Molecular Gas of the Most Massive Spiral Galaxies I: a Case Study of NGC
  5908
  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 $\sim7\times10^9\rm~M_\odot$ and the
total cool gas mass adding atomic hydrogen is
$\sim1.3\times10^{10}\rm~M_\odot$, 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 $M_{\rm vir}\sim10^{13}\rm~M_{\rm \odot}$, 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.
