6,405 research outputs found
Single Star-forming galaxies and Star-forming galaxies in SF+SF and mixed pairs
We compare the SFR of single star-forming galaxies with the SFR of
star-forming galaxies in pairs. Volume-limited samples are compared selected
from the 2dFGRS, applying a maximum magnitude difference criterion. We show
that SF galaxies in SF + SF pairs typically increase their SFR as they get
fainter, whereas this does not happen for SF galaxies in mixed (SF + passive)
pairs. And we provide evidence that differences between single SF and SF in
pairs get more significant when SF galaxies in mixed pairs are excluded from
the pair sample. Our analysis confirms that enhanced SFR and the presence of a
companion galaxy (on 0.5 h^-1 Mpc scale) are correlated quantities, provided
the galaxy is neither too luminous nor too faint, and the triggering galaxy is
itself a SF galaxy.Comment: 6 pages, 3 figures, contributed paper, to be published in ''The
Evolution of Starbursts'' (Bad Honnef 2004), ed. S. Huettemeister & E.
Manthley (Melville:AIP
Dominant galaxies in 2dF groups
We investigate whether the spectral-type of a locally dominant (most
luminous) galaxy can be used to select sets of galaxies that are physically
associated (groups). We assume that passive dominants trace a group-like
potential, and SF-dominants a field-like environment. The group sample includes
988 groups selected in the 2dFGRS applying a maximum magnitude difference
criterion. We find that the average number of passive galaxies associated to a
dominant is larger when the dominant is passive, a result supporting our
assumption that galaxy associations around a passive dominant are reliable
groups. Finally we show that, to reduce the contamination by unbound galaxy
associations (SF-dominant), a >=3 passive-members criterion is more efficient
than a >=6 all-members criterion.Comment: 3 pages, 2 figures, to appear in ''Outskirts of galaxy clusters:
intense life in the suburbs'' ed. A. Diferio, Proceedings of IAU Coll. No.
195, 200
The effect of the environment on the Faber Jackson relation
We investigate the effect of the environment on the Faber Jackson (FJ)
relation, using a sample of 384 nearby elliptical galaxies and estimating
objectively their environment on the typical scale of galaxy clusters. We show
that the intrinsic scatter of the FJ is significantly reduced when ellipticals
in high density environments are compared to ellipticals in low density ones.
This result, which holds on a limited range of overdensities, is likely to
provide an important observational link between scaling relations and formation
mechanisms in galaxies.Comment: accepted by Ap
Optical properties of the NGC 5328 group of galaxies
We present the results of a photometric and spectroscopic study of seven
members of the NGC 5328 group of galaxies, a chain of galaxies spanning over
200 kpc (H_0 = 70 km/s/Mpc). We analyze the galaxy structure and study the
emission line properties of the group members looking for signatures of star
formation and AGN activity. We finally attempt to infer, from the modeling of
line-strength indices, the stellar population ages of the early-type members.
We investigate also the presence of a dwarf galaxy population associated with
the bright members.
The group is composed of a large fraction of early-type galaxies including
NGC 5328 and NGC 5330, two bona fide ellipticals at the center of the group. In
both galaxies no recent star formation episodes are detected by the H_beta vs.
MgFe indices of these galaxies. 2MASX J13524838-2829584 has extremely boxy
isophotes which are believed to be connected to a merging event: line strength
indices suggest that this object probably had a recent star formation episode.
A warped disc component emerges from the model subtracted image of 2MASX
J13530016-2827061 which is interpreted as a signature of an ongoing interaction
with the rest of the group.
Ongoing star formation and nuclear activity is present in the projected
outskirts of the group. The two early-type galaxies 2MASX J13523852-2830444 and
2MASX J13525393-2831421 show spectral signatures of star formation, while a
Seyfert 2 type nuclear activity is detected in MCG -5-33-29.Comment: 18 pages, 12 figures, accepted for publication in MNRA
A new sample of bright galaxy pairs in UZC
We present a new sample of bright galaxy pairs extracted applying an
objective selection code to UZC catalog.The sample is volume limited to
M = -18.9 +5 log and contains 89 galaxy pairs.We analyze the
kinematical, morphological and photometrical properties of galaxies belonging
to this sample. We show that velocity separation, , between
pair members is significantly lower in spiral type (S+S) pairs than in
early-type (E+E) and mixed (E+S) pairs.This indicates that truly isolated
galaxy pairs are more likely to be found among S+S pairs. We show that
ellipticals are rare and underluminous in B and that late spirals (T 4)
are overluminous. We confirm that the formation of bright ellipticals is a
phenomenon linked to group/cluster environment, while galaxy-galaxy interaction
may enhance blue luminosity of disk galaxies through SF phenomena. This last
statement is supported by the presence of strong FIR emission from early
spirals in this sample and by the high frequency of AGN/SB phenomenon, revealed
mainly in pairs of low relative radial velocity separation and showing signs of
interaction.Comment: A & A accepted, 6 pages, 6 figure
A Semantic Hierarchy for Erasure Policies
We consider the problem of logical data erasure, contrasting with physical
erasure in the same way that end-to-end information flow control contrasts with
access control. We present a semantic hierarchy for erasure policies, using a
possibilistic knowledge-based semantics to define policy satisfaction such that
there is an intuitively clear upper bound on what information an erasure policy
permits to be retained. Our hierarchy allows a rich class of erasure policies
to be expressed, taking account of the power of the attacker, how much
information may be retained, and under what conditions it may be retained.
While our main aim is to specify erasure policies, the semantic framework
allows quite general information-flow policies to be formulated for a variety
of semantic notions of secrecy.Comment: 18 pages, ICISS 201
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