27 research outputs found
Ground-based variability surveys towards Centaurus A: worthwhile or not?
Context: Difference imaging has proven to be a powerful technique for
detecting and monitoring the variability of unresolved stellar sources in M 31.
Using this technique in surveys of galaxies outside the Local Group could have
many interesting applications. Aims: The goal of this paper is to test
difference imaging photometry on Centaurus A, the nearest giant elliptical
galaxy, at a distance of 4 Mpc. Methods: We obtained deep photometric data with
the Wide Field Imager at the ESO/MPG 2.2m at La Silla spread over almost two
months. Applying the difference imaging photometry package DIFIMPHOT, we
produced high-quality difference images and detected variable sources. The
sensitivity of the current observational setup was determined through
artificial residual tests. Results: In the resulting high-quality difference
images, we detect 271 variable stars. We find a difference flux detection limit
corresponding to m_R~24.5. Based on a simple model of the halo of Centaurus A,
we estimate that a ground-based microlensing survey would detect in the order
of 4 microlensing events per year due to lenses in the halo. Conclusions:
Difference imaging photometry works very well at the distance of Centaurus A
and promises to be a useful tool for detecting and studying variable stars in
galaxies outside the local group. For microlensing surveys, a higher
sensitivity is needed than achieved here, which would be possible with a large
ground-based telescope or space observatory with wide-field imaging
capabilities.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Stellar populations of classical and pseudo-bulges for a sample of isolated spiral galaxies
In this paper we present the stellar population synthesis results for a
sample of 75 bulges in isolated spiral Sb-Sc galaxies, using the spectroscopic
data from the Sloan Digital Sky Survey and the STARLIGHT code. We find that
both pseudo-bulges and classical bulges in our sample are predominantly
composed of old stellar populations, with mean mass-weighted stellar age around
10 Gyr. While the stellar population of pseudo-bulges is, in general, younger
than that of classical bulges, the difference is not significant, which
indicates that it is hard to distinguish pseudo-bulges from classical bulges,
at least for these isolated galaxies, only based on their stellar populations.
Pseudo-bulges have star formation activities with relatively longer timescale
than classical bulges, indicating that secular evolution is more important in
this kind of systems. Our results also show that pseudo-bulges have a lower
stellar velocity dispersion than their classical counterparts, which suggests
that classical bulges are more dispersion-supported than pseudo-bulges.Comment: 10 pages, 8 figures. Accepted for publication in Astrophysics & Space
Scienc
GHASP : an H alpha kinematic survey of spiral and irregular galaxies. V. Dark matter distribution in 36 nearby spiral galaxies
The results obtained from a study of the mass distribution of 36 spiral
galaxies are presented. The galaxies were observed using Fabry-Perot
interferometry as part of the GHASP survey. The main aim of obtaining high
resolution H alpha 2D velocity fields is to define more accurately the rising
part of the rotation curves which should allow to better constrain the
parameters of the mass distribution. The H alpha velocities were combined with
low resolution HI data from the literature, when available. Combining the
kinematical data with photometric data, mass models were derived from these
rotation curves using two different functional forms for the halo: an
isothermal sphere and an NFW profile. For the galaxies already modeled by other
authors, the results tend to agree. Our results point at the existence of a
constant density core in the center of the dark matter halos rather than a
cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all
types the result already obtained by other authors studying dwarf and LSB
galaxies but would necessitate a larger sample of galaxies to conclude more
strongly. Whatever model is used (ISO or NFW), small core radius halos have
higher central densities, again for all morphological types. We confirm
different halo scaling laws, such as the correlations between the core radius
and the central density of the halo with the absolute magnitude of a galaxy:
low luminosity galaxies have small core radius and high central density. We
find that the product of the central density with the core radius of the dark
matter halo is nearly constant, whatever the model and whatever the absolute
magnitude of the galaxy. This suggests that the halo surface density is
independent from the galaxy type.Comment: 21 pages, 14 figures. MNRAS (accepted october 3rd 2007
Small-scale structure of the galactic cirrus emission
We examined the Fourier power spectrum characteristics of cirrus structures
in 13 sky fields with faint to bright cirrus emission observed with ISOPHOT in
the 90--200µm wavelength range in order to study variations of the
spectral index . We found that varies from field to field with
--5.3 --2.1. It depends on the absolute surface brightness
and on the hydrogen column density. We also found different spectral indices
for the same sky region at different wavelengths. Longer wavelength
measurements show steeper power spectra. This can be explained by the presence
of dust at various temperatures, in particular of a cold extended component.
