453 research outputs found
Disentangling age and metallicity in distant unresolved stellar systems
We present some results of an observational and theoretical study on
unresolved stellar systems based on the Surface Brightness Fluctuations (SBF)
technique. It is shown that SBF magnitudes are a valuable tracer of stellar
population properties, and a reliable distance indicator. SBF magnitudes,
SBF-colors, and SBF-gradients can help to constrain within relatively narrow
limits the metallicity and age of the dominant stellar component in distant
stellar systems, especially if coupled with other spectro-photometric
indicators.Comment: A contributed paper to the Cefalu' (Italy) "Probing Stellar
Populations out to the Distant Universe", 4 pages. To appear as an AIP
Conference Proceedin
Ellsworth, Fair Ellsworth
https://digitalcommons.library.umaine.edu/mmb-me/1541/thumbnail.jp
Detection of Radial Surface Brightness Fluctuation and Color Gradients in elliptical galaxies with ACS
We study surface brightness fluctuations (SBF) in a sample of 8 elliptical
galaxies using Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) data
drawn from the Hubble Space Telescope (HST) archive. SBF magnitudes in the
F814W bandpass, and galaxy colors from F814W, F435W, and F606W images -- when
available -- are presented. Galaxy surface brightness profiles are determined
as well. We present the first SBF--broadband color calibration for the ACS/WFC
F814W bandpass, and (relative) distance moduli estimates for 7 of our galaxies.
We detect and study in detail the SBF variations within individual galaxies
as a probe of possible changes in the underlying stellar populations.
Inspecting both the SBF and color gradients in comparison to model predictions,
we argue that SBF, and SBF-gradients, can in principle be used for unraveling
the different evolutionary paths taken by galaxies, though a more comprehensive
study of this issue would be required. We confirm that the radial variation of
galaxy stellar population properties should be mainly connected to the presence
of radial chemical abundance gradients, with the outer galaxy regions being
more metal poor than the inner ones.Comment: 47 pages, 13 figures, ApJ, accepte
Detection of Surface Brightness Fluctuations in Elliptical Galaxies imaged with the Advanced Camera for Surveys. B- and I-band measurements
Taking advantage of the exceptional capabilities of ACS on board of HST, we
derive Surface Brightness Fluctuation (SBF) measurements in the B and I bands
from images of six elliptical galaxies with . Given the
low S/N ratio of the SBF signal in the blue band images, the reliability of the
measurements is verified both with numerical simulations and experimental data
tests.
This paper presents the first published B- and I-band SBF measurements for
distant ( 20 Mpc) galaxies, essential for the comparisons of the models
to observations of normal ellipticals. By comparing I-band data with our new
Simple Stellar Population (SSP) models we find an excellent agreement and we
confirm that I-band SBF magnitudes are mainly sensitive to the metallicity of
the dominant stellar component in the galaxy, and are not strongly affected by
the contribution of possible secondary stellar components. As a consequence
I-band fluctuations magnitudes are ideal for distance studies. On the other
hand, we show that standard SSP models do not reproduce the B-band SBF
magnitudes of red ((B-I)_0 \gsim 2.1) galaxies in our sample. We explore the
capability of two non--canonical models in properly reproducing the high
sensitivity of B SBF to the presence of even small fractions of bright, hot
stars (metal poor stars, hot evolved stars, etc.). The disagreement is solved
both by taking into account hot (Post--AGB) stars in SSP models and/or by
adopting Composite Stellar Population models. Finally, we suggest a limit value
of the S/N for the B-band SBF signal required to carry out a detailed study of
stellar population properties based on this technique.Comment: ApJ accepte
The luminosity function of the Large Magellanic Cloud globular cluster NGC 1866
We present {\it Hubble Space Telescope} {\it V,I} photometry of the central
region of the LMC cluster NGC 1866, reaching magnitudes as faint as V=27 mag.
We find evidence that the cluster luminosity function shows a strong dependence
on the distance from the cluster center, with a clear deficiency of low
luminosity stars in the inner region. We discuss a {\it global} cluster
luminosity function as obtained from stars in all the investigated region,
which appears in impressive agreement with the prediction from a Salpeter mass
distribution. We also revisit the use of NGC 1866 as a probe for determining
the efficiency of core overshooting, and conclude that a definitive answer to
this question is not possible from this cluster.Comment: AJ accepted, 16 pages, 19 figures, uses aastex.cl
Unsolved Problems about Supernovae
A number of unsolved problems and open questions about the nature and the
properties of supernovae are identified and briefly discussed. Some suggestions
and directions toward possible solutions are also considered.Comment: 7 pages, Proceedings of 'Probing Stellar Populations out to the
Distant Universe', Cefalu, Italy, Sep 7-19, 2008, AIP Conf. Proc. Serie
The He abundance in NGC 1850 A and B: are we observing the early stage of formation of multiple populations in a stellar cluster?
We present the result of a sample of B-stars in the Large Magellanic Cloud
young double stellar cluster NGC 1850 A and NGC 1850 B, observed with the
integral-field spectrograph at the Very Large Telescope, the Multi Unit
Spectroscopic Explorer. We compare the observed equivalent widths (EWs) of four
He lines (4922 , 5015 , 6678
, and 7065 ) with the ones
determined from synthetic spectra computed with different He mass fraction
(Y=0.25, 0.27, 0.30 and 0.35) with the code SYNSPEC, that takes into account
the non-LTE effect. From this comparison, we determined the He mass fraction of
the B stars, finding a not homogeneous distribution. The stars can be divided
in three groups, He-weak (Y 0.24) and the He-normal (0.24 Y
0.26) belonging to the MS of NGC 1850 A, and the He-rich stars
(0.33 Y 0.38) situated in the MS associated to NGC 1850
B. We have analyzed the stellar rotation as possible responsible of the
anomalous features of the He lines in the He-rich stars. We provide a simple
analysis of the differences between the observed EWs and the ones obtained from
the theoretical models with different rotation velocity (V = 0 and 250
Km/s). The resolution of the MUSE spectra do not allow to get a conclusive
result, however our analysis support the He-enhanced hypothesis.Comment: Accepted for publication by MNRAS, 10 pages, 8 figure
Advances on GRB as cosmological tools
Several interesting correlations among Gamma Ray Bursts (GRB) prompt and
afterglow properties have been found in the recent years. Some of these
correlations have been proposed also to standardize GRB energetics to use them
as standard candles in constraining the expansion history of the universe up to
z>6. However, given the still unexplained nature of most of these correlations,
only the less scattered correlations can be used for constraining the
cosmological parameters. The updated E_peak-E_gamma correlation is presented.
Caveats of alternative methods of standardizing GRB energetics are discussed.Comment: 8 parges, AIP conf. proc. "Probing stellar populations out to the
distant universe, Cefalu' 2008" Vol. 1111, pp. 579-58
The formation and evolution of early-type galaxies : solid results and open questions
The most recent results and some of the open key questions on the evolution
of early-type galaxies are reviewed in the general cosmological context of
massive galaxy formation.Comment: 8 pages, invited review at the workshop "Probing Stellar Populations
out to the Distant Universe", Cefalu` (Italy), September 7 - 19, 200
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