947 research outputs found
Access to Stellar Population Models in the Virtual Observatory
A great effort is being made by the international Virtual Observatory
community to build tools ready to be used by scientists. Presently, providing
access to theoretical spectra in general, and synthetic spectra of galaxies in
particular, is a matter of current interest in the Virtual Observatory. Several
ways of accessing such spectra are available. We present two of them for
accessing PEGASE.HR evolutionary synthesis models: HTTP-access to a limited
number of parameters using Simple Spectral Access Protocol (SSAP), and
full-featured WEB-service based access using Common Execution Architecture
(CEA).Comment: 2 pages, 1 figure, to appear in the proceedings of IAU Symposium 241
(Stellar Populations as Building Blocks of Galaxies
SDSSJ150634.27+013331.6: the second compact elliptical galaxy in the NGC5846 group
We report the discovery of the second compact elliptical (cE) galaxy
SDSSJ150634.27+013331.6 in the nearby NGC5846 group by the Virtual Observatory
(VO) workflow . This object (M_B = -15.98 mag, R_e = 0.24 kpc) becomes the
fifth cE where the spatially-resolved kinematics and stellar populations can be
obtained. We used archival HST WFPC2 images to demonstrate that its light
profile has a two-component structure, and integrated photometry from GALEX,
SDSS, UKIDSS, and Spitzer to build the multi-wavelength SED to constraint the
star formation history (SFH). We observed this galaxy with the PMAS IFU
spectrograph at the Calar-Alto 3.5m telescope and obtained two-dimensional maps
of its kinematics and stellar population properties using the full-spectral
fitting technique. Its structural, dynamical and stellar population properties
suggest that it had a massive progenitor heavily tidally stripped by NGC5846.Comment: 5 pages, 4 figure, 1 table. Accepted to MNRAS Letter
Analytical approximations of K-corrections in optical and near-infrared bands
To compare photometric properties of galaxies at different redshifts, the
fluxes need to be corrected for the changes of effective rest-frame wavelengths
of filter bandpasses, called K-corrections. Usual approaches to compute them
are based on the template fitting of observed spectral energy distributions
(SED) and, thus, require multi-colour photometry. Here, we demonstrate that, in
cases of widely used optical and near-infrared filters, K-corrections can be
precisely approximated as two-dimensional low-order polynomials of only two
parameters: redshift and one observed colour. With this minimalist approach, we
present the polynomial fitting functions for K-corrections in SDSS ugriz, UKIRT
WFCAM YJHK, Johnson-Cousins UBVR_cI_c, and 2MASS JHK_s bands for galaxies at
redshifts Z<0.5 based on empirically-computed values obtained by fitting
combined optical-NIR SEDs of a set of 10^5 galaxies constructed from SDSS DR7
and UKIDSS DR5 photometry using the Virtual Observatory. For luminous red
galaxies we provide K-corrections as functions of their redshifts only. In two
filters, g and r, we validate our solutions by computing K-corrections directly
from SDSS DR7 spectra. We also present a K-corrections calculator, a web-based
service for computing K-corrections on-line.Comment: 13 pages, 7 figures, 25 tables. Accepted for publication in MNRAS.
This version contains low-resolution figures. The "K-corrections calculator"
service is available at http://kcor.sai.msu.ru
Dynamical versus Stellar Masses of Ultracompact Dwarf Galaxies in the Fornax Cluster
The origin of ultracompact dwarf (UCD) galaxies, compact extragalactic
stellar systems, is still a puzzle for present galaxy formation models. We
present the comprehensive analysis of high resolution multi-object
spectroscopic data for a sample of 24 Fornax cluster UCDs obtained with VLT
FLAMES. It comprises previously published data for 19 objects (Mieske et al.
2008) which we re-analysed, including 13 with available HST photometric data.
Using Virtual Observatory technologies we found archival HST images for two
more UCDs and then determined their structural properties. For all objects we
derived internal velocity dispersions, stellar population parameters, and
stellar mass-to-light ratios (M/L)* by fitting individual simple stellar
population (SSP) synthetic spectra convolved with a Gaussian against the
observed spectra using the NBursts full spectral fitting technique. For 14
objects we estimated dynamical masses suggesting no dark matter (DM) in 12 of
them and no more than 40 per cent DM mass fraction in the remaining two, in
contrast to findings for several UCDs in the Virgo cluster. Some Fornax UCDs
even have too high values of (M/L)* estimated using the Kroupa stellar initial
mass function (IMF) resulting in negative formally computed DM mass fractions.
