ACS imaging of 25 galaxies in nearby groups and in the field

We present HST/ACS images and color-magnitude diagrams for 25 nearby galaxies with radial velocities V_LG < 500 km/s. Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 2 Mpc to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, are found to be the nearest known representatives of blue compact dwarf (BCD) objects. Using high-quality data on distances and radial velocities of 110 nearby field galaxies, we derive their mean Hubble ratio to be 68 km/(s Mpc) with standard deviation of 15 km/(s Mpc). Peculiar velocities of most of the galaxies, V_pec = V_LG - 68 D, follow a Gaussian distribution with sigma_v = 63 km/s, but with a tail towards high negative values. Our data displays the known correlation between peculiar velocity and galaxy elevation above the Local Supercluster plane. The small observed fraction of galaxies with high peculiar velocities, V_pec < -500 km/s, may be understood as objects associated with nearby groups (Coma I, Eridanus) outside the Local volume.Comment: 17 pages, 8 figures, accepted to A

Distance and mass of the M104 (Sombrero) group

Distances and radial velocities of galaxies in the vicinity of the luminous early-type galaxy M 104 (Sombrero) are used to derive its dark matter mass. Two dwarf galaxies: UGCA 307 and KKSG 30 situated near M 104 were observed with the Advanced Camera for Surveys on the Hubble Space Telescope. The distances $9.03^{+0.84}_{-0.51}$ Mpc (UGCA 307) and $9.72^{+0.44}_{-0.41}$ Mpc (KKSG 30) were determined using the tip of the red giant branch method. These distances are consistent with the dwarf galaxies being satellites of Sombrero. Using radial velocities and projected separations of UGCA 307, KKSG 30, and a third galaxy with an accurate distance (KKSG 29), as well as 12 other assumed companions with less accurate distances, the total mass of M 104 is estimated to be $(1.55\pm0.49) 10^{13} M_{\odot}$. At the K-band luminosity of the Sombrero galaxy of $2.4 10^{11} L_{\odot}$, its total mass-to-luminosity ratio is $M_T/L_K = (65\pm20)M_{\odot}/L_{\odot}$, which is about three times higher than that of luminous bulgeless galaxies.Comment: 6 pages, 5 figures, 3 tables, accepted to Astronomy & Astrophysic

The Hubble flow around the CenA / M83 galaxy complex

We present HST/ACS images and color-magnitude diagrams for 24 nearby galaxies in and near the constellation of Centaurus with radial velocities V_LG < 550 km/s. Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 3.0 Mpc to 6.5 Mpc. The galaxies are concentrated in two spatially separated groups around Cen A (NGC 5128) and M 83 (NGC 5236). The Cen A group itself has a mean distance of 3.76 +/-0.05 Mpc, a velocity dispersion of 136 km/s, a mean harmonic radius of 192 kpc, and an estimated orbital/virial mass of (6.4 - 8.1) x 10^12 M_sun. This elliptical dominated group is found to have a relatively high mass-to-light ratio: M/L_B = 125 M_sun/L_sun. For the M 83 group we derived a mean distance of 4.79 +/-0.10 Mpc, a velocity dispersion of 61 km/s, a mean harmonic radius of 89 kpc, and estimated orbital/virial mass of (0.8 - 0.9) x 10^12 M_sun. This spiral dominated group is found to have a relatively low M/L_B = 34 M_sun/L_sun. The radius of the zero-velocity surface around Cen A lies at R_0 = 1.40 +/-0.11 Mpc, implying a total mass within R_0 of M_T = (6.0 +/-1.4) x 10^12 M_sun. This value is in good agreement with the Cen A virial/orbital mass estimates and provides confirmation of the relatively high M/L_B of this elliptical-dominated group. The centroids of both the groups, as well as surrounding field galaxies, have very small peculiar velocities, < 25 km/s, with respect to the local Hubble flow with H_0 = 68 km/s/Mpc.Comment: 31 pages including 9 figures and 3 tables. Accepted for publication in Astronomical Journal, 133, N0. 2 (February), 200

KKH 22, the first dwarf spheroidal satellite of IC 342

We present observations with the Advanced Camera for Surveys on the Hubble Space Telescope of the nearby dwarf spheroidal galaxy KKH 22 = LEDA 2807114 in the vicinity of the massive spiral galaxy IC 342. We derived its distance of 3.12+-0.19 Mpc using the tip of red giant branch (TRGB) method. We also used the 6 m BTA spectroscopy to measure a heliocentric radial velocity of the globular cluster in KKH22 to be +30+-10 km/s. The dSph galaxy KKH 22 has the V-band absolute magnitude of -12.19 mag and the central surface brightness mu_v,0 = 24.1 mag/sq.arcsec. Both the velocity and the distance of KKH 22 are consistent with the dSph galaxy being gravitationally bound to IC 342. Another nearby dIr galaxy, KKH 34, with a low heliocentric velocity of +106 km/s has the TRGB distance of 7.28+-0.36 Mpc residing in the background with respect to the IC 342 group. KKH 34 has a surprisingly high negative peculiar velocity of -236+-26 km/s.Comment: 7 pages, 6 figures, accepted in A&

A nearby isolated dwarf: star formation and structure of ESO 006-001

Observations with the Hubble Space Telescope unexpectedly revealed that the dwarf galaxy ESO 006-001 is a near neighbor to the Local Group at a distance of 2.70 +- 0.11 Mpc. The stellar population in the galaxy is well resolved into individual stars to a limit of M I ~ -0.5 mag. The dominant population is older than 12 Gyr yet displays a significant range in metallicity of -2 < [Fe/H] < -1, as evidenced by a Red Giant Branch with substantial width. Superimposed on the dominant population are stars on the Main Sequence with ages less than 100 Myr and Helium burning Blue Loop stars with ages of several hundred Myr. ESO 006-001 is an example of a transition dwarf; a galaxy dominated by old stars but one that has experienced limited recent star formation in a swath near the center. No H i gas is detected at the location of the optical galaxy in spite of the evidence for young stars. Intriguingly, an H i cloud with a similar redshift is detected 9 kpc away in projection. Otherwise, ESO 006-001 is a galaxy in isolation with its nearest known neighbor IC 3104, itself a dwarf, at a distance of ~ 500 kpc.Comment: 13 pages, 9 figures, accepted at Ap