Molecular gas in normal late-type galaxies

Abstract

Abstract. We present 12 CO(J = 1-0) line observations of 22 low-luminosity spiral galaxies in the Virgo cluster. These data, together with 244 others available in the literature, allow us to build a large sample that we use to study the molecular gas properties of galaxies spanning a large range of morphological types and luminosities and belonging to different environments (clusters -field). The molecular gas content of the target galaxies is estimated using a luminosity-dependent X = N(H2)/I(CO) conversion factor that has been calibrated on a sample of nearby galaxies. X spans from ∼10 20 mol cm −2 (K km s −1 ) −1 in giant spirals to ∼10 21 mol cm −2 (K km s −1 ) −1 in dwarf irregulars. The value of the X conversion factor is found consistent with a value derived independently from dust masses estimated from FIR fluxes, with a metallicity-dependent dust to gas ratio. The relationships between X and the UV radiation field (as traced by the Hα + [NII]EW), the metallicity and the H band luminosity are analysed. We show that the molecular gas contained in molecular clouds or complexes is of the order of 15% of the total gas on average whatever the luminosity or the Hubble type of the galaxies. We discuss the relation between the star formation rate and the molecular gas content and estimate the average star formation efficiency of late-type galaxies.

Probing the dust properties of galaxies up to submillimetre wavelengths

“…We determine the molecular gas masses using the canonical galactic X CO factor of 2.3 × 10 20 cm −2 /K km s −1 of Strong et al (1988) (Table 1, Value Gal ). The conversion factor from CO to H 2 (X CO ) in galaxies depends on the metallicity (Wilson 1995;Israel 1997Israel , 2000Barone et al 2000;Boselli et al 2002;Strong et al 2004;Leroy et al 2005). Boselli et al (2002) studied X CO in normal late-type galaxies and derived X CO factors ranging from 10 20 cm −2 /K km s −1 in giant spirals to 10 21 cm −2 /K km s −1 in dwarf irregulars.…”

“…The conversion factor from CO to H 2 (X CO ) in galaxies depends on the metallicity (Wilson 1995;Israel 1997Israel , 2000Barone et al 2000;Boselli et al 2002;Strong et al 2004;Leroy et al 2005). Boselli et al (2002) studied X CO in normal late-type galaxies and derived X CO factors ranging from 10 20 cm −2 /K km s −1 in giant spirals to 10 21 cm −2 /K km s −1 in dwarf irregulars. They conclude that using a constant X CO factor to derive the molecular gas mass from CO observations could lead to an underestimation of a factor ∼10.…”

“…They conclude that using a constant X CO factor to derive the molecular gas mass from CO observations could lead to an underestimation of a factor ∼10. We use the relation from Boselli et al (2002) to estimate X CO for each galaxy depending on its metallicity and rescale the molecular gas masses (Table 1, Value B02 ):…”

TheHerschelVirgo Cluster Survey

“…We have used so far the Galactic solar neighborhood value, but there is ample evidence that X CO varies in other galaxies as a function of physical conditions (e.g. Maloney & Black 1988;Israel 1997;Barone et al 2000;Boselli et al 2002;Israel et al 2003;Israel 2005;Bolatto et al 2008;Narayanan et al 2005;Leroy et al 2011;Shetty et al 2011;Narayanan et al 2012;Bell et al 2007). Thus, we consider an alternative conversion factor, which depends on the metallicity, and varies radially following the metallicity gradient along the disk:…”

“…Using the same data of Kenney & Young (1989), Rengarajan & Iyengar (1992) found evidence of a CO deficit in some Virgo spiral, and Vollmer et al (2008) detected in NGC 4522 molecular gas displaced from the galactic disk because of ram pressure. Boselli et al (2002), Fumagalli & Gavazzi (2008) and Fumagalli et al (2009) found a molecular depletion in highly HI deficient galaxies, where the HI is depleted well inside the optical disk. Since not all the HI-deficient galaxies examined in their papers are cluster galaxies, it is of interest to analyze a sample of normal and HI-deficient galaxies in a cluster.…”