For the faintest areas of the far-infrared sky we derive a wavelength
independent spectral index of = --2.30.6 for the cirrus power
spectrum. The application of the correct spectral index is a precondition for
the proper disentanglement of the cirrus foreground component of the Cosmic
Far-Infrared Background and its fluctuations.Comment: 10 pages; 7 figures; Astronomy & Astrophysics, accepte
Rotation Curves of Spiral Galaxies
Rotation curves of spiral galaxies are the major tool for determining the
distribution of mass in spiral galaxies. They provide fundamental information
for understanding the dynamics, evolution and formation of spiral galaxies. We
describe various methods to derive rotation curves, and review the results
obtained. We discuss the basic characteristics of observed rotation curves in
relation to various galaxy properties, such as Hubble type, structure,
activity, and environment.Comment: 40 pages, 6 gif figures; Ann. Rev. Astron. Astrophys. Vol. 39, p.137,
200
Building the cosmic infrared background brick by brick with Herschel/PEP
The cosmic infrared background (CIB) includes roughly half of the energy
radiated by all galaxies at all wavelengths across cosmic time, as observed at
the present epoch. The PACS Evolutionary Probe (PEP) survey is exploited here
to study the CIB and its redshift differential, at 70, 100 and 160 micron,
where the background peaks. Combining PACS observations of the GOODS-S,
GOODS-N, Lockman Hole and COSMOS areas, we define number counts spanning over
more than two orders of magnitude in flux: from ~1 mJy to few hundreds mJy.
Stacking of 24 micron sources and P(D) statistics extend the analysis down to
~0.2 mJy. Taking advantage of the wealth of ancillary data in PEP fields,
differential number counts and CIB are studied up to z=5. Based on these
counts, we discuss the effects of confusion on PACS blank field observations
and provide confusion limits for the three bands considered. The total CIB
surface brightness emitted above PEP 3 sigma flux limits is 4.52 +/- 1.18, 8.35
+/- 0.95 and 9.49 +/- 0.59 [nW/m2/sr] at 70, 100, and 160 micron, respectively.
These values correspond to 58 +/- 7% and 74 +/- 5% of the COBE/DIRBE CIB direct
measurements at 100 and 160 micron. Employing the P(D) analysis, these
fractions increase to ~65% and ~89%. More than half of the resolved CIB was
emitted at redshift z<=1. The 50%-light redshifts lie at z=0.58, 0.67 and 0.73
at the three PACS wavelengths. The distribution moves towards earlier epochs at
longer wavelengths: while the 70 micron CIB is mainly produced by z<=1.0
objects, the contribution of z>1.0 sources reaches 50% at 160 micron. Most of
the CIB resolved in the three PACS bands was emitted by galaxies with infrared
luminosities in the range 1e11-1e12 L(sun).Comment: Proposed for acceptance on A&
ISO's Contribution to the Study of Clusters of Galaxies
Starting with nearby galaxy clusters like Virgo and Coma, and continuing out
to the furthest galaxy clusters for which ISO results have yet been published
(), we discuss the development of knowledge of the infrared and
associated physical properties of galaxy clusters from early IRAS observations,
through the "ISO-era" to the present, in order to explore the status of ISO's
contribution to this field. Relevant IRAS and ISO programmes are reviewed,
addressing both the cluster galaxies and the still-very-limited evidence for an
infrared-emitting intra-cluster medium. ISO made important advances in
knowledge of both nearby and distant galaxy clusters, such as the discovery of
a major cold dust component in Virgo and Coma cluster galaxies, the elaboration
of the correlation between dust emission and Hubble-type, and the detection of
numerous Luminous Infrared Galaxies (LIRGs) in several distant clusters. These
and consequent achievements are underlined and described. We recall that, due
to observing time constraints, ISO's coverage of higher-redshift galaxy
clusters to the depths required to detect and study statistically significant
samples of cluster galaxies over a range of morphological types could not be
comprehensive and systematic, and such systematic coverage of distant clusters
will be an important achievement of the Spitzer Observatory.Comment: 22 pages, 7 figures. Accepted for publication in Space Science
Reviews ISO Special Issue "ISO science legacy - a compact review of ISO major
achievements", Ed.C.Cesarky & A.Salama Updated 23 Aug. 2005 in order to
change some citations from astro-ph nos. to full Journal references after
they were publishe
Spiral arms in near-infrared bands
We investigate the contribution of BrÎłand H2 emission due to young objects
in the arms of spiral galaxies observed in the filter. Out of a sample of
disk galaxies for which we obtained deep surface photometry in broad- and
narrow-band near-infrared filters, we selected two grand design spirals
(NGC 5861, NGC 7412), which clearly have sharp knots along their arms both in
optical and NIR images. For these galaxies we estimate the amount of light
coming from BrÎłand H2 emission and we conclude that it represents only a
few percent of the observed light.
For comparison we used the spiral galaxy NGC 4603, which has high
recessional velocity. In this case the emission lines we study are
practically shifted out of the narrow-band filter. Comparing its flux with
what we found in the two former cases, we conclude that a major
contribution from young objects in comes from continuum radiation
which in the arm regions can amount to 20%
Cosmic flows: University of Hawaii 2.2-m I-band photometry
Within the 'Cosmic Flows' project, I-band photometry of 524 relatively nearby galaxies has been carried out over the course of several years with the University of Hawaii 2.2-m telescope and a camera with a 7.5-arcmin field of view. The primary scientific goal was to provide global magnitudes and inclinations for galaxies for the purpose of measuring distances through the correlation between galaxy luminosities and rotation rates. The 1Ï accuracy on a total magnitude is 0.08 mag. The observations typically extend to 7-8 exponential disc scalelengths, so the data are useful for studies of the structural properties of galaxies