Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

Abstract

We present 12 CO(1−0) and 12 CO(2−1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log(O/H) ∼ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 µm emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H 2 molecular masses, namely a metallicity-dependent CO-to-H 2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses < ∼ 10 9 M , contrary to the atomic hydrogen fraction, M HI /M * , which increases inversely with M * . The flattening of the M H 2 /M * ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both Hi-deficient and Hi-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, τ dep , are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between Hi deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity.

The unusual ISM in blue and dusty gas-rich galaxies (BADGRS)

“…To be certain that these low MH2/ M d ratios are not produced purely by the conversions from observed quantities to dust and H2 mass, we compare the observed quantities in Figure 6. There is a tight relationship between CO emission and 250-µm flux in local galaxies from the HRS (Corbelli et al 2012) and star forming dwarf galaxies in Virgo (Grossi et al 2016). Both the massive metal rich sources from the HRS and the more metal poor Virgo dwarfs lie on the same FCO − F250 relation.…”

“…There are various ways in which corrections for mismatched beams are attempted in the literature (e.g. Braine & Combes 1992;Grossi et al 2016;Cormier et al 2014;Hunt et al 2017). Following Hunt et al (2017), we used the 160µm maps from our Herschel PACS imaging to make a direct estimate of the relative surface brightness of 160µm emission within 22 , 18 and 11 beam sized regions at the locations of our pointings.…”

“…The MH2/ M * ratios for the BADGRS are for the individual pointings; the stellar mass in the area of the IRAM beam is derived from smoothed stellar mass maps created from the g and i SDSS images and the Zibetti et al (2009) stellar mass prescription. From the literature we include the stellar mass selected COLDGASS (Saintonge et al 2011;Huang & Kauffmann 2014), ALLSMOG (Bothwell et al 2014(Bothwell et al , 2016 and HRS (Boselli et al 2014) samples and also the star forming Virgo dwarfs (Grossi et al 2016). Adjustments have been made to literature values in order to bring all measurements onto common scalings, see caption to Fig 7 for details.…”

“…For Log LIR > 11.0, the SFR is estimated using the LIR-SFR relationship of Kennicutt (1998) for a Chabrier (2003) IMF, while for the local samples of less obscured galaxies, a combination of IR, Hα and UV emission is used to capture the total SFR. For BADGRS we have used the FUV and IR emission in the CO regions to estimate the local SFR Grossi et al (2016) to be comparable to magphys as follows. The HRS magphys dust masses were compared to those determined by Grossi et al (2016) using their single MBB β = 2 method.…”

The final data release of ALLSMOG: a survey of CO in typical local low-M∗star-forming galaxies

“…We note that CO observations of galaxies with stellar masses as low as M * ∼ 10 9 M have been recently conducted as part of the Herschel Reference Survey (HRS, Boselli et al 2014a). However, most of the HRS low-M * sources belong to the Virgo cluster and are deficient in Hi because of ram pressure gas stripping and frequent dynamical harassment episodes (Boselli et al 2014b;Grossi et al 2016). …”