Search

International audience

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 Journal

S.-M Niemi, A. Ryś, and H. Salo 1 UCO/Lick Observatory, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA; toloba@ucolick.org 2 Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA; 3 Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands 4 Laboratoire d’Astrophysique de Marseille-LAM, Universite d’Aix-Marseille & CNRS, UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France 5 Astronomisches Rechen-Institut, Zentrum fur Astronomie der Universitat Heidelberg, Monchhofstrase 12-14, D-69120 Heidelberg, Germany 6 Instituto de Astrofisica de Canarias, Via Lactea s/n, La Laguna, Tenerife, Spain 7 Departamento de Astrofisica, Universidad de La Laguna, E-38205, La Laguna, Tenerife, Spain 8 Max Planck Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany 9 Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea 10 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching, Germany 11 Universite Lyon 1, Observatoire de Lyon, Centre de Recherche Astrophysique de Lyon and Ecole Normale Superieure de Lyon, 9 Avenue Charles Andre, F-69230

We present evidence for kinematically decoupled cores (KDCs) in two dwarf early-type (dE) galaxies in the Virgo cluster, VCC 1183 and VCC 1453, studied as part of the SMAKCED stellar absorption-line spectroscopy and imaging survey. These KDCs have radii of 1.8'' (0.14 kpc) and 4.2'' (0.33 kpc), respectively. Each of these KDCs is distinct from the main body of its host galaxy in two ways: (1) inverted sense of rotation; and (2) younger (and possibly more metal-rich) stellar population. The observed stellar population differences are probably associated with the KDC, although we cannot rule out the possibility of intrinsic radial gradients in the host galaxy. We describe a statistical analysis method to detect, quantify the significance of, and characterize KDCs in long-slit rotation curve data. We apply this method to the two dE galaxies presented in this paper and to five other dEs for which KDCs have been reported in the literature. Among these seven dEs, there are four significant KDC detections, two marginal KDC detections, and one dE with an unusual central kinematic anomaly that may be an asymmetric KDC.The frequency of occurence of KDCs and their properties provide important constraints on the formation history of their host galaxies. We discuss different formation scenarios for these KDCs in cluster environments and find that dwarf-dwarf wet mergers or gas accretion can explain the properties of these KDCs. Both of these mechanisms require that the progenitor had a close companion with a low relative velocity. This suggests that KDCs were formed in galaxy pairs residing in a poor group environment or in isolation whose subsequent infall into the cluster quenched star formation., 14 pages, accepted for publication in ApJ