Abstract
We present new 12CO(1-0) observations of 59 late-type galaxies belonging to the Herschel Reference Survey (HRS), a complete K-band-selected, volume-limited (15 <D< 25 Mpc) sample of nearby galaxies spanning a wide range in morphological type and luminosity. We studied different recipes to correct single-beam observations of nearby galaxies of different sizes and inclinations for aperture effects. This was done by comparing single-beam and multiple-beam observations along the major axis, which were corrected for aperture effects using different empirical or analytical prescriptions, to integrated maps of several nearby galaxies, including edge-on systems observed by different surveys. The resulting recipe is an analytical function determined by assuming that late-type galaxies are 3D exponentially declining discs with a characteristic scale length r_CO} = 0.2 r_24.5, where r_24.5 is the optical, g- (or B-) band isophotal radius at the 24.5 mag arcsec^-2 (25 mag arcsec^-2), as well as a scale height z_CO = 1/100 r_24.5. Our new CO data are then combined with those available in the literature to produce the most updated catalogue of CO observations for the HRS, now including 225 out of the 322 galaxies of the complete sample. The 3D exponential disc integration is applied to all the galaxies of the sample to measure their total CO fluxes, which are later transformed into molecular gas masses using a constant and a luminosity-dependent X_CO conversion factor. We also collect HI data for 315 HRS galaxies from the literature and present it in a homogenised form.
Highlights
The Herschel Reference Survey (HRS) is a complete sample of nearby galaxies defined to study the physical properties of the interstellar medium (ISM) in galaxies of different morphological type and luminosity (Boselli et al 2010a)
We present new 12CO(1–0) observations of 59 late-type galaxies belonging to the Herschel Reference Survey (HRS), a complete K-band-selected, volume-limited (15 D 25 Mpc) sample of nearby galaxies spanning a wide range in morphological type and luminosity
The 3D exponential disc integration is applied to all the galaxies of the sample to measure their total CO fluxes, which are later transformed into molecular gas masses using a constant and a luminosity-dependent XCO conversion factor
Summary
The Herschel Reference Survey (HRS) is a complete sample of nearby galaxies defined to study the physical properties of the interstellar medium (ISM) in galaxies of different morphological type and luminosity (Boselli et al 2010a). Dipole rotational transitions are strongly forbidden, making it very hard to directly observe the cold phase of this molecule, which in late-type galaxies has generally a temperature of ∼10 K For this reason, the molecular hydrogen mass is generally determined through observing of the second most aboundant molecule in the cold ISM, carbon monoxide, under the assumption that CO is a good tracer of H2 (Young & Scoville 1991). We adopt the calibration proposed by Boselli et al (2002) based on the H-band luminosity, log XCO [cm−2/(K km s−1)] = −0.38×log LH [LH ] + 24.23 We chose this calibration because it can be determined for all HRS galaxies, for which an H-band magnitude is available from 2MASS. It is a statistically significant, complete sample of nearby galaxies spanning a wide range in morphological type and stellar mass, making it ideally suited to study the cold gas properties of normal galaxies. The interested reader can find the results of other works based on the combined analysis of the Herschel and the other multifrequency data in Cortese et al (2012b; dust scaling relations along the Hubble sequence), Smith et al (2012; dust properties of early-type galaxies), Boselli et al (2012; far infrared colours), Boquien et al (2012, 2013; dust attenuation properties in resolved galaxies) or in the publication of several papers during the science demonstration phase of the instrument (Boselli et al 2010b; Cortese et al 2010; Eales et al 2010; Gomez et al 2010; Pohlen et al 2010; Sauvage et al 2010)
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