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Satellites around Milky Way Analogs: Tension in the Number and Fraction of Quiescent Satellites Seen in Observations versus Simulations

Karunakaran, Ananthan and Spekkens, Kristine and Oman, Kyle A. and Simpson, Christine M. and Fattahi, Azadeh and Sand, David J. and Bennet, Paul and Crnojević, Denija and Frenk, Carlos S. and Gómez, Facundo A. and Grand, Robert J. J. and Jones, Michael G. and Marinacci, Federico and Mutlu-Pakdil, Burçin and Navarro, Julio F. and Zaritsky, Dennis (2021) 'Satellites around Milky Way Analogs: Tension in the Number and Fraction of Quiescent Satellites Seen in Observations versus Simulations.', The Astrophysical Journal Letters, 916 (2). L19.


We compare the star-forming properties of satellites around Milky Way (MW) analogs from the Stage II release of the Satellites Around Galactic Analogs Survey (SAGA-ii) to those from the APOSTLE and Auriga cosmological zoom-in simulation suites. We use archival GALEX UV imaging as a star formation indicator for the SAGA-ii sample and derive star formation rates (SFRs) to compare with those from APOSTLE and Auriga. We compare our detection rates from the NUV and FUV bands to the SAGA-ii Hα detections and find that they are broadly consistent with over 85% of observed satellites detected in all three tracers. We apply the same spatial selection criteria used around SAGA-ii hosts to select satellites around the MW-like hosts in APOSTLE and Auriga. We find very good overall agreement in the derived SFRs for the star-forming satellites as well as the number of star-forming satellites per host in observed and simulated samples. However, the number and fraction of quenched satellites in the SAGA-ii sample are significantly lower than those in APOSTLE and Auriga below a stellar mass of M* ∼ 108 M⊙, even when the SAGA-ii incompleteness and interloper corrections are included. This discrepancy is robust with respect to the resolution of the simulations and persists when alternative star formation tracers are employed. We posit that this disagreement is not readily explained by vagaries in the observed or simulated samples considered here, suggesting a genuine discrepancy that may inform the physics of satellite populations around MW analogs.

Item Type:Article
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Publisher statement:© 2021. The American Astronomical Society. All rights reserved.
Date accepted:22 June 2021
Date deposited:09 November 2021
Date of first online publication:02 August 2021
Date first made open access:09 November 2021

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