71 research outputs found
Lifetime of nuclear velocity dispersion drops in barred galaxies
We have made hydro/N-body simulations with and without star formation to shed
some light on the conditions under which a central kinematically cold stellar
component (characterized by a velocity dispersion drop or \sigma-drop) could be
created in a hot medium (e.g. a bulge) and survive enough time to be observed.
We found that the timescale for a \sigma-drop formation could be short (less
than 500 Myr) whereas its lifetime could be long (more than 1 Gyr) provided
that the central region is continuously or regularly fed by fresh gas which
leads to a continuous star formation activity. Star formation in the central
region, even at a low rate as 1M_{sol} yr^{-1}, is mandatory to sustain a
permanent \sigma-drop by replacing heated particles by new low-\sigma ones. We
moreover show that as soon as star formation is switched off, the \sigma-drop
begins to disappear.Comment: 7 pages, 7 figures, accepted for publication in MNRA
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
The stellar mass to light ratio in the isolated spiral NGC 4414
We present high resolution CO(1-0) interferometric observations and deep HST
B-V-I images of the flocculent isolated Sc type spiral NGC 4414. The goal is to
determine the stellar mass-to-light (M/L) ratio in a galactic disk. The stars
are seen without a dust screen, the central gas mass is very low (undetected),
and we show that the dark matter is negligible in the central regions. We have
developed an axisymmetric analytical gravitational potential model to account
for the central light (mass) profile, the dynamics of the molecular gas in the
highly obscured molecular ring, and the stellar light profile outside the
highly obscured region. The contribution of dark matter is constrained by the
extremely extended HI rotation curve and is small, possibly negligible, at
distances less than 5 -- 7 kpc from the center. Furthermore, the M/L ratios we
derive are low, about 1.5 in I band and 0.5 in K' band. The B and V band M/L
ratios vary greatly due to absorption by dust, reaching 4 in the molecular ring
and decreasing to about 1.6 -- 1.8 at larger radii. This unequivocally shows
that models, like most maximum disk models, assuming constant M/L ratios in an
optical waveband, simply are not appropriate. We illustrate this by making mock
maximum disk models with a constant V band M/L ratio. The key is having the
central light distribution unobscured such that it can be used to trace the
mass. A primitive attempt to determine the intrinsic M/L ratio yields values
close to unity in the B,V, and I bands and slightly below 0.5 in K'.Comment: 12 pages, 10 figures, accepted in A&
FLAIR-II spectroscopy of two DENIS J-band galaxy samples
As a pilot survey for the forthcoming 6dF Galaxy Redshift Survey,
spectroscopy of galaxies selected in J (1.2 mu) with the DENIS survey was
performed at the UKST using the FLAIR II multi-object spectroscope. 69 galaxy
redshifts (z's) were obtained in a high |b| field and an additional 12 z's in a
low latitude (b=-17 deg), obscured field. This study illustrates the
feasibility of obtaining z's with optical spectra on galaxies selected at much
longer wavelengths. It validated a very preliminary algorithm for star/galaxy
separation for high |b| DENIS objects, with 99% reliability for J < 13.9.
However, the FLAIR II z determinations required substantially longer
integration times to achieve 90% completeness than expected from previous
optical surveys at comparable depth. This is mainly due to a degradation in
overall fibre throughput due to known problems with ageing of the
prism-cement-fibre interface with exposure to UV light. Our low |b| (high
extinction) field required 2.5 times more exposure time for less than 50% of
successful z measurements. Among the J <= 13.9 galaxies with measured z's, only
37+/-6% display emission lines, in comparison with 60% of emission-line
galaxies (ELGs) in optical samples of comparable depth. The ELGs are, on
average, 0.5 mag bluer in B-J than non-ELGs of the same luminosity. We confirm
a previous optically-based result that the fraction of ELGs increases rapidly
with decreasing galaxy luminosity. The J-band luminosity function is estimated.
