Astronomy & Astrophysics Division Seminar

Title : Cold gas in AGN host galaxies

Date : 10-10-2019
Time : 16:00:00
Speaker : Dr. J.N.H.S. Aditya (PRL, Ahmedabad)
Area : Astronomy & Astrophysics Division
Venue : Seminar Room # 113/114 (Thaltej Campus)

Abstract

Probing the distribution and kinematical properties of cold gas present in the vicinity of active galactic nuclei (AGN) is critical for assessing the nature of interactions between the AGN and its host galaxy, thereby for understanding the AGN-host galaxy co-evolution. Cold atomic gas (HI) present in the surroundings of AGN can be probed via HI 21 cm absorption, that is an efficient technique for observing systems at high redshifts, z > 0.3. Our earlier studies have established that samples of compact radio sources at high redshifts, z > 1.0, have a significantly low HI 21 cm absorption strength compared to samples at lower redshifts. We find that this discrepancy could be caused by multiple factors: (1) a redshift evolution in the AGN host galaxies, (2) a varied gas covering factor, (3) or a systematic bias in the AGN ultraviolet and/or radio luminosities. We use our recent uGMRT observations, and similar searches available the literature, with the aim to identify the dominant cause among the above, if any. In the talk I will discuss about our recent new detection of HI 21 cm absorption at the highest redshift to date, z = 3.53. The source has an ultraviolet luminosity of 1023 W/Hz, a cut-off above which all the neutral hydrogen in the host galaxy is expected to be completely ionized. Our detection of HI 21 cm absorption in this system is thus a contrary to this hypothesis. Finally, I will talk about the importance of near-IR spectroscopic observations to probe the cold molecular gas in the AGN host galaxies.