Abstract
We make use of sensitive (9.3
μ
Jy beam
−1
rms) 1.2 mm continuum observations from the Atacama Large Millimeter/submillimeter Array (ALMA) Spectroscopic Survey in the Hubble Ultra-Deep Field ...(ASPECS) large program to probe dust-enshrouded star formation from 1362 Lyman-break galaxies spanning the redshift range
z
= 1.5–10 (to ∼7–28
M
⊙
yr
−1
at 4
σ
over the entire range). We find that the fraction of ALMA-detected galaxies in our
z
= 1.5–10 samples increases steeply with stellar mass, with the detection fraction rising from 0% at 10
9.0
M
⊙
to
% at >10
10
M
⊙
. Moreover, on stacking all 1253 low-mass (<10
9.25
M
⊙
) galaxies over the ASPECS footprint, we find a mean continuum flux of −0.1 ± 0.4
μ
Jy beam
−1
, implying a hard upper limit on the obscured star formation rate of <0.6
M
⊙
yr
−1
(4
σ
) in a typical low-mass galaxy. The correlation between the infrared excess (IRX) of UV-selected galaxies (
L
IR
/
L
UV
) and the UV-continuum slope is also seen in our ASPECS data and shows consistency with a Calzetti-like relation at >
and an SMC-like relation at lower masses. Using stellar mass and
β
measurements for
z
∼ 2 galaxies over the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, we derive a new empirical relation between
β
and stellar mass and then use this correlation to show that our IRX–
β
and IRX–stellar mass relations are consistent with each other. We then use these constraints to express the IRX as a bivariate function of
β
and stellar mass. Finally, we present updated estimates of star formation rate density determinations at
z
> 3, leveraging present improvements in the measured IRX and recent probes of ultraluminous far-IR galaxies at
z
> 2.
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.
We investigate the CO excitation and interstellar medium (ISM) conditions in a cold gas mass-selected sample of 22 star-forming galaxies at z = 0.46-3.60, observed as part of the ALMA Spectroscopic ...Survey in the Hubble Ultra Deep Field (ASPECS). Combined with Very Large Array follow-up observations, we detect a total of 34 CO transitions with J = 1 up to 8 (and an additional 21 upper limits, up to J = 10) and 6 and transitions (and 12 upper limits). The CO(2-1) and CO(3-2)-selected galaxies, at and 2.5, respectively, exhibit a range in excitation in their mid-J = 4, 5 and high-J = 7, 8 lines, on average lower than ( -brighter) BzK-color- and submillimeter-selected galaxies at similar redshifts. The former implies that a warm ISM component is not necessarily prevalent in gas mass-selected galaxies at . We use stacking and Large Velocity Gradient models to measure and predict the average CO ladders at z < 2 and z ≥ 2, finding and , respectively. From the models, we infer that the galaxies at z ≥ 2 have intrinsically higher excitation than those at z < 2. This fits a picture in which the global excitation is driven by an increase in the star formation rate surface density of galaxies with redshift. We derive a neutral atomic carbon abundance of , comparable to the Milky Way and main-sequence galaxies at similar redshifts, and fairly high densities (≥104 cm−3), consistent with the low-J CO excitation. Our results imply a decrease in the cosmic molecular gas mass density at z ≥ 2 compared to previous ASPECS measurements.
ABSTRACT We present CO(1-0) observations obtained at the Karl G. Jansky Very Large Array for 14 galaxies with existing CO(3-2) measurements, including 11 galaxies that contain active galactic nuclei ...(AGNs) and three submillimeter galaxies (SMGs). We combine this sample with an additional 15 galaxies from the literature that have both CO(1-0) and CO(3-2) measurements in order to evaluate differences in CO excitation between SMGs and AGN host galaxies, to measure the effects of CO excitation on the derived molecular gas properties of these populations, and to look for correlations between the molecular gas excitation and other physical parameters. With our expanded sample of CO(3-2)/CO(1-0) line ratio measurements, we do not find a statistically significant difference in the mean line ratio between SMGs and AGN host galaxies as can be found in the literature; we instead find for AGN host galaxies and for SMGs (or for both populations combined). We also do not measure a statistically significant difference between the distributions of the line ratios for these populations at the p = 0.05 level, although this result is less robust. We find no excitation dependence on the index or offset of the integrated Schmidt-Kennicutt relation for the two CO lines, and we obtain indices consistent with N = 1 for the various subpopulations. However, including low-z "normal" galaxies increases our best-fit Schmidt-Kennicutt index to . While we do not reproduce correlations between the CO line width and luminosity, we do reproduce correlations between CO excitation and star-formation efficiency.
