863 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 Spatial Clustering of Low Luminosity AGN
We present the first multi-parameter analysis of the narrow line AGN
clustering properties. Estimates of the two-point correlation function (CF)
based on SDSS DR2 data reveal that Seyferts are clearly less clustered than
normal galaxies, while the clustering amplitude (r_0) of LINERs is consistent
with that of the parent galaxy population. The similarities in the host
properties (color and concentration index) of Seyferts and LINERs suggest that
the difference in their r_0 is not driven by the morphology-density relation.
We find that the luminosity of [O I] emission shows the strongest influence on
AGN clustering, with low L([O I]) sources having the highest r_0. This trend is
much stronger than the previously detected dependence on L([O III]), which we
confirm. There is a strong correspondence between the clustering patterns of
objects of given spectral type and their physical properties. LINERs, which
exhibit high r_0, show the lowest luminosities and obscuration levels, and
relatively low gas densities (n_e), suggesting that these objects harbor black
holes that are relatively massive yet weakly active or inefficient in their
accretion, probably due to the insufficiency of their fuel supply. Seyferts,
which have low r_0, are luminous and show large n_e, suggesting that their
black holes are less massive but accrete quickly and efficiently enough to
clearly dominate the ionization. The low r_0 of the H II galaxies can be
understood as a consequence of both the morphology-density and star formation
rate-density relations, however, their spectral properties suggest that their
centers hide amidst large amounts of obscuring material black holes of
generally low mass whose activity remains relatively feeble. Our own Milky Way
may be a typical such case.[abridged]Comment: 27 pages, color figures, some are severely degraded in resolution,
emulateapj. See http://www.physics.drexel.edu/~constant/work/agnclustering.ps
for high resolution version. Accepted to 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
Effects of tidal interactions on the gas flows of elliptical galaxies
During a Hubble time, cluster galaxies may undergo several mutual encounters
close enough to gravitationally perturb their hot, X-ray emitting gas flows. We
ran several 2D, time dependent hydrodynamical models to investigate the effects
of such perturbations on the gas flow inside elliptical galaxies. In
particular, we studied in detail the modifications occurring in the scenario
proposed by D'Ercole et al. (1989), in which the galactic interstellar medium
produced by the aging galactic stellar population, is heated by SNIa at a
decreasing rate. We find that, although the tidal interaction in our models
lasts less than 1 Gyr, its effect extends over several Gyrs. The tidally
induced turbulent flows create dense filaments which cool quickly and accrete
onto the galactic center, producing large spikes in the global Lx. Once this
mechanism starts, it is fed by gravity and amplified by SNIa. In cooling flow
models without supernovae the amplitude of the Lx fluctuations due to the tidal
interaction is substantially reduced. We conclude that, if SNIa significantly
contribute to the energetics of the gas flows in ellipticals, then the observed
spread in the Lx-Lb diagram may be caused, at least in part, by this mechanism.
On the contrary, tidal interactions cannot be responsible for the observed
spread if the pure cooling flow scenario applies (abridged).Comment: 21 pages, 8 figures, to be published in ApJ (main journal
The spectral-type/luminosity and the spectral type/satellite-density relations in the 2dFGRS
We examine the relative fractions of passive (Type 1), quiet-SF (Type 2) and
active-SF (Type 3+4) galaxies as a function of luminosity and number of
neighbours in several volume limited samples selected from the 2dFGRS.
Neighbours are counted within 1 Mpc projected distance, and
1000 km s depth. We apply a maximum magnitude difference criterion and
require neighbours to be fainter than the galaxy itself. We show that, whatever
the environment, passive galaxies dominate in bright samples and active-SF
galaxies in faint samples, whereas quiet-SF galaxies never dominate. We further
show that in bright samples (M -- 5 ) the fraction of passive galaxies grows steadily with fainter neighbour
density, whereas in faint samples a threshold like dependence is observed. This
suggests that the spectral-type / density ( morphology / density)
relation extends to intermediate dense environment, but only in the surrounding
of luminous galaxies and that it reflects an enhancement of the number of
satellites rather than stronger clustering among galaxies themselves. Our
analysis indicates that, in general, luminosity is a good tracer of a galaxy
halo mass and that it dominates over environment (satellite density) in setting
the spectral type mix of a population. However, minority populations exist,
such as luminous SF galaxies and faint passive galaxies, whose luminosity is an
unfair tracer of halo mass.Comment: 8 pages, 5 figur
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
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