The objects with too high (M/L)* ratios compared to the dynamical ones have
relatively short dynamical relaxation timescales, close to the Hubble time or
below. We therefore suggest that their lower dynamical ratios (M/L)dyn are
caused by low-mass star depletion due to dynamical evolution. Overall, the
observed UCD characteristics suggest at least two formation channels: tidal
threshing of nucleated dwarf galaxies for massive UCDs (~10^8 M_sun), and a
classical scenario of red globular cluster formation for lower-mass UCDs (<
10^7 M_sun).Comment: Accepted for publication in MNRAS; 13 pages, 9 figures, 2 table
A universal ultraviolet-optical colour-colour-magnitude relation of galaxies
Although the optical colour-magnitude diagram of galaxies allows one to
select red sequence objects, neither can it be used for galaxy classification
without additional observational data such as spectra or high-resolution
images, nor to identify blue galaxies at unknown redshifts. We show that adding
the near ultraviolet colour to the optical CMD reveals a tight relation in the
three-dimensional colour-colour-magnitude space smoothly continuing from the
"blue cloud" to the "red sequence". We found that 98 per cent of 225,000
low-redshift (Z<0.27) galaxies follow a smooth surface g-r=F(M,NUV-r) with a
standard deviation of 0.03-0.07 mag making it the tightest known galaxy
photometric relation. There is a strong correlation between morphological types
and integrated NUV-r colours. Rare galaxy classes such as E+A or tidally
stripped systems become outliers that occupy distinct regions in the 3D
parameter space. Using stellar population models for galaxies with different
SFHs, we show that (a) the (NUV-r, g-r) distribution is formed by objects
having constant and exponentially declining SFR with different characteristic
timescales; (b) colour evolution for exponentially declining models goes along
the relation suggesting its weak evolution up-to a redshift of 0.9; (c)
galaxies with truncated SFHs have very short transition phase offset from the
relation thus explaining the rareness of E+A galaxies. This relation can be
used as a powerful galaxy classification tool when morphology remains
unresolved. Its mathematical consequence is the photometric redshift estimates
from 3 broad-band photometric points. This approach works better than most
existing photometric redshift techniques applied to multi-colour datasets.
Therefore, the relation can be used as an efficient selection technique for
galaxies at intermediate redshifts (0.3<Z<0.8) using optical imaging surveys.Comment: 15 pages, 10 figures, accepted to MNRAS. This is an updated version
that addresses referee's remarks. All relations have been recomputed using
Petrosian magnitudes. The best-fitting relations in the electronic form are
available at the project web-page: http://specphot.sai.msu.ru/galaxies
Virtual Observatory based identification of AX J194939+2631 as a new cataclysmic variable
We report the discovery of a new cataclysmic variable (CV) among unidentified
objects from the ASCA Galactic Plane Survey made using the Virtual Observatory
data mining. First, we identified AX J194939+2631 with IPHAS
J194938.39+263149.2, the only prominent H-alpha emitter among 400 sources in a
1 arcmin field of the IPHAS survey, then secured as a single faint X-ray source
found in an archival Chandra dataset. Spectroscopic follow-up with the 3.5-m
Calar Alto telescope confirmed its classification as a CV, possibly of magnetic
nature. Our analysis suggests that AX J194939+2631 is a medium distance system
(d ~ 0.6 kpc) containing a late-K or early-M type dwarf as a secondary
component and a partially disrupted accretion disc revealed by the
double-peaked H-alpha line. However, additional deep observations are needed to
confirm our tentative classification of this object as an intermediate polar.Comment: 5 pages, 5 figures, 2 tables, accepted to Astronomy and Astrophysic
The chemical composition of Ultracompact Dwarf Galaxies in the Virgo and Fornax Clusters
We present spectroscopic observations of ultra compact dwarf (UCD) galaxies
in the Fornax and Virgo Clusters made to measure and compare their stellar
populations. The spectra were obtained on the Gemini-North (Virgo) and
Gemini-South (Fornax) Telescopes using the respective Gemini Multi-Object
Spectrographs.
We estimated the ages, metallicities and abundances of the objects from mea-
surements of Lick line-strength indices in the spectra; we also estimated the
ages and metallicities independently using a direct spectral fitting technique.
Both methods re- vealed that the UCDs are old (mean age 10.8 \pm 0.7 Gyr) and
(generally) metal-rich (mean [Fe/H] = -0.8 \pm 0.1). The alpha-element
abundances of the objects measured from the Lick indices are super-Solar.
We used these measurements to test the hypothesis that UCDs are formed by the
tidal disruption of present-day nucleated dwarf elliptical galaxies. The data
are not consistent with this hypothesis because both the ages and abundances
are significantly higher than those of observed dwarf galaxy nuclei (this does
not exclude disruption of an earlier generation of dwarf galaxies). They are
more consistent with the properties of globular star clusters, although at
higher mean metallicity. The UCDs display a very wide range of metallicity
(-1.7 <[Fe/H]< 0.0), spanning the full range of both globular clusters and
dwarf galaxy nuclei.
We confirm previous reports that most UCDs have high metalliticities for
their luminosities, lying significantly above the canonical
metallicitiy-luminosity relation followed by early-type galaxies. In contrast
to previous work we find that there is no significant difference in either the
mean ages or the mean metallicities of the Virgo and Fornax UCD populations.Comment: 15 pages (including references and appendix), 8 figures (including
appendix
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