21-cm absorption from galaxies atz~ 0.3

We report 4 new detections of 21-cm absorption from a systematic search of 21-cm absorption in a sample of 17 strong (Wr(MgII 2796)>1A) intervening MgII absorbers at 0.5<z<1.5. We also present 20-cm milliarcsecond scale maps of 40 quasars having 42 intervening strong MgII absorbers for which we have searched for 21-cm absorption. Combining 21-cm absorption measurements for 50 strong MgII systems from our surveys with the measurements from literature, we obtain a sample of 85 strong MgII absorbers at 0.5<z<1 and 1.1<z<1.5. We present detailed analysis of this sample, taking into account the effect of the varying 21-cm optical depth sensitivity and covering factor associated with the different quasar sight lines. We find that the 21-cm detection rate is higher towards the quasars with flat or inverted spectral index at cm wavelengths. About 70% of 21-cm detections are towards the quasars with linear size, LS<100 pc. The 21-cm absorption lines having velocity widths, DeltaV>100 km/s are mainly seen towards the quasars with extended radio morphology at arcsecond scales. However, we do not find any correlation between the integrated 21-cm optical depth or DeltaV with the LS measured from the milliarcsecond scale images. All this can be understood if the absorbing gas is patchy with a typical correlation length of ~30-100 pc. We show that within the measurement uncertainty, the 21-cm detection rate in strong MgII systems is constant over 0.5<z<1.5, i.e., over ~30% of the total age of universe. We show that the detection rate can be underestimated by up to a factor 2 if 21-cm optical depths are not corrected for the partial coverage estimated using milliarcsecond scale maps. Since stellar feedback processes are expected to diminish the filling factor of cold neutral medium over 0.5<z<1, this lack of evolution in the 21-cm detection rate in strong MgII absorbers is intriguing. [abridged]

This review summarizes recent studies of the cold neutral hydrogen gas associated with galaxies probed via the HI 21-cm absorption line. HI 21-cm absorption against background radio-loud quasars is a powerful tool to study the neutral gas distribution and kinematics in foreground galaxies from kilo-parsec to parsec scales. At low redshifts (z<0.4), it has been used to characterize the distribution of high column density neutral gas around galaxies and study the connection of this gas with the galaxy's optical properties. The neutral gas around galaxies has been found to be patchy in distribution, with variations in optical depth observed at both kilo-parsec and parsec scales. At high redshifts (z>0.5), HI 21-cm absorption has been used to study the neutral gas in metal or Lyman-alpha absorption-selected galaxies. It has been found to be closely linked with the metal and dust content of the gas. Trends of various properties like incidence, spin temperature and velocity width of HI 21-cm absorption with redshift have been studied, which imply evolution of cold gas properties in galaxies with cosmic time. Upcoming large blind surveys of HI 21-cm absorption with next generation radio telescopes are expected to determine accurately the redshift evolution of the number density of HI 21-cm absorbers per unit redshift and hence understand what drives the global star formation rate density evolution.

We present the results from our 21-cm absorption survey of a sample of 5 quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range 0.03<zg<0.18, selected from the SDSS. The HI 21-cm absorption was searched towards the 9 sight lines with impact parameters ranging from 10 to 55 kpc using GMRT. 21-cm absorption was detected only in one case i.e. towards the Quasar (zq=2.625 SDSS J124157.54+633241.6)-galaxy (zg=0.143 SDSS J124157.26+633237.6) pair with the impact parameter 11 kpc. The quasar sight line in this case pierces through the stellar disk of a galaxy having near solar metallicity (i.e (O/H)+12=8.7) and star formation rate uncorrected for dust attenuation of 0.1 M_odot/yr. The quasar spectrum reddened by the foreground galaxy is well fitted with the Milky Way extinction curve (with an Av of 0.44) and the estimated HI column density is similar to the value obtained from 21-cm absorption assuming spin temperature of 100K. Combining our sample with the z<0.1 data available in the literature, we find the detectability of 21-cm absorption with integrated optical depth greater than 0.1 km\s to be 50% for the impact parameter less than 20 kpc. Using the surface brightness profiles and relationship between the optical size and extent of the HI disk known for nearby galaxies, we conclude that in most of the cases of 21-cm absorption non-detection, the sight lines may not be passing through the HI gas. We also find that in comparison to the absorption systems associated with these QGPs, z<1 DLAs with 21-cm absorption detections have lower CaII equivalent widths despite having higher 21-cm optical depths and smaller impact parameters. This suggests that the current sample of DLAs may be a biased population that avoids sight lines through dusty star-forming galaxies. A systematic survey of QGPs is needed to confirm these findings and understand the nature of 21-cm absorbers.

