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Register a new userNOEMA Observations of a Molecular Cloud in the low-metallicity Galaxy Kiso 5639
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A giant star-forming region in a metal-poor dwarf galaxy has been observed in optical lines with the 10-m Gran Telescopio Canarias and in the emission line of CO(1-0) with the NOEMA mm-wave interferometer. The metallicity was determined to be 12+log(O/H)=7.83+-0.09, from which we estimate a conversion factor of alpha_CO~100 Msun/pc2/(K km/s) and a molecular cloud mass of ~2.9x10^7 Msun. This is an enormous concentration of molecular mass at one end of a small galaxy, suggesting a recent accretion. The molecular cloud properties seem normal: the surface density, 120 Msun/pc2, is comparable to that of a standard giant molecular cloud, the clouds virial ratio of ~1.8 is in the star-formation range, and the gas consumption time, 0.5 Gyr, at the present star formation rate is typical for molecular regions. The low metallicity implies that the cloud has an average visual extinction of only 0.8 mag, which is close to the threshold for molecule formation. With such an extinction threshold, molecular clouds in metal-poor regions should have high surface densities and high internal pressures. If high pressure is associated with the formation of massive clusters, then metal-poor galaxies such as dwarfs in the early universe could have been the hosts of metal-poor globular clusters.
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We find evidence for the impact of infalling, low-metallicity gas on the Galactic disk. This is based on FUV absorption line spectra, 21-cm emission line spectra, and FIR mapping to estimate the abundance and physical properties of IV21 (IVC135+54-45), a galactic intermediate-velocity molecular cloud (IVMC) that lies ~300 pc above the disk. The metallicity of IV21 was estimated using observations toward the sdB star PG1144+615, located at a projected distance of 16 pc from the clouds densest core, by measuring ion and HI column densities for comparison with known solar abundances. Despite the clouds bright FIR emission and large column densities of molecular gas as traced by CO, we find that it has a sub-solar metallicity of log(Z/Z_Sun)=-0.43 +/- 0.12dex. IV21 is thus the first known sub-solar metallicity cloud in the solar neighborhood. In contrast, most intermediate-velocity clouds (IVC) have near-solar metallicities and are believed to originate in the Galactic Fountain. The clouds low metallicity is also atypical for Galactic molecular clouds, especially in the light of the bright FIR emission which suggest a substantial dust content. The measured I_100mu/N(HI) ratio is a factor of three below the average found in high latitude HI clouds within the solar neighborhood. We argue that IV21 represents the impact of an infalling, low-metallicity high-velocity cloud (HVC) that is mixing with disk gas in the lower Galactic halo.
Cold atomic hydrogen clouds are the precursors of molecular clouds. Due to self-absorption, the opacity of cold atomic hydrogen may be high, and this gas may constitute an important mass component of the interstellar medium (ISM). Atomic hydrogen gas can be cooled to temperatures much lower than found in the cold neutral medium (CNM) through collisions with molecular hydrogen. In this paper, we search for HI Narrow Self-Absorption (HINSA) features in the Large Magellanic Cloud (LMC) as an indicator of such cold HI clouds, and use the results to quantify atomic masses and atomic-to-molecular gas ratio. Our search for HINSA features was conducted towards molecular clouds in the LMC using the ATCA+Parkes HI survey and the MAGMA CO survey. HINSA features are prevalent in the surveyed sightlines. This is the first detection of HINSA in an external galaxy. The HINSA-HI/$rm{H}_{2}$ ratio in the LMC varies from 0.5e{-3} to 3.4e{-3} (68% interval), with a mean value of $(1.31 pm 0.03)$e{-3}, after correcting for the effect of foreground HI gas. This is similar to the Milky Way value and indicates that similar fractions of cold gas exist in the LMC and the Milky Way, despite their differing metallicities, dust content and radiation fields. The low ratio also confirms that, as with the Milky Way, the formation timescale of molecular clouds is short. The ratio shows no radial gradient, unlike the case for stellar metallicity. No correlation is found between our results and those from previous HI absorption studies of the LMC.
