Contribution of HI-bearing ultra-diffuse galaxies to the cosmic number density of galaxies

¹ Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía, Granada 18008, Spain
e-mail: mjones@iaa.es
² Kapteyn Astronomical Institute, University of Groningen, Landleven 12, Groningen 9747AD, The Netherlands
³ Credit Risk Modeling Department, Coöperative Rabobank U.A., Croeselaan 18, Utrecht 3521CB, The Netherlands
⁴ Department of Astronomy & Astrophysics, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
⁵ Cornell Center for Astrophysics and Planetary Science, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA
⁶ Department of Physics & Astronomy, Valparaiso University, Valparaiso, IN 46383, USA
⁷ Department of Physics & Astronomy, San José State University, One Washington Square, San Jose, CA 95192, USA
⁸ University of California Observatories, 1156 High Street, Santa Cruz, CA 95064, USA
⁹ Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA
¹⁰ Center for Computational Astrophysics, Flatiron Institute, 162 5th Ave, New York, NY 10010, USA
¹¹ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, Dwingeloo 7900AA, The Netherlands

Received: 4 December 2017
Accepted: 22 January 2018

Abstract

We estimate the cosmic number density of the recently identified class of HI-bearing ultra-diffuse sources (HUDs) based on the completeness limits of the ALFALFA survey. These objects have HI masses approximately in the range 8.5 < logM_HI∕M_⊙ < 9.5, average r-band surface brightnesses fainter than 24 mag arcsec⁻², half-light radii greater than 1.5 kpc, and are separated from neighbours by at least 350 kpc. In this work we demonstrate that they contribute at most ~6% of the population of HI-bearing dwarfs detected by ALFALFA (with similar HI masses), have a total cosmic number density of (1.5 ± 0.6) × 10⁻³ Mpc⁻³, and an HI mass density of (6.0 ± 0.8) × 10⁵ M_⊙ Mpc⁻³. We estimate that this is similar to the total cosmic number density of ultra-diffuse galaxies (UDGs) in groups and clusters, and conclude that the relation between the number of UDGs hosted in a halo and the halo mass must have a break below M₂₀₀ ~ 10¹² M_⊙ in order to account for the abundance of HUDs in the field. The distribution of the velocity widths of HUDs rises steeply towards low values, indicating a preference for slow rotation rates compared to the global HI-rich dwarf population. These objects were already included in previous measurements of the HI mass function, but have been absent from measurements of the galaxy stellar mass function owing to their low surface brightness. However, we estimate that due to their low number density the inclusion of HUDs would constitute a correction of less than 1%. Comparison with the Santa Cruz semi-analytic model shows that it produces HI-rich central UDGs that have similar colours to HUDs, but that these UDGs are currently produced in a much greater number. While previous results from this sample have favoured formation scenarios where HUDs form in high spin-parameter halos, comparisons with recent results which invoke that formation mechanism reveal that this model produces an order of magnitude more field UDGs than we observe in the HUD population, and these have an occurrence rate (relative to other dwarfs) that is approximately double what we observe. In addition, the colours of HUDs are bluer than predicted, although we suspect this is due to a systematic problem in reproducing the star formation histories of low-mass galaxies rather than being specific to the ultra-diffuse nature of these sources.