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.
We present a CO and atomic fine-structure line-luminosity function analysis using the ALMA Spectroscopic Survey (ASPECS) in the Hubble Ultra Deep Field. ASPECS consists of two spatially overlapping ...mosaics that cover the entire ALMA 3 mm and 1.2 mm bands. We combine the results of a line-candidate search of the 1.2 mm data cube with those previously obtained from the 3 mm cube. Our analysis shows that ∼80% of the line flux observed at 3 mm arises from CO(2-1) or CO(3-2) emitters at z = 1-3 ("cosmic noon"). At 1.2 mm, more than half of the line flux arises from intermediate-J CO transitions (Jup = 3-6); ∼12% from neutral carbon lines; and <1% from singly ionized carbon, C ii. This implies that future C ii intensity mapping surveys in the epoch of reionization will need to account for a highly significant CO foreground. The CO luminosity functions probed at 1.2 mm show a decrease in the number density at a given line luminosity (in units of L′) at increasing Jup and redshift. Comparisons between the CO luminosity functions for different CO transitions at a fixed redshift reveal subthermal conditions on average in galaxies up to z ∼ 4. In addition, the comparison of the CO luminosity functions for the same transition at different redshifts reveals that the evolution is not driven by excitation. The cosmic density of molecular gas in galaxies, H2, shows a redshift evolution with an increase from high redshift up to z ∼ 1.5 followed by a factor ∼6 drop down to the present day. This is in qualitative agreement with the evolution of the cosmic star formation rate density, suggesting that the molecular gas depletion time is approximately constant with redshift, after averaging over the star-forming galaxy population.
ABSTRACT We make use of deep 1.2 mm continuum observations (12.7 Jy beam−1 rms) of a 1 arcmin2 region in the Hubble Ultra Deep Field to probe dust-enshrouded star formation from 330 Lyman-break ...galaxies spanning the redshift range z = 2-10 (to ∼2-3 M yr−1 at 1 over the entire range). Given the depth and area of ASPECS, we would expect to tentatively detect 35 galaxies, extrapolating the Meurer z ∼ 0 IRX-β relation to z ≥ 2 (assuming dust temperature Td ∼ 35 K). However, only six tentative detections are found at z 2 in ASPECS, with just three at >3 . Subdividing our z = 2-10 galaxy samples according to stellar mass, UV luminosity, and UV-continuum slope and stacking the results, we find a significant detection only in the most massive (>109.75 M ) subsample, with an infrared excess (IRX = LIR/LUV) consistent with previous z ∼ 2 results. However, the infrared excess we measure from our large selection of sub-L∗ (<109.75 M ) galaxies is 0.34 (bootstrap and formal uncertainties) and 0.18 at z = 2-3 and z = 4-10, respectively, lying below even an IRX-β relation for the Small Magellanic Cloud (95% confidence). These results demonstrate the relevance of stellar mass for predicting the IR luminosity of z 2 galaxies. We find that the evolution of the IRX-stellar mass relationship depends on the evolution of the dust temperature. If the dust temperature increases monotonically with redshift ( ) such that Td ∼ 44-50 K at z ≥ 4, current results are suggestive of little evolution in this relationship to z ∼ 6. We use these results to revisit recent estimates of the z ≥ 3 star formation rate density.
