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Register a new userEvolution of cold gas at 2<z<5: a blind search for HI and OH absorption lines towards mid-infrared color selected radio-loud AGNs
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We present results from a spectroscopically blind search for associated and intervening HI 21-cm and OH 18-cm absorption lines towards 88 AGNs at $2le zle5$ using the upgraded Giant Metrewave Radio Telescope (uGMRT). The sample of AGNs with 1.4 GHz spectral luminosity in the range, $10^{27 - 29.3}$ W/Hz, is selected using mid-infrared colors and closely resembles the distribution of the underlying quasar population. The search for associated or proximate absorption, defined to be within 3000 km/s of the AGN redshift, led to one HI 21-cm absorption detection (M1540-1453; $z_{abs}$= 2.1139). This is only the fourth known absorption at $z>2$. The detection rate ($1.6^{+3.8}_{-1.4}$%) suggests low covering factor of cold neutral medium (CNM; T$sim$100 K) associated with these powerful AGNs. The intervening absorption line search, with a sensitivity to detect CNM in damped Ly$alpha$ systems (DLAs), has comoving absorption path lengths of $Delta$X = 130.1 and 167.7 for HI and OH, respectively. The corresponding number of absorber per unit comoving path lengths are $le$0.014 and $le$0.011, respectively. The former is at least 4.5 times lower than that of DLAs and consistent with the CNM cross-section estimated using H$_2$ and CI absorbers at $z>2$. Our AGN sample is optically fainter compared to the quasars used to search for DLAs in the past. In our optical spectra obtained using the Southern African Large Telescope (SALT) and the Nordic Optical Telescope (NOT), we detect 5 intervening (redshift path$sim9.3$) and 2 proximate DLAs. This is slightly excessive compared to the statistics based on optically selected quasars. The non-detection of HI 21-cm absorption from these DLAs suggests small CNM covering fraction around galaxies at $z>2$.
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H{sc i} absorption studies of active galaxies enable us to probe their circumnuclear regions and the general interstellar medium, and study the supply of gas which may trigger the nuclear activity. In this paper, we investigate the detection rate of H{sc i} absorption on the nature of radio galaxies based on their emission-line spectra, nature of the host galaxies based on the textit{WISE} colours and their radio structure, which may help understand the different accretion modes. We find significant difference in distributions of W2$-$W3 colour for sources with H{sc i} absorption detections and non-detections. We report a high detection rate of H{sc i} absorption in the galaxies with textit{WISE} infrared colours W2$-$W3 $>$ 2, which is typical of gas-rich systems, along with a compact radio structure. The H{sc i} detection rate for low-excitation radio galaxies (LERGs) with W2$-$W3 $>$ 2 and compact radio structure is high (70.6$pm$20.4 %). In HERGs, compact radio structure in the nuclear or circumnuclear region could give rise to absorption by gas in the dusty torus in addition to gas in the interstellar medium. However, higher specific star formation rate (sSFR) for the LERGs with W2$-$W3 $>$ 2 suggests that H{sc i} absorption may be largely due to star-forming gas in their hosts. LERGs with extended radio structure tend to have significantly lower values of W2$-$W3 compared to those with compact structure. Extended radio sources and those with W2$-$W3 $<$ 2 have low H{sc i} detection rates.
We present details of the Automated Radio Telescope Imaging Pipeline (ARTIP) and results of a sensitive blind search for HI and OH absorbers at $z<0.4$ and $z<0.7$, respectively. ARTIP is written in Python 3.6, extensively uses the Common Astronomy Software Application (CASA) tools and tasks, and is designed to enable the geographically-distributed MeerKAT Absorption Line Survey (MALS) team to collaboratively process large volumes of radio interferometric data. We apply it to the first MALS dataset obtained using the 64-dish MeerKAT radio telescope and 32K channel mode of the correlator. With merely 40 minutes on target, we present the most sensitive spectrum of PKS1830-211 ever obtained and characterize the known HI ($z=0.19$) and OH ($z=0.89$) absorbers. We further demonstrate ARTIPs capabilities to handle realistic observing scenarios by applying it to a sample of 72 bright radio sources observed with the upgraded Giant Metrewave Radio Telescope (uGMRT) to blindly search for HI and OH absorbers. We estimate the numbers of HI and OH absorbers per unit redshift to be $n_{21}(zsim0.18)<$0.14 and $n_{rm OH}(zsim0.40)<$0.12, respectively, and constrain the cold gas covering factor of galaxies at large impact parameters (50 kpc $<rho<$ 150 kpc) to be less than 0.022. Due to the small redshift path, $Delta zsim$13 for HI with column density$>5.4times10^{19}$ cm$^{-2}$, the survey has probed only the outskirts of star-forming galaxies at $rho>30$ kpc. MALS with the expected $Delta zsim10^{3-4}$ will overcome this limitation and provide stringent constraints on the cold gas fraction of galaxies in diverse environments over $0<z<1.5$.