The experiment to detect the global epoch of reionization signature (EDGES) collaboration reported the detection of a line at 78 MHz in the sky-averaged spectrum due to neutral hydrogen (\\ion{H}{i}) 21-cm hyperfine absorption of cosmic microwave background (\\cmb) photons at redshift z∼ 17. This requires that the spin temperature of \\ion{H}{i} be coupled to the kinetic temperature of the gas at this redshift through the scattering of \\lya photons emitted by massive stars. To explain the experimental result, star formation needs to be sufficiently efficient at z∼ 17 and this can be used to constrain models in which small-scale structure formation is suppressed (\\dmf models), either due to dark matter free-streaming or non-standard inflationary dynamics. We combine simulations of structure formation with a simple recipe for star formation to investigate whether these models emit enough Lyman-α photons to reproduce the experimental signal for reasonable values of the star formation efficiency, f⋆. We find that a thermal warm dark matter (\\wdm) model with mass mWDM∼ 4.3 keV is consistent with the timing of the signal for f⋆ ≲ 2%. The exponential growth of structure around z∼ 17 in such a model naturally generates a sharp onset of the absorption. A warmer model with mWDM∼3 keV requires a higher star formation efficiency, f⋆∼ 6%, which is a factor of few above predictions of current star formation models and observations of satellites in the Milky Way. However, uncertainties in the process of star formation at these redshifts do not allow to derive strong constrains on such models using 21-cm absorption line. The onset of the 21-cm absorption is generally slower in \\dmf than observed in cold dark matter (\\cdm) models, unless some process significantly suppresses star formation in halos with masses below ~108h-1M⊙.

Quasar (QSO) absorption spectra provide an extremely useful probe of possible cosmological variation in various physical constants. Comparison of H i 21-cm absorption with corresponding molecular (rotational) absorption spectra allows us to constrain variation in , where α is the fine-structure constant and gp is the proton g-factor. We analyse spectra of two QSOs, PKS 1413+135 and TXS 0218+357, and derive values of at absorption redshifts of and 0.6847 by simultaneous fitting of the H i 21-cm and molecular lines. We find and respectively, indicating an insignificantly smaller y in the past. We compare our results with other constraints from the same two QSOs given recently by Drinkwater et al. and Carilli et al., and with our recent optical constraints, which indicated a smaller α at higher redshifts.

We present a detailed study of the QSO-galaxy pair [SDSS J163956.35+112758.7 (zq = 0.993) and SDSS J163956.38+112802.1 (zg = 0.079)] based on observations carried out using the Giant Meterwave Radio Telescope (GMRT), the Very Large Baseline Array (VLBA), the Sloan Digital Sky Survey (SDSS) and the ESO New Technology Telescope (NTT). We show that the interstellar medium of the galaxy probed by the QSO line of sight has near-solar metallicity (12+log(O/H) = 8.47+/-0.25) and dust extinction (E(B-V) 0.83+/-0.11) typical of what is usually seen in translucent clouds. We report the detection of absorption in the \lambda 6284 diffuse interstellar band (DIB) with a rest equivalent width of 1.45+/-0.20\AA. Our GMRT spectrum shows a strong 21-cm absorption at the redshift of the galaxy with an integrated optical depth of 15.70+/-0.13 km/s. Follow-up VLBA observations show that the background radio source is resolved into three components with a maximum projected separation of 89 pc at the redshift of the galaxy. One of these components is too weak to provide useful HI 21-cm absorption information. The integrated HI optical depth towards the other two components are higher than that measured in our GMRT spectrum and differ by a factor 2. By comparing the GMRT and VLBA spectra we show the presence of structures in the 21-cm optical depth on parsec scales. We discuss the implications of such structures for the spin-temperature measurements in high-z damped Lyman-alpha systems. The analysis presented here suggests that this QSO-galaxy pair is an ideal target for studying the DIBs and molecular species using future observations in optical and radio wavebands.