We present high-resolution (sub-parsec) observations of a giant molecular cloud in the nearest star-forming galaxy, the Large Magellanic Cloud. ALMA Band 6 observations trace the bulk of the molecular gas in $^{12}$CO(2-1) and high column density regions in $^{13}$CO(2-1). Our target is a quiescent cloud (PGCC G282.98-32.40, which we refer to as the Planck cold cloud or PCC) in the southern outskirts of the galaxy where star-formation activity is very low and largely confined to one location. We decompose the cloud into structures using a dendrogram and apply an identical analysis to matched-resolution cubes of the 30 Doradus molecular cloud (located near intense star formation) for comparison. Structures in the PCC exhibit roughly 10 times lower surface density and 5 times lower velocity dispersion than comparably sized structures in 30 Dor, underscoring the non-universality of molecular cloud properties. In both clouds, structures with relatively higher surface density lie closer to simple virial equilibrium, whereas lower surface density structures tend to exhibit super-virial line widths. In the PCC, relatively high line widths are found in the vicinity of an infrared source whose properties are consistent with a luminous young stellar object. More generally, we find that the smallest resolved structures (leaves) of the dendrogram span close to the full range of line widths observed across all scales. As a result, while the bulk of the kinetic energy is found on the largest scales, the small-scale energetics tend to be dominated by only a few structures, leading to substantial scatter in observed size-linewidth relationships.
Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations, they are molecular with H_2 the dominant species and CO the best available tracer. When the abundances of carbon and oxygen are low compared to hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies, CO forms slowly and is easily destroyed, so it is difficult for it to accumulate inside dense clouds. Here we report interferometric observations of CO clouds in the local group dwarf irregular galaxy Wolf-Lundmark-Melotte (WLM), which has a metallicity that is 13% of the solar value and 50% lower than the previous CO detection threshold. The clouds are tiny compared to the surrounding atomic and H_2 envelopes, but they have typical densities and column densities for CO clouds in the Milky Way. The normal CO density explains why star clusters forming in dwarf irregulars have similar densities to star clusters in giant spiral galaxies. The low cloud masses suggest that these clusters will also be low mass, unless some galaxy-scale compression occurs, such as an impact from a cosmic cloud or other galaxy. If the massive metal-poor globular clusters in the halo of the Milky Way formed in dwarf galaxies, as is commonly believed, then they were probably triggered by such an impact.
We present Atacama Large Millimeter/submillimeter Array observations of CO lines and dust continuum emission of the source RCSGA 032727--132609, a young $z=1.7$ low-metallicity starburst galaxy. The CO(3-2) and CO(6-5) lines, and continuum at rest-frame $450,mu m$ are detected and show a resolved structure in the image plane. We use the corresponding lensing model to obtain a source plane reconstruction of the detected emissions revealing intrinsic flux density of $S_{450,mu m}=23.5_{-8.1}^{+26.8}$ $mu$Jy and intrinsic CO luminosities $L_{rm CO(3-2)}=2.90_{-0.23}^{+0.21}times10^{8}$ ${rm K,km,s^{-1},pc^{2}}$ and $L_{rm CO(6-5)}=8.0_{-1.3}^{+1.4}times10^{7}$ ${rm K,km,s^{-1},pc^{2}}$. We used the resolved properties in the source plane to obtain molecular gas and star-formation rate surface densities of $Sigma_{rm H2}=16.2_{-3.5}^{+5.8},{rm M}_{odot},{rm pc}^{-2}$ and $Sigma_{rm SFR}=0.54_{-0.27}^{+0.89},{rm M}_{odot},{rm yr}^{-1},{rm kpc}^{-2}$ respectively. The intrinsic properties of RCSGA 032727--132609 show an enhanced star-formation activity compared to local spiral galaxies with similar molecular gas densities, supporting the ongoing merger-starburst phase scenario. RCSGA 032727--132609 also appears to be a low--density starburst galaxy similar to local blue compact dwarf galaxies, which have been suggested as local analogs to high-redshift low-metallicity starburst systems. Finally, the CO excitation level in the galaxy is consistent with having the peak at ${rm J}sim5$, with a higher excitation concentrated in the star-forming clumps.
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