19 research outputs found
Oxygen abundances in low surface-brightness galaxies
Recent theories predict that some protogalaxies, in low-density environments of the field, are contracting and interacting so slowly that global star formation can be delayed until today. These systems should be gas rich and have low surface-brightness. Blue compact galaxies (BCG's) and other compact HII region galaxies currently experiencing a burst of star formation are good candidates of truly young galaxies (in the sense that global star formation recently has been initiated). If they really are young, they ought to have a recent phase when their brightness was much lower than in the bursting phase. No claims of observations of such proto-BCG's exist. Observations of galaxies in their juvenile phases would undoubtedly be of great interest, e.g. the determination of the primordial helium abundance would improve. A proper place to search for young nearby galaxies could be among blue low surface-brightness galaxies (BLSBG's) in the local field. The study of low surface-brightness galaxies (LSBG's) as a group began relatively recently. They are galaxies with extraordinary properties both as individuals and as a group. A few years ago we started an optical study of a sample of BLSBG's selected from the ESO/Uppsala catalogue. Results of spectroscopic observations obtained on a subsample - 8 galaxies - of our selection are reported. The HII region oxygen chemical abundances and its relation to the blue absolute magnitude and surface-brightness is investigated
Spitzer Observations of Low Luminosity Isolated and Low Surface Brightness Galaxies
We examine the infrared properties of five low surface brightness galaxies
(LSBGs) and compare them with related but higher surface brightness galaxies,
using Spitzer Space Telescope images and spectra. All the LSBGs are detected in
the 3.6 and 4.5um bands, representing the stellar population. All but one are
detected at 5.8 and 8.0um, revealing emission from hot dust and aromatic
molecules, though many are faint or point-like at these wavelengths. Detections
of LSBGs at the far-infrared wavelengths, 24, 70, and 160um, are varied in
morphology and brightness, with only two detections at 160um, resulting in
highly varied spectral energy distributions. Consistent with previous
expectations for these galaxies, we find that detectable dust components exist
for only some LSBGs, with the strength of dust emission dependent on the
existence of bright star forming regions. However, the far-infrared emission
may be relatively weak compared with normal star-forming galaxies.Comment: 20 pages, 8 figures, accepted to Ap
Low Surface Brightness Galaxies around the HDF-S: II. Distances and volume densities
With this study we aim at the spectroscopic verification of a photometrically
selected sample of Low Surface Brightness (LSB) galaxy candidates in a field
around the Hubble Deep Field-South (HDF-S). The sample helps to extend the
parameter space for LSB galaxies to lower central surface brightnesses and to
provide better estimates on the volume densities of these objects. To derive
redshifts for the LSB candidates, long-slit spectra were obtained covering a
spectral range from 3400{\AA} to 7500{\AA}. The observations have been obtained
using the ESO 3.6m telescope, equipped with the EFOSC2 spectrograph. From the
measured radial velocities, distances could be estimated. With this distance
information, it is possible to differentiate between true LSB galaxies and
higher redshift High Surface Brightness (HSB) galaxies which may contaminate
the sample. A correction for the surface brightnesses can then be applied,
accounting for the cosmological dimming effect (``Tolman Dimming''). We show
that ~70% of the LSB candidates, selected based on their location in the
color-color space, are real LSB galaxies. Their position in the color-color
diagrams, therefore, indicate that the LSB galaxies have a different stellar
population mix resulting from a different star formation history compared to
HSBs. Our LSB galaxy sample consists only of large disk galaxies with
scale-length between 2.5kpc and 7.3kpc. We confirm the flat central surface
brightness distribution of previous surveys and extend this distribution down
to central surface brightnesses of 27 B mag arcsec^-2.Comment: 12 pages, 20 figures, accepted by A&
Galactic Evolution along the Hubble Sequence
A generalization of the multiphase chemical evolution model applied to a wide
set of theoretical galaxies is shown. This set of models has been computed by
using the so-called Universal Rotation Curve from Persic, Salucci & Steel to
calculate the radial mass distributions of each theoretical galaxy. By assuming
that the molecular cloud and star formation efficiencies depend on the
morphological type of each galaxy, we construct a bi-parametric grid of models
whose results are valid in principle for any spiral galaxy, of given maximum
rotation velocity or total mass, and morphological type.Comment: Proceedings of the Euroconference "The Evolution of Galaxies. III..."
(Kiel 2002
Chemical evolution of starburst galaxies: How does star formation proceed?
