94 research outputs found
Homogeneous Photometry VI: Variable Stars in the Leo I Dwarf Spheroidal Galaxy
We have characterized the pulsation properties of 164 candidate RR Lyrae
variables (RRLs) and 55 candidate Anomalous and/or short-period Cepheids in Leo
I dwarf spheroidal galaxy. On the basis of its RRLs Leo I is confirmed to be an
Oosterhoff-intermediate type galaxy, like several other dwarfs. We show that in
their pulsation properties, the RRLs representing the oldest stellar population
in the galaxy are not significantly different from those of five other nearby,
isolated dwarf spheroidal galaxies. A similar result is obtained when comparing
them to RR Lyrae stars in recently discovered ultra-faint dwarf galaxies. We
are able to compare the period distributions and period-amplitude relations for
a statistically significant sample of ab type RR Lyrae stars in dwarf galaxies
(~1300stars) with those in the Galactic halo field (~14,000stars) and globular
clusters (~1000stars). Field RRLs show a significant change in their period
distribution when moving from the inner (dG14kpc)
halo regions. This suggests that the halo formed from (at least) two dissimilar
progenitors or types of progenitor. Considered together, the RRLs in classical
dwarf spheroidal and ultra-faint dwarf galaxies-as observed today-do not appear
to follow the well defined pulsation properties shown by those in either the
inner or the outer Galactic halo, nor do they have the same properties as RRLs
in globular clusters. In particular, the samples of fundamental-mode RRLs in
dwarfs seem to lack High Amplitudes and Short Periods ("HASP":AV>1.0mag and P
<0.48d) when compared with those observed in the Galactic halo field and
globular clusters. The observed properties of RRLs do not support the idea that
currently existing classical dwarf spheroidal and ultra-faint dwarf galaxies
are surviving representative examples of the original building blocks of the
Galactic halo.Comment: 49 pages in referee format, 12 figure
Spatial dependence of the Star Formation History in the Central Regions of the Fornax Dwarf Spheroidal Galaxy
We present the Star Formation History (SFH) and the age-metallicity relation
(AMR) in three fields of the Fornax dwarf spheroidal galaxy. They sample a
region spanning from the centre of the galaxy to beyond one core radius, which
allows studying galactocentric gradients. In all the cases, we found stars as
old as 12 Gyr, together with intermediate-age and young stellar populations.
The last star formation events, as young as 1 Gyr old, are mainly located in
the central region, which may indicate that the gas reservoir in the outer
parts of the galaxy would have been exhausted earlier than in the centre or
removed by tidal interactions. The AMR is smoothly increasing in the three
analyzed regions and similar to each other, indicating that no significant
metallicity gradient is apparent within and around the core radius of Fornax.
No significant traces of global UV-reionization or local SNe feedback are
appreciated in the early SFH of Fornax.
Our study is based on FORS1@VLT photometry as deep as I~24.5 and the
IAC-star/IAC-pop/MinnIAC suite of codes for the determination of the SFH in
resolved stellar populations.Comment: 13 pages, 8 figures, 4 table
On the use of asymmetric PSF on NIR images of crowded stellar fields
We present data collected using the camera PISCES coupled with the Firt Light
Adaptive Optics (FLAO) mounted at the Large Binocular Telescope (LBT). The
images were collected using two natural guide stars with an apparent magnitude
of R<13 mag. During these observations the seeing was on average ~0.9". The AO
performed very well: the images display a mean FWHM of 0.05 arcsec and of 0.06
arcsec in the J- and in the Ks-band, respectively. The Strehl ratio on the
quoted images reaches 13-30% (J) and 50-65% (Ks), in the off and in the central
pointings respectively. On the basis of this sample we have reached a J-band
limiting magnitude of ~22.5 mag and the deepest Ks-band limiting magnitude ever
obtained in a crowded stellar field: Ks~23 mag.
J-band images display a complex change in the shape of the PSF when moving at
larger radial distances from the natural guide star. In particular, the stellar
images become more elongated in approaching the corners of the J-band images
whereas the Ks-band images are more uniform. We discuss in detail the strategy
used to perform accurate and deep photometry in these very challenging images.
In particular we will focus our attention on the use of an updated version of
ROMAFOT based on asymmetric and analytical Point Spread Functions.
The quality of the photometry allowed us to properly identify a feature that
clearly shows up in NIR bands: the main sequence knee (MSK). The MSK is
independent of the evolutionary age, therefore the difference in magnitude with
the canonical clock to constrain the cluster age, the main sequence turn off
(MSTO), provides an estimate of the absolute age of the cluster. The key
advantage of this new approach is that the error decreases by a factor of two
when compared with the classical one. Combining ground-based Ks with space
F606W photometry, we estimate the absolute age of M15 to be 13.70+-0.80 Gyr.Comment: 15 pages, 7 figures, presented at the SPIE conference 201
The ACS LCID project. IX. Imprints of the early Universe in the radial variation of the star formation history of dwarf galaxies
Based on Hubble Space Telescope observations from the Local Cosmology from
Isolated Dwarfs project, we present the star formation histories, as a function
of galactocentric radius, of four isolated Local Group dwarf galaxies: two dSph
galaxies, Cetus and Tucana, and two transition galaxies (dTrs), LGS-3 and
Phoenix. The oldest stellar populations of the dSphs and dTrs are, within the
uncertainties, coeval () at all galactocentric radii. We find that
there are no significative differences between the four galaxies in the
fundamental properties (such as the normalized star formation rate or
age-metallicity relation) of their outer regions (radii greater than four
exponential scale lengths); at large radii, these galaxies consist exclusively
of old () metal-poor stars. The duration of star formation in
the inner regions vary from galaxy to galaxy, and the extended central star
formation in the dTrs produces the dichotomy between dSph and dTr galaxy types.
