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Register a new userMolecular depletion times and the CO-to-H2 conversion factor in metal-poor galaxies
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Tracing molecular hydrogen content with carbon monoxide in low-metallicity galaxies has been exceedingly difficult. Here we present a new effort, with IRAM 30-m observations of 12CO(1-0) of a sample of 8 dwarf galaxies having oxygen abundances ranging from 12+logO/H=7.7 to 8.4. CO emission is detected in all galaxies, including the most metal-poor galaxy of our sample (0.1 Zsun); to our knowledge this is the largest number of 12CO(1-0) detections ever reported for galaxies with 12+logO/H<=8 (0.2 Zsun) outside the Local Group. We calculate stellar masses (Mstar) and star-formation rates (SFRs), and analyze our results by combining our observations with galaxy samples from the literature. Extending previous results for a correlation of the molecular gas depletion time, tau(dep), with Mstar and specific SFR (sSFR), we find a variation in tau(dep) of a factor of 200 or more (from <50 Myr to 10 Gyr) over a spread of 1000 in sSFR and Mstar. We exploit the variation of tau(dep) to constrain the CO-to-H2 mass conversion factor alpha(CO) at low metallicity, and assuming a power-law variation find alpha(CO) propto (Z/Zsun)^1.9, similar to results based on dust continuum measurements compared with gas mass. By including HI measurements, we show that the fraction of total gas mass relative to the baryonic mass is higher in galaxies that are metal poor, of low mass, and of high sSFR. Finally, comparisons of the data with star-formation models of the molecular gas phases suggest that, at metallicities Z/Zsun<=0.2, there are some discrepancies with model predictions.
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We derive the CO-to-H2 conversion factor, X_CO = N(H2)/I_CO, across the Perseus molecular cloud on sub-parsec scales by combining the dust-based N(H2) data with the I_CO data from the COMPLETE Survey. We estimate an average X_CO ~ 3 x 10^19 cm^-2 K^-1 km^-1 s and find a factor of ~3 variations in X_CO between the five sub-regions in Perseus. Within the individual regions, X_CO varies by a factor of ~100, suggesting that X_CO strongly depends on local conditions in the interstellar medium. We find that X_CO sharply decreases at Av < 3 mag but gradually increases at Av > 3 mag, with the transition occurring at Av where I_CO becomes optically thick. We compare the N(HI), N(H2), I_CO, and X_CO distributions with two models of the formation of molecular gas, a one-dimensional photodissociation region (PDR) model and a three-dimensional magnetohydrodynamic (MHD) model tracking both the dynamical and chemical evolution of gas. The PDR model based on the steady state and equilibrium chemistry reproduces our data very well but requires a diffuse halo to match the observed N(HI) and I_CO distributions. The MHD model generally matches our data well, suggesting that time-dependent effects on H2 and CO formation are insignificant for an evolved molecular cloud like Perseus. However, we find interesting discrepancies, including a broader range of N(HI), likely underestimated I_CO, and a large scatter of I_CO at small Av. These discrepancies likely result from strong compressions/rarefactions and density fluctuations in the MHD model.
We present kiloparsec (kpc) spatial resolution maps of the CO-to-H2 conversion factor (alpha_co) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for alpha_co and DGR by assuming that the DGR is approximately constant on kpc scales. With this assumption, we can combine maps of dust mass surface density, CO integrated intensity and HI column density to solve for both alpha_co and DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high resolution far-IR maps from the Herschel key program KINGFISH, 12CO J=(2-1) maps from the IRAM 30m large program HERACLES and HI 21-cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our alpha_co results on the more typically used 12CO J=(1-0) scale and show using literature measurements that variations in the line ratio do not effect our results. In total, we derive 782 individual solutions for alpha_co and DGR. On average, alpha_co = 3.1 Msun pc^-2 (K km s^-1)^-1 for our sample with a standard deviation of 0.3 dex. Within galaxies we observe a generally flat profile of alpha_co as a function of galactocentric radius. However, most galaxies exhibit a lower alpha_co in the central kpc---a factor of ~2 below the galaxy mean, on average. In some cases, the central alpha_co value can be factors of 5 to 10 below the standard Milky Way (MW) value of alpha_co,MW =4.4 Msun pc^-2 (K km s^-1)^-1. While for alpha_co we find only weak correlations with metallicity, DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate alpha_co for studies of nearby galaxies.