We compute chemical evolution models to constrain the mode and the history of
star formation in starburst galaxies as a whole, i.e. over a large range of
mass and metallicity. To this end, we investigate the origin of the dispersion
observed in the evolution of both nitrogen-to-oxygen abundance ratio and galaxy
luminosity as a function of metallicity for a large sample of starburst
galaxies. We find that the variation of the star formation efficiency, in the
framework of continuous star formation models, produce a scatter equivalent to
what is observed in the N/O versus O/H diagram for low-mass HII galaxies only.
However, continous star formation models are unable to reproduce i) the scatter
observed for massive starburst and UV-selected galaxies in the N/O versus O/H
relation, and ii) the scatter in the luminosity versus O/H scaling relation
observed for the whole sample of starburst galaxies. The dispersion associated
with the distribution of N/O as a function of metallicity, for both low-mass
and massive galaxies, is well explained in the framework of bursting star
formation models. It is interpreted as a consequence of the time-delay between
the ejection of nitrogen and that of oxygen into the ISM. These models also
reproduce the spread observed in the luminosity-metallicity relation.
Metal-rich spiral galaxies differ from metal-poor ones by a higher star
formation efficiency and starburst frequency. Low-mass galaxies experienced a
few bursts of star formation whereas massive spiral galaxies experienced
numerous and extended powerful starbursts (abridged version).Comment: 9 pages, 4 figures. Accepted for publication in A&
Properties of Galaxies in and around Voids
Two surveys for intrinsically faint galaxies towards nearby voids have been
conducted at the MPI f\"ur Astronomie, Heidelberg. One selected targets from a
new diameter limited () catalog with morphological criteria while
the other used digitized objective prism Schmidt plates to select mainly HII
dwarf galaxies. For some 450 galaxies, redshifts and other optical data were
obtained. We studied the spatial distribution of the sample objects, their
luminosity function, and their intrinsic properties. Most of the galaxies
belong to already well known sheets and filaments. But we found about a dozen
highly isolated galaxies in each sample (nearest neighborhood distance ). These tend to populate additional structures and are not
distributed homogeneously throughout the voids. As our results on 'void
galaxies' still suffer from small sample statistics, I also tried to combine
similar existing surveys of nearby voids to get further hints on the larger
structure and on the luminosity function of the isolated galaxies. No
differences in the luminosity function of sheet and void galaxies could be
found. The optical and infrared properties of both samples are in the normal
range for samples dominated by late-type dwarfs. Follow-up HI studies show that
the isolated dwarfs in both samples have unusual high amount of neutral gas for
a given luminosity.Comment: 10 pages, 4 figures, latex, to appear in the proceedings of the
'Ringberg workshop on Large Scale Structure', hold Sep. 23-28, 199
A large sample of low surface brightness disc galaxies from the SDSS- II. Metallicities in surface brightness bins
We study the spectroscopic properties of a large sample of Low Surface
Brightness galaxies (LSBGs) (with B-band central surface brightness mu0(B)>22
mag arcsec^(-2)) selected from the Sloan Digital Sky Survey Data Release 4
(SDSS-DR4) main galaxy sample. A large sample of disk-dominated High Surface
Brightness galaxies (HSBGs, with mu0(B)<22 mag arcsec^(-2)) are also selected
for comparison simultaneously. To study them in more details, these sample
galaxies are further divided into four subgroups according to mu0(B) (in units
of mag arcsec^(-2)): vLSBGs (24.5-22.75),iLSBGs (22.75-22.0), iHSBGs
(22.0-21.25), and vHSBGs (<21.25). The diagnostic diagram from spectral
emission-line ratios shows that the AGN fractions of all the four subgroups are
small (<9%). The 21,032 star-forming galaxies with good quality spectroscopic
observations are further selected for studying their dust extinction,
strong-line ratios, metallicities and stellar mass-metallicities relations. The
vLSBGs have lower extinction values and have less metal-rich and massive
galaxies than the other subgroups. The oxygen abundances of our LSBGs are not
as low as those of the HII regions in LSBGs studied in literature, which could
be because our samples are more luminous, and because of the different
metallicity calibrations used. We find a correlation between 12+log(O/H) and
mu0(B) for vLSBGs, iLSBGs and iHSBGs but show that this could be a result of
correlation between mu0(B) and stellar mass and the well-known mass-metallicity
relation. This large sample shows that LSBGs span a wide range in metallicity
and stellar mass, and they lie nearly on the stellar mass vs. metallicity and
N/O vs. O/H relations of normal galaxies. This suggests that LSBGs and HSBGs
