LOW CO LUMINOSITIES IN DWARF GALAXIES

By A. Schruba, A. Leroy, F. Walter, F. Bigiel, E. Brinks, W. D. de Blok, C. Kramer, Erik Rosolowsky, K. Sandstrom, K. Schuster and 3 more. 2012

196 citations recorded by the source corpus.

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 LCO 2–1 = (3–28) × 106 K km s−1 pc2. The other 11 galaxies remain undetected in CO even in the stacked images and have LCO 2–1 ≲ (0.4–8) × 106 K km s−1 pc2. We combine our sample of dwarf galaxies with a large sample of spiral galaxies from the literature to study scaling relations of LCO with MB and metallicity. We find that dwarf galaxies with metallicities of Z ≈ 1/2–1/10 Z☉ have LCO of 2–4 orders of magnitude smaller than massive spiral galaxies and that their LCO per unit LB is 1–2 orders of magnitude smaller. A comparison with tracers of star formation (FUV and 24 μm) shows that LCO 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 LCO/SFR ratio is due to the fact that the CO-to-H2 conversion factor, αCO, changes significantly in low-metallicity environments. Assuming that a constant H2 depletion time of τdep = 1.8 Gyr holds in dwarf galaxies (as found for a large sample of nearby spirals) implies αCO 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 αCO at low metallicity is consistent with previous studies, in particular those of Local Group dwarf galaxies that model dust emission to constrain H2 masses. Even though it is difficult to parameterize the dependence of αCO 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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/deca1432084073bd194738e760680a307deffa2c

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Citations in the corpus

Dusty Star Forming Galaxies at High Redshift (2014, background): “Further studies have shown that indeed there is quite a bit of ‘‘CO-dark’’ molecular gas in these systems and other low-metallicity regions in galaxies, and that they are not simply molecular gas-poor (e.g. McQuinn et al., 2012; Schruba et al., 2012; Blanc et al., 2013).”
Cold gas properties of the Herschel Reference Survey - II. Molecular and total gas scaling relations (2014, background): “The same THINGS team has recently made several efforts to extend their analysis to different physical conditions, such as those encountered in the outskirt of extended discs (Bigiel et al. 2010), in Hi-dominated regions (Schruba et al. 2012), or in metal-poor regions (Bolatto et al. 2011).”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, methodology): “Constraining αCO in more extreme conditions would require different techniques (see Schruba et al. 2012, for more discussion).”
THE SMALL SCATTER OF THE BARYONIC TULLY–FISHER RELATION (2015, background): “CO emission is often undetected in low-mass, metal-poor galaxies (Schruba et al. 2012).”
MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES (2013, methodology): “In Section 4.1 we show that these low apparent τmoldep originate from low-mass, low-metallicity systems (see also Schruba et al. 2011; Krumholz et al. 2011; Schruba et al. 2012).”
A fundamental relation between the metallicity, gas content, and stellar mass of local galaxies (2013, methodology): “In this work, we have adopt a metallicity-dependent conversion factor, as given by Schruba et al. (2012): log αCO = log(A) + N (12 + log O/H) − 8.7, (1) with A = 8.2 ± 1.0 and N = −2.8 ± 0.2.”
Gas-to-dust mass ratios in local galaxies over a 2 dex metallicity range (2013, background): “First, detection of CO in low-metallicity galaxies is challenging: sensitivity has limited CO detections to galaxies with 12 + log(O/H) & 8.0 (e.g., Leroy et al. 2009; Schruba et al. 2012).”
The CO-to-H2 Conversion Factor (2013, methodology): “Schruba et al. (2012) applied this technique to estimate XCO as function of metallicity in a sample of nearby galaxies.”
THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS (2012, result): “In addition to our galaxies we also include a sample of local normal galaxies for which we have robust M∗ and SFR estimates from Leroy et al. (2008) and αCO measurements from Schruba et al. (2012).”
REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “…et al. (2012) subsequently were able to show that the conversion factor of high-z MS galaxies scales with gas-phase metallicity in a similar manner as the negative power laws observed for local galaxies (e.g., Wilson 1995, Israel 1997, Boselli et al. 2002, Leroy et al. 2011, Schruba et al. 2012).”
An Overview of the Dwarf Galaxy Survey (2013, background): “…to reconcile with the molecular gas necessary to fuel star formation and a valiant effort has been made in determining a recipe for the CO (J ¼ 1 0)- to-H2 conversion factor in low metallicity galaxies (e.g., Leroy et al. 2011; Schruba et al. 2012; Bigiel et al. 2011, and references within).”
HIGH-RESOLUTION MASS MODELS OF DWARF GALAXIES FROM LITTLE THINGS (2015, background): “Here, we do not correct for the effect of molecular hydrogen (H2) since low metallicities in dwarf galaxies can induce only a small fraction of the gaseous component in the form of H2 (e.g., Leroy et al. 2007; Leroy et al. 2011; Schruba et al. 2012).”