38 research outputs found
Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. III. SBS 1415+437
We present a detailed optical spectroscopic and B,V,I,Halpha photometric
study of the metal-deficient cometary blue compact dwarf (BCD) galaxy SBS
1415+437. We derive an oxygen abundance 12+log(O/H)=7.61+/-0.01 and 7.62+/-0.03
(Z =Zsun/20) in the two brightest H II regions, among the lowest in BCDs. The
helium mass fractions in these regions are Y=0.246+/-0.003 and 0.243+/-0.010.
Four techniques based on the equivalent widths of the hydrogen emission and
absorption lines, the spectral energy distribution and the colours of the
galaxy are used to put constraints on the age of the stellar population in the
low-surface-brightness (LSB) component of the galaxy, assuming two limiting
cases of star formation (SF), the case of an instantaneous burst and that of a
continuous SF with a constant or a variable star formation rate (SFR). The
spectroscopic and photometric data for different regions of the LSB component
are well reproduced by a young stellar population with an age t<250 Myr,
assuming a small extinction in the range A(V)= 0-0.6 mag. Assuming no
extinction, we find that the upper limit for the mass of the old stellar
population, formed between 2.5 Gyr and 10 Gyr, is not greater than ~(1/20 - 1)
of that of the stellar population formed during the last ~250 Myr. Depending on
the region considered, this also implies that the SFR in the most recent SF
period must be 20 to 1000 times greater than the SFR at ages > 2.5 Gyr. We
compare the photometric and spectroscopic properties of SBS 1415+437 with those
of a sample of 26 low-metallicity dwarf irregular and BCD galaxies. We show
that there is a clear trend for the stellar LSB component of lower-metallicity
galaxies to be bluer. This trend cannot be explained only by metallicity
effects. There must be also a change in the age of the stellar populations.Comment: 16 pages, 9 figures, Accepted for publication in A&
The primordial Helium-4 abundance determination: systematic effects
By extrapolating to O/H = N/H = 0 the empirical correlations Y-O/H and Y-N/H
defined by a relatively large sample of ~ 45 Blue Compact Dwarfs (BCDs), we
have obtained a primordial 4Helium mass fraction Yp= 0.2443+/-0.0015 with dY/dZ
= 2.4+/-1.0. This result is in excellent agreement with the average Yp=
0.2452+/-0.0015 determined in the two most metal-deficient BCDs known, I Zw 18
(Zsun/50) and SBS 0335-052 (Zsun/41), where the correction for He production is
smallest. The quoted error (1sigma) of < 1% is statistical and does not include
systematic effects. We examine various systematic effects including collisional
excitation of Hydrogen lines, ionization structure and temperature fluctuation
effects, and underlying stellar HeI absorption, and conclude that combining all
systematic effects, our Yp may be underestimated by ~ 2-4%. Taken at face
value, our Yp implies a baryon-to-photon number ratio eta = 4.7x10^-10 and a
baryon mass fraction Omega_b h^2_{100} = 0.017+/-0.005 (2sigma), consistent
with the values obtained from deuterium and Cosmic Microwave Background
measurements. Correcting Yp upward by 2-4% would make the agreement even
better.Comment: 12 pages, 5 PS figures, to appear in "Matter in the Universe", ed P.
Jetzer, K. Pretzl and R. von Steiger, Kluwer, Dordrecht (2002
Computability of simple games: A characterization and application to the core
The class of algorithmically computable simple games (i) includes the class
of games that have finite carriers and (ii) is included in the class of games
that have finite winning coalitions. This paper characterizes computable games,
strengthens the earlier result that computable games violate anonymity, and
gives examples showing that the above inclusions are strict. It also extends
Nakamura's theorem about the nonemptyness of the core and shows that computable
games have a finite Nakamura number, implying that the number of alternatives
that the players can deal with rationally is restricted.Comment: 35 pages; To appear in Journal of Mathematical Economics; Appendix
added, Propositions, Remarks, etc. are renumbere
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
Determining the initial helium abundance of the Sun
We determine the dependence of the initial helium abundance and the
present-day helium abundance in the convective envelope of solar models (\yim
and \ysm respectively) on the parameters that are used to construct the
models. We do so by using reference standard solar models to compute the
power-law coefficients of the dependence of \yim and \ysm on the input
parameters. We use these dependencies to determine the correlation between
\yim and \ysm and use this correlation to eliminate uncertainties in \yim
from all solar model input parameters except the microscopic diffusion rate. We
find an expression for \yim that depends only on \ysm and the diffusion
rate. By adopting the helioseismic determination of solar surface helium
abundance, \ysms= 0.2485\pm0.0035, and an uncertainty of 20% for the
diffusion rate, we find that the initial solar helium abundance, \yims, is
independently of the reference standard solar models (and
particularly on adopted the solar abundances) used in the derivation of the
correlation between \yim and \ysm. When non-standard solar models with
extra mixing are used, then we derive \yims = 0.273 \pm 0.006. In both cases,
the derived \yims value is higher than that directly derived from solar model
calibrations when the low metalicity solar abundances (e.g. by Asplund et al.)
