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Register a new userEDGE: The origin of scatter in ultra-faint dwarf stellar masses and surface brightnesses
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We demonstrate how the least luminous galaxies in the Universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass ($sim text{1.5} times 10^{9} , text{M}_{odot}$) dark matter halo from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using genetically modified initial conditions. Earlier forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter ($geq$ 1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2 % of its final dynamical mass before reionization, rapidly quenching in-situ star formation. The bulk of its final stellar mass is later grown by dry mergers, depositing stars in the galaxys outskirts and hence expanding its effective radius. This mechanism constitutes a new formation scenario for highly diffuse ($text{r}_{1 /2} sim 820 , text{pc}$, $sim 32 , text{mag arcsec}^2$), metal-poor ($big[ mathrm{Fe}, / mathrm{H} big]= -2.9$), ultra-faint ($mathcal{M}_V= -5.7$) dwarf galaxies within the reach of next-generation low surface brightness surveys.
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We present Magellan/M2FS, VLT/GIRAFFE, and Gemini South/GMOS spectroscopy of the newly discovered Milky Way satellite Reticulum II. Based on the spectra of 25 Ret II member stars selected from Dark Energy Survey imaging, we measure a mean heliocentric velocity of 62.8 +/- 0.5 km/s and a velocity dispersion of 3.3 +/- 0.7 km/s. The mass-to-light ratio of Ret II within its half-light radius is 470 +/- 210 Msun/Lsun, demonstrating that it is a strongly dark matter-dominated system. Despite its spatial proximity to the Magellanic Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by 199 and 83 km/s, respectively, suggesting that it is not gravitationally bound to the Magellanic system. The likely member stars of Ret II span 1.3 dex in metallicity, with a dispersion of 0.28 +/- 0.09 dex, and we identify several extremely metal-poor stars with [Fe/H] < -3. In combination with its luminosity, size, and ellipticity, these results confirm that Ret II is an ultra-faint dwarf galaxy. With a mean metallicity of [Fe/H] = -2.65 +/- 0.07, Ret II matches Segue~1 as the most metal-poor galaxy known. Although Ret II is the third-closest dwarf galaxy to the Milky Way, the line-of-sight integral of the dark matter density squared is log J = 18.8 +/- 0.6 Gev^2/cm^5 within 0.2 degrees, indicating that the predicted gamma-ray flux from dark matter annihilation in Ret II is lower than that of several other dwarf galaxies.
The heaviest metals found in stars in most ultra-faint dwarf (UFD) galaxies in the Milky Way halo are generally underabundant by an order of magnitude or more when compared with stars in the halo field. Among the heavy elements produced by n-capture reactions, only Sr and Ba can be detected in red giant stars in most UFD galaxies. This limited chemical information is unable to identify the nucleosynthesis process(es) responsible for producing the heavy elements in UFD galaxies. Similar [Sr/Ba] and [Ba/Fe] ratios are found in three bright halo field stars, BD-18 5550, CS 22185-007, and CS 22891-200. Previous studies of high-quality spectra of these stars report detections of additional n-capture elements, including Eu. The [Eu/Ba] ratios in these stars span +0.41 to +0.86. These ratios and others among elements in the rare earth domain indicate an r-process origin. These stars have some of the lowest levels of r-process enhancement known, with [Eu/H] spanning -3.95 to -3.32, and they may be considered nearby proxies for faint stars in UFD galaxies. Direct confirmation, however, must await future observations of additional heavy elements in stars in the UFD galaxies themselves.
I show that a recently discovered star cluster near the center of the ultra-faint dwarf galaxy Eridanus II provides strong constraints on massive compact halo objects (MACHOs) of >~5 M_sun as the main component of dark matter. MACHO dark matter will dynamically heat the cluster, driving it to larger sizes and higher velocity dispersions until it dissolves into its host galaxy. The stars in compact ultra-faint dwarf galaxies themselves will be subject to the same dynamical heating; the survival of at least ten such galaxies places independent limits on MACHO dark matter of masses >~10 M_sun. Both Eri IIs cluster and the compact ultra-faint dwarfs are characterized by stellar masses of just a few thousand M_sun and half-light radii of 13 pc (for the cluster) and ~30 pc (for the ultra-faint dwarfs). These systems close the ~20--100 M_sun window of allowed MACHO dark matter and combine with existing constraints from microlensing, wide binaries, and disk kinematics to rule out dark matter composed entirely of MACHOs from ~10$^{-7}$ M_sun up to arbitrarily high masses.
Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; ${rm M}_{rm V}$ $sim -2$ or ${rm M}_{star}$ $sim 10^{2}$ at $z=0$) had ultra-violet (UV) luminosities of ${rm M}_{rm UV}$ $sim -3$ to $-6$ during reionization ($zsim6-10$). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep ($alphalesssim-2$) to ${rm M}_{rm UV}$ $sim -3$, then: (i) the ancestors of UFDs produced $>50$% of UV flux from galaxies; (ii) galaxies can maintain reionization with escape fractions that are $>$2 times lower than currently-adopted values; (iii) direct HST and JWST observations may detect only $sim10-50$% of the UV light from galaxies; (iv) the cosmic star formation history increases by $gtrsim4-6$ at $zgtrsim6$. Significant flux from UFDs, and resultant tensions with LG dwarf galaxy counts, are reduced if the high-redshift UVLF turns over. Independent of the UVLF shape, the existence of a large population of UFDs requires a non-zero luminosity function to ${rm M}_{rm UV}$ $sim -3$ during reionization.
Aims. We use stellar line-of-sight velocities to constrain the dark matter-density profile of Eridanus 2, an ultra-faint dwarf galaxy ($M_mathrm{V} = -7.1$, $M_* approx 9 times 10^4,M_odot$). We furthermore derive constraints on fundamental properties of self-interacting and fuzzy dark matter scenarios. Methods. We present new observations of Eridanus 2 from MUSE-Faint, a survey of ultra-faint dwarf galaxies with MUSE on the Very Large Telescope, and determine line-of-sight velocities for stars inside the half-light radius. Combined with literature data, we have 92 stellar tracers out to twice the half-light radius. We constrain models of cold dark matter, self-interacting dark matter, and fuzzy dark matter with these tracers, using CJAM and pyGravSphere for the dynamical analysis. Results. We find substantial evidence for cold dark matter over self-interacting dark matter and weak evidence for fuzzy dark matter over cold dark matter. We find a virial mass $M_{200} sim 10^8,M_odot$ and astrophysical factors $J(alpha_mathrm{c}^J) sim 10^{11},M_odot^2,mathrm{kpc}^{-5}$ and $D(alpha_mathrm{c}^D) sim 10^2$-$10^{2.5},M_odot,mathrm{kpc}^{-2}$. We do not resolve a core ($r_mathrm{c} < 47,mathrm{pc}$, 68-% level) or soliton ($r_mathrm{sol} < 7.2,mathrm{pc}$, 68-% level). These limits are equivalent to an effective self-interaction coefficient $fGamma < 2.2 times 10^{-29},mathrm{cm}^3,mathrm{s}^{-1},mathrm{eV}^{-1},c^2$ and a fuzzy-dark-matter particle mass $m_mathrm{a} > 4.0 times 10^{-20},mathrm{eV},c^{-2}$. The constraint on self-interaction is complementary to those from gamma-ray searches. The constraint on fuzzy-dark-matter particle mass is inconsistent with those obtained for larger dwarf galaxies, suggesting that the flattened density profiles of those galaxies are not caused by fuzzy dark matter. (Abridged)
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