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EPoS |
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EPoS Contribution
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Unraveling Molecular Cloud Populations and Lifecycles with LEGO and the Haystack Telescope
Jens Kauffmann MIT, Westford, MA, US | |
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ALMA, and in future the ngVLA, spend substantial fractions of their time on constraining the details of the star formation process in galaxies. However, most of this important work must be done using indirect methods, as detail in extragalactic molecular clouds cannot be meaningfully resolved with physical beam sizes that span several parsec at extragalactic distances. The LEGO Large Program on the IRAM 30m-telescope, which images molecular clouds in the Milky Way, provides important data to “calibrate” these indirect methods, in many cases for the first time on the basis of observational data instead of theoretical models.
We provide an overview and status update on LEGO, which obtains the most comprehensive wide-field spectroscopic views of molecular clouds available to date. The survey has by now imaged two dozen clouds at frequencies 70–115 GHz. This work samples numerous astrophysically important molecules like CO, HCN, CS, and N2H+, and it generates for the first time a substantial sample of cloud-integrated line luminosities for these species. Emission from these molecules is, for example, widely used to probe the density and temperature structure of extragalactic molecular clouds. The LEGO data now can be used to test these methods used for such in well-studied galactic clouds, and to use this information to provide extragalactic work with improved and validated data analysis methods. LEGO has for example already shown that emission from HCN is not a straightforward tracer of dense gas in galaxies, in contrast to what is frequently assumed in extragalactic research. Here we present new data on well-studied dense and cold molecular clouds that are representative of the onset of star formation — and that are essentially devoid of HCN emission, thus further questioning the use of HCN as a tracer of dense gas. We also discuss how LEGO equips us with tools needed to unravel the molecular cloud populations in galaxies. Specifically, LEGO provides us with the cloud-averaged line luminosities needed to learn how different “types” of molecular clouds (e.g., very dense ones, or those disrupted by star formation feedback) can be identified from line ratios alone, and this knowledge can then be used to constrain the composition of cloud populations (e.g., relative ratio of clouds with and without star formation feedback) that would explain spectra observed in other galaxies. Initial results from LEGO reveal a rich diversity in cloud-averaged line ratios, highlighting the power of line ratios to constrain the properties of molecular clouds. We finally present how we plan to use the upgraded Haystack 37m-Telescope to expand this work to large sections of the Galactic Plane. We envision to make this instrument accessible to external users, and are therefore very keen to learn which studies the EPoS community would like to pursue with this telescope. | |
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| Caption: The G11 Infrared Dark Cloud as seen by Spitzer (background) and LEGO (inset). The absence of bright HCN emission in this cloud of very high density questions assumptions commonly made in extragalactic research. | |
| Collaborators: LEGO Collaboration |
Relevant topic(s): Extra-Galactic SF Feedback Molecular Clouds |
| Relevant Big Question: How do collapse and feedback conspire to produce low star formation efficiencies? | |