Antlia B: A faint dwarf galaxy member of the NGC 3109 association

We report the discovery of Antlia B, a faint dwarf galaxy at a projected distance of $\sim$72 kpc from NGC 3109 ($M_{V}$$\sim$$-$15 mag), the primary galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The tip of the red giant branch distance to Antlia B is $D$=1.29$\pm$0.10 Mpc, which is consistent with the distance to NGC 3109. A qualitative analysis indicates the new dwarf's stellar population has both an old, metal poor red giant branch ($\gtrsim$10 Gyr, [Fe/H]$\sim$$-$2), and a younger blue population with an age of $\sim$200-400 Myr, analogous to the original Antlia dwarf, another likely satellite of NGC 3109. Antlia B has \ion{H}{1} gas at a velocity of $v_{helio,HI}$=376 km s$^{-1}$, confirming the association with NGC 3109 ($v_{helio}$=403 km s$^{-1}$). The HI gas mass (M$_{HI}$=2.8$\pm$0.2$\times$10$^{5}$ M$_{\odot}$), stellar luminosity ($M_{V}$=$-$9.7$\pm$0.6 mag) and half light radius ($r_{h}$=273$\pm$29 pc) are all consistent with the properties of dwarf irregular and dwarf spheroidal galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy. The discovery of Antlia B is the initial result from a Dark Energy Camera survey for halo substructure and faint dwarf companions to NGC 3109 with the goal of comparing observed substructure with expectations from the $\Lambda$+Cold Dark Matter model in the sub-Milky Way regime.Comment: 7 pages, 3 figures. Submitted to ApJ

A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies

[abridged] We present the results of a spectroscopic survey of the recently discovered faint Milky Way satellites Boo, UMaI, UMaII and Wil1. Using the DEIMOS spectrograph on Keck, we have obtained samples that contain from 15 to 85 probable members of these satellites for which we derive radial velocities precise to a few km/s down to i~21-22. About half of these stars are observed with a high enough S/N to estimate their metallicity to within \pm0.2 dex. From this dataset, we show that UMaII is the only object that does not show a clear radial velocity peak. However, the measured systemic radial velocity (v_r=115\pm5 km/s) is in good agreement with recent simulations in which this object is the progenitor of the recently discovered Orphan Stream. The three other satellites show velocity dispersions that make them highly dark-matter dominated systems. In particular the Willman 1 object is not a globular cluster given its metallicity scatter over -2.0<[Fe/H]<-1.0 and is therefore almost certainly a dwarf galaxy or dwarf galaxy remnant. We measure a radial velocity dispersion of only 4.3_{-1.3}^{+2.3} km/s around a systemic velocity of -12.3\pm2.3 km/s which implies a mass-to-light ratio of ~700 and a total mass of ~5x10^5 Msun for this satellite, making it the least massive satellite galaxy known to date. Such a low mass could mean that the 10^7 Msun limit that had until now never been crossed for Milky Way and Andromeda satellite galaxies may only be an observational limit and that fainter, less massive systems exist within the Local Group. However, more modeling and an extended search for potential extra-tidal stars are required to rule out the possibility that these systems have not been significantly heated by tidal interaction.Comment: 24 pages, 11 figures, MNRAS accepte

Simulating Diffuse Light in Galaxy Clusters

Using N-body simulations, we have modeled the production and evolution of low surface brightness, diffuse intra-cluster light (ICL) in galaxy clusters. By creating simulated observations of the clusters we have measured the evolution of the ICL luminosity throughout the dynamical history of the clusters. We find that ICL production tends to occur in short, discrete events, which correlate very strongly with strong, small-scale interactions and accretions between groups within the clusters.Comment: 5 pages, 2 figures; includes 1 color figure. To appear in ESO Astrophysics Symposia: "Groups of Galaxies in the Nearby Universe

