Circumnuclear and infalling H i gas in a merging galaxy pair at z = 0.123

Abstract

Using long-slit optical spectra obtained with the 2-m telescope at IUCAA Girawali Observatory, we show that the radio source J094221.98+062335.2 (z = 0.123) is associated with a galaxy pair undergoing a major merger. Its companion is a normal star-forming galaxy infalling with a velocity of 185 km s −1 at a projected separation of 4.8 kpc. Using the Westerbork Synthesis Radio Telescope (WSRT) and Giant Metrewave Radio Telescope (GMRT) we detect a strong H i 21-cm absorption at the systemic redshift of the radio galaxy with N (H i)∼ 9 × 10 21 cm −2 for an assumed spin-temperature of 100 K. Such a strong H i 21-cm absorption is rare and has been seen only in a few compact radio sources associated with similar merging galaxy pairs. Milliarcsecond resolution Very Long Baseline Array (VLBA) observations resolve the radio source into a compact symmetric object with the hotspot separation of 89 pc. The 21-cm absorption is detected in the VLBA spectra towards both the radio lobes albeit with a strong optical depth gradient. We show that the strong 21-cm absorption is consistent with it being arising from a clumpy circumnuclear disk/torus. We also detect two weaker absorption lines redshifted with respect to the radio source in the WSRT/GMRT spectrum. They probably represent cold (i.e. T 10 4 K) H i gas falling into the radio source. The presence of high concentration of H i gas in the circumnuclear regions and signature of infalling cold gas allows us to conjecture that the young radio source may have been triggered by the gas infall due to the ongoing merger.

Prevalence of neutral gas in centres of merging galaxies

“…The absorption data are also not of uniform spectral resolution, coverage and sensitivity. So all of the available spectra may not be sensitive to detect narrow weak infalling absorption as seen in the case of SDSS J094221.98+062335.2 (Srianand et al 2015), or shallow highly blueshifted absorption as seen in the case of Mrk 231 (Morganti et al 2016). Furthermore, the current absorption spectra provide information about the neutral gas present in the merging systems along our line of sight to the radio continuum emission.…”

“…This suggests that mergers can trigger inflow of large volumes of neutral gas to the central regions of galaxies. For example, based on the high concentration of H i gas in the circumnuclear region of a radio source undergoing merger and signature of infalling cold gas found in sub-arcsecond-scale spectroscopy, Srianand et al (2015) have conjectured that the radio source may have been triggered by the gas infall due to the ongoing merger. However, the H i 21-cm absorption from mergers are reported in the literature either as individual detections or as part of samples where good quality optical imaging and spectroscopic data are not available.…”

Discovery of OH Absorption from a Galaxy at z ∼ 0.05: Implications for Large Surveys with SKA Pathfinders

“…We also included the associated 21-cm absorption from the merging pair J0942+0623. This is one of the strongest H I 21-cm absorbers (N (H I)∼10 22 cm −2 ; Srianand et al 2015), and hence, a promising candidate for molecular line search. Five quasars, namely 3C232, Q0248+430, J1042+0748, J1241+6332 and J1243+4043, were observed with the WSRT in Maxi-short configuration in December 2010.…”

Atomic and molecular absorption in redshifted radio sources

“…3), which is used as the FWHM to obtain the integrated optical depth limit, thus giving the N H I f /T spin limit per channel (see Curran 2012). Allison et al (2012Allison et al ( , 2015; Yan et al (2012Yan et al ( , 2016; Geréb et al (2015); Srianand et al (2015); Aditya et al (2016Aditya et al ( , 2017. 8 The absorption redshift of z abs = 1.343 (Kanekar et al 2009) corresponds to a velocity offset of ∆v = 64 km s −1 relative to z QSO = 1.3425 and so we deem this as associated rather than intervening.…”