29 research outputs found
Statistical properties of the GALEX spectroscopic stellar sample
The GALEX General Data Release 4/5 includes 174 spectroscopic tiles, obtained
from slitless grism observations, for a total of more than 60,000 ultraviolet
spectra. We have determined statistical properties of the sample of GALEX
stars. We have defined a suitable system of spectroscopic indices, which
measure the main mid-UV features at the GALEX low spectral resolution and we
have employed it to determine the atmospheric parameters of of stars in the
range 4500<Teff<9000 K. Our preliminary results indicate that the sample is
formed by a majority of main sequence F- and G-type stars, with metallicity
[M/H]>-1 dex.Comment: 9 pages, 9 figures, accepted for publication in Astrophysics & Space
Science, UV universe special issu
The MeerKAT Fornax Survey
We present the science case and observations plan of the MeerKAT Fornax Survey, an HI and radio continuum survey of the Fornax galaxy cluster to be carried out with the SKA precursor MeerKAT. Fornax is the second most massive cluster within 20 Mpc and the largest nearby cluster in the southern hemisphere. Its low X-ray luminosity makes it representative of the environment where most galaxies live and where substantial galaxy evolution takes place. Fornax's ongoing growth makes it an excellent laboratory for studying the assembly of clusters, the physics of gas accretion and stripping in galaxies falling in the cluster, and the connection between these processes and the neutral medium in the cosmic web. We will observe a region of 12 deg2 reaching a projected distance of 1.5 Mpc from the cluster centre. This will cover a wide range of environment density out to the outskirts of the cluster, where gas-rich in-falling groups are found. We will: study the HI morphology of resolved galaxies down to a column density of a few times 1e+19 cm−2 at a resolution of 1 kpc; measure the slope of the HI mass function down to M(HI) 5e+5 M(sun); and attempt to detect HI in the cosmic web reaching a column density of 1e+18 cm−2 at a resolution of 10 kpc
Extrapolating SMBH correlations down the mass scale: the case for IMBHs in globular clusters
Empirical evidence for both stellar mass black holes M_bh<10^2 M_sun) and
supermassive black holes (SMBHs, M_bh>10^5 M_sun) is well established.
Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is
correlated with the bulge mass, and even more strongly with the central stellar
velocity dispersion sigma_c, the `M-sigma' relation. On the other hand,
evidence for "intermediate-mass" black holes (IMBHs, with masses in the range
1^2 - 10^5 M_sun) is relatively sparse, with only a few mass measurements
reported in globular clusters (GCs), dwarf galaxies and low-mass AGNs. We
explore the question of whether globular clusters extend the M-sigma
relationship for galaxies to lower black hole masses and find that available
data for globular clusters are consistent with the extrapolation of this
relationship. We use this extrapolated M-sigma relationship to predict the
putative black hole masses of those globular clusters where existence of
central IMBH was proposed. We discuss how globular clusters can be used as a
constraint on theories making specific predictions for the low-mass end of the
M-sigma relation.Comment: 14 pages, 3 figures, accepted for publication in Astrophysics and
Space Science; fixed typos and a quote in Sec.
The Planetary Nebula Luminosity Function at the Dawn of Gaia
The [O III] 5007 Planetary Nebula Luminosity Function (PNLF) is an excellent
extragalactic standard candle. In theory, the PNLF method should not work at
all, since the luminosities of the brightest planetary nebulae (PNe) should be
highly sensitive to the age of their host stellar population. Yet the method
appears robust, as it consistently produces < 10% distances to galaxies of all
Hubble types, from the earliest ellipticals to the latest-type spirals and
irregulars. It is therefore uniquely suited for cross-checking the results of
other techniques and finding small offsets between the Population I and
Population II distance ladders. We review the calibration of the method and
show that the zero points provided by Cepheids and the Tip of the Red Giant
Branch are in excellent agreement. We then compare the results of the PNLF with
those from Surface Brightness Fluctuation measurements, and show that, although
both techniques agree in a relative sense, the latter method yields distances
that are ~15% larger than those from the PNLF. We trace this discrepancy back
to the calibration galaxies and argue that, due to a small systematic error
associated with internal reddening, the true distance scale likely falls
between the extremes of the two methods. We also demonstrate how PNLF
measurements in the early-type galaxies that have hosted Type Ia supernovae can
help calibrate the SN Ia maximum magnitude-rate of decline relation. Finally,
we discuss how the results from space missions such as Kepler and Gaia can help
our understanding of the PNLF phenomenon and improve our knowledge of the
physics of local planetary nebulae.Comment: 12 pages, invited review at the conference "The Fundamental Cosmic
Distance Scale: State of the Art and Gaia Perspective", to appear in
Astrophysics and Space Scienc
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
The MeerKAT Fornax Survey. I. Survey description and first evidence of ram pressure in the Fornax galaxy cluster
The MeerKAT Fornax Survey maps the distribution and kinematics of atomic
neutral hydrogen gas (HI) in the nearby Fornax galaxy cluster using the MeerKAT
telescope. The 12 deg^2 survey footprint covers the central region of the
cluster out to ~ Rvir and stretches out to ~ 2 Rvir towards south west to
include the NGC 1316 galaxy group. The HI column density sensitivity (3 sigma
