ALBERT

Description: To test the theoretical understanding that finding bright CO emission depends primarily on dust shielding, we investigate the relationship between CO emission ( I CO ) and the amount of dust (estimated from infrared emission and expressed as ‘ A V ’) across the Large Magellanic Cloud (LMC), the Small Magellanic Cloud, and the Milky Way. We show that at our common resolution of 10 pc scales, I CO given a fixed line of sight A V is similar across all three systems despite the difference in metallicity. We find some evidence for a secondary dependence of I CO on radiation field; in the LMC, I CO at a given A V is smaller in regions of high T dust , perhaps because of an increased photodissociating radiation field. We suggest a simple but useful picture in which the CO-to-H 2 conversion factor ( X CO ) depends on two separable factors: (1) the distribution of gas column densities, which maps to an extinction distribution via a dust-to-gas ratio; and (2) the dependence of I CO on A V . Assuming that the probability distribution function (PDF) of local Milky Way clouds is universal, this approach predicts a dependence of ${X_{\rm CO}}$ on Z between Z –1 and Z –2 above about a third solar metallicity. Below this metallicity, CO emerges from only the high column density parts of the cloud and so depends very sensitively on the adopted PDF and the H 2 /H  i prescription. The PDF of low-metallicity clouds is thus of considerable interest and the uncertainty associated with even an ideal prescription for X CO at very low metallicity will be large.

Description: We combine Spitzer and Herschel data of the star-forming region N11 in the Large Magellanic Cloud (LMC) to produce detailed maps of the dust properties in the complex and study their variations with the interstellar-medium conditions. We also compare Atacama Pathfinder EXperiment/Large APEX Bolometer Camera (APEX/LABOCA) 870 μm observations with our model predictions in order to decompose the 870 μm emission into dust and non-dust [free–free emission and CO(3–2) line] contributions. We find that in N11, the 870 μm can be fully accounted for by these three components. The dust surface density map of N11 is combined with H  i and CO observations to study local variations in the gas-to-dust mass ratios. Our analysis leads to values lower than those expected from the LMC low-metallicity as well as to a decrease of the gas-to-dust mass ratio with the dust surface density. We explore potential hypotheses that could explain the low ‘observed’ gas-to-dust mass ratios (variations in the X CO factor, presence of CO-dark gas or of optically thick H  i or variations in the dust abundance in the dense regions). We finally decompose the local spectral energy distributions (SEDs) using a principal component analysis (i.e. with no a priori assumption on the dust composition in the complex). Our results lead to a promising decomposition of the local SEDs in various dust components (hot, warm, cold) coherent with that expected for the region. Further analysis on a larger sample of galaxies will follow in order to understand how unique this decomposition is or how it evolves from one environment to another.

Description: We estimate the parameters of the Kennicutt–Schmidt (KS) relationship, linking the star formation rate ( SFR ) to the molecular gas surface density ( mol ), in the Survey Toward Infrared-Bright Nearby Galaxies sample of nearby disc galaxies using a hierarchical Bayesian method. This method rigorously treats measurement uncertainties, and provides accurate parameter estimates for both individual galaxies and the entire population. Assuming standard conversion factors to estimate SFR and mol from the observations, we find that the KS parameters vary between galaxies, indicating that no universal relationship holds for all galaxies. The KS slope of the whole population is 0.76, with the 2 range extending from 0.58 to 0.94. These results imply that the molecular gas depletion time is not constant, but varies from galaxy-to-galaxy, and increases with the molecular gas surface density. Therefore, other galactic properties besides just mol affect SFR , such as the gas fraction or stellar mass. The non-universality of the KS relationship indicates that a comprehensive theory of star formation must take into account additional physical processes that may vary from galaxy to galaxy.

Description: Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts 〈z〉 of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today's massive spiral galaxies. The slow decrease between z approximately 2 and z approximately 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Tacconi, L J -- Genzel, R -- Neri, R -- Cox, P -- Cooper, M C -- Shapiro, K -- Bolatto, A -- Bouche, N -- Bournaud, F -- Burkert, A -- Combes, F -- Comerford, J -- Davis, M -- Schreiber, N M Forster -- Garcia-Burillo, S -- Gracia-Carpio, J -- Lutz, D -- Naab, T -- Omont, A -- Shapley, A -- Sternberg, A -- Weiner, B -- England -- Nature. 2010 Feb 11;463(7282):781-4. doi: 10.1038/nature08773.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Max-Planck-Institut fur extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany. linda@mpe.mpg.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20148033" target="_blank"〉PubMed〈/a〉

Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Bolatto, Alberto D -- England -- Nature. 2012 Jun 13;486(7402):199-200. doi: 10.1038/486199a.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/22699607" target="_blank"〉PubMed〈/a〉

