400 research outputs found
Pairwise dwarf galaxy formation and galaxy downsizing: some clues from extremely metal-poor Blue Compact Dwarf galaxies
Some of the extremely metal-poor Blue Compact Dwarf galaxies (XBCDs) in the
nearby universe form galaxy pairs with remarkably similar properties. This fact
points to an intriguing degree of synchronicity in the formation history of
these binary dwarf galaxies and raises the question as to whether some of them
form and co-evolve pairwise (or in loose galaxy groups), experiencing recurrent
mild interactions and minor tidally induced star formation episodes throughout
their evolution. We argue that this hypothesis offers a promising conceptual
framework for the exploration of the retarded previous evolution and recent
dominant formation phase of XBCDs.Comment: To appear in the proceedings of the JENAM 2010 Symposium "Dwarf
Galaxies: Keys to Galaxy Formation and Evolution" (Lisbon, 9-10 September
2010), P. Papaderos, S. Recchi, G. Hensler (eds.), Springer Verlag (2011), in
pres
The continuous rise of bulges out of galactic disks
(abridged) This study revolves around dmB, a new distance- and
extinction-independent measure of the contribution by stellar populations older
than 9 Gyr to the mean r-band surface brightness of the bulge component in 135
late-type galaxies (LTGs) from the CALIFA survey, spanning a range of 2.6 dex
and 3 dex in total and bulge stellar mass (M*T~10^(8.9-11.5) M_solar and
M*B~10^(8.3-11.3) M_solar, respectively). The main insight from this study is
that LTG bulges form a continuous sequence of increasing dmB with increasing
M*T, M*B, stellar mass surface density S* and mass-weighted age and
metallicity: high-dmB bulges are the oldest, densest and most massive ones, and
vice versa. Furthermore, we find that the bulge-to-disk age and metallicity
contrast, as well as the bulge-to-disk mass ratio increase with M*T, raising
from, respectively, ~0 Gyr, 0 dex and 0.25 to ~3 Gyr, ~0.3 dex and 0.67 across
the mass range covered by our sample. Whereas gas excitation in lower-mass
bulges is invariably dominated by star formation (SF), LINER- and
Seyfert-specific emission-line ratios were exclusively documented in high-mass,
high-S* bulges. The continuity both in the properties of LTG bulges themselves
and in their age and metallicity contrast to their parent disks suggests that
these components evolve alongside in a concurrent process that leads to a
continuum of physical and evolutionary characteristics. Our results are
consistent with a picture where bulge growth in LTGs is driven by a
superposition of quick-early and slow-secular processes, the relative
importance of which increases with M*T. These processes, which presumably
combine in situ SF in the bulge and inward migration of material from the disk,
are expected to lead to a non-homologous radial growth of S* and a trend for an
increasing Sersic index with increasing galaxy mass.Comment: 24 pages, accepted for publication in A&
Fitting Analysis using Differential Evolution Optimization (FADO): Spectral population synthesis through genetic optimization under self-consistency boundary conditions
The goal of population spectral synthesis (PSS) is to decipher from the
spectrum of a galaxy the mass, age and metallicity of its constituent stellar
populations. This technique has been established as a fundamental tool in
extragalactic research. It has been extensively applied to large spectroscopic
data sets, notably the SDSS, leading to important insights into the galaxy
assembly history. However, despite significant improvements over the past
decade, all current PSS codes suffer from two major deficiencies that inhibit
us from gaining sharp insights into the star-formation history (SFH) of
galaxies and potentially introduce substantial biases in studies of their
physical properties (e.g., stellar mass, mass-weighted stellar age and specific
star formation rate). These are i) the neglect of nebular emission in spectral
fits, consequently, ii) the lack of a mechanism that ensures consistency
between the best-fitting SFH and the observed nebular emission characteristics
of a star-forming (SF) galaxy. In this article, we present FADO (Fitting
Analysis using Differential evolution Optimization): a conceptually novel,
publicly available PSS tool with the distinctive capability of permitting
identification of the SFH that reproduces the observed nebular characteristics
of a SF galaxy. This so-far unique self-consistency concept allows us to
significantly alleviate degeneracies in current spectral synthesis. The
innovative character of FADO is further augmented by its mathematical
foundation: FADO is the first PSS code employing genetic differential evolution
optimization. This, in conjunction with other unique elements in its
mathematical concept (e.g., optimization of the spectral library using
artificial intelligence, convergence test, quasi-parallelization) results in
