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Register a new userThe HI content of star-forming galaxies at z = 0.24
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We use observations from the Giant Metrewave Radio Telescope (GMRT) to measure the atomic hydrogen gas content of star-forming galaxies at z = 0.24 (i.e. a look-backtime of ~3 Gyr). The sample of galaxies studied were selected from Halpha-emitting field galaxies detected in a narrow-band imaging survey with the Subaru Telescope. The Anglo-Australian Telescope was used to obtain precise optical redshifts for these galaxies. We then coadded the HI 21 cm emission signal for all the galaxies within the GMRT spectral line data cube. From the coadded signal of 121 galaxies, we measure an average atomic hydrogen gas mass of (2.26 +- 0.90)*10^9 solar masses. We translate this HI signal into a cosmic density of neutral gas at z = 0.24 of Omega_gas = (0.91 +- 0.42)*10^-3. This is the current highest redshift at which Omega_gas has been constrained from 21 cm emission and our value is consistent with that estimated from damped Lyman-alpha systems around this redshift. We also find that the correlations between the Halpha luminosity and the radio continuum luminosity and between the star formation rate and the HI gas content in star-forming galaxies at z = 0.24 are consistent with the correlations found at z = 0. These two results suggest that the star formation mechanisms in field galaxies ~3 Gyr ago were not substantially different from the present, even though the star formation rate is 3 times higher.
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We make a search for Halpha emitting galaxies at z=0.24 in the Subaru Deep Field (SDF) using the archival data set obtained with the Subaru Telescope. We carefully select Halpha emitters in the narrowband filter NB816, using B, V, Rc, i, and z broad-band colors. We obtain a sample of 258 emitting galaxies with observed equivalent widths of (Halpha+[NII]6548,6584) greater than 12 angstrom. We also analyze a sample of Halpha emitters taken from the Sloan Digital Sky Survey (SDSS) to constrain the luminous end of Halpha luminosity function. Using the same selection criteria as for the SDF, and after excluding AGNs, we obtain 317 Halpha emitting star-forming galaxies. Combining these two samples of Halpha emitters found in both SDF and SDSS, we derive a Halpha luminosity function with best-fit Schechter function parameters of alpha = -1.31^+0.17_-0.17, log phi^* = -2.46^+0.34_-0.40 Mpc^-3, log L^* = 41.99^+0.08_-0.07 ergs s^-1. An extinction-corrected Halpha luminosity density is 4.45^+2.96_-1.75 x 10^39 ergs s^-1 Mpc^-3. Using the Kennicutt relation between the Halpha luminosity and star formation rate, the star formation rate density in the survey volume is estimated as 0.035^+0.024_-0.014 M_sun yr^-1 Mpc^-3. The angular two-point correlation function of Halpha emitters over 875 arcmin^2 at z = 0.24 is well fitted by a power-law form with w(theta) = 0.047^+0.017_-0.013 theta^-0.66 +- 0.08, corresponding to the correlation function of xi(r) = (r/2.6^+1.0_-0.8 Mpc)^(-1.66 +- 0.08). The small correlation length of Halpha emitters may imply the weak clustering of active star-forming galaxies.
We study the molecular gas content of 24 star-forming galaxies at $z=3-4$, with a median stellar mass of $10^{9.1}$ M$_{odot}$, from the MUSE Hubble Ultra Deep Field (HUDF) Survey. Selected by their Lyman-alpha-emission and H-band magnitude, the galaxies show an average EW $approx 20$ angstrom, below the typical selection threshold for Lyman Alpha Emitters (EW $> 25$ angstrom), and a rest-frame UV spectrum similar to Lyman Break Galaxies. We use rest-frame optical spectroscopy from KMOS and MOSFIRE, and the UV features observed with MUSE, to determine the systemic redshifts, which are offset from Lyman alpha by 346 km s$^{-1}$, with a 100 to 600 km s$^{-1}$ range. Stacking CO(4-3) and [CI](1-0) (and higher-$J$ CO lines) from the ALMA Spectroscopic Survey of the HUDF (ASPECS), we determine $3sigma$ upper limits on the line luminosities of $4.0times10^{8}$ K km s$^{-1}$pc$^{2}$ and $5.6times10^{8}$ K km s$^{-1}$pc$^{2}$, respectively (for a 300 km s$^{-1}$ linewidth). Stacking the 1.2 mm and 3 mm dust continuum flux densities, we find a $3sigma$ upper limits of 9 $mu$Jy and $1.2$ $mu$Jy, respectively. The inferred gas fractions, under the assumption of a Galactic CO-to-H$_{2}$ conversion factor and gas-to-dust ratio, are in tension with previously determined scaling relations. This implies a substantially higher $alpha_{rm CO} ge 10$ and $delta_{rm GDR} ge 1200$, consistent with the sub-solar metallicity estimated for these galaxies ($12 + log(O/H) approx 7.8 pm 0.2$). The low metallicity of $z ge 3$ star-forming galaxies may thus make it very challenging to unveil their cold gas through CO or dust emission, warranting further exploration of alternative tracers, such as [CII].