(Abridged) We present the results of a systematic GBT and GMRT survey for 21-cm absorption in a sample of 10 DLAs at 2<z_abs<3.4. Analysis of L-band VLBA images of the background QSOs are also presented. We detect 21-cm absorption in only one DLA (at z_abs = 3.1745 towards J1337+3152). Combining our data with the data from the literature (a sample of 28 DLAs) and assuming the measured core fraction at milliarcsecond scale to represent the gas covering factor, we find that the HI gas in DLAs at z> 2 is predominantly constituted by WNM. The detection rate of 21-cm absorption seems to be higher for systems with higher N(HI) or metallicity. However, no clear correlation is found between the integrated 21-cm optical depth (or spin temperature) and either N(HI), metallicity or velocity spread of the low ionization species. There are 13 DLAs in our sample for which high resolution optical spectra covering the expected wavelength range of H_2 absorption are available. We report the detection of H_2 molecules in the z_abs = 3.3871 21-cm absorber towards J0203+1134 (PKS 0201+113). In 8 cases, neither H_2 nor 21-cm absorption are detected. The lack of 21-cm and H_2 absorption in these systems can be explained if most of the HI in these DLAs originate from low density high temperature gas. In one case we have a DLA with 21-cm absorption not showing H_2 absorption. In two cases, both species are detected but do not originate from the same velocity component. In the remaining 2 cases 21-cm absorption is not detected despite the presence of H_2 with evidence for the presence of cold gas. All this is consistent with the idea that the H_2 components seen in DLAs are compact (with sizes of < 15 pc) and contain only a small fraction (i.e typically <10%) of the total N(HI) measured in the DLAs.

We report the detection of 21-cm and H2 absorption lines in the same DLA system (log N(HI)=21.36+-0.10) at zabs=3.17447 towards SDSSJ133724+315254 (z=3.174). We estimate the spin temperature of the gas to be, Ts~600 K, intermediate between the expected values for cold and warm neutral media. This suggests that the HI absorption originates from a mixture of different phases. The total molecular fraction is low, f=10^-7, and H2 rotational level populations are not in equilibrium. The average abundance of the alpha-elements is, [S/H]=-1.45. N and Fe are found underabundant with respect to alpha-elements by ~1.0 dex and ~0.5 dex respectively. Using photoionization models we conclude that the gas is located more than 270 kpc away from the QSO. While the position of 21-cm absorption line coincides with the H2 velocity profile, their centroid are shifted by 2.7+-1.0 km/s from each other. However, the position of the strongest metal absorption component matches the position of the 21-cm absorption line within 0.5 km/s. From this, we constrain the variation of the combination of fundamental constants x=alpha^2 Gp/mu, Delta x/x=-(1.7+-1.7)x10^-6. This system is unique as we can at the same time have an independent constrain on mu using H2 lines. However only Werner band absorption lines are seen and the range of sensitivity coefficients is too narrow to provide a stringent constraint: Delta mu/mu <= 4.0x10^-4. The VLT/UVES spectrum reveals another DLA at zabs=3.16768 with log N(HI)=20.41+-0.15 and low metallicity, [Si/H]=-2.68+-0.11. We derive log N(DI)/N(HI)=-(4.93+-0.15) in this system. This is a factor of two smaller than the value expected from the best fitted value of Omega_b from the WMAP 5 yr data. This confirms the presence of astration of deuterium even at very low metallicity. [abridged]

We re-examine the case for numerous invisible low surface brightness galaxies (LSBGs), rather than rare but huge galactic haloes, as the absorbers responsible for a large fraction of the metal-line systems in the spectra of distant QSOs. In particular we find that the supposed best evidence for huge haloes, i.e. the direct observation of giant galaxies at projected distances of a few Holmberg radii from the QSO line of sight, can be explained by the normal clustering of galaxies near an (invisible) LSBG absorber directly in the line of sight to the QSO. We further point out that the most recent observations of the LSBG population indicate that we should expect a very high total cross-section for this population compared to the conventional population of 'normal' galaxies. In addition, adoption of the LSBG hypothesis removes several difficulties which occur in the 'conventional' picture, such as the unaccountably high average luminosity of the giant absorbers, the enormous size of the haloes required and the lack of correlation of the metal-line equivalent width and the QSO-giant galaxy separation. Perhaps most compelling of all, though, is the fact that the LSBG hypothesis simply explains the fact that searches for bright galaxies near QSOs with metal-line systems usually succeed (there is usually a bright galaxy somewhere near the absorbing LSBG), while searches for metal-line systems in QSOs projected a few Holmberg radii from a giant galaxy usually fail (no neighbouring LSBG happens to be in precisely the right place).

