Oversized Gas Clumps in an Extremely Metal-poor Molecular Cloud Revealed by ALMA’s Parsec-scale Maps

Abstract

Metals are thought to have profound effects on the internal structures of molecular clouds in which stars are born. The absence of metals is expected to prevent gas from efficient cooling and fragmentation in theory. However, this effect has not yet been observed at low metallicity environments, such as in the early Universe and local dwarf galaxies, because of the lack of high spatial resolution maps of gas. We carried out ALMA observations of the carbon monoxide (CO) J=2-1 emission line at 1.4-parsec resolutions of a molecular cloud in DDO 70 at 7% solar metallicity, the most metal-poor galaxy currently known with a CO detection. In total, five clumps have been identified and they are found to follow more or less the Larson's law. Since the CO emission exists in regions with visual extinction A V around 1.0, we converted this A V to the gas mass surface density using a gas-to-dust ratio of 4,594±2,848 for DDO 70. We found that the CO clumps in DDO 70 exhibit significantly larger (on average four times) sizes than those at the same gas mass surface densities in massive star-formation regions of the Milky Way. The existence of such large clumps appears to be consistent with theoretical expectations that gas fragmentation in low metallicity clouds is suppressed. While our observation is only for one cloud in the galaxy, if it is representative, the above result implies suppressed gas fragmentation during the cloud collapse and star formation in the early Universe.

12CO and 13CO observation of the low-metallicity dwarf galaxy DDO 154

“…which is three orders of magnitude larger than the typical MW value. This is consistent with previous estimates (Wilson 1995 ;Arimoto et al 1996 ;Israel 1997 ;Leroy et al 2011 ;Bolatto et al 2013 ;Hunt et al 2023 ) extrapolated to low metallicity, but much higher than recent estimates based on actual CO detection with interferometers in dwarf irregular galaxies with similar metallicity, which give X CO values of only several times the MW value (Rubio et al 2015 ;Shi et al 2020 ). These estimates based on interferometric CO detection assume that CO is virialized, and indicates the conversion factor limited to the region where CO is detected.…”

“…The cloud size R was assumed to be 50 pc, but since we assume in subsection 3.1 that the cloud has a velocity width of V = 2 km s −1 , we change the cloud size accordingly. All detected clouds in WLM have radii of less than 6 pc (Rubio et al 2015 ), and those in Sextans B have radii less than 3 pc (Shi et al 2020 ), both roughly following the classical size-line width relation Solomon et al ( 1987 ). Here we assume that the cloud has a radius of 5 pc.…”

“…The non-detection does not allow for a measurement of V . K11 assumed a velocity width of 10 km s −1 , but in light of recent interferometric CO detections at similar metallicity (Rubio et al 2015 ;Shi et al 2020 ) which have found that CO line-widths are smaller than 2 km s −1 , we take V = 2 km s −1 as a conservative value. The CO intensity upper limits are shown in figure 2 .…”

“…8 (Rubio et al 2015 ) and Sextans B at 12 + log [O / H] = 7 . 7 (Shi et al 2016(Shi et al , 2020 have been successful. For both galaxies, the studies find that molecular clouds are systematically smaller than MW clouds, their radii ranging from 1 to 4 pcs, but roughly follow the same size-line width relation (Solomon et al 1987 ).…”

The Interstellar Medium of Dwarf Galaxies

“…An even more extreme object, the irregular galaxy Wolf-Lundmark-Melotte (WLM) with 12 + log(O/H) ≈ 7.8, studied by [79], revealed 10 isolated CO J = 1→0 clumps with properties similar to those encountered in NGC 6822. One of the most extreme CO detected galaxies with respect to metallicity (12 + log(O/H) = 7.5; [80]), Sextans B (also known as DDO 70), has been mapped with ALMA by [81]. Using a linear resolution of 1.4 pc for this source located in the outskirts of the Local Group revealed five clumps, accounting for the bulk of the single-dish emission and following Larson's size-line width relation for the disk of the Milky Way.…”