2012AJ....143..138S

We present maps of ¹²COJ = 2-1 emission covering the entire star-forming disks of 16 nearby dwarf galaxies observed by the IRAM HERACLES survey. The data have 13'' angular resolution, ∼250 pc at our average distance of D = 4 Mpc, and sample the galaxies by 10-1000 resolution elements. We apply stacking techniques to perform the first sensitive search for CO emission in dwarf galaxies outside the Local Group ranging from individual lines of sight, stacking over IR-bright regions of embedded star formation, and stacking over the entire galaxy. We detect five galaxies in CO with total CO luminosities of L_ CO 2-1_= (3-28)x10⁶ K.km/s.pc2. The other 11 galaxies remain undetected in CO even in the stacked images and have L_ CO 2-1_ ≲ (0.4-8) x10⁶ K.km/s.pc2. We combine our sample of dwarf galaxies with a large sample of spiral galaxies from the literature to study scaling relations of L_CO with M_Band metallicity. We find that dwarf galaxies with metallicities of Z ~ 1/2-1/10 Z_☉ have L_COof 2-4 orders of magnitude smaller than massive spiral galaxies and that their L_CO per unit L_B is 1-2 orders of magnitude smaller. A comparison with tracers of star formation (FUV and 24 µm) shows that L_CO per unit star formation rate (SFR) is 1-2 orders of magnitude smaller in dwarf galaxies. One possible interpretation is that dwarf galaxies form stars much more efficiently: we argue that the low L_CO/SFR ratio is due to the fact that the CO-to-H₂ conversion factor, α_CO, changes significantly in low-metallicity environments. Assuming that a constant H₂ depletion time of τ_dep= 1.8 Gyr holds in dwarf galaxies (as found for a large sample of nearby spirals) implies α_CO values for dwarf galaxies with Z ~ 1/2-1/10 Z_☉ that are more than one order of magnitude higher than those found in solar metallicity spiral galaxies. Such a significant increase of α_CO at low metallicity is consistent with previous studies, in particular those of Local Group dwarf galaxies that model dust emission to constrain H₂masses. Even though it is difficult to parameterize the dependence of α_CO on metallicity given the currently available data, the results suggest that CO is increasingly difficult to detect at lower metallicities. This has direct consequences for the detectability of star-forming galaxies at high redshift, which presumably have on average sub-solar metallicity.