We report results from a Giant Metrewave Radio Telescope search for "associated" redshifted HI 21 cm absorption from 24 active galactic nuclei (AGNs), at 1.1 < z < 3.6, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. 22 out of 23 sources with usable data showed no evidence of absorption, with typical 3σ optical depth detection limits of ≈ 0.01 at a velocity resolution of ≈ 30 km s −1 . A single tentative absorption detection was obtained at z ≈ 3.530 towards TXS 0604+728. If confirmed, this would be the highest redshift at which HI 21 cm absorption has ever been detected.Including 29 CJF sources with searches for redshifted HI 21 cm absorption in the literature, mostly at z < 1, we construct a sample of 52 uniformly-selected flat-spectrum sources. A Peto-Prentice two-sample test for censored data finds (at ≈ 3σ significance) that the strength of HI 21 cm absorption is weaker in the high-z sample than in the low-z sample; this is the first statistically significant evidence for redshift evolution in the strength of HI 21 cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the HI 21 cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at ≈ 3.4σ significance. The fact that the higher-luminosity AGNs of the sample typically lie at high redshifts implies that it is currently not possible to break the degeneracy between AGN luminosity and redshift evolution as the primary cause of the low HI 21 cm opacities in high-redshift, high-luminosity active galactic nuclei.

We report the first detections of associated Hi 21 cm absorption in Gigahertz-peakedspectrum (GPS) sources at high redshifts, z > 1, using the Giant Metrewave Radio Telescope (GMRT). Our GMRT search for associated Hi 21 cm absorption in a sample of 12 GPS sources yielded two new detections of absorption, towards TXS 1200+045 at z = 1.226 and TXS 1245−197 at z = 1.275, and five non-detections. These are only the sixth and seventh detections of associated Hi 21 cm absorption in active galactic nuclei (AGNs) at z > 1. Both Hi 21 cm absorption profiles are wide, with velocity spans between nulls of ≈ 600 km s −1 (TXS 1200+045) and ≈ 1100 km s −1 (TXS 1245−197). In both absorbers, the large velocity spread of the absorption and its blueshift from the AGN, suggests that it arises in outflowing neutral gas, perhaps driven by the radio jets to high velocities. We derive mass outflow rates ofṀ ≈ 32 M ⊙ yr −1 (TXS 1200+045) andṀ ≈ 18 M ⊙ yr −1 (TXS 1245−197), comparable to the mass outflow rates seen earlier in low-redshift active galactic nuclei.

We report a Giant Metrewave Radio Telescope (GMRT) survey for associated Hi 21cm absorption from 50 active galactic nuclei (AGNs), at z ≈ 0.04 − 3.01, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. Clean spectra were obtained towards 40 sources, yielding two new absorption detections, at z = 0.229 towards TXS 0003+380 and z = 0.333 towards TXS 1456+375, besides confirming an earlier detection, at z = 1.277 towards TXS 1543+480. There are 92 CJF sources, at 0.01 z 3.6, with searches for associated Hi 21-cm absorption, by far the largest uniformly-selected AGN sample with searches for such absorption. We find weak (≈ 2σ) evidence for a lower detection rate of Hi 21-cm absorption at high redshifts, with detection rates of 28 +10 −8 % and 7 +6 −4 % in the low-z (z < z med ) and high-z (z > z med ) sub-samples, respectively. We use two-sample tests to find that the strength of the Hi 21-cm absorption in the AGNs of our sample depends on both redshift and AGN luminosity, with a lower detection rate and weaker absorption at high redshifts and high ultraviolet/radio AGN luminosities. Unfortunately, the luminosity bias in our sample, with high-luminosity AGNs arising at high redshifts, implies that it is not currently possible to identify whether redshift evolution or AGN luminosity is the primary cause of the weaker absorption in high-z, high-luminosity AGNs. We find that the strength of Hi 21-cm absorption does not depend on AGN colour, suggesting that dust extinction is not the main cause of reddening in the CJF sample.