key improvements with respect to computational efficiency and uniqueness of the
best-fitting SFHs.Comment: 25 pages, 12 figures, A&A accepte
Impact of an AGN featureless continuum on estimation of stellar population properties
The effect of the featureless power-law (PL) continuum of an active galactic
nucleus (AGN) on the estimation of physical properties of galaxies with optical
population spectral synthesis (PSS) remains largely unknown. With this in mind,
we fit synthetic galaxy spectra representing a wide range of galaxy star
formation histories (SFHs) and including distinct PL contributions of the form
with the PSS code STARLIGHT to study to which
extent various inferred quantities (e.g. stellar mass, mean age, and mean
metallicity) match the input. The synthetic spectral energy distributions
(SEDs) computed with our evolutionary spectral synthesis code include an AGN PL
component with and a fractional contribution to the monochromatic flux at 4020 \AA. At the
empirical AGN detection threshold that we
previously inferred in a pilot study on this subject, our results show that the
neglect of a PL component in spectral fitting can lead to an overestimation by
2 dex in stellar mass and by up to 1 and 4 dex in the light-
and mass-weighted mean stellar age, respectively, whereas the light- and
mass-weighted mean stellar metallicity are underestimated by up to 0.3
and 0.6 dex, respectively. Other fitting set-ups including either a
single PL or multiple PLs in the base reveal, on average, much lower
unsystematic uncertainties of the order of those typically found when fitting
purely stellar SEDs with stellar templates, however, reaching locally up to
1, 3 and 0.4 dex in mass, age and metallicity, respectively. Our results
underscore the importance of an accurate modelling of the AGN spectral
contribution in PSS fits as a minimum requirement for the recovery of the
physical and evolutionary properties of stellar populations in active galaxies.Comment: 18 pages, 22 figures, accepted for publication in A&
Chandra Observations of the Three Most Metal-Deficient Blue Compact Dwarf Galaxies known in the Local Universe, SBS 0335-052, SBS 0335-052W, and I Zw 18
We present an X-ray study of the three most metal-deficient blue compact
dwarf (BCD) galaxies known in the local Universe, based on deep Chandra
observations of SBS 0335-052 (0.025 solar abundance), SBS 0335-052W (0.02 solar
abundance) and I Zw 18 (0.02 solar abundance). All three are detected, with
more than 90% of their X-ray emission arising from point-like sources. The
0.5-10.0 keV luminosities of these point sources are in the range
(1.3-8.5)x1e39 erg/s. We interpret them to be single or a collection of
high-mass X-ray binaries, the luminosities of which may have been enhanced by
the low metallicity of the gas. There are hints of faint extended diffuse X-ray
emission in both SBS 0335-052 and I Zw 18, probably associated with the
superbubbles visible in both BCDs. The spectrum of I Zw 18 shows a OVIII
hydrogen-like emission line. The best spectral fit gives an O overabundance of
the gas in the X-ray point source by a factor of ~7 with respect to the Sun, or
a factor of ~350 with respect to the O abundance determined for the HII region.Comment: emulateapj.cls used, 7 pages, 7 figures + 1 table, accepted for
publication in Ap
An investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies
(abridged) We present 8.2m VLT spectroscopic observations of 28 HII regions
in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations
of 38 HII regions in 28 emission-line galaxies. These emission-line galaxies
were selected mainly from the Data Release 6 (DR6) of the Sloan Digital Sky
Survey (SDSS) as metal-deficient galaxy candidates. We collect photometric and
high-quality spectroscopic data for a large uniform sample of star forming
galaxies including new observations. Our aim is to study the
luminosity-metallicity (L-Z) relation for nearby galaxies, especially at its
low-metallicity end and compare it with that for higher-redshift galaxies. From
our new observations we find that the oxygen abundance in 61 out of the 66 HII
regions of our sample ranges from 12+logO/H=7.05 to 8.22. Our sample includes
27 new galaxies with 12+logO/H<7.6 which qualify as extremely metal-poor
star-forming galaxies (XBCDs). Among them are 10 HII regions with
12+logO/H<7.3. The new sample is combined with a further 93 low-metallicity
galaxies with accurate oxygen abundance determinations from our previous
studies, yielding in total a high-quality spectroscopic data set of 154 HII
regions. 9000 more galaxies with oxygen abundances, based mainly on the
Te-method, are compiled from the SDSS. Our data set spans a range of 8 mag with
respect to its absolute magnitude in SDSS g (-12>Mg>-20) and nearly 2 dex in
its oxygen abundance (7.0<12+logO/H<8.8), allowing us to probe the L-Z relation
in the nearby universe down to the lowest currently studied metallicity level.