The dTr galaxies show prominent radial stellar population gradients: the
centers of these galaxies host young () populations while the age
of the last formation event increases smoothly with increasing radius. This
contrasts with the two dSph galaxies. Tucana shows a similar, but milder,
gradient, but no gradient in age is detected Cetus. For the three galaxies with
significant stellar population gradients, the exponential scale length
decreases with time. These results are in agreement with outside-in scenarios
of dwarf galaxy evolution, in which a quenching of the star formation toward
the center occurs as the galaxy runs out of gas in the outskirts.Comment: Accepted to be published in Ap
The ACS LCID Project XI. On the early time resolution of LG dwarf galaxy SFHs: Comparing the effects of reionization in models with observations
The analysis of the early star formation history (SFH) of nearby galaxies,
obtained from their resolved stellar populations is relevant as a test for
cosmological models. However, the early time resolution of observationally
derived SFHs is limited by several factors. Thus, direct comparison of
observationally derived SFHs with those derived from theoretical models of
galaxy formation is potentially biased. Here we investigate and quantify this
effect. For this purpose, we analyze the duration of the early star formation
activity in a sample of four Local Group dwarf galaxies and test whether they
are consistent with being true fossils of the pre-reionization era; i.e., if
the quenching of their star formation occurred before cosmic reionization by UV
photons was completed. Two classical dSph (Cetus and Tucana) and two dTrans
(LGS-3 and Phoenix) isolated galaxies with total stellar masses between
to M have been studied. Accounting
for time resolution effects, the SFHs peak as much as 1.25 Gyr earlier than the
optimal solutions. Thus, this effect is important for a proper comparison of
model and observed SFHs. It is also shown that none of the analyzed galaxies
can be considered a true-fossil of the pre-reionization era, although it is
possible that the {\it outer regions} of Cetus and Tucana are consistent with
quenching by reionization.Comment: To be published by the Ap
Dark halo structure in the Carina dwarf spheroidal galaxy: joint analysis of multiple stellar components
Photometric and spectroscopic observations of the Carina dSph revealed that
this galaxy contains two dominant stellar populations of different age and
kinematics. The co-existence of multiple populations provides new constraints
on the dark halo structure of the galaxy, because different populations should
be in equilibrium in the same dark matter potential well. We develop
non-spherical dynamical models including such multiple stellar components and
attempt to constrain the properties of the non-spherical dark halo of Carina.
We find that Carina probably has a larger and denser dark halo than found in
previous works and a less cuspy inner dark matter density profile, even though
the uncertainties of dark halo parameters are still large due to small volume
of data sample. Using our fitting results, we evaluate astrophysical factors
for dark matter annihilation and decay and find that Carina should be one of
the most promising detectable targets among classical dSph galaxies. We also
calculate stellar velocity anisotropy profiles for both stellar populations and
find that they are both radially anisotropic in the inner regions, while in the
outer regions the older population becomes more tangentially biased than the
intermediate one. This is consistent with the anisotropy predicted from tidal
effects on the dynamical structure of a satellite galaxy and thereby can be
considered as kinematic evidence for the tidal evolution of Carina.Comment: 14 pages, 7 figures and 3 tables. Accepted for publication in MNRA
The galaxy "missing dark matter" NGC1052-DF4 is undergoing tidal disruption
The existence of long-lived galaxies lacking dark matter represents a
challenge to our understanding of how galaxies form. Here, we present evidence
that explains the lack of dark matter in one of such galaxies: NGC1052-DF4.
Deep optical imaging of the system has detected tidal tails in this object
caused by its interaction with its neighbouring galaxy NGC1035. As stars are
more centrally concentrated than the dark matter, the tidal stripping will
remove a significant percentage of the dark matter before affecting the stars
of the galaxy. Only ~7% of the stellar mass of the galaxy is in the tidal
tails, suggesting that the stars of NGC1052-DF4 are starting only now to be
affected by the interaction, while the percentage of remaining dark matter is
<1%. This naturally explains the low content of dark matter inferred for this
galaxy and reconciles these type of galaxies with our current models of galaxy
formation.Comment: 18 pages, 9 figures, accepted for publication in ApJ; The main
results of the paper are shown in figures 5 and
Stellar photometry with Multi Conjugate Adaptive Optics
We overview the current status of photometric analyses of images collected
with Multi Conjugate Adaptive Optics (MCAO) at 8-10m class telescopes that
operated, or are operating, on sky. Particular attention will be payed to
resolved stellar population studies. Stars in crowded stellar systems, such as
globular clusters or in nearby galaxies, are ideal test particles to test AO
performance. We will focus the discussion on photometric precision and accuracy
reached nowadays. We briefly describe our project on stellar photometry and
astrometry of Galactic globular clusters using images taken with GeMS at the
Gemini South telescope. We also present the photometry performed with DAOPHOT
suite of programs into the crowded regions of these globulars reaching very
faint limiting magnitudes Ks ~21.5 mag on moderately large fields of view (~1.5
arcmin squared). We highlight the need for new algorithms to improve the
modeling of the complex variation of the Point Spread Function across the field
of view. Finally, we outline the role that large samples of stellar standards
plays in providing a detailed description of the MCAO performance and in
precise and accurate colour{magnitude diagrams.Comment: 17 pages, 12 figures, SPIE 201
- …