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

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.

💡 Research Summary

This paper presents a comprehensive theoretical prediction for the population of dwarf galaxies (stellar mass 10⁶ ≤ M★/M⊙ ≤ 10⁹) in the nearby Universe, focusing on the volume between 1 Mpc and 25 Mpc from Milky Way (MW)–like hosts. The authors use the highest‑resolution run of the IllustrisTNG suite, TNG50‑1, which offers a 50 cMpc box, baryonic mass resolution of 8.5 × 10⁴ M⊙, and a dark‑matter particle mass of 4.5 × 10⁵ M⊙. Within this simulation they identify three Local‑Group (LG) analogues, each consisting of a MW‑like galaxy and an M31‑like companion, following the criteria of (i) disk‑dominated morphology, (ii) stellar mass 10⁵·⁵–10¹¹·² M⊙, (iii) host halo mass M200c < 10¹³ M⊙, and (iv) isolation from other massive galaxies within 500 kpc. The pair must be separated by 500–1000 kpc and have a negative radial velocity relative to each other, reproducing the dynamical configuration of the real MW–M31 system.

For each MW analogue the authors recenter the coordinate system, define a “galactic north” using the net stellar angular momentum, and impose a Zone of Avoidance (ZoA) by removing all subhalos with galactic latitude |b| < 10° in the plane perpendicular to that angular momentum vector. This mimics the observational blind spot caused by Milky Way dust and stars. To ensure reliable statistics at the low‑mass end they retain only subhalos containing at least 25 stellar particles (≈ 2 × 10⁵ M⊙), thereby guaranteeing that galaxies down to 10⁶ M⊙ are resolved.

The analysis is performed in three radial shells: D < 1 Mpc (the immediate Local Volume), 1 < D < 10 Mpc, and 1 < D < 25 Mpc. Cumulative number counts are derived in bins of Δlog M★ = 0.2 dex. Within 1 < D < 10 Mpc the simulation contains roughly 1 200 dwarfs with M★ ≥ 10⁶ M⊙, of which about 800 exceed 10⁹ M⊙. Extending to 25 Mpc yields ≈ 12 000 dwarfs in the full mass range. The stellar‑mass function is well described by a Schechter form with a faint‑end slope α ≈ ‑1.5, indicating a steep rise toward low masses.

To assess observational completeness the authors compare these predictions with the 50 Mpc Galaxy Catalog. For the critical low‑mass interval 10⁶–10⁷ M⊙ (corresponding roughly to –13 ≲ Mr ≲ ‑10), the catalog recovers only ~23 % of the simulated dwarfs within 10 Mpc and a mere ~4 % within 25 Mpc. This demonstrates that current surveys are dramatically incomplete for the faintest dwarfs beyond the Local Group.

A striking result concerns quiescent (quenched) dwarfs. The TNG50 sample contains about eight times more quenched dwarfs than are present in the observed catalog. Most of these are satellites of relatively massive hosts (10⁸–10¹¹ M⊙), but a non‑negligible fraction (≥ 15 % of the total dwarf population) are field dwarfs that have already ceased star formation. This implies that the observed dwarf population is biased toward star‑forming systems, and that a substantial red dwarf component awaits discovery.

Environmental breakdown shows that, in the 1–25 Mpc shell, ≈ 60 % of dwarfs are truly isolated field galaxies, ≈ 30 % are satellites of MW‑like or M31‑like hosts, and the remaining ≈ 10 % reside in small groups or clusters. The fraction of dwarfs removed by the ZoA is modest (≈ 10 % at the lowest masses) but non‑negligible, providing a useful correction factor for future all‑sky surveys.

The authors discuss implications for upcoming wide‑field facilities. LSST, with its planned 18 000 deg² coverage, r‑band depth of 27.5 mag, and surface‑brightness limit near 30 mag arcsec⁻², should be able to detect roughly 770 additional dwarfs in the 10 Mpc volume and about 9 500 in the 25 Mpc volume that are currently missing from catalogs. Euclid and the Roman Space Telescope, with their near‑infrared capabilities, will complement LSST by probing slightly higher redshifts and providing robust photometric redshifts for faint dwarfs.

In summary, this work leverages the high resolution and sufficient volume of TNG50 to deliver the first statistically robust prediction for the number, mass distribution, star‑formation activity, and environmental dependence of dwarf galaxies out to 25 Mpc from MW‑like hosts. By quantifying the severe incompleteness of existing catalogs and forecasting the yield of next‑generation surveys, the paper provides essential guidance for observational strategies aimed at completing the census of low‑mass galaxies in the nearby Universe.