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Published December 1, 2016 | Published + Submitted
Journal Article Open

ALMA Reveals Weak [N II] Emission in "Typical" Galaxies and Intense Starbursts at z = 5–6


We report interferometric measurements of [N II] 205 μm fine-structure line emission from a representative sample of three galaxies at z = 5–6 using the Atacama Large (sub)Millimeter Array (ALMA). These galaxies were previously detected in [C II] and far-infrared continuum emission and span almost two orders of magnitude in star formation rate (SFR). Our results show at least two different regimes of ionized interstellar medium properties for galaxies in the first billion years of cosmic time, separated by their L_([C II])/L_([N II]) ratio. We find extremely low [N II] emission compared to [C II] (L_([C II])/L_([N II]) = 68^(+200)_(-28)) from a "typical" ~L_*(UV) star-forming galaxy, likely directly or indirectly (by its effect on the radiation field) related to low dust abundance and low metallicity. The infrared-luminous modestly star-forming Lyman-break galaxy (LBG) in our sample is characterized by an ionized-gas fraction (L_([C II])/L_([N II]) ≾ 20) typical of local star-forming galaxies and shows evidence for spatial variations in its ionized-gas fraction across an extended gas reservoir. The extreme SFR, warm and compact dusty starburst AzTEC-3 shows an ionized fraction higher than expected given its SFR surface density (L_([C II])/L_([N II]) = 22 ± 8) suggesting that [N II] dominantly traces a diffuse ionized medium rather than star-forming H II regions in this type of galaxy. This highest redshift sample of [N II] detections provides some of the first constraints on ionized and neutral gas modeling attempts and on the structure of the interstellar medium at z = 5–6 in "normal" galaxies and starbursts.

Additional Information

© 2016 The American Astronomical Society. Received 2016 July 8; revised 2016 August 24; accepted 2016 September 15; published 2016 November 29. We thank the anonymous referee for a helpful and constructive report and T.K. Daisy Leung for helpful discussion. D.R. and R.P. acknowledge support from the National Science Foundation under grant number AST-1614213 to Cornell University. R.P. acknowledges support through award SOSPA3-008 from the NRAO. A.K. acknowledges support by the Collaborative Research Council 956, sub-project A1, funded by the Deutsche Forschungsgemeinschaft (DFG). D.R. acknowledges the hospitality at the Aspen Center for Physics and the Kavli Institute for Theoretical Physics during part of the writing of this manuscript. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00928.S, 2011.0.00064.S, 2012.1.00523.S, 2011.0.00006.SV, and 2013.1.00745.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ.

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Published - Pavesi_2016_ApJ_832_151.pdf

Submitted - 1607.02520v3.pdf


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