HIDES -- I. The population and diversity of HI-rich 'dark' galaxies in the Hestia and Auriga simulations

TL;DR

This paper investigates HI-rich 'dark' galaxies (HIDEs) using the constrained Local Group mathcing simulations Hestia and high-resolution Auriga runs. By selecting halos with $M_{\mathrm{HI}} > 10^5\ M_\odot$ and $M_g > -10$, the authors identify 89 HIDEs (49 in Hestia, 34 in Auriga L3, 6 in Auriga L2) and show their properties converge across resolutions, with typical halo masses around $M_{200} \sim 10^{9.5}\ M_\odot$, gas masses $M_{\mathrm{gas}} \sim 10^{7.4}\ M_\odot$, HI masses $M_{\mathrm{HI}} \sim 10^{6.5}\ M_\odot$, and stellar masses $M_* \sim 10^{5.6}\ M_\odot$, hosting old stellar populations and low metallicities. The study finds substantial scatter in HI density profiles and the $M_{\mathrm{HI}}-M_*$ relation that cannot be explained by halo mass or concentration alone; environmental processes such as ram pressure stripping, past mergers, and stellar feedback are key in shaping HIDEs, with some objects surviving near dense halos up to $\sim 300$ kpc from Milky Way–mass neighbors (e.g., analogues to Cloud-9). An empirical number-density fit, $n = 0.25 \left(d_{\mathrm{MW}}/1\ \mathrm{Mpc}\right)^{-1.4}$ Mpc$^{-3}$, extends to $3.7$ Mpc to support observational forecasts. The work demonstrates that both mass assembly history and environmental history drive the formation and diversity of HIDEs, bridging simulations and observations in the low-mass, HI-rich regime of galaxy formation.

Abstract

We present our investigation of HI-rich 'Dark' galaxiEs in Simulations (HIDES), specifically using the Hestia and Auriga simulations in this work. We select galaxies that are faint ($M_g > -10$) and contain sufficient HI ($M_\mathrm{HI} > 10^5\,M_\odot$), and identify 89 such objects, only one of which is completely starless. Their demographics generally converge across simulations of different resolution, with $M_{200} \sim 10^{9.5}\,M_\odot$, $M_\mathrm{gas} \sim 10^{7.4}\,M_\odot$, $M_\mathrm{HI} \sim 10^{6.5}\,M_\odot$, $M_\mathrm{*} \sim 10^{5.6}\,M_\odot$, low gas metallicity, little or no current star formation, and a mean stellar age of $\sim$ 11 Gyr, and with some of them can survive in dense environments as close as $\sim$ 300 kpc from a Milky-Way mass neighbor. We find a large scatter in their HI density profiles and $M_\mathrm{HI} - M_\mathrm{*}$ relation, which cannot be fully explained by current halo mass or concentration, but can be attributed to ram pressure stripping in dense environments, past mergers, and stellar feedback. In particular, close encounters with massive halos and dense environments can reshape the HI content, which may explain the asymmetric HI map of an intriguing observed analogue, Cloud-9. An empirical fit, $n = 0.25 \left(d_\mathrm{MW}/{1\,\mathrm{Mpc}}\right)^{-1.4}\, \mathrm{Mpc}^{-3}$, based on their number density extended to 3.7 Mpc in constrained local volume simulations, is also provided to aid observational forecasts. We conclude that both mass assembly history and environmental history play a crucial role in the formation and subsequent diversity of these galaxies.

Figures (6)

