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Register a new userOn the Stellar Kinematics and Mass of the Virgo Ultra-Diffuse Galaxy VCC 1287
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Here, we present a kinematical analysis of the Virgo cluster ultra-diffuse galaxy (UDG) VCC 1287 based on data taken with the Keck Cosmic Web Imager (KCWI). We confirm VCC 1287s association both with the Virgo cluster and its globular cluster (GC) system, measuring a recessional velocity of $1116 pm 2 mathrm{km s^{-1}}$. We measure a stellar velocity dispersion ($19 pm 6 mathrm{km s^{-1}}$) and infer both a dynamical mass ($1.11^{+0.81}_{-0.81} times 10^{9} mathrm{M_{odot}}$) and mass to light ratio ($13^{+11}_{-11}$) within the half light radius (4.4 kpc). This places VCC 1287 slightly above the well established relation for normal galaxies, with a higher mass to light ratio for its dynamical mass than normal galaxies. We use our dynamical mass, and an estimate of GC system richness, to place VCC 1287 on the GC number -- dynamical mass relation, finding good agreement with a sample of normal galaxies. Based on a total halo mass derived from GC counts, we then infer that VCC 1287 likely resides in a cored or low concentration dark matter halo. Based on the comparison of our measurements to predictions from simulations, we find that strong stellar feedback and/or tidal effects are plausibly the dominant mechanisms in the formation of VCC 1287. Finally, we compare our measurement of the dynamical mass with those for other UDGs. These dynamical mass estimates suggest relatively massive halos and a failed galaxy origin for at least some UDGs.
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The so-called ultra-diffuse galaxy NGC~1052-DF2 was announced to be a galaxy lacking dark matter based on a spectroscopic study of its constituent globular clusters. Here we present the first spectroscopic analysis of the stellar body of this galaxy using the MUSE integral-field spectrograph at the (ESO) Very Large Telescope. The MUSE datacube simultaneously provides DF2s stellar velocity field and systemic velocities for seven globular clusters (GCs). We further discovered three planetary nebulae (PNe) that are likely part of this galaxy. While five of the clusters had velocities measured in the literature, we were able to confirm the membership of two more candidates through precise radial velocity measurements, which increases the measured specific frequency of GCs in DF2. The mean velocity of the diffuse stellar body, 1792.9$^{-1.8}_{+1.4}$~kms, is consistent with the mean globular cluster velocity. We detect a weak but significant velocity gradient within the stellar body, with a kinematic axis close to the photometric major-axis, making it a prolate-like rotator. We estimate a velocity dispersion from the clusters and PNe of $sigma_{mathrm{int}}=10.6^{+3.9}_{-2.3}$~kms. The velocity dispersion $sigma_{rm{DF2}star}$(re) for the stellar body within one effective radius is $10.8^{-4.0}_{+3.2}$~kms. Considering various sources of systemic uncertainties this central value varies between 5 and 13~kms, and we conservatively report a 95% confidence upper limit to the dispersion within one re of 21~kms. We provide updated mass estimates based on these dispersions corresponding to the different distances to NGC~1052-DF2 that have been reported in the recent literature.
Ultra diffuse galaxies (UDGs) have the sizes of giant galaxies but the luminosities of dwarfs. A key to understanding their origins comes from their total masses, but their low surface brightnesses ($mu(V) geq$ 25.0) generally prohibit dynamical studies. Here we report the first such measurements for a UDG (VCC~1287 in the Virgo cluster), based on its globular cluster system dynamics and size. From 7 GCs we measure a mean systemic velocity $v_{rm sys}$ = 1071$^{+14}_{-15}$ km/s, thereby confirming a Virgo-cluster association. We measure a velocity dispersion of 33$^{+16}_{-10}$ km/s within 8.1 kpc, corresponding to an enclosed mass of $(4.5 pm 2.8)times10^{9}$ $M_{odot}$ and a $g$-band mass-to-light ratio of $(M/L)_g = 106^{+126}_{-54}$. From the cumulative mass curve, along with the GC numbers, we estimate a virial mass of $sim8times10^{10}$ $M_{odot}$, yielding a dark-to-stellar mass fraction of $sim3000$. We show that this UDG is an outlier in $M_{rm star} - M_{rm halo}$ relations, suggesting extreme stochasticity in relatively massive star-forming halos in clusters. Finally, we discuss how counting GCs offers an efficient route to determining virial masses for UDGs.
The ultra-diffuse galaxy in the NGC 5846 group (NGC 5846_UDG1) was shown to have a large number of globular cluster (GC) candidates from deep imaging as part of the VEGAS survey. Recently, Muller et al. published a velocity dispersion, based on a dozen of its GCs. Within their quoted uncertainties, the resulting dynamical mass allowed for either a dark matter free or a dark matter dominated galaxy. Here we present spectra from KCWI which reconfirms membership of the NGC 5846 group and reveals a stellar velocity dispersion for UDG1 of $sigma_{GC}$ = 17 $pm$ 2 km/s. Our dynamical mass, with a reduced uncertainty, indicates a very high contribution of dark matter within the effective radius. We also derive an enclosed mass from the locations and motions of the GCs using the Tracer Mass Estimator, finding a similar mass inferred from our stellar velocity dispersion. We find no evidence that the galaxy is rotating and is thus likely pressure-supported. The number of confirmed GCs, and the total number inferred for the system ($sim$45), suggest a total halo mass of $sim2 times 10^{11}$ M$_{odot}$. A cored mass profile is favoured when compared to our dynamical mass. Given its stellar mass of 1.1$times$10$^{8}$ M$_{odot}$, NGC 5846_UDG1 appears to be an ultra-diffuse galaxy with a dwarf-like stellar mass and an overly massive halo.
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.
We describe the structural, stellar population and gas properties of the nearest Ultra Diffuse Galaxy (UDG) discovered so far: UGC2162 (z=0.00392; R$_{e,g}$=1.7$(pm$0.2) kpc; $mu_g(0)$=24.4$pm$0.1 mag/arcsec$^2$; g-i=0.33$pm$0.02). This galaxy, located at a distance of 12.3($pm$1.7) Mpc, is a member of the M77 group. UGC2162 has a stellar mass of $sim$2($^{+2}_{-1}$)$times$10$^7$ M$_odot$ and is embedded within a cloud of HI gas $sim$10 times more massive: $sim$1.9($pm$0.6)$times$10$^8$ M$_odot$. Using the width of its HI line as a dynamical proxy, the enclosed mass within the inner R$sim$5 kpc is $sim$4.6($pm$0.8)$times$10$^9$ M$_odot$ (i.e. M/L$sim$200). The estimated virial mass from the cumulative mass curve is $sim$8($pm$2)$times$10$^{10}$ M$_odot$. Ultra-deep imaging from the IAC Stripe82 Legacy Project show that the galaxy is irregular and has many star forming knots, with a gas-phase metallicity around one-third of the solar value. Its estimated Star Formation Rate (SFR) is $sim$0.01 M$_odot$/yr. This SFR would double the stellar mass of the object in $sim$2 Gyr. If the object were to stop forming stars at this moment, after a passive evolution, its surface brightness would become extremely faint: $mu_g(0)$$sim$27 mag/arcsec$^2$ and its size would remain large R$_{e,g}$$sim$ 1.8 kpc. Such faintness would make it almost undetectable to most present-day surveys. This suggests that there could be an important population of M$_{star}$$sim$10$^7$ M$_odot$ dark galaxies in rich environments (depleted of HI gas) waiting to be discovered by current and future ultra-deep surveys.
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