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Published February 1, 2017 | Submitted + Published
Journal Article Open

Dense Molecular Gas Tracers in the Outflow of the Starburst Galaxy NGC 253


We present a detailed study of a molecular outflow feature in the nearby starburst galaxy NGC 253 using ALMA. We find that this feature is clearly associated with the edge of NGC 253's prominent ionized outflow, has a projected length of ~300 pc, with a width of ~50 pc, and a velocity dispersion of ~40 km s^(−1), which is consistent with an ejection from the disk about 1 Myr ago. The kinematics of the molecular gas in this feature can be interpreted (albeit not uniquely) as accelerating at a rate of 1 km s^(−1) pc^(−1). In this scenario, the gas is approaching an escape velocity at the last measured point. Strikingly, bright tracers of dense molecular gas (HCN, CN, HCO^+, CS) are also detected in the molecular outflow: we measure an HCN(1–0)/CO(1–0) line ratio of ~1/10 in the outflow, similar to that in the central starburst region of NGC 253 and other starburst galaxies. By contrast, the HCN/CO line ratio in the NGC 253 disk is significantly lower (~1/30), similar to other nearby galaxy disks. This strongly suggests that the streamer gas originates from the starburst, and that its physical state does not change significantly over timescales of ~1 Myr during its entrainment in the outflow. Simple calculations indicate that radiation pressure is not the main mechanism for driving the outflow. The presence of such dense material in molecular outflows needs to be accounted for in simulations of galactic outflows.

Additional Information

© 2017 The American Astronomical Society. Received 2016 August 8; revised 2016 December 16; accepted 2016 December 28; published 2017 February 1. We thank the referee for excellent comments that improved the paper. D.S.M. acknowledges partial support by the National Science Foundation through grant AST-1009620. S.V. acknowledges NSF grant AST-1009583. A.D.B. acknowledges visiting support by the Alexander von Humboldt Foundation, and support by the National Science Foundation through a CAREER grant AST-0955836 and AST-1412419. E.C.O. is supported by the National Science Foundation through grant AST-1312006. This paper makes use of the following ALMA data: ADS/JAO.ALMA #2011.0.00172.S, #2012.1.00108.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). Some of the HST data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST), others were acquired under program HST-GO-13730 with support provided by NASA through a grant from the STScI. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. The Mopra radio telescope is part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a National Facility managed by CSIRO. Based on observations carried out under project number 209-14 with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

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Published - Walter_2017_ApJ_835_265.pdf

Submitted - 1701.05040v1.pdf


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