We use observations made with the Giant Metrewave Radio Telescope (GMRT) to probe the neutral hydrogen (HI) gas content of field galaxies in the VIMOS VLT Deep Survey (VVDS) 14h field at $z approx 0.32$. Because the HI emission from individual galaxies is too faint to detect at this redshift, we use an HI spectral stacking technique using the known optical positions and redshifts of the 165 galaxies in our sample to co-add their HI spectra and thus obtain the average HI mass of the galaxies. Stacked HI measurements of 165 galaxies show that 95 per cent of the neutral gas is found in blue, star-forming galaxies. Among these galaxies, those having lower stellar mass are more gas-rich than more massive ones. We apply a volume correction to our HI measurement to evaluate the HI gas density at $z approx 0.32$ as $Omega_{HI}=(0.50pm0.18) times 10^{-3}$ in units of the cosmic critical density. This value is in good agreement with previous results at z < 0.4, suggesting no evolution in the neutral hydrogen gas density over the last $sim 4$ Gyr. However the $z approx 0.32$ gas density is lower than that at $z sim 5$ by at least a factor of two.
We measure a relation between the depth of four prominent rest-UV absorption complexes and metallicity for local galaxies and verify it up to z~3. We then apply this relation to a sample of 224 galaxies at 3.5 < z < 6.0 (<z> = 4.8) in COSMOS, for which unique UV spectra from DEIMOS and accurate stellar masses from SPLASH are available. The average galaxy population at z~5 and log(M/Msun) > 9 is characterized by 0.3-0.4 dex (in units of 12+log(O/H)) lower metallicities than at z~2, but comparable to z~3.5. We find galaxies with weak/no Ly-alpha emission to have metallicities comparable to z~2 galaxies and therefore may represent an evolved sub-population of z~5 galaxies. We find a correlation between metallicity and dust in good agreement with local galaxies and an inverse trend between metallicity and star-formation rate (SFR) consistent with observations at z~2. The relation between stellar mass and metallicity (MZ relation) is similar to z~3.5, however, there are indications of it being slightly shallower, in particular for the young, Ly-alpha emitting galaxies. We show that, within a bathtub approach, a shallower MZ relation is expected in the case of a fast (exponential) build-up of stellar mass with an e-folding time of 100-200 Myr. Due to this fast evolution, the process of dust production and metal enrichment as a function of mass could be more stochastic in the first billion years of galaxy formation compared to later times.
We report a Giant Metrewave Radio Telescope (GMRT) search for HI 21cm emission from a large sample of star-forming galaxies at $z approx 1.18 - 1.34$, lying in sub-fields of the DEEP2 Redshift Survey. The search was carried out by co-adding (stacking) the HI 21cm emission spectra of 857 galaxies, after shifting each galaxys HI 21cm spectrum to its rest frame. We obtain the $3sigma$ upper limit S$_{rm{HI}} < 2.5 mu$Jy on the average HI 21cm flux density of the 857 galaxies, at a velocity resolution of $approx 315$ km s$^{-1}$. This yields the $3sigma$ constraint M$_{rm{HI}} < 2.1 times 10^{10} times left[Delta {rm V}/315 rm{km/s} right]^{1/2} textrm{M}_odot$ on the average HI mass of the 857 stacked galaxies, the first direct constraint on the atomic gas mass of galaxies at $z > 1$. The implied limit on the average atomic gas mass fraction (relative to stars) is ${rm M}_{rm GAS}/{rm M}_* < 0.5$, comparable to the cold molecular gas mass fraction in similar star-forming galaxies at these redshifts. We find that the cosmological mass density of neutral atomic gas in massive star-forming galaxies at $z approx 1.3$ is $Omega_{rm GAS} < 3.7 times 10^{-4}$, significantly lower than $Omega_{rm GAS}$ estimates in both galaxies in the local Universe and damped Lyman-$alpha$ absorbers at $z geq 2.2$. Massive blue star-forming galaxies thus do not appear to dominate the neutral atomic gas content of the Universe at $z approx 1.3$.
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