Deep Herschel imaging and 12CO(2-1) line luminosities from the IRAM PdBI are combined for a sample of 17 galaxies at z>1 from the GOODS-N field. The sample includes galaxies both on and above the main sequence (MS) traced by star-forming galaxies in the SFR-M* plane. The far-infrared data are used to derive dust masses, Mdust. Combined with an empirical prescription for the dependence of the gas-to-dust ratio on metallicity (GDR), the CO luminosities and Mdust values are used to derive for each galaxy the CO-to-H2 conversion factor, alpha_co. Like in the local Universe, the value of alpha_co is a factor of ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We also uncover a relation between alpha_co and dust temperature (Tdust; alpha_co decreasing with increasing Tdust) as obtained from modified blackbody fits to the far-infrared data. While the absolute normalization of the alpha_co(Tdust) relation is uncertain, the global trend is robust against possible systematic biases in the determination of Mdust, GDR or metallicity. Although we cannot formally distinguish between a step and a smooth evolution of alpha_co with the dust temperature, we can conclude that in galaxies of near-solar metallicity, a critical value of Tdust=30K can be used to determine whether the appropriate alpha_co is closer to the starburst value (1.0 Msun(K kms pc^2)^-1, if Tdust>30K) or closer to the Galactic value (4.35 Msun (K kms pc^2)^-1, if Tdust<30K). This indicator has the great advantage of being less subjective than visual morphological classifications of mergers/SFGs, which can be difficult at high z because of the clumpy nature of SFGs. In the absence of far-infrared data, the offset of a galaxy from the main sequence (i.e., log[SSFR(galaxy)/SSFR_MS(M*,z)]) can be used to identify galaxies requiring the use of an alpha_co conversion factor lower than the Galactic value.
We show that the XCO factor, which converts the CO luminosity into the column density of molecular hydrogen has similar values for dense, fully molecular gas and for diffuse, partially molecular gas. We discuss the reasons of this coincidence and the consequences for the understanding of the interstellar medium.
Studying the molecular component of the interstellar medium in metal-poor galaxies has been challenging because of the faintness of carbon monoxide emission, the most common proxy of H2. Here we present new detections of molecular gas at low metallicities, and assess the physical conditions in the gas through various CO transitions for 8 galaxies. For one, NGC 1140 (Z/Zsun ~ 0.3), two detections of 13CO isotopologues and atomic carbon, [CI](1-0), and an upper limit for HCN(1-0) are also reported. After correcting to a common beam size, we compared 12CO(2-1)/12CO(1-0) (R21) and 12CO(3-2)/12CO(1-0) (R31) line ratios of our sample with galaxies from the literature and find that only NGC 1140 shows extreme values (R21 ~ R31 ~ 2). Fitting physical models to the 12CO and 13CO emission in NGC 1140 suggests that the molecular gas is cool (kinetic temperature Tkin<=20 K), dense (H2 volume density nH2 >= $10^6$ cm$^{-3}$), with moderate CO column density (NCO ~ $10^{16}$ cm$^{-2}$) and low filling factor. Surprisingly, the [12CO]/[13CO] abundance ratio in NGC 1140 is very low (~ 8-20), lower even than the value of 24 found in the Galactic Center. The young age of the starburst in NGC 1140 precludes 13C enrichment from evolved intermediate-mass stars; instead we attribute the low ratio to charge-exchange reactions and fractionation, because of the enhanced efficiency of these processes in cool gas at moderate column densities. Fitting physical models to 12CO and [CI](1-0) emission in NGC 1140 gives an unusually low [12CO]/[12C] abundance ratio, suggesting that in this galaxy atomic carbon is at least 10 times more abundant than 12CO.
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