Our high latitude field displays a concentration of galaxies at cz ~ 38000 km/s
suggesting a possible supercluster. A radial velocity is reported for a galaxy
lying near the projected centre of the Abell 1434 cluster of galaxies, for
which no cluster z is currently available.Comment: Accepted in PASA, 21 pages, 9 figure
Determination of the cosmic far-infrared background level with the ISOPHOT instrument
The cosmic infrared background (CIRB) consists mainly of the integrated light
of distant galaxies. In the far-infrared the current estimates of its surface
brightness are based on the measurements of the COBE satellite. Independent
confirmation of these results is still needed from other instruments. In this
paper we derive estimates of the far-infrared CIRB using measurements made with
the ISOPHOT instrument aboard the ISO satellite. The results are used to seek
further confirmation of the CIRB levels that have been derived by various
groups using the COBE data. We study three regions of very low cirrus emission.
The surface brightness observed with the ISOPHOT instrument at 90, 150, and 180
um is correlated with hydrogen 21 cm line data from the Effelsberg radio
telescope. Extrapolation to zero hydrogen column density gives an estimate for
the sum of extragalactic signal plus zodiacal light. The zodiacal light is
subtracted using ISOPHOT data at shorter wavelengths. Thus, the resulting
estimate of the far-infrared CIRB is based on ISO measurements alone. In the
range 150 to 180 um, we obtain a CIRB value of 1.08+-0.32+-0.30 MJy/sr quoting
statistical and systematic errors separately. In the 90 um band, we obtain a
2-sigma upper limit of 2.3 MJy/sr. The estimates derived from ISOPHOT
far-infrared maps are consistent with the earlier COBE results.Comment: Accepted for publication in A&A, 17 page
Long slit spectroscopy of a sample of isolated spirals with and without an AGN
We present the kinematical data obtained for a sample of active (Seyfert) and
non active isolated spiral galaxies, based on long slit spectra along several
position angles in the Halpha line region and, in some cases, in the Ca triplet
region as well. Gas velocity distributions are presented, together with a
simple circular rotation model that allows to determine the kinematical major
axes. Stellar velocity distributions are also shown. The main result is that
active and control galaxies seem to be equivalent in all kinematical aspects.
For both subsamples, the departure from pure circular rotation in some galaxies
can be explained by the presence of a bar and/or of a spiral arm. They also
present the same kind of peculiarities, in particular, S-shape structures are
quite common near the nuclear regions. They define very similar Tully-Fisher
relations. Emission line ratios are given for all the detected HII regions; the
analysis of the [NII]/Halpha metallicity indicator shows that active and
non-active galaxies have indistinguishable disk metallicities. These results
argue in favour of active and non-active isolated spiral galaxies having
essentially the same properties, in agreement with our previous results based
on the analysis of near infrared images. It appears now necessary to confirm
these results on a larger sample.Comment: 35 pages, 54 figures, Accepted for publication in Astronomy &
Astrophysics The full paper with its figures is available on the anonymous
account of ftp.iap.fr in /home/ftp/pub/from_users/durret/marquez.ps.gz (999
kb
The European Large Area ISO Survey IX: the 90 micron luminosity function from the Final Analysis sample
We present the 90 micron luminosity function of the Final Analysis of the
European Large Area ISO Survey (ELAIS), extending the sample size of our
previous analysis (paper IV) by about a factor of 4. Our sample extends to
z=1.1, around 50 times the comoving volume of paper IV, and 10^{7.7} <
h^{-2}L/Lsun < 10^{12.5}. From our optical spectroscopy campaigns of the
northern ELAIS 90 mircon survey (7.4 deg^2 in total, to S(90um)>70mJy), we
obtained redshifts for 61% of the sample (151 redshifts) to B<21 identified at
7 microns, 15 microns, 20cm or with bright (B<18.5) optical identifications.