ABSTRACT We present the rationale for and the observational description of ASPECS: the ALMA SPECtroscopic Survey in the Hubble Ultra-Deep Field (UDF), the cosmological deep field that has the deepest ...multi-wavelength data available. Our overarching goal is to obtain an unbiased census of molecular gas and dust continuum emission in high-redshift (z > 0.5) galaxies. The ∼1′ region covered within the UDF was chosen to overlap with the deepest available imaging from the Hubble Space Telescope. Our ALMA observations consist of full frequency scans in band 3 (84-115 GHz) and band 6 (212-272 GHz) at approximately uniform line sensitivity ( 2 × 109 K km s−1 pc2), and continuum noise levels of 3.8 Jy beam−1 and 12.7 Jy beam−1, respectively. The molecular surveys cover the different rotational transitions of the CO molecule, leading to essentially full redshift coverage. The C ii emission line is also covered at redshifts . We present a customized algorithm to identify line candidates in the molecular line scans and quantify our ability to recover artificial sources from our data. Based on whether multiple CO lines are detected, and whether optical spectroscopic redshifts as well as optical counterparts exist, we constrain the most likely line identification. We report 10 (11) CO line candidates in the 3 mm (1 mm) band, and our statistical analysis shows that <4 of these (in each band) are likely spurious. Less than one-third of the total CO flux in the low-J CO line candidates are from sources that are not associated with an optical/NIR counterpart. We also present continuum maps of both the band 3 and band 6 observations. The data presented here form the basis of a number of dedicated studies that are presented in subsequent papers.
We present the results from the 1.2 mm continuum image obtained as part of the Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field. The 1.2 mm continuum ...image has a size of 2.9 (4.2) arcmin2 within a primary beam response of 50% (10%) and an rms value of . We detect 35 sources at high significance (Fidelity ≥0.5); 32 have well-characterized near-infrared Hubble Space Telescope counterparts. We estimate the 1.2 mm number counts to flux levels of in two different ways: we first use the detected sources to constrain the number counts and find a significant flattening of the counts below S ∼ 0.1 mJy. In a second approach, we constrain the number counts using a probability of deflection statistics (P(D)) analysis. For this latter approach, we describe new methods to accurately measure the noise in interferometric imaging (employing jackknifing in the cube and in the visibility plane). This independent measurement confirms the flattening of the number counts. Our analysis of the differential number counts shows that we are detecting ∼93% (∼100% if we include the lower fidelity detections) of the total continuum dust emission associated with galaxies in the Hubble Ultra Deep Field. The ancillary data allow us to study the dependence of the 1.2 mm number counts on redshift (z = 0−4), galaxy dust mass ( ), stellar mass ( ), and star formation rate ( ). In an accompanying paper we show that the number counts are crucial to constrain galaxy evolution models and the understanding of star-forming galaxies at high redshift.
We analyze the interstellar medium properties of a sample of 16 bright CO line emitting galaxies identified in the ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) Large Program. ...This CO−selected galaxy sample is complemented by two additional CO line emitters in the UDF that are identified based on their Multi-Unit Spectroscopic Explorer (MUSE) optical spectroscopic redshifts. The ASPECS CO−selected galaxies cover a larger range of star formation rates (SFRs) and stellar masses compared to literature CO emitting galaxies at z > 1 for which scaling relations have been established previously. Most of ASPECS CO-selected galaxies follow these established relations in terms of gas depletion timescales and gas fractions as a function of redshift, as well as the SFR-stellar mass relation ("galaxy main sequence"). However, we find that ∼30% of the galaxies (5 out of 16) are offset from the galaxy main sequence at their respective redshift, with ∼12% (2 out of 16) falling below this relationship. Some CO-rich galaxies exhibit low SFRs, and yet show substantial molecular gas reservoirs, yielding long gas depletion timescales. Capitalizing on the well-defined cosmic volume probed by our observations, we measure the contribution of galaxies above, below, and on the galaxy main sequence to the total cosmic molecular gas density at different lookback times. We conclude that main-sequence galaxies are the largest contributors to the molecular gas density at any redshift probed by our observations (z ∼ 1−3). The respective contribution by starburst galaxies above the main sequence decreases from z ∼ 2.5 to z ∼ 1, whereas we find tentative evidence for an increased contribution to the cosmic molecular gas density from the passive galaxies below the main sequence.
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