HST UV Spectroscopy of the Dwarf Starburst Galaxy Pox 186

Creators: Rogers, Noah S. J.¹; Scarlata, Claudia M.¹; Skillman, Evan D.¹; Eggen, Nathan R.¹; Jaskot, Anne E.²; Mehta, Vihang^{3, 4}; Cannon, John M.⁵

1. University of Minnesota
2. Williams College
3. Infrared Processing and Analysis Center
4. California Institute of Technology
5. Macalester College

Abstract

Studying the galaxies responsible for reionization is often conducted through local reionization-era analogs; however, many of these local analogs are too massive to be representative of the low-mass star-forming galaxies that are thought to play a dominant role in reionization. The local, low-mass dwarf starburst galaxy Pox 186 is one such system with physical conditions representative of a reionization-era starburst galaxy. We present deep ultraviolet (UV) spectroscopy of Pox 186 to study its stellar population and ionization conditions and to compare these conditions to other local starburst galaxies. The new Cosmic Origins Spectrograph data are combined with archival observations to cover ∼1150–2000 Å and allow for an assessment of Pox 186's stellar population, the relative enrichment of C and O, and the escape of ionizing photons. We detect significant Lyα and low-ionization state absorption features, indicative of previously undetected neutral gas in Pox 186. The C/O relative abundance, log(C/O) = −0.62 ± 0.02, is consistent with other low-metallicity dwarf galaxies and suggests a comparable star formation history in these systems. We compare UV line ratios in Pox 186 to those of dwarf galaxies and photoionization models, and we find excellent agreement for the ratios utilizing the intense C iii], O iii], and double-peaked C iv lines. However, the UV and optical He ii emission is faint and distinguishes Pox 186 from other local starburst dwarf galaxies. We explore mechanisms that could produce faint He ii, which have implications for the low-mass reionization-era galaxies that may have similar ionization conditions.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We thank the referee for providing a careful review of this manuscript, which helped refine its discussion and broaden its scope. We thank Simon Gazagnes for sharing his SPS code and for the useful discussion concerning the fitting procedures. We also thank Danielle Berg for her insightful discussions and for sharing her experience using COS observations. This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program Nos. 16294, 16071, and 16445. Support for program No. 16294 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. J.M.C. is supported by NSF grant AST-2009894. The authors acknowledge Nimisha Kumari for developing their observing program Nos. 16071 and 16445. The Mexican Million Models Database, 3MdB, grid of photoionization models used in the UV emission-line analysis is available at https://sites.google.com/site/mexicanmillionmodels.

Facilities

HST (COS) - Hubble Space Telescope satellite, Gemini:Gillett (GMOS) - .

Data Availability

All of the HST data used in this paper can be found in MAST:10.17909/41cb-0v53.

Software References

AstroPy (Astropy Collaboration et al. 2013,2018, 2022), dustmaps (Green 2018), matplotlib (Hunter 2007), NumPy (Harris et al. 2020), PyNeb (Luridiana et al. 2012, 2015), SciPy (Virtanen et al. 2020), SpectRes (Carnall2017).

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