184 research outputs found
A New Approach to the Study of Stellar Populations in Early-Type Galaxies: K-band Spectral Indices and an Application to the Fornax Cluster
New measurements of K-band spectral features are presented for eleven
early-type galaxies in the nearby Fornax galaxy cluster. Based on these
measurements, the following conclusions have been reached: (1) in galaxies with
no signatures of a young stellar component, the K-band Na I index is highly
correlated with both the optical metallicity indicator [MgFe]' and central
velocity dispersion; (2) in the same galaxies, the K-band Fe features saturate
in galaxies with sigma > 150 km/s while Na I (and [MgFe]') continues to
increase; (3) [Si/Fe] (and possibly [Na/Fe]) is larger in all observed Fornax
galaxies than in Galactic open clusters with near-solar metallicity; (4) in
various near-IR diagnostic diagrams, galaxies with signatures of a young
stellar component (strong Hbeta, weak [MgFe]') are clearly separated from
galaxies with purely old stellar populations; furthermore, this separation is
consistent with the presence of an increased number of M-giant stars (most
likely to be thermally pulsating AGB stars); (5) the near-IR diagrams discussed
here seem as efficient for detecting putatively young stellar components in
early-type galaxies as the more commonly used age/metallicity diagnostic plots
using optical indices (e.g Hbeta vs. [MgFe]').Comment: 47 pages, 16 figures, ApJ accepte
The Planetary Mass Companion 2MASS1207-3932 B: Temperature, Mass and Evidence for an Edge-On Disk
We present J-band imaging and H+K-band low-resolution spectroscopy of
2MASS1207-3932 AB, obtained with VLT NACO. For the putative planetary mass
secondary, we find J = 20.0+/-0.2 mag. The HK spectra of both components imply
low gravity, and a dusty atmosphere for the secondary. Comparisons to synthetic
spectra yield Teff_A ~ 2550+/-150K, and Teff_B ~ 1600+/-100K, consistent with
their late-M and mid-to-late L types. For these Teff, and an age of 5-10 Myrs,
evolutionary models imply M_A ~ 24+/-6 M_Jup and M_B ~ 8+/-2 M_Jup. Independent
comparisons of these models to the observed colors, spanning ~I to L', also
yield the same masses and temperatures. Our primary mass agrees with other
recent analyses; however, our secondary mass, while still in the planetary
regime, is 2-3 times larger than claimed previously. This discrepancy can be
traced to the luminosities: while the absolute photometry and Mbol of the
primary agree with theoretical predictions, the secondary is ~ 2.5+/-0.5 mag
fainter than expected in all bands from I to L' and in Mbol. This accounts for
the much lower secondary mass (and temperature) derived earlier. We argue that
this effect is highly unlikely to result from a variety of model-related
problems, and is instead real. This conclusion is bolstered by the absence of
any luminosity problems in either the primary, or in AB Pic B which we also
analyse. We therefore suggest grey extinction in 2M1207B, due to occlusion by
an edge-on circum-secondary disk. This is consistent with the observed
properties of edge-on disks around T Tauri stars, and with the known presence
of a high-inclination evolved disk around the primary. Finally, the system's
implied mass ratio of ~0.3 suggests a binary-like formation scenario.
(abridged)Comment: Accepted by The Astrophysical Journal, 43 pages text + 16 figs + 1
tabl
Deep near-infrared luminosity function of a cluster of galaxies at z=0.3
The deep near-infrared luminosity function of AC118, a cluster of galaxies at
z=0.3, is presented. AC118 is a bimodal cluster, as evidenced both by our
near-infrared images of lensed galaxies, by public X-ray Rosat images and by
the spatial distribution of bright galaxies. Taking advantage of the extension
and depth of our data, which sample an almost unexplored region in the depth
vs. observed area diagram, we derive the luminosity function (LF), down to the
dwarf regime (M*+5), computed in several cluster portions. The overall LF,
computed on a 2.66 Mpc2 areas (H_0=50 km/s/Mpc), has an intermediate slope
(alpha=-1.2). However, the LF parameters depend on the surveyed cluster region:
the central concentration has 2.6^{+5.1}_{-1.7} times more bright galaxies and
5.3^{+7.2}_{-2.3} times less dwarfs per typical galaxy than the outer region,
which includes galaxies at an average projected distance of ~580 kpc (errors
are quoted at the 99.9 % confidence level). The LF in the secondary AC118 clump
is intermediate between the central and outer one. In other words, the
near-infrared AC118 LF steepens going from high to low density regions. At an
average clustercentric distance of ~580 kpc, the AC118 LF is statistically
indistinguishable from the LF of field galaxies at similar redshift, thus
suggesting that the hostile cluster environment plays a minor role in shaping
the LF at large clustercentric distances, while it strongly affects the LF at
higher galaxy density.Comment: ApJ, in press. The whole paper with all high resolution images is
available at http://www.na.astro.it/~andreon/listapub.htm
Properties of mm galaxies: Constraints from K-band blank fields
We have used the IRAM Plateau de Bure mm interferometer to locate with
subarcsecond accuracy the dust emission of three of the brightest 1.2mm sources
in the NTT Deep Field (NDF) selected from our 1.2mm MAMBO survey at the IRAM
30m telescope. We combine these results with deep B to K imaging and VLA
interferometry. Strikingly, none of the three accurately located mm galaxies
MMJ120546-0741.5, MMJ120539-0745.4, and MMJ120517-0743.1 has a K-band
counterpart down to the faint limit of K>21.9. This implies that these three
galaxies are either extremely obscured and/or are at very high redshifts
(z>~4). We combine our results with literature data for 11 more (sub)mm
galaxies that are identified with similar reliability. In terms of their K-band
properties, the sample divides into three roughly equal groups: (i) undetected
to K~22, (ii) detected in the near-infrared but not the optical and (iii)
detected in the optical with the possibility of optical follow-up spectroscopy.
