently most reliable measurement of the cosmic density of cold gas in galaxies at early epochs, i.e., the cold gas history of the universe, as determined over a large cosmic volume of ∼375,000 Mpc3. Our measurements are in agreement with an increase of the cold gas density from z ∼ 0 to z ∼ 2-3, followed by a possible decline toward z ∼ 5-7. These findings are consistent with recent surveys based on higher-J CO line measurements, upon which COLDz improves in terms of statistical uncertainties by probing ∼50-100 times larger areas and in the reliability of total gas mass estimates by probing the low-J CO lines accessible to the VLA. Our results thus appear to suggest that the cosmic star formation rate density follows an increased cold molecular gas content in galaxies toward its peak about 10 billion years ago, and that its decline toward the earliest epochs is likely related to a lower overall amount of cold molecular gas (as traced by CO) bound in galaxies toward the first billion years after the Big Bang.
A key goal of the Stage IV dark energy experiments Euclid, LSST, and WFIRST is to measure the growth of structure with cosmic time from weak lensing analysis over large regions of the sky. Weak ...lensing cosmology will be challenging: in addition to highly accurate galaxy shape measurements, statistically robust and accurate photometric redshift (photo-z) estimates for billions of faint galaxies will be needed in order to reconstruct the three-dimensional matter distribution. Here we present an overview of and initial results from the Complete Calibration of the Color-Redshift Relation (C3R2) survey, which is designed specifically to calibrate the empirical galaxy color-redshift relation to the Euclid depth. These redshifts will also be important for the calibrations of LSST and WFIRST. The C3R2 survey is obtaining multiplexed observations with Keck (DEIMOS, LRIS, and MOSFIRE), the Gran Telescopio Canarias (GTC; OSIRIS), and the Very Large Telescope (VLT; FORS2 and KMOS) of a targeted sample of galaxies that are most important for the redshift calibration. We focus spectroscopic efforts on undersampled regions of galaxy color space identified in previous work in order to minimize the number of spectroscopic redshifts needed to map the color-redshift relation to the required accuracy. We present the C3R2 survey strategy and initial results, including the 1283 high-confidence redshifts obtained in the 2016A semester and released as Data Release 1.
Abstract
The redshift range
z
= 4–6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. ...The study of galaxies in this redshift range is therefore important to understanding early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star formation history (SFH) of 221
z
∼ 4.5 main-sequence galaxies at
by comparing their ultra-violet (UV) continuum, H
α
luminosity, and H
α
equivalent-width (EW). The H
α
properties are derived from the
Spitzer
3.6
μ
m−4.5
μ
m broadband color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between H
α
- and UV-derived luminosities and star formation rates (SFRs). About half of the galaxies show a significant excess in H
α
compared to expectations from a constant smooth SFH. We also find a tentative anticorrelation between H
α
EW and stellar mass, ranging from 1000 Å at
to below 100 Å at
. Consulting models suggests that most
z
∼ 4.5 galaxies had a burst of star formation within the last 50 Myr, increasing their SFRs by a factor of >5. The most massive galaxies on the other hand might decrease their SFRs and may be transitioning to a quiescent stage by
z
= 4. We identify differential dust attenuation (
f
) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing H
α
EW (reaching values similar to high-
z
galaxies), we predict that
f
approaches unity at
z
> 4, consistent with the extrapolation of measurements out to
z
= 2.
Using the Very Large Array and ALMA, we have obtained CO(2-1), C ii, and N ii line emission and multiple dust continuum measurements in a sample of "normal" galaxies at z = 5-6. We report the ...highest-redshift detection of low-J CO emission from a Lyman break galaxy, at z ∼ 5.7. The CO line luminosity implies a massive molecular gas reservoir of (1.3 0.3)( CO/4.5 M (K km s−1 pc2)−1) × 1011 M , suggesting low star formation efficiency with a gas depletion timescale of order ∼1 Gyr. This efficiency is much lower than traditionally observed in z 5 starbursts, indicating that star-forming conditions in main-sequence galaxies at z ∼ 6 may be comparable to those of normal galaxies probed up to z ∼ 3 to date but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a main-sequence galaxy at z ∼ 5.3 with an approximately three times lower star formation rate, perhaps implying a high CO conversion factor, as typically found in low-metallicity galaxies. For a sample including both CO targets, we also find faint N ii 205 m emission relative to C ii in all but the most IR-luminous "normal" galaxies at z = 5-6, implying more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties suggest that low metallicity may be common in typical ∼1010 M galaxies at z = 5-6. While a fraction of main-sequence star formation in the first billion yr may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.