Description: The under-abundance of very massive galaxies in the Universe is frequently attributed to the effect of galactic winds. Although ionized galactic winds are readily observable, most of the expelled mass (that is, the total mass flowing out from the nuclear region) is likely to be in atomic and molecular phases that are cooler than the ionized phases. Expanding molecular shells observed in starburst systems such as NGC 253 (ref. 12) and M 82 (refs 13, 14) may facilitate the entrainment of molecular gas in the wind. Although shell properties are well constrained, determining the amount of outflowing gas emerging from such shells and the connection between this gas and the ionized wind requires spatial resolution better than 100 parsecs coupled with sensitivity to a wide range of spatial scales, a combination hitherto not available. Here we report observations of NGC 253, a nearby starburst galaxy (distance approximately 3.4 megaparsecs) known to possess a wind, that trace the cool molecular wind at 50-parsec resolution. At this resolution, the extraplanar molecular gas closely tracks the Halpha filaments, and it appears to be connected to expanding molecular shells located in the starburst region. These observations allow us to determine that the molecular outflow rate is greater than 3 solar masses per year and probably about 9 solar masses per year. This implies a ratio of mass-outflow rate to star-formation rate of at least 1, and probably approximately 3, indicating that the starburst-driven wind limits the star-formation activity and the final stellar content.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Bolatto, Alberto D -- Warren, Steven R -- Leroy, Adam K -- Walter, Fabian -- Veilleux, Sylvain -- Ostriker, Eve C -- Ott, Jurgen -- Zwaan, Martin -- Fisher, David B -- Weiss, Axel -- Rosolowsky, Erik -- Hodge, Jacqueline -- England -- Nature. 2013 Jul 25;499(7459):450-3. doi: 10.1038/nature12351.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Astronomy, Laboratory for Millimeter-wave Astronomy, and Joint Space Institute, University of Maryland, College Park, Maryland 20742, USA. bolatto@astro.umd.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/23887428" target="_blank"〉PubMed〈/a〉

Description: Galaxies observed at redshift z 〉 6, when the Universe was less than a billion years old, thus far very rarely show evidence of the cold dust that accompanies star formation in the local Universe, where the dust-to-gas mass ratio is around one per cent. A prototypical example is the galaxy Himiko (z = 6.6), which--a mere 840 million years after the Big Bang--is forming stars at a rate of 30-100 solar masses per year, yielding a mass assembly time of about 150 x 10(6) years. Himiko is thought to have a low fraction (2-3 per cent of the Sun's) of elements heavier than helium (low metallicity), and although its gas mass cannot yet be determined its dust-to-stellar mass ratio is constrained to be less than 0.05 per cent. The local dwarf galaxy I Zwicky 18, which has a metallicity about 4 per cent that of the Sun's and is forming stars less rapidly (assembly time about 1.6 x 10(9) years) than Himiko but still vigorously for its mass, is also very dust deficient and is perhaps one of the best analogues of primitive galaxies accessible to detailed study. Here we report observations of dust emission from I Zw 18, from which we determine its dust mass to be 450-1,800 solar masses, yielding a dust-to-stellar mass ratio of about 10(-6) to 10(-5) and a dust-to-gas mass ratio of 3.2-13 x 10(-6). If I Zw 18 is a reasonable analogue of Himiko, then Himiko's dust mass must be around 50,000 solar masses, a factor of 100 below the current upper limit. These numbers are quite uncertain, but if most high-z galaxies are more like Himiko than like the very-high-dust-mass galaxy SDSS J114816.64 + 525150.3 at z approximately 6, which hosts a quasar, then our prospects for detecting the gas and dust inside such galaxies are much poorer than hitherto anticipated.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Fisher, David B -- Bolatto, Alberto D -- Herrera-Camus, Rodrigo -- Draine, Bruce T -- Donaldson, Jessica -- Walter, Fabian -- Sandstrom, Karin M -- Leroy, Adam K -- Cannon, John -- Gordon, Karl -- England -- Nature. 2014 Jan 9;505(7482):186-9. doi: 10.1038/nature12765. Epub 2013 Dec 8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉1] Department of Astronomy, Laboratory for Millimeter-wave Astronomy and Joint Space Institute, University of Maryland, College Park, Maryland 20742, USA [2] Centre for Astrophysics and Supercomputing, Swinburne University, PO Box 218, Hawthorn, Victoria 3122, Australia. ; Department of Astronomy, Laboratory for Millimeter-wave Astronomy and Joint Space Institute, University of Maryland, College Park, Maryland 20742, USA. ; Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA. ; Max-Planck-Institut fur Astronomie, Konigstuhl 17, Heidelberg 69117, Germany. ; National Radio Astronomy Observatory, Charlottesville, Virginia 22903, USA. ; Department of Physics and Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105, USA. ; Space Telescope Science Institute, Baltimore, Maryland 21218, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24317694" target="_blank"〉PubMed〈/a〉