2019ApJ...877....3L

We present IRAM 30 m observations of molecular lines of CO and its isotopologues from the massive spiral galaxy NGC 5908 selected from the CGM-MASS sample. ¹²CO J = 1 - 0, ¹²CO J = 2 - 1, and ¹³CO 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 ∼7×10⁹M_☉ and the total cool gas mass adding atomic hydrogen is ∼1.3×10¹⁰M_☉, 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_vir∼10¹³M_☉, putting it among the most massive isolated spiral galaxies. The ¹²CO/¹³CO J = 1 - 0, ¹²CO 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.