• Figure 1: The baryon mass vs. halo mass relation (left panel) and HI mass vs. stellar mass relation (right panel) of galaxies in the hestia simulation. Left: colored dots: halos colored by their stellar mass, black dots: completely starless halos; black solid line: median baryon mass; black dashed line: cosmic mean baryonic fraction; green lines: the median baryonic mass of halos that contain stars (solid: hestia, dotted: Benitez-Llambay2017); blue lines: median stellar mass (solid: hestia, dotted: Benitez-Llambay2017, dash-dotted: observational results of Moster2013 via abundance matching). Right: gray dots: halos colored with the $g$-band magnitude; black dots: completely starless halos (i.e., $M_*=0$); red stars: observational results of Cloud-9 and Leo T, dash-dotted lines: extrapolated $M_\mathrm{HI}$ - $M_*$ fits obtained from observations of more massive galaxies. In hestia, We do not find completely starless halos with HI masses comparable to Cloud-9 and Leo T, therefore we select HIDEs (HI-rich 'dark' halos, red-circled dots) as our sample instead, which have a great amount of HI ($M_\mathrm{HI}>10^5\,M_\odot$) and are faint enough to possibly 'hide' themselves from wide-field optical surveys ($M_g > -10$); HIDEs selected with the same criteria from Auriga L3 and Auriga L2 are overplotted (red-circled squares and triangles, respectively) for comparison.
• Figure 2: Histograms of the general properties of HIDEs in hestia (red) and Auriga L3 (blue), with the vertical dashed lines showing the median values. In the bottom left panel, the red dotted line shows the distribution of distances to the Milky Way, which differs from the minimum distances to either M31 or the Milky Way shown with red solid lines, and there is no need to distinguish these definitions in Auriga as there is only one host galaxy. HESITA and Auriga HIDEs have largely converged properties, with $M_{200} \sim 10^{9.5}\,M_\odot$, $M_\mathrm{gas} \sim 10^{7.4}\,M_\odot$, $M_\mathrm{HI} \sim 10^{6.5}\,M_\odot$, $M_\mathrm{*} \sim 10^{5.6}\,M_\odot$, low metallicity gas due to the low stellar mass, low or no star formation at present, and old mean stellar age $\sim 11$ Gyr. The difference in distance distribution is mainly due to the different volume sizes of two simulation suites.
• Figure 3: The spatial distribution of HIDEs. Left: projected dark matter density field, with solid, dashed, and dotted circles representing HIDEs, M31 and the Milky Way, the boundary of zoom-in region respectively. Right: number density of HIDEs as a function of radius, averaged among different realizations. Red solid line: the smaller distance to either M31 and the Milky Way in hestia; red dotted line: the distance to the Milky Way in hestia; blue line: the distance to the central host galaxy in Auriga L3; error bars: Poisson errors; error with arrows: lower limit due to incomplete volumes; gray line: 0.1 times the number density of halo in the relevant mass range $[10^9,\,10^{10}]\, M_\odot$. The radial distribution ca be fitted with a simple power law function (black dotted line): $n = 0.25 \left(d_\mathrm{MW}/1\,\mathrm{Mpc}\right)^{-1.4}\mathrm{Mpc}^{-3}$.
• Figure 4: Gas phase diagrams and projected density field, with each row representing one case study (Halo 26, 34, 35) at $z=0$. First column: gas temperature ($T$) vs. hydrogen number density ($n_\mathrm{H}$) of all gas particles inside the halo virial radius. The color of the dots represents the HI mass fraction of gas particles; dots on the right side with black edges represent gas particles that meet the star formation criteria. Dashed lines: the $T-n_\mathrm{H}$ relation fitted in Benitez-Llambay2017 for Apostle RELHICs. Second column: local dark matter density map, with dashed, dash-dotted, and dotted cyan circles representing the 1, 3, and 5 times the virial radius respectively, and the environmental dark matter overdensity $\delta_\mathrm{3r_{200}-5r_{200}}$ is measured in the shell between the latter two. White dashed circles indicate nearby more massive halos, and the distance and radius of the nearest one are marked as $d_\mathrm{large}$ and $r_\mathrm{200,\,large}$. Third column: zoom-in view of gas density map. Fourth column: further zoom-in view of HI density map, with red solid circles marking 0.15$r_{200}$ as a reference, and red dotted circles marking the contour of HI column density $N_\mathrm{HI} = 10^{18},\ 10^{19},$ and $10^{20}\ \mathrm{cm}^{-2}$ from outside toward center.
• Figure 5: Inner structures of 3 HIDEs, with each column representing one case study (the same cases as Fig. \ref{['fig:fig4']}). First row: the profiles of gravitational acceleration (solid lines: acceleration from total mass; dotted lines: acceleration from dark matter mass; dashed lines: acceleration of halos with fitted NFW profile). Second and third rows: red (blue) dots: gas (HI) density and temperature of each gas particle; red dashed lines: gas profile predicted by the Benitez-Llambay2017 model based on the halo mass and fitted concentration; black dotted lines: cosmic mean hydrogen density; black dash-dotted lines: halo virial temperature.