are adopted in the models.Comment: 18 pages, including 3 figures and 2 tables. Accepted for publication
in Ap
Unveiling the nature of the "Green Pea" galaxies
We review recent results on the oxygen and nitrogen chemical abundances in
extremely compact, low-mass starburst galaxies at redshifts between 0.1-0.3
recently named to as "Green Pea" galaxies. These galaxies are genuine
metal-poor galaxies ( one fifth solar) with N/O ratios unusually high for
galaxies of the same metallicity. In combination with their known general
properties, i.e., size, stellar mass and star-formation rate, these findings
suggest that these objects could be experiencing a short and extreme phase in
their evolution. The possible action of both recent and massive inflow of gas,
as well as stellar feedback mechanisms are discussed here as main drivers of
the starburst activity and their oxygen and nitrogen abundances.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
Revisiting Delta Y/Delta Z from multiple main sequences in Globular Clusters: insight from nearby stars
For nearby K dwarfs, the broadening of the observed Main Sequence at low
metallicities is much narrower than expected from isochrones with the standard
helium-to-metal enrichment ratio DY/DZ=2. Though the latter value fits well the
Main Sequence around solar metallicity, and agrees with independent
measurements from HII regions as well as with theoretical stellar yields and
chemical evolution models, a much higher DY/DZ~10 is necessary to reproduce the
broadening observed for nearby subdwarfs. This result resembles, on a milder
scale, the very high DY/DZ estimated from the multiple Main Sequences in Omega
Cen and NGC 2808. Although not "inverted" as in omega Cen, where the metal-rich
Main Sequence is bluer than the metal-poor one, the broadening observed for
nearby subdwarfs is much narrower than stellar models predict for a standard
helium content. We use this empirical evidence to argue that a revision of
lower Main Sequence stellar models, suggested from nearby stars, could
significantly reduce the helium content inferred for the subpopulations of
those globular clusters. A simple formula based on empirically calibrated
homology relations is constructed, for an alternative estimate of DY/DZ in
multiple main sequences. We find that, under the most favourable assumptions,
the estimated helium content for the enriched populations could decrease from
Y~0.4 to as low as Y~0.3.Comment: 15 pages, 12 figures, in press on MNRA
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
Bino Dark Matter and Big Bang Nucleosynthesis in the Constrained E6SSM with Massless Inert Singlinos
We discuss a new variant of the E6 inspired supersymmetric standard model
(E6SSM) in which the two inert singlinos are exactly massless and the dark
matter candidate has a dominant bino component. A successful relic density is
achieved via a novel mechanism in which the bino scatters inelastically into
heavier inert Higgsinos during the time of thermal freeze-out. The two massless
inert singlinos contribute to the effective number of neutrino species at the
time of Big Bang Nucleosynthesis, where the precise contribution depends on the
mass of the Z' which keeps them in equilibrium. For example for mZ' > 1300 GeV
we find Neff \approx 3.2, where the smallness of the additional contribution is
due to entropy dilution. We study a few benchmark points in the constrained
E6SSM with massless inert singlinos to illustrate this new scenario.Comment: 24 pages, revised for publication in JHE
Searching for sterile neutrinos in ice
Oscillation interpretation of the results from the LSND, MiniBooNE and some
other experiments requires existence of sterile neutrino with mass eV
and mixing with the active neutrinos . It has
been realized some time ago that existence of such a neutrino affects
significantly the fluxes of atmospheric neutrinos in the TeV range which can be
tested by the IceCube Neutrino Observatory. In view of the first IceCube data
release we have revisited the oscillations of high energy atmospheric neutrinos
in the presence of one sterile neutrino. Properties of the oscillation
probabilities are studied in details for various mixing schemes both
analytically and numerically. The energy spectra and angular distributions of
the events have been computed for the simplest mass, and
mixing schemes and confronted with the IceCube data. An
illustrative statistical analysis of the present data shows that in the
mass mixing case the sterile neutrinos with parameters required by
LSND/MiniBooNE can be excluded at about level. The
mixing scheme, however, can not be ruled out with currently available IceCube
data.Comment: 41 pages, 16 figures. Accepted for publication in JHEP. Minor changes
from the previous versio