We report a deep Giant Metrewave Radio Telescope search for Galactic H i 21-cm absorption towards the quasar B0438−436, yielding the detection of wide, weak H i 21-cm absorption, with a velocity-integrated H i 21-cm optical depth of 0.0188 ± 0.0036 km s−1. Comparing this with the H i column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 ± 365 K, one of the highest measured in the Galaxy. This is consistent with most of the H i along the sightline arising in the stable warm neutral medium. The low-peak H i 21-cm optical depth towards B0438−436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H i 21-cm absorption and emission spectra. This yields a gas kinetic temperature of $T_{\rm k} \le (4910 \pm \lt t\gt 1900)$ K, and a spin temperature of $T_{\rm s} = (1000 \pm 345)$ K for the gas that gives rise to the H i 21-cm absorption. Our data are consistent with the H i 21-cm absorption arising from either the stable WNM, with $T_{\rm s} \ll T_{\rm k}$, $\rm T_k \approx 5000$ K, and little penetration of the background Lyman-α radiation field into the neutral hydrogen, or the unstable neutral medium, with $T_{\rm s} \approx T_{\rm k} \approx 1000\,{\rm K}$.

Aims. We present results of a blind search for Galactic H I 21-cm absorption lines toward 19 130 radio sources brighter than 1 mJy at 1.4 GHz, using 390 pointings of the MeerKAT Absorption Line Survey (MALS), each pointing centered on a source brighter than 200 mJy. The spectral resolution, the median spatial resolution, and the median 3σ optical depth sensitivity (τ3σ) are 5.5 km s−1, ~ 9″, and 0.381, respectively. We used the spectra of the central sources and the other off-axis radio sources within the telescope pointings to constrain the properties of H I gas in the local interstellar medium (LISM) of the Galaxy. Methods. Through an automated procedure, we detected 3640 H I absorption features over ~800 deg2. This represents the largest Galactic H I absorption line catalog to date. We used H I 21-cm emission line measurements from HI4PI, an all sky single-dish survey, and far-infrared maps from COBE/DIRBE and IRAS/ISSA in addition to the Gaussian decomposition of the HI4PI into cold (CNM), lukewarm (LNM), and warm (WNM) neutral medium phases for our analyses. Results. We find a strong linear correlation with a coefficient of 0.84 between the H I 21-cm emission line column densities (NHI) and the visual extinction (AV) measured toward the pointing center, along with the confinement of the absorption features to a narrow range in radial velocities (−25&lt; vLSR[km s−1]&lt;+25). This implies that the detected absorption lines form a homogeneous sample of H I clouds in the LISM. For central sight lines (median τ3σ=0.008), the detection rate is 82±5%. All the central MALS sight lines with H I absorption have NHI(CNM) + NHI(LNM) ≥ NHI(WNM). The H I 21-cm absorption optical depth is linearly correlated to NHI and AV, with a correlation coefficient in excess of 0.8 up to NHI ≃ 2 · 1021 cm−2 or, equivalently, AV ≃ 1 mag. Above this threshold, AV traces the total hydrogen content, and consequently, AV and the single-dish NHI scale, differently. The slopes of NHI distributions of central sight lines with H I 21-cm absorption detections and non-detection differ at &gt;2σ. A similar difference is observed for H2 detections and non-detections in damped Lyman-alpha systems at z≳1.8, implying that turbulence-driven WNM-to-CNM conversion is the common governing factor for the presence of H I 21-cm and H2 absorption. Through a comparison of central and off-axis absorption features, we find the optical depth variations (Δτ) to be higher for pointings centered on regions with a higher NHI and CNM fraction. However, no such dependence is observed for the covering fraction of the absorbing structures over 0.1–10 pc. The slope (2.327 ± 0.153) of root mean square (rms) fluctuations in optical depth variations in the quiescent gas associated with LISM is shallower than the earlier measurements in the disk. The densities (20–30 cm−3) inferred from |Δτ| at the median separation (1.5 pc) of the sample are typical of the CNM values. The negligible (median ~0 km s−1) velocity shifts between central and off-axis absorbers are in line with the hypothesis that the CNM/LNM clouds freeze out of the extended WNM phase.

