1,058 research outputs found
SPITZER IRS spectra of Virgo early type galaxies: detection of stellar silicate emission
We present high signal to noise ratio Spitzer Infrared Spectrograph
observations of 17 Virgo early-type galaxies. The galaxies were selected from
those that define the colour-magnitude relation of the cluster, with the aim of
detecting the silicate emission of their dusty, mass-losing evolved stars. To
flux calibrate these extended sources we have devised a new procedure that
allows us to obtain the intrinsic spectral energy distribution and to
disentangle resolved and unresolved emission within the same object. We have
found that thirteen objects of the sample (76%) are passively evolving galaxies
with a pronounced broad silicate feature which is spatially extended and likely
of stellar origin, in agreement with model predictions. The other 4 objects
(24%) are characterized by different levels of activity. In NGC 4486 (M 87) the
line emission and the broad silicate emission are evidently unresolved and,
given also the typical shape of the continuum, they likely originate in the
nuclear torus. NGC 4636 shows emission lines superimposed on extended (i.e.
stellar) silicate emission, thus pushing the percentage of galaxies with
silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by
polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central
unresolved region. A more detailed analysis of our sample, with updated models,
will be presented in a forthcoming paper.Comment: 6 pages; ApJ Letters, accepte
A spectroscopic study of NGC 6251 and its companion galaxies
Measurements of the velocities of galaxies thought to be associated with the
giant radio galaxy NGC 6251 confirm the presence of a poor cluster with a
systemic redshift of z= 0.0244 +/- 0.0004 and a line-of-sight velocity
dispersion of sigma_{z}= 283 (+109, -52) km/s. This suggests a cluster
atmosphere temperature of T = 0.7 (+0.6, -0.2) keV, which is not enough to
confine the radio jet by gas pressure. The core of NGC 6251 shows strong
emission lines of [O III] and H alpha + [N II], but there is no evidence for
line emission from the jet (detected in optical continuum by Keel (1988)).Comment: 7 pages, 2 figures, to be published in MNRA
Observations of the Hubble Deep Field with the Infrared Space Observatory. I. Data reduction, maps and sky coverage
We present deep imaging at 6.7 micron and 15 micron from the CAM instrument
on the Infrared Space Observatory (ISO), centred on the Hubble Deep Field
(HDF). These are the deepest integrations published to date at these
wavelengths in any region of sky. We discuss the observation strategy and the
data reduction. The observed source density appears to approach the CAM
confusion limit at 15 micron, and fluctuations in the 6.7 micron sky background
may be identifiable with similar spatial fluctuations in the HDF galaxy counts.
ISO appears to be detecting comparable field galaxy populations to the HDF, and
our data yields strong evidence that future IR missions (such as SIRTF, FIRST
and WIRE) as well as SCUBA and millimetre arrays will easily detect field
galaxies out to comparably high redshifts.Comment: 7 pages, LaTeX (using mn.sty), 9 figures included as GIFs. Gzipped
Postscipt version available from http://artemis.ph.ic.ac.uk/hdf/papers/ps/.