No abstract

Observational searches for intermediate-mass black holes (IMBHs; 102–106 M ⊙) include relatively isolated dwarf galaxies. For those that host active galactic nuclei (AGNs), the IMBH nature may be discerned through the accretion–jet activity. We present radio observations of four AGN-hosting dwarf galaxies, which potentially harbor IMBHs. Very large array (VLA) observations indicate steep spectra (indices of −0.63 to −1.05) between 1.4 and 9 GHz. However, a comparison with the 9 GHz in-band spectral index shows a steepening for GH047 and GH158 (implying older/relic emission) and flattening for GH106 and GH163 (implying recent activity). Overlapping emission regions in the VLA 1.4 GHz and our very long baseline array (VLBA) 1.5 GHz observations, and possibly symmetric pc-scale extensions, are consistent with recent activity in the latter two. Using the compact VLBA radio luminosity, X-ray luminosity (probing the accretion activity), and the black hole masses, all AGNs are found to lie on the empirical fundamental plane relation. The four AGNs are radio-quiet with relatively higher Eddington ratios (0.04–0.32) and resemble X-ray binaries during spectral state transitions that entail an outflow ejection. Furthermore, the radio to X-ray luminosity ratio log R X of −3.9 to −5.6 in these four sources support the scenarios that include corona mass ejection from the accretion disk and wind activity. The growth to kpc-scales likely proceeds along a similar trajectory to young AGNs and peaked spectrum sources. These complex clues can thus aid in the detection and monitoring of IMBHs in the nearby universe.

We report a uGMRT detection of H i 21-cm absorption associated with the radio source 8C 0604+728, at z = 3.52965. The source is at the highest redshift at which associated H i 21-cm absorption has been discovered to date, surpassing earlier known absorber at z ≈ 3.39. We estimate ultraviolet luminosities of $\rm (3.2 \pm 0.1) \times 10^{23}~W~Hz^{-1}$ and $\rm (6.2 \pm 0.2)\times 10^{23}~W~Hz^{-1}$, and ionising photon rates of $\rm (1.8 \pm 0.1) \times 10^{56}~s^{-1}$ and $\rm (5.0 \pm 0.1) \times 10^{56}~s^{-1}$, using data at different epochs; the source shows year-scale variability in both its luminosity and photon rate. The luminosity and photon rate at later epochs are ≈6.2 and ≈1.7 times higher than thresholds suggested in the literature above which all the neutral hydrogen in the AGN host galaxy is expected to be ionised. The detection demonstrates that neutral hydrogen can survive in the host galaxies of AGNs with high ultraviolet luminosities. We estimate a high equivalent width ratio of 15.2 for the Lyα and HeII emission lines detected in the optical spectrum, that is consistent with AGN photoionisation models. However, a significant contribution from young stellar populations to the excess Lyα flux cannot be ruled out.

Studies of high redshift radio galaxies can shed light on the activity of active galactic nuclei (AGN) in massive elliptical galaxies, and on the assembly and evolution of galaxy clusters in the Universe. J1606+3124 has been tentatively identified as a radio galaxy at a redshift of 4.56, at an era of one-tenth of the current age of the Universe. Very long baseline interferometry (VLBI) images show a compact triple structure with a size of 68 parsecs. The radio properties of J1606+3124, including the edge-brightening morphology, peaked GHz radio spectrum, slow variability, and low jet speed, consistently indicate that it is a compact symmetric object (CSO). The radio source size and expansion rate of the hotspots suggest that J1606+3124 is a young (kinematic age of ∼3600 years) radio source. Infrared observations reveal a gas- and dust-rich host galaxy environment, which may hinder the growth of the jet; however, the ultra-high jet power of J1606+3124 gives it an excellent chance to grow into a large-scale double-lobe radio galaxy. If its redshift and galaxy classification can be confirmed by further optical spectroscopic observations, J1606+3124 will be the highest redshift CSO galaxy known to date.

I report detections of four new Hi 21-cm absorbers associated with sources at intermediate redshifts, 0.7 < z < 1.0. The sources are part of a sample of 11 radio-loud galaxies, all at 0.7 < z < 1.0, that were searched for associated Hi 21-cm absorption using uGMRT. Previously, just four such absorbers were known in the literature at these redshifts; the current observations have increased the total to eight. The results indicate that the detection fraction at intermediate redshifts could be as high as that at lower redshifts, ≈ 30%, on contrary to a much lower detection fraction observed in samples at z > 1. Three detections show strong blueshifted features, indicating cold gas outflows. These three sources also tentatively show excess [O ii] line luminosity compared to a bulk of the remaining sample, possibly suggesting that the hosts of these AGNs harbour different environments, either due to interaction with the radio jets or due to excess star formation in the host galaxy. Further, a cold Hi mass outflow rate of ≈ 78 M ⊙ yr −1 , assuming T s = 1000 K, is estimated for the detection towards SDSS J014652.79-015721.2, at z = 0.95904, which is the highest till date in comparison to similar estimates available in the literature.