We present the results from our survey of HI 21-cm absorption, using GMRT, VLA and WSRT, in a sample of 55 z<0.4 galaxies towards radio sources with impact parameters (b) in the range ~0-35 kpc. In our primary sample (defined for statistical analyses) of 40 quasar-galaxy-pairs (QGPs), probed by 45 sightlines, we have found seven HI 21-cm absorption detections, two of which are reported here for the first time. Combining our primary sample with measurements having similar optical depth sensitivity ($\int\tau dv$ <= 0.3 km/s) from the literature, we find a weak anti-correlation (rank correlation coefficient = -0.20 at 2.42sigma level) between $\int\tau dv$ and b, consistent with previous literature results. The covering factor of HI 21-cm absorbers (C_21) is estimated to be 0.24 (+0.12/-0.08) at b <= 15 kpc and 0.06 (+0.09/-0.04) at b = 15-35 kpc. $\int\tau dv$ and C_21 show similar declining trend with radial distance along the galaxy's major axis and distances scaled with the effective HI radius. There is also tentative indication that most of the HI 21-cm absorbers could be co-planar with the extended HI discs. No significant dependence of $\int\tau dv$ and C_21 on galaxy luminosity, stellar mass, colour and star formation rate is found, though the HI 21-cm absorbing gas cross-section may be larger for the luminous galaxies. The higher detection rate (by a factor of ~4) of HI 21-cm absorption in z<1 DLAs compared to the QGPs indicates towards small covering factor and patchy distribution of cold gas clouds around low-z galaxies.

The redshifted 21-cm and Mg II absorption lines observed in the QSO 3C 286 and the BL Lacertae object 0735+178, respectively, are analyzed to determine the location of the absorbing gas. We first describe evidence which supports the presence of neutral hydrogen intrinsic to QSOs, and then examine excitation conditions influencing the formation of 21-cm absorption lines. Due to the high spin temperature T/subs/ resulting from the bright 21-cm continuum, the 21-cm absorption line observed in 3C 286 can arise close to the QSO only if L$alpha$ radiation determines the excitation of the hyperfine levels. The 21-cm line could also form in an intervening galaxy. The absence of redshifted Mg II absorption in the spectrum of 3C 286 places and upper limit of 3 x 10$sup -3$/T/ subs/ on the Mg$sup +$/H I. ratio in the column of absorbing H I. This is compatible with a solar abundance ratio of Mg/H; but if the observed upper limit to Mg II absorption were reduced by a factor 2, this would require Mg$sup +$/H I<10$sup -2$ (Mg/H)/sub sun/. An analysis of the Mg II absorption lines seen in 0735+178 leads to two possibilities: (a) a large column density of Mg$sup +$ ionsmore » and low velocity dispersion; or (b) a low column density and large velocity dispersion. Whereas (b) is kinematically inconsistent with an intervening galaxy, (a) is not. We propose that 21-cm absorption features be searched for in the 30-cm continuum of 0735+178. If no absorption is seen, and (a) applies, then the spin temperature must be much higher than that observed in absorbing clouds in our Galaxy, thus making an intervening galaxy implausible. (AIP)« less

A strong H I 21-cm absorption line is identified toward PKS 1413+135 at the redshift of the host galaxy and argued to indicate that the IR-optical cutoff of the BL Lac object's spectrum is caused by extinction. The BL Lac object is shown to have strong neutral 21-cm absorption at the redshift range z = 0.24671 +/- 0.00001, and the implied H I column density is derived. The H I covering factor is assumed to be not more than 0.1, suggesting that a high degree of extinction exists in the galaxy. The host galaxy is theorized to be a spiral of type later than S0 based on the redshift of the H I absorption line and the host. The statistical and observational conclusions are found to support the theories of Stock et al. (1992) regarding the red spectrum of the object.

We investigate the nature of the star formation law at low gas surface densities using a sample of 19 low surface brightness (LSB) galaxies with existing HI maps in the literature, UV imaging from the Galaxy Evolution Explorer satellite, and optical images from the Sloan Digital Sky Survey. All of the LSB galaxies have (NUV-r) colors similar to those for higher surface brightness star-forming galaxies of similar luminosity indicating that their average star formation histories are not very different. Based upon four LSB galaxies with both UV and FIR data, we find FIR/UV ratios significantly less than one, implying low amounts of internal UV extinction in LSB galaxies. We use the UV images and HI maps to measure the star formation rate and hydrogen gas surface densities within the same region for all of the galaxies. The LSB galaxy star formation rate surface densities lie below the extrapolation of the power law fit to the star formation rate surface density as a function of the total gas density for higher surface brightness galaxies. Although there is more scatter, the LSB galaxies also lie below a second version of the star formation law in which the star formation rate surface density is correlated with the gas density divided by the orbital time in the disk. The downturn seen in both star formation laws is consistent with theoretical models that predict lower star formation efficiencies in LSB galaxies due to the declining molecular fraction with decreasing density.