Abstract We present the first estimate, based on direct H i 21 cm observations, of the H i mass function (H i MF) of star-forming galaxies at z ≈ 1, obtained by combining our measurement of the scaling relation between H i mass ( M H i ) and B -band luminosity ( M B ) of star-forming galaxies with a literature estimate of the B -band luminosity function at z ≈ 1. We determined the M H i – M B relation by using the GMRT-CAT z 1 survey of the DEEP2 fields to measure the average H i mass of blue galaxies at z = 0.74–1.45 in three separate M B subsamples. This was done by separately stacking the H i 21 cm emission signals of the galaxies in each subsample to detect, at (3.5–4.4) σ significance, the average H i 21 cm emission of each subsample. We find that the M H i – M B relation at z ≈ 1 is consistent with that at z ≈ 0. We combine our estimate of the M H i – M B relation at z ≈ 1 with the B -band luminosity function at z ≈ 1 to determine the H i MF at z ≈ 1. We find that the number density of galaxies with M H i > 10 10 M ⊙ (higher than the knee of the local H i mass function) at z ≈ 1 is a factor of ≈4–5 higher than that at z ≈ 0, for a wide range of assumed scatters in the M H i – M B relation. We rule out the hypothesis that the number density of galaxies with M H i > 10 10 M ⊙ remains unchanged between z ≈ 1 and z ≈ 0 at ≳99.7% confidence. This is the first statistically significant evidence for evolution in the H i MF of galaxies from the epoch of cosmic noon.
We derive the specific baryonic angular momentum of five gas-rich dwarf galaxies from H i kinematics complemented by stellar mass profiles. Since the gas mass of these galaxies is much larger than the stellar mass, the angular momentum can be determined with relatively little uncertainty arising from the uncertainties in the stellar mass-to-light ratio. We compare the relation between the specific baryonic angular momentum (j) and the total baryonic mass (M) for these galaxies with that found for spiral galaxies. Our combined sample explores the j–M plane over three orders of magnitude in baryon mass. We find that our sample dwarf has significantly higher specific angular momentum than expected from the relation found for spiral galaxies. The probability that these gas-rich dwarf galaxies follow the same relation as spirals is found to be <10−6. This implies a difference in the evolution of angular momentum in these galaxies compared to larger ones. We suggest that this difference could arise due to one or more of the following: a lower baryon fraction in dwarf galaxies, particularly that arising from preferential outflows low angular momentum gas as found in high-resolution simulations that include baryonic feedback; 'cold mode' anisotropic accretion from cosmic filaments. Our work reinforces the importance of the j–M plane in understanding the evolution of galaxies.
Abstract Measurements of the atomic hydrogen (H i ) properties of high-redshift galaxies are critical to understanding the decline in the star formation rate (SFR) density of the universe after its peak ≈8–11 Gyr ago. Here, we use ≈510 hr of observations with the upgraded Giant Metrewave Radio Telescope to measure the dependence of the average H i mass of star-forming galaxies at z = 0.74–1.45 on their average stellar mass and redshift by stacking their H i 21 cm emission signals. We divide our sample of 11,419 main-sequence galaxies at z = 0.74–1.45 into two stellar-mass ( M * ) subsamples, with M * > 10 10 M ⊙ and M * < 10 10 M ⊙ , and obtain clear detections, at >4.6 σ significance, of the stacked H i 21 cm emission in both subsamples. We find that galaxies with M * > 10 10 M ⊙ , which dominate the decline in the cosmic SFR density at z ≲ 1, have H i reservoirs that can sustain their SFRs for only a short period, 0.86 ± 0.20 Gyr, unless their H i is replenished via accretion. We also stack the H i 21 cm emission from galaxies in two redshift subsamples, at z = 0.74–1.25 and z = 1.25–1.45, again obtaining clear detections of the stacked H i 21 cm emission signals, at >5.2 σ significance in both subsamples. We find that the average H i mass of galaxies with 〈 M * 〉 ≈ 10 10 M ⊙ declines steeply over a period of ≈1 billion years, from (33.6 ± 6.4) × 10 9 M ⊙ at 〈 z 〉 ≈ 1.3 to (10.6 ± 1.9) × 10 9 M ⊙ at 〈 z 〉 ≈ 1.0, i.e., by a factor ≳3. We thus find direct evidence that accretion of H i onto star-forming galaxies at z ≈ 1 is insufficient to replenish their H i reservoirs and sustain their SFRs, thus resulting in the decline in the cosmic SFR density 8 billion years ago.
