Abstract Details
| Name: J. N. H. S. Aditya Affiliation: National Centre for Radio Astrophysics Conference ID: ASI2016_838 Title : Cold Gas in High Redshift Galaxies Authors and Co-Authors : Nissim Kanekar National Centre for Radio Astrophysics, TIFR Abstract Type : Oral Abstract Category : Extragalactic astronomy Abstract : We have conducted a survey using Giant Metrewave Radio Telescope to search for “associated†redshifted HI 21 cm absorption from 76 active galactic nuclei (AGNs), selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. Out of 63 sources which have usable data 17 sources are at 0 < z < 0.5, 39 are at 1.1 < z < 1.5 and 7 are at z ~ 3.5. We have obtained detections of HI 21 cm absorption in 4 sources, out of which one is a tentative detection towards TXS 0604+728 at z = 3.53. If confirmed, this would be the highest redshift at which HI 21 cm absorption has been detected till date. Including 29 CJF sources with searches for redshifted HI 21 cm absorption in the literature, mostly at z < 1, we construct a sample of 92 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 (ref Aditya et al.2015, MNRAS). 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 further plan to observe 36 sources of the CJF sample in 0.4 < z < 1.0, which have high AGN luminosities, using GBT (~21 hrs allocated), to break this degeneracy. Additionally, we have observed 13 Gigahertz peaked spectrum (GPS) sources, 9 at z < 0.4 and 4 at 1.1 < z < 1.5, using GMRT, to test if the same results are applicable in a total sample of 58 GPS sources (remaining 45 sources are selected from literature). The data analysis of these sources is in progress. |