Further information on ISO-HDF project can be found at
http://artemis.ph.ic.ac.uk/hdf
Observations of the Hubble Deep Field with the Infrared Space Observatory V. Spectral energy distributions starburst models and star formation history
We have modelled the spectral energy distributions of the 13 Hubble Deep Field (HDF) galaxies reliably detected by the Infrared Space Observatoiy (ISO). For two galaxies the emission detected by ISO is consistent with being starlight or the infrared 'cirrus' in the galaxies. For the remaining 11 galaxies there is a clear mid-infrared excess, which we interpret as emission from dust associated with a strong starburst. 10 of these galaxies are spirals or interacting pairs, while the remaining one is an elliptical with a prominent nucleus and broad emission lines. We give a new discussion of how the star formation rate can be deduced from the far-infrared luminosity, and derive star formation rates for these galaxies of 8-1000ø M¿ yr-1, where ø takes account of the uncertainty in the initial mass function. The HDF galaxies detected by ISO are clearly forming stars at a prodigious rate compared with nearby normal galaxies. We discuss the implications of our detections for the history of star and heavy element formation in the Universe. Although uncertainties in the calibration, reliability of source detection, associations and starburst models remain, it is clear that dust plays an important role in star formation out to redshift 1 at least
Observational Mass-to-Light Ratio of Galaxy Systems: from Poor Groups to Rich Clusters
We study the mass-to-light ratio of galaxy systems from poor groups to rich
clusters, and present for the first time a large database for useful
comparisons with theoretical predictions. We extend a previous work, where B_j
band luminosities and optical virial masses were analyzed for a sample of 89
clusters. Here we also consider a sample of 52 more clusters, 36 poor clusters,
7 rich groups, and two catalogs, of about 500 groups each, recently identified
in the Nearby Optical Galaxy sample by using two different algorithms. We
obtain the blue luminosity and virial mass for all systems considered. We
devote a large effort to establishing the homogeneity of the resulting values,
as well as to considering comparable physical regions, i.e. those included
within the virial radius. By analyzing a fiducial, combined sample of 294
systems we find that the mass increases faster than the luminosity: the linear
fit gives M\propto L_B^{1.34 \pm 0.03}, with a tendency for a steeper increase
in the low--mass range. In agreement with the previous work, our present
results are superior owing to the much higher statistical significance and the
wider dynamical range covered (about 10^{12}-10^{15} M_solar). We present a
comparison between our results and the theoretical predictions on the relation
between M/L_B and halo mass, obtained by combining cosmological numerical
simulations and semianalytic modeling of galaxy formation.Comment: 25 pages, 12 eps figures, accepted for publication in Ap
The Properties of Poor Groups of Galaxies: I. Spectroscopic Survey and Results
We use multi-fiber spectroscopy of 12 nearby, poor groups of galaxies to
address whether the groups are bound systems or chance projections of galaxies
along the line-of-sight, why the members of each group have not already merged
to form a single galaxy, despite the groups' high galaxy densities, short
crossing times, and likely environments for galaxy-galaxy mergers, and how
galaxies might evolve in these groups, where the collisional effects of the
intra-group gas and the tidal influences of the global potential are weaker
than in rich clusters. We conclude the following. (1) The nine groups with
diffuse X-ray emission (cf. Paper II) are bound systems with at least 20-50
group members. (2) Galaxies in each X-ray-detected group have not all merged
together, because a significant fraction of the group mass lies outside of the
galaxies and in a common halo. (3) Unlike cD galaxies in some rich clusters,
the giant, brightest elliptical in each X-ray group lies in the center of the
group potential, suggesting that such galaxies may form first in poor groups.
(4) In some groups, the fraction and recent star formation histories of the
early types are consistent with those in rich clusters, suggesting that the
effects of cluster environment on these galaxies are relatively unimportant at
the current epoch. (Abridged)Comment: 58 pages, AASLaTeX with 9 figures. Table 1 available on request. To
appear with Paper II in Ap
Joint Cosmological Formation of QSOs and Bulge-dominated Galaxies
Older and more recent pieces of observational evidence suggest a strong
connection between QSOs and galaxies; in particular, the recently discovered
correlation between black hole and galactic bulge masses suggests that QSO
activity is directly connected to the formation of galactic bulges. The
cosmological problem of QSO formation is analyzed in the framework of an
analytical model for galaxy formation; for the first time a joint comparison
with galaxy and QSO observables is performed. In this model it is assumed that
the same physical variable which determines galaxy morphology is able to
modulate the mass of the black hole responsible for QSO activity. Both halo
spin and the occurence of a major merger are considered as candidates to this
role. The predictions of the model are compared to available data for the
type-dependent galaxy mass functions, the star-formation history of elliptical
galaxies, the QSO luminosity function and its evolution (including the obscured
objects contributing to the hard-X-ray background), the mass function of
dormant black holes and the distribution of black-hole -- bulge mass ratios. A
good agreement with observations is obtained if the halo spin modulates the
efficiency of black-hole formation, and if the galactic halos at have
shone in an inverted order with respect to the hierarchical one (i.e., stars
and black holes in bigger galactic halos have formed before those in smaller
ones). This inversion of hierarchical order for galaxy formation, which
reconciles galaxy formation with QSO evolution, is consistent with many pieces
of observational evidence.Comment: 20 pages, figures included, mn.sty, in press on MNRAS, fig 6 changed
(new data added at z=4.4
- …