61 research outputs found
Rebirth of Novae as Distance Indicators Due to Efficient Large Telescopes
Nova outbursts are the result of strong thermonuclear runaways on the surface
of a white dwarf accreting Hydrogen-rich material from a small mass companion.
These giant explosions cause the star to increase its brightness by hundreds of
thousands of times then making these objects powerful standard candles useful
to measure the extragalactic distances.We have used the Very Large Telescope,
located in the Chilean Atacama desert, to search for novae in NGC 1316--an
early type galaxy in the Fornax cluster. We discovered 4 novae with 3h of
observing time. The use of 8-10m class telescopes coupled with new detectors,
can dramatically improve the efficiency of nova searches in extragalactic
systems.Comment: 4 pages + 2 plot
Using globular clusters to test gravity in the weak acceleration regime: NGC 7099
A test of Newton's law of gravity in the low acceleration regime using
globular clusters is presented. New results for the core collapsed globular
cluster NGC 7099 are given. The run of the gravitational potential as a
function of distance is probed studying the velocity dispersion profile of the
cluster, as derived from a set of 125 radial velocities with accuracy better
than 1 km/s. The velocity dispersion profile is traced up to ~18 pc from the
cluster center. The dispersion is found to be maximal at the center, then
decrease until 10+-2 pc from the center, well inside the cluster tidal radius
of 42 pc. After that the dispersion remains constant with average value
2.2+-0.3 km/s. Assuming for NGC 7099 a total V mag of M(V)=-7.43 mags and
mass-to-light ratio M/L=1, the acceleration at 10 pc from the center is 1.1e-8
cm/s/s. Thus, the flattening of the velocity dispersion profile occurs for a
value of the internal acceleration of gravity fully consistent with a_0=1.2e-8
cm/s/s observed in galaxies. This new result for NGC 7099 brings to 4 the
clusters with velocity dispersion profile probing acceleration below a_0. All
four have been found to have a flat dispersion profile at large radii where the
acceleration is below a_0, mimicking qualitatively and quantitatively
elliptical galaxies. Whether this indicates a failure of Newtonian dynamics in
the low acceleration limit or some more conventional dynamical effect (e.g.,
tidal heating) is still unclear. However, the similarities emerging between
very different globular clusters, as well as between globular clusters and
elliptical galaxies seem to favor the first of these two possibilities.Comment: Accepted for publication in A&A Letters. Four pages in tota
Using globular clusters to test gravity in the weak acceleration regime
We report on the results from an ongoing program aimed at testing Newton's
law of gravity in the low acceleration regime using globular clusters. It is
shown that all clusters studied so far do behave like galaxies, that is, their
velocity dispersion profile flattens out at large radii where the acceleration
of gravity goes below 1e-8 cm/s/s, instead of following the expected Keplerian
fall off. In galaxies this behavior is ascribed to the existence of a dark
matter halo. Globular clusters, however, do not contain dark matter, hence this
result might indicate that our present understanding of gravity in the weak
regime of accelerations is incomplete and somehow incorrect.Comment: As published on the European Southern Observatory "the Messenger",
Num. 128, June 2007. Seven pages, 4 figures, 2 table
The Lyman alpha forest of the high-z quasar 0000-263
Medium-resolution (delta(v) = 45 km/s) optical spectra of the bright, high-redshift (z = 4.1) quasar 0000-263 taken at the ESO 3.5-m NTT telescope were analyzed to determine the distribution of column densities, velocities and line widths of the Lyman-alpha forest absorption components. The values of NH, b, and z were determined by fitting Voigt profiles to the lines, and convolving with a Gaussian instrumental response function. Over 350 components with log N(sub H) greater than 13.2 were identified. An analysis of the dependence of the number of components with z reveals that the number evolution of components obeys the power law dN/dz varies as (1+z)(sup gamma), where gamma = 0.5 +/- 0.4 for the sample of 182 lines with log N(sub H) greater than 14.0. The distribution of component strengths is found to obey f(N(sub H)) varies as N(sub h)(sup -beta), where beta = -1.55 for components with log(N(sub H)) is greater than 14.7, and beta = -0.68 for the components with log(N(sub H)) greater than 13.5. A distinct break in the f(N(sub H)) histogram is also observed, at log(N(sub H)) is approximately 14.7. The results are briefly considered in the context of theoretical models of quasar Lyman alpha clouds and their evolution
Globular Clusters as a Test for Gravity in the Weak Acceleration Regime
Non-baryonic Dark Matter (DM) appears in galaxies and other cosmic structures
when and only when the acceleration of gravity, as computed considering only
baryons, goes below a well defined value a0=1.2e-8 cm/s/s. This might indicate
a breakdown of Newton's law of gravity (or inertia) below a0, an acceleration
smaller than the smallest probed in the solar system. It is therefore important
to verify whether Newton's law of gravity holds in this regime of
accelerations. In order to do this, one has to study the dynamics of objects
that do not contain significant amounts of DM and therefore should follow
Newton's prediction for whatever small accelerations. Globular clusters are
believed, even by strong supporters of DM, to contain negligible amounts of DM
and therefore are ideal for testing Newtonian dynamics in the low acceleration
limit. Here, we discuss the status of an ongoing program aimed to do this test.
Compared to other studies of globular clsuters, the novelty is that we trace
the velocity dispersion profile of globular clusters far enough from the center
to probe gravitational accelerations well below a0. In all three clusters
studied so far the velocity dispersion is found to remain constant at large
radii rather than follow the Keplerian falloff. On average, the flattening
occurs at the radius where the cluster internal acceleration of gravity is
1.8+-0.4 x 10^{-8} cm/s/s, fully consistent with MOND predictions.Comment: Comments: To Appear in the proceeding of the "First crisis in
cosmology" conference, published in the American Institute of Physiscs'
proceedings series, Vol. 822. (includes 3 pages and 1 fig
The distance to NGC1316 (Fornax A): yet another curious case
The distance of NGC1316, the brightest galaxy in Fornax, is an interesting
test for the cosmological distance scale. First, because Fornax is the 2nd
largest cluster of galaxies at <~25 Mpc after Virgo and, in contrast to Virgo,
has a small line-of-sight depth; and second, because NGC1316 is the galaxy with
the largest number of detected SNeIa, giving the opportunity to test the
consistency of SNeIa distances internally and against other indicators.
We measure SBF mags in NGC1316 from ground and space-based imaging data,
providing a homogeneous set of measurements over a wide wavelength interval.
The SBF, coupled with empirical and theoretical calibrations, are used to
estimate the distance to the galaxy. We present the first B-band SBF
measurements of NGC1316 and use them together with the optical and near-IR SBF
data to analyze the properties of field stars.
Our distance modulus m-M=31.59 +-0.05(stat) +-0.14(sys), when placed in a
consistent Cepheid distance scale, agrees with the results from other
indicators. However, our result is ~17% larger than the most recent estimate
based on SNeIa. Possible explanations for this disagreement are the
uncertainties on internal extinction, or calibration issues. Concerning the
stellar population analysis, we confirm earlier results from other indicators:
the field stars in NGC1316 are dominated by a solar metallicity, intermediate
age component. A substantial mismatch exists between B-band SBF models and
data, a behavior that can be accounted for by an enhanced percentage of hot
horizontal branch stars.
Our study of the SBF distance to NGC1316, and the comparison with distances
from other indicators, raises some concern about the homogeneity between the
calibrations of different indicators. If not properly placed in the same
reference scale, significant differences can occur, with dramatic impact on the
cosmological distance ladder.Comment: 21 pages, 13 figures; A&A accepte
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
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