have not had dramatically different star formation and chemical enrichment
histories.Comment: 14 pages, 11 figures, accepted for publication in MNRA
Stellar populations in the nuclei of late-type spiral galaxies
(Abridged) As part of an ongoing effort to study the stellar nuclei of very
late-type, bulge-less spirals, we present results from a high-resolution
spectroscopic survey of nine such nuclear star clusters, undertaken with
VLT/UVES. We fit the spectra with population synthesis models and measure
Lick-type indices to determine mean luminosity-weighted ages, which range from
4.1*10^7 to 1.1*10^10 years and are insensitive to assumed metallicity or
internal extinction. The average metallicity of nuclear clusters in late-type
spirals is slightly sub-solar ( = 0.015) but shows significant scatter. The
fits also show that the nuclear cluster spectra are best described by a mix of
several generations of stars. This is supported by the fact that only models
with composite stellar populations yield mass-to-light ratios that match those
obtained from dynamical measurements. The last star formation episode was on
average 34 Myr ago, while all clusters experienced some star formation in the
last 100 Myr. We thus conclude that the nuclear clusters undergo repeated
episodes of star formation. The robustness with respect to possible
contamination from the underlying galaxy disk is demonstrated by comparison to
spectra obtained with HST/STIS. Combining these results with those from Walcher
et al. (2005), we have thus shown that the stellar nuclei of these bulge-less
galaxies are massive and dense star clusters that form stars recurrently until
the present day. This unique set of properties is likely due to the central
location of these clusters in their host galaxies.Comment: ApJ submitted (original submission Oct 19, 2005, present version
includes changes based on referee recommendations). 53 pages, 12 figures, 8
table
Metallicity Effects on Dust Properties in Starbursting Galaxies
We present infrared observations of 66 starburst galaxies over a wide range
of oxygen abundances, to measure how metallicity affects their dust properties.
The data include imaging and spectroscopy from the Spitzer Space Telescope,
supplemented by groundbased near-infrared imaging. We confirm a strong
correlation of aromatic emission with metallicity, with a threshold at a
metallicity [12+log(O/H)]~8. The large scatter in both the metallicity and
radiation hardness dependence of this behavior implies that it is not due to a
single effect, but to some combination. We show that the far-infrared color
temperature of the large dust grains increases towards lower metallicity,
peaking at a metallicity of 8 before turning over. We compute dust masses and
compare them to HI masses from the literature to derive the gas to dust ratio,
which increases by nearly 3 orders of magnitude between solar metallicity and a
metallicity of 8, below which it flattens out. The abrupt change in aromatic
emission at mid-infrared wavelengths thus appears to be reflected in the
far-infrared properties, indicating that metallicity changes affect the
composition of the full range of dust grain sizes that dominate the infrared
emission. In addition, we find that the ratio L(8 micron)/L(TIR), important for
calibrating 24 micron measurements of high redshift galaxies, increases
slightly as the metallicity decreases from ~solar to ~50% of solar, and then
decreases by an order of magnitude with further decreases in metallicity.
Although the great majority of galaxies show similar patterns of behavior as
described above, there are three exceptions, SBS 0335-052E, Haro 11, and SHOC
391. Their infrared SEDs are dominated energetically by the mid-IR near 24
micron rather than by the 60 - 200 micron region. (Abridged)Comment: 34 pages, 11 figures, accepted to Ap
Star forming dwarf galaxies
Star forming dwarf galaxies (SFDGs) have a high gas content and low
metallicities, reminiscent of the basic entities in hierarchical galaxy
formation scenarios. In the young universe they probably also played a major
role in the cosmic reionization. Their abundant presence in the local volume
and their youthful character make them ideal objects for detailed studies of
the initial stellar mass function (IMF), fundamental star formation processes
and its feedback to the interstellar medium. Occasionally we witness SFDGs
involved in extreme starbursts, giving rise to strongly elevated production of
super star clusters and global superwinds, mechanisms yet to be explored in
more detail. SFDGs is the initial state of all dwarf galaxies and the relation
to the environment provides us with a key to how different types of dwarf
galaxies are emerging. In this review we will put the emphasis on the exotic
starburst phase, as it seems less important for present day galaxy evolution
but perhaps fundamental in the initial phase of galaxy formation.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres