55 research outputs found
The discovery of blue-cored dwarf early-type galaxies in isolated environments
The presence of blue-cored dwarf early-type galaxies (dE(bc)s) in
high-density environments supports the scenario of the transformation of
infalling late-type galaxies into quiescent dwarf early-type galaxies by
environmental effects. While low-density environments lacking environmental
processes could not be relevant to the formation of dE(bc)s, we discovered a
large sample of rare dE(bc)s in isolated environments at z < 0.01 using the
NASA-Sloan Atlas catalog. Thirty-two isolated dE(bc)s were identified by visual
inspection of the Sloan Digital Sky Survey images and g - r color profiles. We
found that (1) isolated dE(bc)s exhibit similar structural parameters to
dE(bc)s in the Virgo cluster; (2) based on the ultraviolet-r color-magnitude
relation, color gradients, and optical emission lines of dE(bc)s, isolated
dE(bc)s show more vigorous, centrally concentrated SF compared to their
counterparts in the Virgo cluster; (3) at a given stellar mass, isolated
dE(bc)s tend to have a larger fraction of gas mass than their Virgo
counterparts. We discuss a scenario of episodic SF sustained by gas accretion,
suggested by Sanchez Almeida et al., in which the star-bursting blue compact
dwarf galaxy (BCD)-quiescent BCD (QBCD) cycle can be repeated during the Hubble
time. We suggest that, in this cadence, isolated dE(bc)s might be QBCDs at pre-
or post-BCD stages. Our results imply that dE(bc)s comprise a mixture of
objects with two types of origins, nature or nurture, depending on their
environment.Comment: 8 pages, 5 figures, accepted for publication in Astrophysical Journa
The Extended Virgo Cluster Catalog
We present a new catalog of galaxies in the wider region of the Virgo
cluster, based on the Sloan Digital Sky Survey (SDSS) Data Release 7. The
Extended Virgo Cluster Catalog (EVCC) covers an area of 725 deg^2 or 60.1
Mpc^2. It is 5.2 times larger than the footprint of the classical Virgo Cluster
Catalog (VCC) and reaches out to 3.5 times the virial radius of the Virgo
cluster. We selected 1324 spectroscopically targeted galaxies with radial
velocities less than 3000 kms^-1. In addition, 265 galaxies that have been
missed in the SDSS spectroscopic survey but have available redshifts in the
NASA Extragalactic Database are also included. Our selection process secured a
total of 1589 galaxies of which 676 galaxies are not included in the VCC. The
certain and possible cluster members are defined by means of redshift
comparison with a cluster infall model. We employed two independent and
complementary galaxy classification schemes: the traditional morphological
classification based on the visual inspection of optical images and a
characterization of galaxies from their spectroscopic features. SDSS u, g, r,
i, and z passband photometry of all EVCC galaxies was performed using Source
Extractor. We compare the EVCC galaxies with the VCC in terms of morphology,
spatial distribution, and luminosity function. The EVCC defines a comprehensive
galaxy sample covering a wider range in galaxy density that is significantly
different from the inner region of the Virgo cluster. It will be the foundation
for forthcoming galaxy evolution studies in the extended Virgo cluster region,
complementing ongoing and planned Virgo cluster surveys at various wavelengths.Comment: 69 pages, 29 figures, 4 tables, accepted for publication in the ApJ
The properties of early-type galaxies in the Ursa Major cluster
Using SDSS-DR7 and NASA/IPAC Extragalactic Database spectroscopic data, we
identify 166 galaxies as members of the Ursa Major cluster with Mr < -13.5 mag.
We morphological classify all galaxies by means of carefully inspecting g-, r-,
i-band colour and monochromatic images. We show that the Ursa Major cluster is
dominated by late-type galaxies, but also contains a significant number of
early- type galaxies, particularly in the dwarf regime. We present further
evidence for the existence of several subgroups in the cluster, consistent with
previous findings. The early-type fraction is found to correlate with the mass
of the subgroup. We also investigate environmental effects by comparing the
properties of the Ursa Major early-type dwarf galaxies to those of the Virgo
cluster. In contrast to the Virgo, the red sequence of the Ursa Major cluster
is only sparsely populated in the optical and ultraviolet colour-magnitude
relations. It also shows a statistically significant gap between -18 < Mr < -17
mag, i.e. the Ursa Major cluster lacks early-type dwarf galaxies at the bright
end of their luminosity function. We discover that the majority of early-type
dwarf galaxies in the Ursa Major cluster have blue cores with hints of recent
or ongoing star formation. We suggest that gravitational tidal interactions can
trigger central blue star forming regions in early-type dwarfs. After that,
star formation would only fade completely when the galaxies experience ram
pressure stripping or harassment, both of which are nearly absent in the Ursa
Major cluster.Comment: 19 pages, 18 figures, 2 tables, Accepted for publication in MNRA
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