The most ancient spiral galaxy: a 2.6-Gyr-old disk with a tranquil velocity field

Yuan, T., Richard, J., Gupta, A., Federrath, C., Sharma, S., Groves, B. A., Kewley, L. J., Cen, R., Birnboim, Y., Fisher, D. B., 2017

The Astrophyiscal Journal, 850, 61  [ ADS link ]  [ PDF ]

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Abstract

We report an integral-field spectroscopic (IFS) observation of a gravitationally lensed spiral galaxy A1689B11 at redshift z = 2.54. It is the most ancient spiral galaxy discovered to date and the second kinematically confirmed spiral at z >~ 2. Thanks to gravitational lensing, this is also by far the deepest IFS observation with the highest spatial resolution (~400 pc) on a spiral galaxy at a cosmic time when the Hubble sequence is about to emerge. After correcting for a lensing magnification of 7.2 +/- 0.8, this primitive spiral disk has an intrinsic star formation rate of 22 +/- 2 solar masses per year, a stellar mass of 109.8 +/- 0.3 solar masses and a half-light radius of 2.6 +/- 0.7 kpc, typical of a main-sequence star-forming (SF) galaxy at z ~ 2. However, the H-alpha kinematics show a surprisingly tranquil velocity field with an ordered rotation (200 +/- 12 km/s) and uniformly small velocity dispersions (with an average of 23 +/- 4 km/s and in the outer−disk of about 15 +/- 2 km/s). The low gas velocity dispersion is similar to local spiral galaxies and is consistent with the classic density wave theory where spiral arms form in dynamically cold and thin disks. We speculate that A1689B11 belongs to a population of rare spiral galaxies at z >~ 2 that mark the formation epoch of thin disks. Future observations with JWST will greatly increase the sample of these rare galaxies and unveil the earliest onset of spiral arms.
The most ancient spiral galaxy.

The most ancient spiral galaxy discovered to date.

Acknowledgements

We thank the anonymous referee for his/her report, which helped to restructure and improve the work significantly. This work is a tribute to the late Peter McGregor who built NIFS/Gemini and taught TY how to reduce the NIFS data. This research was conducted by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. TY thanks useful discussions with Ken Freeman, Lars Hernquist, Enrico Teodoro, Yusuke Fujimoto, Ben Davis and the GEARS3D group. TY acknowledges the support from the ASTRO 3D fellowship. BG gratefully acknowledges the support of the Australian Research Council as the recipient of a Future Fellowship (FT140101202). CF acknowledges funding provided by the ARC Discovery Pro jects (grants DP150104329 and DP170100603). JR acknowledges support from the ERC starting grant 336736-CALENDS. LK gratefully acknowledges support from an Australian Research Council (ARC) Laureate Fellowship (FL150100113). RC acknowledges grants NNX12AF91G and AST15-15389. YB acknowledges ISF grant 1059/14. We thank Stuart Ryder for his assistance with the Gemini observations. The data for this work is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministe- rio de Ciencia, Tecnologia e Innovacion Productiva (Argentina), and Ministerio da Ciencia, Tecnologia e Inovacao (Brazil). The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community.

© C. Federrath 2018