Analysis of Galaxies at the Extremes: Failed Galaxy Progenitors in the MAGNETICUM Simulations

There is increasing observational evidence for a “failed galaxy” formation pathway for ultra-diffuse galaxies (UDGs) at low redshift however they currently lack simulated counterparts. In this work we attempt to identify dark matter halos at high redshift within the MAGNETICUM cosmological simulations that could plausibly be their progenitors. To this end we build a toy model of passive galaxy evolution within the stellar mass–halo mass relation to trace z = 0 observations of UDGs back to their z = 2 locations. We identify a population of 443 galaxies that match these parameter space positions within the simulation. In addition, we build two comparison samples within the simulation that follow the stellar mass–halo mass relationship at z = 2, one of which is stellar mass matched (with varying smaller halo masses) and the other is halo mass matched (with varying larger stellar masses) to our sample. We identify that our “failed galaxy” progenitor candidates have 1) flatter, cored dark matter halos; 2) more extended stellar bodies; 3) a larger fraction of their gas in the outskirts of their halos; 4) lower metallicities and 5) higher star formation rates than the control samples. Findings 1 ) and 2) are similar to low redshift observations of UDGs. Finding 3) will aid the removal of gas and permanent quenching of star formation which is a requirement of the “failed galaxy” formation scenario. The low metallicities of finding 4) match what is observed in low redshift “failed galaxy” UDGs. Comparing the high star formation rates of finding 5) to recent JWST observations suggests that a starburst would naturally explain the high globular cluster richness of the UDGs. Many of the properties we find for these “failed galaxy” progenitors can be explained by an assembly bias of their dark matter halo to later formation times. We conclude by proposing an observational test of this scenario where the fraction of “failed galaxy” UDGs is expected to increase with environmental density.