Physical Properties of Molecular Clouds at 2 pc Resolution in the Low-metallicity Dwarf Galaxy NGC 6822 and the Milky Way

Abstract

We present the Atacama Large Millimeter/submillimeter Array survey of CO(2-1) emission from the 1/5 solar metallicity, Local Group dwarf galaxy NGC6822. We achieve high (  » 0. 9 2 pc) spatial resolution while covering a large area: four 250 pc × 250 pc regions that encompass~2 3 of NGC6822ʼs star formation. In these regions, we resolve~150 compact CO clumps that have small radii (∼2-3 pc), narrow line width (~1 km s −1 ), and low filling factor across the galaxy. This is consistent with other recent studies of low-metallicity galaxies, but here shown with a15 larger sample. At parsec scales, CO emission correlates with m 8 m emission better than with m 24 m emission and anticorrelates with Hα, so that polycyclic aromatic hydrocarbon emission may be an effective tracer of molecular gas at low metallicity. The properties of the CO clumps resemble those of similar-size structures in Galactic clouds except of slightly lower surface brightness and with CO-to-H 2 ratio ∼1-2× the Galactic value. The clumps exist inside larger atomic-molecular complexes with masses typical for giant molecular clouds. Using dust to trace H 2 for the entire complex, we find the CO-to-H 2 ratio to be -20 25 the Galactic value, but with strong dependence on spatial scale and variations between complexes that may track their evolutionary state. The H 2 -to-H I ratio is low globally and only mildly above unity within the complexes. The ratio of star formation rate to H 2 is -3 5 higher in the complexes than in massive disk galaxies, but after accounting for the bias from targeting star-forming regions, we conclude that the global molecular gas depletion time may be as long as in massive disk galaxies.

The Physical Origin of Long Gas Depletion Times in Galaxies

“…During this vigorous evolution, gas cycles between non-starforming and star-forming stages and spends only t sf ∼ 5 − 15 Myr in the star-forming stage on each cycle (see the bottom panel of Figure 7), which is consistent with the short lifetimes derived for observed GMCs (e.g., Kawamura et al 2009;Murray 2011;Schruba et al 2017).…”

“…Likewise, when depletion time is estimated on larger scales, the scatter in τ dep decreases as we average over the distribution of local t . This is indeed observed (Schruba et al 2010(Schruba et al , 2017, although some of the obtained variation may be due to observational effects Kruijssen & Longmore 2014).…”

“…This number of cycles is also consistent with the typical depletion times of star-forming gas, τ ≡ 1/t −1 sf , and lifetimes, t sf , derived for observed star-forming regions, N c = τ /t sf . Specifically, depletion times of gas in observed star-forming regions are estimated to be t ∼ 50 − 500 Myr (e.g., Evans et al 2009Evans et al , 2014Lada et al 2010Lada et al , 2012Heiderman et al 2010;Gutermuth et al 2011;Schruba et al 2017); for t sf ∼ 5 − 15 Myr quoted above, these t give N c ∼ (50 − 500)/(5 − 15) ∼ 3 − 100. Such N c are in the ballpark of the N c range that we estimate for the gas in our simulations (see the bottom panel of Figure 7), although we note that the number of cycles in our simulation can be somewhat overestimated due to the artificially sharp threshold in the definition of star-forming gas.…”

“…The local depletion time in observed star-forming regions is t ∼ 40 − 500 Myr (e.g., Evans et al 2009Evans et al , 2014Lada et al 2010Lada et al , 2012Heiderman et al 2010;Gutermuth et al 2011;Schruba et al 2017). Thus, although the scatter is significant, typical values of t are considerably smaller than the global depletion time of molecular gas, τ dep,H2 ∼ 1 − 3 Gyr.…”

“…As pointed out by Kruijssen & Longmore (2014), a model considering different evolutionary stages and corresponding chemical phases of the ISM gas can be used to interpret the dependence of the depletion time and its scatter on the averaging scale (see, e.g., Schruba et al 2017). This dependence in observed galaxies can also be used as a stringent test of the star formation and feedback implementation in galaxy formation models.…”