The L-Z relation established on the basis of the present sample is consistent
with previous ones obtained for emission-line galaxies.Comment: 27 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
Luminous Compact Blue Galaxies up to z~1 in the HST Ultra Deep Field: I. Small galaxies, or blue centers of massive disks?
We analyze 26 Luminous Compact Blue Galaxies (LCBGs) in the HST/ACS Ultra
Deep Field (UDF) at z ~ 0.2-1.3, to determine whether these are truly small
galaxies, or rather bright central starbursts within existing or forming large
disk galaxies. Surface brightness profiles from UDF images reach fainter than
rest-frame 26.5 B mag/arcsec^2 even for compact objects at z~1. Most LCBGs show
a smaller, brighter component that is likely star-forming, and an extended,
roughly exponential component with colors suggesting stellar ages >~ 100 Myr to
few Gyr. Scale lengths of the extended components are mostly >~ 2 kpc, >1.5-2
times smaller than those of nearby large disk galaxies like the Milky Way.
Larger, very low surface brightness disks can be excluded down to faint
rest-frame surface brightnesses (>~ 26 B mag/arcsec^2). However, 1 or 2 of the
LCBGs are large, disk-like galaxies that meet LCBG selection criteria due to a
bright central nucleus, possibly a forming bulge. These results indicate that
>~ 90% of high-z LCBGs are small galaxies that will evolve into small disk
galaxies, and low mass spheroidal or irregular galaxies in the local Universe,
assuming passive evolution and no significant disk growth. The data do not
reveal signs of disk formation around small, HII-galaxy-like LCBGs, and do not
suggest a simple inside-out growth scenario for larger LCBGs with a disk-like
morphology. Irregular blue emission in distant LCBGs is relatively extended,
suggesting that nebular emission lines from star-forming regions sample a major
fraction of an LCBG's velocity field.Comment: 11 pages, 2 figures, AASTeX; accepted for publication in
Astrophysical Journal Letter
SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies
(abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations
of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS
0335-052W. Our aim is to derive element abundances in different HII regions of
this unique system of galaxies and to study spatial abundance variations. We
determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon
and iron. The oxygen abundance in the brighter eastern galaxy varies in the
range 7.11 to 7.32 in different HII regions supporting previous findings and
suggesting the presence of oxygen abundance variations on spatial scales of
~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is
7.22+/-0.07, consistent with previous determinations.Three other HII regions
are much more metal-poor with an unprecedently low oxygen abundance of
12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14
(region No.4). These are the lowest oxygen abundances ever derived in
emission-line galaxies. Helium abundances derived for the brightest HII regions
of both galaxies are mutually consistent. We derive weighted mean He mass
fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI
emissivities. The N/O abundance ratio in both galaxies is slightly higher than
that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in
extremely metal-deficient galaxies could increase with decreasing metallicity.Comment: 20 pages, 11 figures, accepted for pulication in Astronomy and
Astrophysic
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