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Date and Time of the Query: 2019-06-25 T21:06:48 PDT
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For refcode 2012AJ....143..138S:
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2012AJ....143..138S Low CO Luminosities in Dwarf Galaxies Schruba, Andreas; Leroy, Adam K.; Walter, Fabian; Bigiel, Frank; Brinks, Elias; de Blok, W. J. G.; Kramer, Carsten; Rosolowsky, Erik; Sandstrom, Karin; Schuster, Karl; Usero, Antonio; Weiss, Axel; Wiesemeyer, Helmut Abstract. We present maps of ^12^COJ = 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) x 10^6^ K km s^--1^ pc^2^. The other 11 galaxies remain undetected in CO even in the stacked images and have L_CO 2-1_ <~ (0.4-8) x 10^6^ K km s^--1^ pc^2^. 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_B_ and metallicity. We find that dwarf galaxies with metallicities of Z ~ 1/2-1/10 Z_&sun;_ have L_CO_ of 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 micron) 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-H_2_ conversion factor, {alpha}_CO_, changes significantly in low-metallicity environments. Assuming that a constant H_2_ depletion time of tau_dep_ = 1.8 Gyr holds in dwarf galaxies (as found for a large sample of nearby spirals) implies {alpha}_CO_ values for dwarf galaxies with Z ~ 1/2-1/10 Z_&sun;_ that are more than one order of magnitude higher than those found in solar metallicity spiral galaxies. Such a significant increase of {alpha}_CO_ at low metallicity is consistent with previous studies, in particular those of Local Group dwarf galaxies that model dust emission to constrain H_2_ masses. Even though it is difficult to parameterize the dependence of {alpha}_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. Key words: galaxies: ISM, ISM: molecules, radio lines: galaxies
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