Dwarf Galaxy Number Counts within 25 Mpc: Predictions from Local Group Analogues in TNG50

TL;DR

This study uses TNG50 simulations to predict the distribution and properties of dwarf galaxies within 25 Mpc, highlighting significant observational incompleteness and the potential for future discoveries.

Contribution

It provides the first detailed predictions of dwarf galaxy populations around Local Group analogues in a cosmological simulation, guiding upcoming observational efforts.

Findings

Approximately 1,000 and 12,000 dwarf galaxies within 10 and 25 Mpc, respectively.

Current observations are only 23% complete for low-mass dwarfs within 10 Mpc.

Simulations predict eight times more quiescent dwarf galaxies than observed.

Abstract

The modern generation of wide-field galaxy surveys, such as LSST, Euclid and Roman, will enable studies of dwarf galaxies $(10^6 \leq M_\ast / M_\odot \leq 10^9)$ beyond the Local Group (LG) in unprecedented detail. Improved theoretical understanding of this population is necessary to guide these observations, since predictions in this regime are generally limited to specific environments like the LG. We present predictions for the population of dwarf galaxies from the TNG50 run of the IllustrisTNG suite of cosmological hydrodynamical simulations, focusing on the environments within $1 < D / \mathrm{Mpc} < 25$ of LG analogues at $z = 0$. In the simulated sample, there are $\sim 1,000$ and $\sim 12,000$ dwarf galaxies within $10$ and $25$ Mpc, respectively. We compare our results with the 50 Mpc Galaxy Catalog and estimate that current observations are highly incomplete at low masses: for $10^6 \leq M_\ast / M_\odot \leq 10^7$ $(-13 \lesssim M_r \lesssim -10)$, we find completeness fractions of $\sim 23 \%$ within $10$ Mpc and $\sim 4 \%$ within $25$ Mpc. The simulated galaxies below the completeness limits of the observations exist in a range of environments, with notable populations of field dwarfs at all distances and satellites around centrals with masses $10^8 \lesssim M_\ast / M_\odot \lesssim 10^{11}$ within $10-25$ Mpc. We find that there are $\sim 8$ times more quiescent dwarf galaxies in the TNG50 sample than are currently cataloged. Our results suggest that upcoming observations should uncover a substantial population of dwarf galaxies, and that $\gtrsim 15 \%$ of these will be red, currently quenched galaxies in the field.