We present the first estimate of the HI mass function (HIMF) of star-forming galaxies at $z\approx1$, obtained by combining our measurement of the scaling relation between HI mass ($M_{HI}$) and B-band luminosity ($M_B$) of star-forming galaxies with literature estimates of the B-band luminosity function at $z\approx1$. We determined the $M_{HI}-M_B$ relation by using the GMRT-CATz1 survey of the DEEP2 fields to measure the average HI mass of blue galaxies at $z=0.74-1.45$ in three separate $M_B$ subsamples. This was done by separately stacking the HI 21 cm emission signals of the galaxies in each subsample to detect, at (3.5-4.4)$\sigma$ significance, the average HI 21 cm emission of each subsample. We find that the $M_{HI}-M_B$ relation at $z\approx1$ is consistent with that at $z\approx0$. We combine our estimate of the $M_{HI}-M_B$ relation at $z\approx1$ with the B-band luminosity function at $z\approx1$ to determine the HIMF at $z\approx1$. We find that the number density of galaxies with $M_{HI}>10^{10} M_\odot$ (higher than the knee of the local HIMF) at $z\approx1$ is a factor of $\approx4-5$ higher than that at $z\approx0$, for a wide range of assumed scatters in the $M_{HI}-M_B$ relation. We rule out the hypothesis that the number density of galaxies with $M_{HI}>10^{10} M_\odot$ remains unchanged between $z \approx 1$ and $z\approx0$ at $\gtrsim99.7$\% confidence. This is the first statistically significant evidence for evolution in the HIMF of galaxies from the epoch of cosmic noon.
We describe the design, data analysis, and basic results of the Giant Metrewave Radio Telescope Cold-HI AT $z\approx1$ (GMRT-CAT$z$1) survey, a 510-hour upgraded GMRT HI 21 cm emission survey of galaxies at $z=0.74-1.45$ in the DEEP2 survey fields. The GMRT-CAT$z$1 survey is aimed at characterising HI in galaxies during and just after the epoch of peak star-formation activity in the Universe, a key epoch in galaxy evolution. We obtained high-quality HI 21 cm spectra for 11,419 blue star-forming galaxies at $z=0.74-1.45$, in seven pointings on the DEEP2 subfields. We detect the stacked HI 21 cm emission signal of the 11,419 star-forming galaxies, which have an average stellar mass of $M_* \approx 10^{10} M_\odot$, at $7.1\sigma$ statistical significance, obtaining an average HI mass of $\langle M_{HI}\rangle=(13.7\pm1.9)\times10^{9} M_\odot$. This is significantly higher than the average HI mass of $\langle M_{HI} \rangle=(3.96 \pm 0.17)\times10^{9} M_\odot$ in star-forming galaxies at $z \approx 0$ with an identical stellar-mass distribution. We stack the rest-frame 1.4 GHz continuum emission of our 11,419 galaxies to infer an average star-formation rate (SFR) of $8.07\pm0.82 M_\odot yr^{-1}$. Combining our average HI mass and average SFR estimates yields an HI depletion timescale of $1.70\pm0.29$ Gyr, for star-forming galaxies at $z\approx1$, $\approx3$ times lower than that of local galaxies. We thus find that, although main-sequence galaxies at $z\approx1$ have a high HI mass, their short HI depletion timescale is likely to cause quenching of their star-formation activity in the absence of rapid gas accretion from the circumgalactic medium.
