Revisiting the Group-Dominant Elliptical NGC 5044 in the Radio Band: Continuum Emission and Detection of HI Absorption

Abstract

We present new MeerKAT L-band (continuum and HI) and upgraded Giant Metrewave Radio Telescope (300-850 MHz) observations of the archetypal cool-core group-dominant early-type galaxy NGC 5044. Our new continuum images reveal diffuse, steep spectrum ($\hat{I}{\pm }_{0.99\mathrm{GHz}}^{1.56\mathrm{GHz}}=-1.53\pm 0.6$) radio emission extending about 25 kpc around the unresolved radio core. The observed radio emission overlaps with the known X-ray cavities, but is not confined to them. We also find the first direct evidence of neutral atomic gas in NGC 5044, in the form of a 3.8$\ddot{I}ƒ$ significant two-component HI absorption line seen against the emission of the active nucleus. The peak velocities are well correlated with the previously reported CO(2-1) absorption, but the HI lines are moderately broader, spanning velocities from $265\mathrm{km}{\mathrm{s}}^{-1}$ to $305\mathrm{km}{\mathrm{s}}^{-1}$. We do not detect HI emission, but place an upper limit of $M_{HI}<5.4\times 10^7{M}_{\odot }$ in the central 15 arcsec (2.2 kpc) of the galaxy. This is significantly less than the estimated molecular gas content, and implies a molecular-to-atomic mass ratio of $\ge$1.7:1, consistent with these gas phases forming through cooling from the hot intra-group medium. We also constrain the spin temperature to $T_{\mathrm{spin}}\le 950\mathrm{K}$, indicating that the detected HI is in the cold neutral phase.

17 pages, 8 figures, Submitted to ApJ