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

TL;DR

This work tackles the puzzle of failed-galaxy ultra-diffuse galaxies by seeking plausible high-redshift progenitors in the MAGNETICUM simulations. It combines a toy passive-evolution model within the $M_*$–$M_{200\mathrm{crit}}$ relation with a robust halo-based selection to identify $z=2$ candidates and constructs two control samples for comparison. The analysis reveals that the proposed progenitors tend to reside in flatter, cored halos, host more extended stellar distributions, harbor more gas at large radii, exhibit lower metallicities, and experience elevated star-formation rates, consistent with rapid early assembly and eventual quenching. The findings point to assembly bias and environmental effects as key drivers and suggest that the fraction of failed-galaxy UDGs should rise with environmental density, offering observable tests with cluster and group environments and JWST-era GC formation signatures.

Abstract

There is increasing observational evidence for a failed galaxy formation pathway for some ultradiffuse galaxies (UDGs) at low redshift however they currently lack simulated counterparts. We attempt to identify dark matter halos at high redshift within the MAGNETICUM cosmological simulations that could plausibly be their progenitors. 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. 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 those 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 that the fraction of failed galaxy UDGs is expected to increase with environmental density.

Figures (7)

• Figure 1: The z=2 stellar mass--halo mass relationship. Grey points are individual galaxies in the MAGNETICUM simulation at z=2, with the black line their 5000-point running median. Green squares are individual data points from the study of $z=2$ Ly$\alpha$ emitters by Kusakabe2018 with the orange star their weighted-average. MAGNETICUM is accurately reproducing these observations. Red triangles are UDG observations at z$\approx$0. UDG halo masses ($M_{\rm200crit}$) are estimated from their GC counts. Cyan lines correspond to the expected factor of 2.5 mass growth due to the pseudo-evolution in dark matter halos between z=2 and z=0. Blue lines correspond to 40% stellar mass loss of a passively evolving system between z=2 and z=0. Magenta lines are the combination of both effects with one end tracing the expected position of the z=0 UDGs if traced back to z=2. UDG halo mass estimates at z=0 are consistent with galaxies at z=2 in the MAGNETICUM simulations that have evolved passively since this redshift.
• Figure 2: The stellar mass--halo mass relationship at $z=2$ in the MAGNETICUM simulations. The underlying colours indicate the number of haloes ($N_{hal}$) within the simulation at each point as shown in the colour bar. Dashed black lines indicate where there are at least 50 stellar (horizontal) or dark matter (vertical) particles. From this, we select three samples of galaxies to study the properties of progenitor failed galaxy UDGs: 1) we select galaxies with low stellar masses for their halo mass that reside in the region expected for a failed galaxy progenitor (black), 2) we select galaxies of similar stellar mass but lower halo mass, consistent with the main stellar mass -- halo mass relationship as a first control sample (red) and 3) we select galaxies with normal stellar masses for the failed galaxy progenitors' halo masses as a second control sample (blue). Fractional histograms of these samples and the total population are included above and to the right of the main plot. We will use these three samples throughout the remainder of the paper.
• Figure 3: The baryonic mass--halo mass relationship at $z=2$ in the MAGNETICUM simulations. Colouring and style follow Fig. \ref{['fig:selection']}. Note that there is a large overlap in the UDG candidate sample (in black) and the halo mass matched sample (in blue). While many of the failed galaxy UDG candidates are outlying in the stellar mass --- halo mass relationship, they follow the baryonic mass--halo mass relationship. This is suggestive that these dark matter halos have not been starved of material to form stars, they are merely failing to. If this trend continues to $z=0$ they will become "failed galaxies".
• Figure 4: The stacked density profiles of the candidates. Along the top row, from the left to the right they are: the total mass profile, the dark matter mass profile, the stellar mass profile and the gas mass profile. In the top row, a horizontal black dashed line indicates 200 times the critical density of the Universe at z=2. In MAGNETICUM different particle species have different softening lengths. As such, gray-shaded areas show regions within one times the softening length of each particle species (with $\epsilon_\mathrm{stars,\,z=2}=0.33$kpc lying outside of the plotted range). Coloured bands are the $1\sigma$-bounds generated via bootstrapping the particles of each set of candidates $100$ times. The lower panels are the same as the first, but the profiles for each sample have been normalised by their density at 2 kpc. The failed galaxy progenitor candidates are in black, the stellar mass-matched sample is in red and the halo mass-matched sample is in blue. "Failed galaxy" UDG candidates tend to have: 1) flatter, cored dark matter halos, 2) a slightly more extended stellar body at the same stellar mass and 3) a larger relative quantity of gas in the outskirts of their halo compared to our two control samples.
• Figure 5: Left: The combined star formation histories of the three galaxy samples as identified in Figure \ref{['fig:selection']}. Compared to galaxies in a similar mass halo, candidate "failed galaxy" UDGs are forming later. Centre Left: The metallicity of the stellar particles in each of the candidate and control samples. Medians with 1-$\sigma$ uncertainties are shown as points above each histogram, as is the average $z=2.2$ stellar metallicity for a galaxy of $M_*=5\cdot10^{8}M_\odot$ as given by Ma2016. The large number of particles at [Z/Z$_\odot$]$=-6$ is caused by star particles made from pristine, non-enriched gas in the simulation. On average the candidate failed galaxy UDGs have lower metallicity than the other control samples. Centre Right: A histogram of the current star formation rates of galaxies in each of the three samples. Medians with 1-$\sigma$ uncertainties are shown as points above each histogram. For the stellar mass matched sample the lower 1-$\sigma$ uncertainty is poorly defined due to the large number of galaxies that are not forming stars. We thus indicate the lower uncertainty with an arrow. The candidate failed galaxy UDGs have star formation rates much greater than the stellar mass-matched sample, and more similar to the halo mass-matched sample. As GC production is closely tied to star formation rates, and the candidate failed galaxies are the most strongly starforming galaxies for their stellar mass at this redshift, it is likely that they are also forming GCs with high efficiency. Right: The mass assembly of the dark matter halos with redshift. While UDG halos begin with similar total masses to the stellar mass-matched sample, they accrete dark matter mass much faster which results in a total mass at $z=2$ equivalent to the halo mass-matched sample. They have assembled this total mass much later than the halo mass matched sample, thus we confirm their late assembly.