Molecular Gas of the Most Massive Spiral Galaxies I: a Case Study of NGC 5908

Abstract

We present IRAM 30m observations of molecular lines of CO and its isotopologues from the massive spiral galaxy NGC 5908 selected from the CGM-MASS sample. ${}^{12}$CO $J=1-0$, ${}^{12}$CO $J=2-1$, and ${}^{13}$CO $J=1-0$ lines have been detected in most of the positions along the galactic disk. The total molecular gas mass of NGC 5908 is $\sim 7\times 10^9{\mathrm{M}}_{\odot }$ and the total cool gas mass adding atomic hydrogen is $\sim 1.3\times 10^{10}{\mathrm{M}}_{\odot }$, comparable to the upper limit of the mass of the X-ray emitting hot gas in the halo. Modeling the rotation curves constructed with all three CO lines indicates that NGC 5908 has a dark matter halo mass of $M_{\mathrm{vir}}\sim 10^{13}{\mathrm{M}}_{\odot }$, putting it among the most massive isolated spiral galaxies. The ${}^{12}$CO/${}^{13}$CO $J=1-0$, ${}^{12}$CO $J=2-1$/$J=1-0$ line ratios and the estimated molecular gas temperature all indicate normal but non-negligible star formation in this fairly gas-rich massive isolated spiral galaxy, consistent with the measured star formation intensity and surface densities. The galaxy is probably at an early evolutionary stage after a fast growth stage with mergers and/or starbursts, with plenty of leftover cool gas, relatively high SFR, low hot CGM cooling rate, and low X-ray emissivity.

13 pages, 8 figures, 2 tables, accepted for publication by AJ

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