over 25 km/s) ranges from 5e+19/cm^2 at a resolution of ~ 10" (~ 1 kpc at the
20 Mpc distance of Fornax) down to ~ 1e+18/cm^2 at ~ 1' (~ 6 kpc), and slightly
below this level at the lowest resolution of ~ 100" (~ 10 kpc). The HI mass
sensitivity (3 sigma over 50 km/s) is 6e+5 Msun. The HI velocity resolution is
1.4 km/s. In this paper we describe the survey design and HI data processing,
and we present a sample of six galaxies with long, one-sided, star-less HI
tails (of which only one was previously known) radially oriented within the
cluster and with measurable internal velocity gradients. We argue that the
joint properties of the HI tails represent the first unambiguous evidence of
ram pressure shaping the distribution of HI in the Fornax cluster. The
disturbed optical morphology of all host galaxies supports the idea that the
tails consist of HI initially pulled out of the galaxies' stellar body by tidal
forces. Ram pressure was then able to further displace the weakly bound HI and
give the tails their present direction, length and velocity gradient.Comment: Astronomy & Astrophysics, accepted. Data available at the MeerKAT
Fornax Survey website https://sites.google.com/inaf.it/meerkatfornaxsurve
Mask Connectivity by Viscous Closings:Linking Merging Galaxies without Merging Double Stars
Second-generation connectivity opened the path to the use of mask images to freely define connectivity among the image components. In theory, any image could be treated as a mask image that defines a certain connectivity. This creates a new problem in terms of which image to use. In this paper, clustering masks suitable for the analysis of astronomical images are discussed. The connectivity defined by such masks must be capable of preserving faint structures like the filaments that link merging galaxies while separating neighboring stars. In this way, the actual morphology of the objects of interest is kept. This is useful for proper segmentation. We show that viscous mathematical morphology operators have a superior performance and create appropriate connectivity masks that can deal with the characteristic features of astronomical images
Star formation histories of Coma cluster galaxies matched to simulated orbits hint at quenching around first pericenter
Context. The star formation in galaxies in present-day clusters has almost entirely been shut down, but the exact mechanism that quenched these galaxies is still uncertain. Aims. By tracing the orbital and star formation histories of galaxies within the Coma cluster, we seek to understand the role of the high-density cluster environment in quenching these galaxies. Methods. We combine star formation histories extracted from high-signal-to-noise spectra of 11 early-type galaxies around the center of the Coma cluster with probability distributions for their orbital parameters obtained using an N-body simulation to connect their orbital and star formation histories. Results. We find that all 11 galaxies likely quenched near their first pericentric approach. Higher stellar mass galaxies (log(M⋆/M⊙) > 10) had formed a higher fraction of their stellar mass (more than ∼90%) than their lower mass counterparts (∼80−90%) by the time they fell into the cluster (when they cross 2.5rvir). We find that the expected infall occurred around z ∼ 0.6, followed by the first pericentric passage ∼4 Gyr later. Galaxies in our sample formed a significant fraction of their stellar mass, up to 15%, between infall and first pericenter, and had assembled more than ∼98% of their cumulative stellar mass by first pericenter. Conclusions. Unlike previous low-redshift studies that suggest that star formation continues until about first apocenter or later, the high percentage of stellar mass already formed by first pericenter in our sample galaxies points to star formation ceasing within a gigayear after the first pericentric passage. We consider the possible physical mechanisms driving quenching and find that our results resemble the situation in clusters at z ∼ 1, where active stripping of gas (ram-pressure or tidally driven) seems to be required to quench satellites by their first pericentric passage. However, a larger sample will be required to conclusively account for the unknown fraction of preprocessed satellites in the Coma cluster
Comparison of density estimation methods for astronomical datasets
Context. Galaxies are strongly influenced by their environment. Quantifying the galaxy density is a difficult but critical step in studying the properties of galaxies.
Aims. We aim to determine differences in density estimation methods and their applicability in astronomical problems. We study the performance of four density estimation techniques: k-nearest neighbors (kNN), adaptive Gaussian kernel density estimation (DEDICA), a special case of adaptive Epanechnikov kernel density estimation (MBE), and the Delaunay tessellation field estimator (DTFE).
Methods. The density estimators are applied to six artificial datasets and on three astronomical datasets, the Millennium Simulation and two samples from the Sloan Digital Sky Survey. We compare the performance of the methods in two ways: first, by measuring the integrated squared error and Kullback-Leibler divergence of each of the methods with the parametric densities of the datasets (in case of the artificial datasets); second, by examining the applicability of the densities to study the properties of galaxies in relation to their environment (for the SDSS datasets).
Results. The adaptive kernel based methods, especially MBE, perform better than the other methods in terms of calculating the density properly and have stronger predictive power in astronomical use cases.
Conclusions. We recommend the modified Breiman estimator as a fast and reliable method to quantify the environment of galaxies.