The Space Density of Intermediate-redshift, Extremely Compact, Massive Starburst Galaxies

We present a measurement of the intrinsic space density of intermediate-redshift (z ∼ 0.5), massive (M * ∼ 1011 M ⊙), compact (R e ∼ 100 pc) starburst (ΣSFR ∼ 1000 M ⊙ yr−1 kpc−1) galaxies with tidal features indicative of them having undergone recent major mergers. A subset of them host kiloparsec-scale, > 1000 km s−1 outflows and have little indication of AGN activity, suggesting that extreme star formation can be a primary driver of large-scale feedback. The aim for this paper is to calculate their space density so we can place them in a better cosmological context. We do this by empirically modeling the stellar populations of massive, compact starburst galaxies. We determine the average timescale on which galaxies that have recently undergone an extreme nuclear starburst would be targeted and included in our spectroscopically selected sample. We find that massive, compact starburst galaxies targeted by our criteria would be selectable for ∼148−24+27 Myr and have an intrinsic space density nCS∼(1.1−0.3+0.5)×10−6Mpc−3 . This space density is broadly consistent with our z ∼ 0.5 compact starbursts being the most extremely compact and star-forming low-redshift analogs of the compact star-forming galaxies in the early universe, as well as them being the progenitors to a fraction of intermediate-redshift, post-starburst, and compact quiescent galaxies.