We report the serendipitous discovery of a dusty, starbursting galaxy at z = 5.667 (hereafter called CRLE) in close physical association with the "normal" main-sequence galaxy HZ10 at z = 5.654. CRLE ...was identified by detection of C ii, N ii, and CO(2-1) line emission, making it the highest-redshift, most luminous starburst in the COSMOS field. This massive, dusty galaxy appears to be forming stars at a rate of at least 1500 M yr−1 in a compact region only ∼3 kpc in diameter. The dynamical and dust emission properties of CRLE suggest an ongoing merger driving the starburst, which is in a potentially intermediate stage relative to other known dusty galaxies at the same epoch. The ratio of C ii to N ii may suggest that an important (∼15%) contribution to the C ii emission comes from a diffuse ionized gas component, which could be more extended than the dense, starbursting gas. CRLE appears to be located in a significant galaxy overdensity at the same redshift, potentially associated with a large-scale cosmic structure recently identified in a Lyman -emitter survey. This overdensity suggests that CRLE and HZ10 reside in a protocluster environment, offering the tantalizing opportunity to study the effect of a massive starburst on protocluster star formation. Our findings support the interpretation that a significant fraction of the earliest galaxy formation may occur from the inside out, within the central regions of the most massive halos, while rapidly evolving into the massive galaxy clusters observed in the local universe.
We report the detection of CO(J = 2 → 1) emission from three massive dusty starburst galaxies at z > 5 through molecular line scans in the NSF's Karl G. Jansky Very Large Array (VLA) CO Luminosity ...Density at High Redshift (COLDz) survey. Redshifts for two of the sources, HDF 850.1 (z = 5.183) and AzTEC-3 (z = 5.298), were previously known. We revise a previous redshift estimate for the third source GN10 (z = 5.303), which we have independently confirmed through detections of CO J = 1 → 0, 5 → 4, 6 → 5, and C ii 158 m emission with the VLA and the NOrthern Extended Milllimeter Array. We find that two currently independently confirmed CO sources in COLDz are "optically dark", and that three of them are dust-obscured galaxies at z > 5. Given our survey area of ∼60 arcmin2, our results appear to imply a ∼6-55 times higher space density of such distant dusty systems within the first billion years after the Big Bang than previously thought. At least two of these z > 5 galaxies show star formation rate surface densities consistent with so-called "maximum" starbursts, but we find significant differences in CO excitation between them. This result may suggest that different fractions of the massive gas reservoirs are located in the dense, star-forming nuclear regions-consistent with the more extended sizes of the C ii emission compared to the dust continuum and higher C ii-to-far-infrared luminosity ratios in those galaxies with lower gas excitation. We thus find substantial variations in the conditions for star formation between z > 5 dusty starbursts, which typically have dust temperatures that are ∼57% 25% warmer than starbursts at z = 2-3 due to their enhanced star formation activity.
The Complete Calibration of the Color-Redshift Relation (C3R2) survey is a multi-institution, multi-instrument survey that aims to map the empirical relation of galaxy color to redshift to i ∼ 24.5 ...(AB), thereby providing a firm foundation for weak lensing cosmology with the Stage IV dark energy missions Euclid and WFIRST. Here we present 3171 new spectroscopic redshifts obtained in the 2016B and 2017A semesters with a combination of DEIMOS, LRIS, and MOSFIRE on the Keck telescopes.13 The observations come from all of the Keck partners: Caltech, NASA, the University of Hawaii, and the University of California. Combined with the 1283 redshifts published in DR1, the C3R2 survey has now obtained and published 4454 high-quality galaxy redshifts. We discuss updates to the survey design and provide a catalog of photometric and spectroscopic data. Initial tests of the calibration method performance are given, indicating that the sample, once completed and combined with extensive data collected by other spectroscopic surveys, should allow us to meet the cosmology requirements for Euclid, and make significant headway toward solving the problem for WFIRST. We use the full spectroscopic sample to demonstrate that galaxy brightness is weakly correlated with redshift once a galaxy is localized in the Euclid or WFIRST color space, with potentially important implications for the spectroscopy needed to calibrate redshifts for faint WFIRST and LSST sources.