We report a $\approx 400$-hour Giant Metrewave Radio Telescope (GMRT) search for HI 21 cm emission from star-forming galaxies at $z = 1.18-1.39$ in seven fields of the DEEP2 Galaxy Survey. Including data from an earlier 60-hour GMRT observing run, we co-added the HI 21 cm emission signals from 2,841 blue star-forming galaxies that lie within the full-width at half-maximum of the GMRT primary beam. This yielded a $5.0\sigma$ detection of the average HI 21 cm signal from the 2,841 galaxies at an average redshift $\langle z \rangle \approx 1.3$, only the second detection of HI 21 cm emission at $z\ge1$. We obtain an average HI mass of $\langle {\rm M_{HI}} \rangle=(3.09 \pm 0.61) \times 10^{10}\ {\rm M}_\odot$ and an HI-to-stellar mass ratio of $2.6\pm0.5$, both significantly higher than values in galaxies with similar stellar masses in the local Universe. We also stacked the 1.4 GHz continuum emission of the galaxies to obtain a median star-formation rate (SFR) of $14.5\pm1.1\ {\rm M}_\odot \textrm{yr}^{-1}$. This implies an average HI depletion timescale of $\approx 2$ Gyr for blue star-forming galaxies at $z\approx 1.3$, a factor of $\approx 3.5$ lower than that of similar local galaxies. Our results suggest that the HI content of galaxies towards the end of the epoch of peak cosmic SFR density is insufficient to sustain their high SFR for more than $\approx 2$ Gyr. Insufficient gas accretion to replenish the HI could then explain the observed decline in the cosmic SFR density at $z< 1$.
Abstract We use the Giant Metrewave Radio Telescope Cold-H i AT z ≈ 1 (CAT z 1) survey, a 510 hr H i 21 cm emission survey of galaxies at z = 0.74–1.45, to report the first measurements of atomic hydrogen (H i ) scaling relations at z ≈ 1. We divide our sample of 11,419 blue star-forming galaxies at z ≈ 1 into three stellar-mass ( M * ) subsamples and obtain detections (at ≥4 σ significance) of the stacked H i 21 cm emission signal from galaxies in all three subsamples. We fit a power-law relation to the measurements of the average H i mass ( M HI ) in the three stellar-mass subsamples to find that the slope of the M HI – M * relation at z ≈ 1 is consistent with that at z ≈ 0. However, we find that the M HI – M * relation has shifted downwards from z ≈ 1 to z ≈ 0, by a factor of 3.54 ± 0.48. Further, we find that the H i depletion timescales ( t dep,HI ) of galaxies in the three stellar-mass subsamples are systematically lower than those at z ≈ 0, by factors of ≈2–4. We divide the sample galaxies into three specific star formation rate (sSFR) subsamples, again obtaining ≥4 σ detections of the stacked H i 21 cm emission signal in all three subsamples. We find that the relation between the ratio of H i mass to stellar mass and the sSFR evolves between z ≈ 1 and z ≈ 0. Unlike the efficiency of conversion of molecular gas to stars, which does not evolve significantly with redshift, we find that the efficiency with which H i is converted to stars is much higher for star-forming galaxies at z ≈ 1 than those at z ≈ 0.
We report the discovery of two remarkable high-opacity HI 21cm absorbers against low-luminosity active galactic nuclei (AGNs), at $z = 1.2166$ towards J0229+0044 and at $z=1.1630$ towards J0229+0053. The absorbers were detected in an unbiased Giant Metrewave Radio Telescope survey for HI 21cm absorption against radio sources in the DEEP2 survey fields, covering $z \approx 0.73-1.53$, and including sources without known redshifts. The velocity-integrated HI 21cm optical depths are $(74.2 \pm 7.8)$ km s$^{-1}$ (J0229+0044) and $(78.41 \pm 0.81)$ km s$^{-1}$ (J0229+0053), higher than that of any known redshifted HI 21cm absorber at $z > 0.12$, and implying high H{\sc i} column densities, $> 10^{22}$ cm$^{-2}$. The emission redshift of J0229+0044 is consistent with the HI 21cm absorption redshift, while the strength and velocity spread of the absorption against J0229+0053 suggest that it too arises from gas in the AGN environment: both absorbers are thus likely to be "associated" systems. The two AGNs have low rest-frame 1.4 GHz radio and 1215 Angstrom ultraviolet luminosities ($\lesssim 10^{26.1}$ W Hz$^{-1}$ and $\lesssim 10^{21.7}$ W Hz$^{-1}$, respectively), both significantly lower than the typical luminosities of AGNs against which HI 21cm searches have hitherto been carried out at $z \gtrsim 1$. The paucity of HI 21cm absorbers at $z \gtrsim 1$ may be due to a luminosity bias in high-$z$ AGN samples that have been searched for HI 21cm absorption, where the high AGN ultraviolet luminosity affects physical conditions in its environment, ionizing the neutral hydrogen.
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