21 research outputs found
VLT Spectroscopy of Globular Clusters in Low Surface Brightness Dwarf Galaxies
We present VLT/FORS2 spectroscopic observations of globular clusters (GCs) in
five low surface brightness (LSB) dwarf galaxies: KK211 and KK221, which are
both dwarf spheroidal satellites (dSph) of NGC 5128, dSph KK84 located close to
the isolated S0 galaxy NGC 3115, and two isolated dwarf irregular (dIrr)
galaxies UGC 3755 and ESO 490-17. Our sample is selected from the Sharina et
al. (2005) database of Hubble Space Telescope WFPC2 photometry of GC candidates
in dwarf galaxies. For objects with accurate radial velocity measurements we
confirm 26 as genuine GCs out of the 27 selected candidates from our WFPC2
survey. Lick absorption line indices in the spectra of confirmed GCs and the
subsequent comparison with SSP model predictions show that all confirmed GCs in
dSphs are old, except GC KK211-3-149 (6 +/- 2 Gyr), which we consider to be the
nucleus of KK211. GCs in UGC 3755 and ESO 490-17 show a large spread in ages
ranging from old objects (t > 10 Gyr) to clusters with ages around 1 Gyr. Most
of our sample GCs have low metallicities [Z/H] <= -1. Two relatively metal-rich
clusters with [Z/H] ~ -0.3 are likely to be associated with NGC 3115. Our
sample GCs show in general a complex distribution of alpha-element enhancement
with a mean [alpha/Fe]=0.19 +/-0.04 derived with the chi2 minimization
technique and 0.18+/-0.12 dex computed with the iterative approach. These
values are slightly lower than the mean [alpha/Fe]=0.29+/-0.01 for typical
Milky Way GCs. We compare other abundance ratios with those of Local Group GCs
and find indications for systematic differences in N and Ca abundance. The
specific frequencies, S_N, of our sample galaxies are in line with the
predictions of a simple mass-loss model for dwarf galaxies and compare well
with S_N values found for dwarf galaxies in nearby galaxy clusters.Comment: accepted for publication in Ap
Statistical analysis of dwarf galaxies and their globular clusters in the Local Volume
Morphological classification of dwarf galaxies into early and late type,
though can account for some of their origin and characteristics but does not
help to study their formation mechanism. So an objective classification using
Principal Component analysis together with K means Cluster Analysis of these
dwarf galaxies and their globular clusters is carried out to overcome this
problem. It is found that the classification of dwarf galaxies in the Local
Volume is irrespective of their morphological indices. The more massive (MV 0 <
-13.7) galaxies evolve through self-enrichment and harbor dynamically less
evolved younger globular clusters (GCs) whereas fainter galaxies (MV 0 > -13.7)
are influenced by their environment in the star formation process.Comment: 22 pages, 10 figures, 5 tables, published in Ap
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
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
Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005
We present HST WFPC2 imaging of the nearby low-surface-brightness dwarf
spheroidal galaxy ESO 410-G005, which has been resolved into stars for the
first time. The resulting color-magnitude diagram for about 2500 stars shows a
red giant branch branch with a tip at I=(22.4+-0.15), which yields a distance
of D=(1.9+-0.2) Mpc. ESO 410-G005 is found to be metal-poor with a mean
metallicity of (-1.8+-0.4) dex estimated from its red giant branch. Upper
asymptotic giant branch stars appear to be present near the center of the
galaxy, indicative of a substantial, centrally concentrated intermediate-age
population, unless these objects are artifacts of crowding. Previous studies
did not detect ESO 410-G005 in H alpha or in HI. ESO 410-G005 is a probable
member of the Sculptor group. Its linear separation from the nearest spiral,
NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to
615 kpc depending on the assumed distance of NGC 55. ESO 410 G005 appears to be
a relatively isolated dSph within the Sculptor group. Its absolute magnitude,
Mv = (-12.1+-0.2) mag, its central surface brightness, mu_V = (22.7+-0.1)
mag/arcsec^2, and its mean metallicity, [Fe/H] = (-1.8+-0.4) dex, follow the
trend observed for dwarf galaxies in the Local Group. (abridged)Comment: Accepted for publication in the Astrophysical Journal, Vol. 542 (Oct
20). 23 pages in AASTEX style, 9 figures, partially in gif format to save
spac
Uncovering the formation of ultra-compact dwarf galaxies by multivariate statistical analysis
We present a statistical analysis of the properties of a large sample of
dynamically hot old stellar systems, from globular clusters to giant
ellipticals, which was performed in order to investigate the origin of
ultra-compact dwarf galaxies. The data were mostly drawn from Forbes et al.
(2008). We recalculated some of the effective radii, computed mean surface
brightnesses and mass-to-light-ratios, estimated ages and metallicities. We
completed the sample with globular clusters of M31. We used a multivariate
statistical technique (K-Means clustering), together with a new algorithm (Gap
Statistics) for finding the optimum number of homogeneous sub-groups in the
sample, using a total of six parameters (absolute magnitude, effective radius,
virial mass-to-light ratio, stellar mass-to-light ratio and metallicity). We
found six groups. FK1 and FK5 are composed of high- and low-mass elliptical
galaxies respectively. FK3 and FK6 are composed of high-metallicity and
low-metallicity objects, respectively, and both include globular clusters and
ultra-compact dwarf galaxies. Two very small groups, FK2 and FK4, are composed
of Local Group dwarf spheroidals. Our groups differ in their mean masses and
virial mass-to-light ratios. The relations between these two parameters are
also different for the various groups. The probability density distributions of
metallicity for the four groups of galaxies is similar to that of the globular
clusters and UCDs. The brightest low-metallicity globular clusters and
ultra-compact dwarf galaxies tend to follow the mass-metallicity relation like
elliptical galaxies. The objects of FK3 are more metal-rich per unit effective
luminosity density than high-mass ellipticals.Comment: Accepted for publication in Astrophysical Journa
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