We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3$-$2) or (4$-$3) emission from the fields of 12 high-metallicity ([M/H]~$geq -0.72$,dex) damped Lyman-$alpha$ absorbers (DLAs) at $z approx 1.7-2.6$. We detected CO emission from galaxies in the fields of five DLAs (two of which have been reported earlier), obtaining high molecular gas masses, $rm M_{mol} approx (1.3 - 20.7) times (alpha_{rm CO}/4.36) times 10^{10} ; M_odot$. The impact parameters of the CO emitters to the QSO sightline lie in the range $b approx 5.6-100$~kpc, with the three new CO detections having $b lesssim 15$~kpc. The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with [M/H]~$gtrsim -0.3$,dex. The high inferred molecular gas masses may be explained by a combination of a stellar mass-metallicity relation and a high molecular gas-to-stars mass ratio in high-redshift galaxies; the DLA galaxies identified by our CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice-versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and C[{sc ii}]~158$mu$m detection rate in high-$z$, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-$z$ DLAs. The H{sc i}-absorption criterion identifying DLAs selects the entire high-$z$ galaxy population, including dusty and UV-bright galaxies, in a wide range of environments.
We present Giant Metrewave Radio Telescope (GMRT) 323 MHz radio continuum observations toward 13 radio-loud quasars at $z>5$, sampling the low-frequency synchrotron emission from these objects. Among the 12 targets successfully observed, we detected 10 above $4sigma$ significance, while 2 remain undetected. All of the detected sources appear as point sources. Combined with previous radio continuum detections from the literature, 9 quasars have power-law spectral energy distributions throughout the radio range; for some the flux density drops with increasing frequency while it increases for others. Two of these sources appear to have spectral turnover. For the power-law-like sources, the power-law indices have a positive range between 0.18 and 0.67 and a negative values between $-0.90$ and $-0.27$. For the turnover sources, the radio peaks around $sim1$ and $sim10$ GHz in the rest frame, the optically thin indices are $-0.58$ and $-0.90$, and the optically thick indices are 0.50 and 1.20. A magnetic field and spectral age analysis of SDSS J114657.59+403708.6 at $z=5.01$ may indicate that the turnover is not caused by synchrotron self-absorption, but rather by free-free absorption by the high-density medium in the nuclear region. Alternatively, the apparent turnover may be an artifact of source variability. Finally, we calculated the radio loudness $R_{2500rm, AA}$ for our sample, which spans a very wide range from 12$^{+13}_{-13}$ to 4982$^{+279}_{-254}$.
We report results from a deep search for redshifted HI 21cm absorption from eight damped Lyman-$alpha$ absorbers (DLAs) detected in our earlier optical survey for DLAs towards radio-loud quasars. HI 21cm absorption was detected from the $z = 2.192$ DLA towards TXS2039+187, only the sixth case of such a detection at $z > 2$, while upper limits on the HI 21cm optical depth were obtained in six other DLAs at $z > 2$. Our detection of HI 21cm absorption in the eighth system, at $z = 2.289$ towards TXS0311+430, has been reported earlier. We also present high spatial resolution images of the background quasars at frequencies close to the redshifted HI 21cm line frequency, allowing us to estimate the covering factor of each DLA, and then determine its spin temperature $T_s$. For three non-detections of HI 21cm absorption, we obtain strong lower limits on the spin temperature, $T_s gtrsim 790$ K, similar to the bulk of the high-$z$ DLA population; three other DLAs yield weak lower limits, $T_s > 140-400$ K. However, for the two DLAs with detections of HI 21cm absorption, the derived spin temperatures are both low $T_s = (160 pm 35) times (f/0.35)$ K for the $z = 2.192$ DLA towards TXS2039+187 and $T_s = (72 pm 18) times (f/0.52)$ K for the $z = 2.289$ DLA towards TXS0311+430. These are the first two DLAs at $z > 1$ with $T_s$ values comparable to those obtained in local spiral galaxies. Based on the observed correlations between spin temperature and metallicity and velocity spread and metallicity in DLAs, we suggest that the hosts of the two absorbers are likely to be massive, high-metallicity galaxies.
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