ABSTRACT
We perform a comprehensive analysis of the redshift evolution of the rest-UV spectra of star-forming galaxies out to z ∼ 5. We combine new z ∼ 5 measurements of H i Ly α and low- and ...high-ionization interstellar metal absorption features with comparable measurements at z ∼ 2–4. We measure the equivalent widths of interstellar absorption features using stacked spectra in bins of Ly α equivalent width, performing corrections to Ly α strengths based on a model for the transmission of the intergalactic medium. We find a strong correlation between decreasing low-ionization absorption strength and increasing Ly α emission strength over the redshift range z ∼ 2–5, suggesting that both of these quantities are fundamentally linked to neutral gas covering fraction. At the highest Ly α equivalent widths, we observe evolution at z ∼ 5 towards greater Ly α emission strength at fixed low-ionization absorption strength. If we interpret the non-evolving relationship of Ly α emission strength and low-ionization line strength at z ∼ 2−4 as primarily reflecting the radiative transfer of Ly α photons, this evolution at z ∼ 5 suggests a higher intrinsic production rate of Ly α photons than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Ly α emission strength, interstellar absorption strength, and dust reddening. We perform additional analysis in bins of stellar mass, star formation rate, UV luminosity, and age, examining how the relationships between galaxy properties and Ly α emission evolve towards higher redshift. We conclude that increasing intrinsic Ly α photon production and strong detection of nebular C iv emission (signalling lower metallicity) at z ∼ 5 indicate an elevated ionized photon production efficiency (ξion).
ABSTRACT We report interferometric measurements of N ii 205 m fine-structure line emission from a representative sample of three galaxies at z = 5-6 using the Atacama Large (sub)Millimeter Array ...(ALMA). These galaxies were previously detected in C ii and far-infrared continuum emission and span almost two orders of magnitude in star formation rate (SFR). Our results show at least two different regimes of ionized interstellar medium properties for galaxies in the first billion years of cosmic time, separated by their ratio. We find extremely low N ii emission compared to C ii ( ) from a "typical" star-forming galaxy, likely directly or indirectly (by its effect on the radiation field) related to low dust abundance and low metallicity. The infrared-luminous modestly star-forming Lyman-break galaxy (LBG) in our sample is characterized by an ionized-gas fraction ( typical of local star-forming galaxies and shows evidence for spatial variations in its ionized-gas fraction across an extended gas reservoir. The extreme SFR, warm and compact dusty starburst AzTEC-3 shows an ionized fraction higher than expected given its SFR surface density ( ) suggesting that N ii dominantly traces a diffuse ionized medium rather than star-forming H ii regions in this type of galaxy. This highest redshift sample of N ii detections provides some of the first constraints on ionized and neutral gas modeling attempts and on the structure of the interstellar medium at z = 5-6 in "normal" galaxies and starbursts.
Abstract
We investigate Ly
α
, O
iii
λ
5007, H
α
, and C
ii
158
μ
m emission from 1124 galaxies at
z
= 4.9–7.0. Our sample is composed of 1092 Ly
α
emitters (LAEs) at
z
= 4.9, 5.7, 6.6, and 7.0 ...identified by Subaru/Hyper-Suprime-Cam (HSC) narrowband surveys covered by
Spitzer
Large Area Survey with Hyper-Suprime-Cam (SPLASH) and 34 galaxies at
z
= 5.148–7.508 with deep ALMA C
ii
158
μ
m data in the literature. Fluxes of strong rest-frame optical lines of O
iii
and H
α
(H
β
) are constrained by significant excesses found in the SPLASH 3.6 and 4.5
μ
m photometry. At
z
= 4.9, we find that the rest-frame H
α
equivalent width and the Ly
α
escape fraction
f
Ly
α
positively correlate with the rest-frame Ly
α
equivalent width
. The
correlation is similarly found at
z
∼ 0–2, suggesting no evolution of the correlation over
z
≃ 0–5. The typical ionizing photon production efficiency of LAEs is log(
ξ
ion
/Hz erg
−1
) ≃ 25.5, significantly (60%–100%) higher than those of LBGs at a given UV magnitude. At
z
= 5.7–7.0, there exists an interesting turnover trend that the O
iii
/H
α
flux ratio increases in
and then decreases out to
. We also identify an anticorrelation between a ratio of C
ii
luminosity to star formation rate (
L
C
ii
/SFR) and
at the >99% confidence level.. We carefully investigate physical origins of the correlations with stellar-synthesis and photoionization models and find that a simple anticorrelation between
and metallicity explains self-consistently all of the correlations of Ly
α
, H
α
, O
iii
/H
α
, and C
ii
identified in our study, indicating detections of metal-poor (∼0.03
Z
⊙
) galaxies with
.
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