On the Adromeda to Milky Way mass-ratio

We have explored the hypothesis that the total mass-ratio of the two main galaxies of the Local Group: Andromeda Galaxy(M31) and the Milky Way (MW) can be constrained measuring the tidal force induced by the surrounding mass distribution, M31 included, on the MW. We argue that the total mass-ratio between the two groups can be approximated, at least qualitatively, finding the tidal radius where the internal binding force of the MW balances the external tidal force acting on it. Since M31 is the massive tidal perturber of the local environment, we have used a wide range of M31 to MW mass-ratio combinations to compute the corresponding tidal radii. Of them, only few match the distance of the zero-tidal shell i.e. the shell identified observationally by the outermost dwarf galaxies which do not show any sign of tidal effects. This is the key to constrain the best mass-ratio interval of the two galaxies. Our results favour a solution where the mass-ratio ranges from 2 to 3 implying a massive predominance of M31.Comment: 8 pages, 2 figures, Accepted for publication in Mon. Not. of Royal Astron. So

At the upper palaeolithic - mesolithic boundary: Revision of the human remains from riparo fredian (MOLAZZANA, LUCCA, ITALY)

Pleistocene and early Holocene human fossils in Tuscany are very few and poorly described. Any new information is thus an important contribution to our knowledge of the peopling of this region. Here we present a revision of the human fossil remains from the Riparo Fredian, a site located in Garfagnana and first published by Boschian et al., (1995). The authors described the human remains of the site pointing out the presence of 39 isolated human teeth (19 maxillary and 20 mandibular) and "fragments of one adult humerus, of a child's femur and of a youngster's ulna", considered as belonging to six individuals at least. A reanalysis of the human remains indicated that several specimens were incorrectly identified. It was thus deemed important to revise the identification of each fossil and their interpretation. The revision of human remains from Riparo Fredian has led to several changes in their anatomical identification with respect to the original publications. Of the 39 teeth previously described, the analysis revealed that two of them belonged to non-human animals, and 18 were mistakenly identified. A new, correct identification is provided for each of them. Also, two human teeth not described in the original papers have been identified. The anatomical identification of the post-cranial remains has been confirmed for two out of the three specimens. The minimum number of individuals, based on the dental remains, is confirmed as at least 5, but most probably 6, although with a different allocation of teeth to individual specimens. The age at death of the six individuals has also been reassessed, indicating the presence of two infants, two young adults and two mature adults

Hubble Space Telescope Imaging of the Ultra-Compact High Velocity Cloud AGC 226067: A stripped remnant in the Virgo Cluster

We analyze the optical counterpart to the ultra-compact high velocity cloud AGC 226067, utilizing imaging taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The color magnitude diagram of the main body of AGC 226067 reveals an exclusively young stellar population, with an age of $\sim$7--50 Myr, and is consistent with a metallicity of [Fe/H]$\sim$$-$0.3 as previous work has measured via HII region spectroscopy. Additionally, the color magnitude diagram is consistent with a distance of $D$$\approx$17 Mpc, suggesting an association with the Virgo cluster. A secondary stellar system located $\sim$1.6' ($\sim$8 kpc) away in projection has a similar stellar population. The lack of an old red giant branch ($\gtrsim$5 Gyr) is contrasted with a serendipitously discovered Virgo dwarf in the ACS field of view (Dw J122147+132853), and the total diffuse light from AGC~226067 is consistent with the luminosity function of the resolved $\sim$7--50 Myr stellar population. The main body of AGC~226067 has a $M_{V}$=$-$11.3$\pm$0.3, or $M_{stars}$=5.4$\pm$1.3$\times$10$^4$ $M_{\odot}$ given the stellar population. We searched 20 deg$^2$ of imaging data adjacent to AGC~226067 in the Virgo Cluster, and found two similar stellar systems dominated by a blue stellar population, far from any massive galaxy counterpart -- if this population has similar star formation properties as AGC~226067, it implies $\sim$0.1 $M_{\odot}$ yr$^{-1}$ in Virgo intracluster star formation. Given its unusual stellar population, AGC~226067 is likely a stripped remnant and is plausibly the result of compressed gas from the ram pressure stripped M86 subgroup ($\sim$350 kpc away in projection) as it falls into the Virgo Cluster.Comment: 9 pages, 4 figures, 1 table. Accepted by the Astrophysical Journa