I. Babyk | Searcharxiv

arXiv2024

A Deep Dive into the NGC 741 Galaxy Group: Insights into a Spectacular Head-Tail Radio Galaxy from VLA, MeerKAT, uGMRT and LOFAR

We present deep, wideband multifrequency radio observations (144 MHz$-$8 GHz) of the remarkable galaxy group NGC 741, which yield crucial insights into the interaction between the infalling head-tail radio galaxy (NGC 742) and the main group. Our new data provide an unprecedentedly detailed view of the NGC 741-742 system, including the shock cone, disrupted jets from NGC 742, the long ($\sim$ 255 kpc) braided southern radio tail, and eastern lobe-like structure, and reveal, for the first time, complex radio filaments throughout the tail and lobe, and a likely vortex ring behind the shock cone. The cone traces the bow shock caused by the supersonic ($\mathcal{M}\sim2$) interaction between the head-tail radio galaxy NGC 742 and the intragroup medium (IGrM) while the ring may have been formed by interaction between the NGC 742 shock and a previously existing lobe associated with NGC 741. This interaction plausibly compressed and re-accelerated the radio plasma. We estimate that shock-heating by NGC 742 has likely injected $\sim$2-5$\times$10$^{57}$ erg of thermal energy into the central 10 kpc cooling region of the IGrM, potentially affecting the cooling and feedback cycle of NGC 741. A comparison with Chandra X-ray images shows that some of the previously detected thermal filaments align with radio edges, suggesting compression of the IGrM as the relativistic plasma of the NGC 742 tail interacts with the surrounding medium. Our results highlight that multi-frequency observations are key to disentangling the complex, intertwined origins of the variety of radio features seen in the galaxy group NGC 741, and the need for simulations to reproduce all the detected features.

astro-ph.GA↗

arXiv2017

The Origin of Molecular Clouds In Central Galaxies

We present an analysis of 55 central galaxies in clusters and groups with molecular gas masses and star formation rates lying between $10^{8}-10^{11}\ M_{\odot}$ and $0.5-270$ $M_{\odot}\ yr^{-1}$, respectively. We have used Chandra observations to derive profiles of total mass and various thermodynamic variables. Molecular gas is detected only when the central cooling time or entropy index of the hot atmosphere falls below $\sim$1 Gyr or $\sim$35 keV cm$^2$, respectively, at a (resolved) radius of 10 kpc. This indicates that the molecular gas condensed from hot atmospheres surrounding the central galaxies. The depletion timescale of molecular gas due to star formation approaches 1 Gyr in most systems. Yet ALMA images of roughly a half dozen systems drawn from this sample suggest the molecular gas formed recently. We explore the origins of thermally unstable cooling by evaluating whether molecular gas becomes prevalent when the minimum of the cooling to free-fall time ratio ($t_{\rm cool}/t_{\rm ff}$) falls below $\sim10$. We find: 1) molecular gas-rich systems instead lie between $10 < min(t_{\rm cool}/t_{\rm ff}) < 25$, where $t_{\rm cool}/t_{\rm ff}=25$ corresponds approximately to cooling time and entropy thresholds $t_{\rm cool} \lesssim 1$ Gyr and 35 keV~cm$^2$, respectively, 2) $min(t_{\rm cool}/t_{\rm ff}$) is uncorrelated with molecular gas mass and jet power, and 3) the narrow range $10 < min(t_{\rm cool}/t_{\rm ff}) < 25$ can be explained by an observational selection effect. These results and the absence of isentropic cores in cluster atmospheres are in tension with "precipitation" models, particularly those that assume thermal instability ensues from linear density perturbations in hot atmospheres. Some and possibly all of the molecular gas may instead have condensed from atmospheric gas lifted outward either by buoyantly-rising X-ray bubbles or merger-induced gas motions.

astro-ph.GA↗