The selection function is well-defined, permitting the construction of the 90
micron luminosity function of the Final Analysis catalogue in the ELAIS
northern fields, which is in excellent agreement with our Preliminary Analysis
luminosity function in the ELAIS S1 field from paper IV. The luminosity
function is also in good agreement with the IRAS-based prediction of Serjeant &
Harrison (2004), which if correct requires luminosity evolution of (1+z)^{3.4
+/- 1.0} for consistency with the source counts. This implies an evolution in
comoving volume averaged star formation rate at z<~1 consistent with that
derived from rest-frame optical and ultraviolet surveys.Comment: MNRAS accepted. 7 pages, 5 figures. Uses BoxedEPS (included
The detection of stellar velocity dispersion drops in the central regions of five isolated Seyfert spirals
We analyze the kinematics of the central regions of five isolated Seyfert
spiral galaxies from the DEGAS sample (four with new data presented in this
paper, IC184, UGC3223, NGC2639, NGC6814, and NGC6951 from our previous data),
by using long slit spectroscopy in the CaII triplet range (at ~ 8600 A)
obtained with a 4m-class telescope. A drop of the velocity dispersions in the
innermost 1-3 arcsec is observed in four of them, and hinted in the remaining
galaxy (NGC6814). The available HST images for our sample together with another
nine galaxies with reported velocity dispersion drops, are also used to
investigate the presence of morphological inner structures at the scales of the
kinematical drops. Evidence for disk-like shapes is found in 12 out of the 14
cases. The only exceptions are NGC6814 and NGC6951. Existing N-body simulations
including stars, gas and star formation predict that such a drop is most
probably due to a young stellar population born from dynamically cold gas
accreted in a circumnuclear disk formed during an episode of central gas
accretion driven by a bar. The equivalent widths of the Calcium triplet lines
for our 5 galaxies have been measured. Even if the profiles could be formally
consistent with constant EW(CaT) values, they seem to indicate the presence of
a local maximum in the regions corresponding spatially to the drops; if
confirmed, this would imply the presence of a different stellar population,
whose properties could help constraining the models.Comment: A&A accepted for publicatio
Dark and luminous matter in the NGC 3992 group of galaxies, I. The large barred spiral NGC 3992
Detailed neutral hydrogen observations have been obtained of the large barred
spiral galaxy NGC 3992 and its three small companion galaxies, UGC 6923, UGC
6940, and UGC 6969. For the main galaxy, the HI distribution is regular with a
low level radial extension outside the stellar disc. However, at exactly the
region of the bar, there is a pronounced central HI hole in the gas
distribution. Likely gas has been transported inwards by the bar and because of
the emptyness of the hole no large accretion events can have happened in recent
galactic times. The gas kinematics is very regular and it is demonstrated that
the influence of the bar potential on the velocity field is negligible. A
precise and extended rotation curve has been derived showing some distinct
features which can be explained by the non-exponential radial light
distribution of NGC 3992. The decomposition of the rotation curve gives a
slight preference for a sub maximal disc, though a range of disc contributions,
up to a maximum disc situation fits nearly equally well. For such a maximum
disc contribution, which might be expected in order to generate and maintain
the bar, the required mass-to-light ratio is large but not exceptional.Comment: accepted for publication in Astronomy and Astrophysics. A copy with
high resolution graphics will shortly become available at
http://www.astro.rug.nl/Preprints/preprints.htm
Near-infrared spectroscopy of stellar populations in nearby spiral galaxies
We present high spatial resolution, medium spectral resolution near-infrared
(NIR) H- and K-band long-slit spectroscopy for a sample of 29 nearby (z < 0.01)
inactive spiral galaxies, to study the composition of their NIR stellar
populations. These spectra contain a wealth of diagnostic stellar absorption
lines, e.g. MgI 1.575 micron, SiI 1.588 micron, CO (6-3) 1.619 micron, MgI
1.711 micron, NaI 2.207 micron, CaI 2.263 micron and the 12CO and 13CO
bandheads longward of 2.29 micron. We use NIR absorption features to study the
stellar population and star formation properties of the spiral galaxies along
the Hubble sequence, and we produce the first high spatial resolution NIR
HK-band template spectra for low redshift spiral galaxies along the Hubble
sequence. These templates will find applications in a variety of galaxy
studies. The strength of the absorption lines depends on the luminosity and/or
temperature of stars and, therefore, spectral indices can be used to trace the
stellar population of galaxies. The entire sample testifies that the evolved
red stars completely dominate the NIR spectra, and that the hot young star
contribution is virtually nonexistent.Comment: 13 pages, 5 figures. Accepted to MNRAS. arXiv admin note: text
overlap with arXiv:astro-ph/040313
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