We find a trend in this sample between near-infrared to submm and submm to
radio spectral indices, which in comparison to spectral energy distributions
(SEDs) of low redshift infrared luminous galaxies suggests that the most
plausible primary factor causing the extreme near-infrared faintness of our
objects is their high redshift. We show that the near-infrared to radio SEDs of
the sample are inconsistent with SEDs that resemble local far-infrared cool
galaxies with moderate luminosities, which were proposed in some models of the
submm sky. We briefly discuss the implications of the results for our
understanding of galaxy formation.Comment: aastex, 5 figures. Accepted by Ap
A brown dwarf desert for intermediate mass stars in Sco OB2?
We present JHK observations of 22 intermediate-mass stars in Sco OB2,
obtained with VLT/NACO. The survey was performed to determine the status of
(sub)stellar candidate companions of A and late-B members. The distinction
between companions and background stars is by a comparison with isochrones and
statistical arguments. We are sensitive to companions in the separation range
0.1''-11'' (13-1430 AU) and K<17. We detect 62 secondaries of which 18 are
physical companions (3 new), 11 candidates, and 33 background stars. The
companion masses are in the range 0.03<M<1.19 Msun, with mass ratios
0.06<q<0.55. We include in our sample a subset of 9 targets with multi-color
ADONIS observations from Kouwenhoven et al. (2005). In the ADONIS survey
secondaries with K12 as
background stars. Our multi-color analysis demonstrates that the simple K=12
criterion correctly classifies the secondaries in ~80% of the cases. We
reanalyse the total ADONIS/NACO sample and conclude that of the 176
secondaries, 25 are physical companions, 55 are candidates, and 96 are
background stars. Although we are sensitive and complete to brown dwarfs as
faint as K=14 in the separation range 130-520 AU, we detect only one, giving a
brown dwarf companion fraction of 0.5% (M>30 MJ). However, the number of brown
dwarfs is consistent with an extrapolation of the stellar companion mass
distribution. This indicates that the physical mechanism for the formation of
brown dwarfs around intermediate mass stars is similar to that of stellar
companions, and that the embryo ejection mechanism does not need to be invoked
in order to explain the small number of brown dwarf companions among these
stars.Comment: 29 pages, 9 figures, accepted by A&
VLT-ISAAC near-IR Spectroscopy of ISO selected Hubble Deep Field South Galaxies
We report the results of near-infrared VLT-ISAAC spectroscopy of a sample of
12 galaxies at z = 0.4-1.4, drawn from the ISOCAM survey of the Hubble Deep
Field South. We find that the rest frame R-band spectra of the ISOCAM galaxies
resemble those of powerful dust-enshrouded starbursts. Halpha emission is
detected in 11 out of 12 objects down to a flux limit of 7x10^(-17) erg/cm^2/s,
corresponding to a luminosity limit of 10^41 erg/s at z = 0.6, (for an Ho = 50
and Omega = 0.3 cosmology). From the Halpha luminosities in these galaxies we
derive estimates of the star formation rate in the range 2--50 Mo/yr for
stellar masses 1--100 Mo. The raw Halpha-based star formation rates are an
order of magnitude or more lower than SFR(FIR) estimates based on ISOCAM LW3
fluxes. If the Halpha emission is corrected for extinction the median offset is
reduced to a factor of 3. The sample galaxies are part of a new population of
optically faint but infrared--luminous active starburst galaxies, which are
characterized by an extremely high rate of evolution with redshift up to z~1.5
and expected to contribute significantly to the cosmic far-IR extragalactic
background.Comment: Accepted for publication in the Astrophysical Journal Letters, 16
pages, 2 figure
Statistical mechanics of topological phase transitions in networks
We provide a phenomenological theory for topological transitions in
restructuring networks. In this statistical mechanical approach energy is
assigned to the different network topologies and temperature is used as a
quantity referring to the level of noise during the rewiring of the edges. The
associated microscopic dynamics satisfies the detailed balance condition and is
equivalent to a lattice gas model on the edge-dual graph of a fully connected
network. In our studies -- based on an exact enumeration method, Monte-Carlo
simulations, and theoretical considerations -- we find a rich variety of
topological phase transitions when the temperature is varied. These transitions
signal singular changes in the essential features of the global structure of
the network. Depending on the energy function chosen, the observed transitions
can be best monitored using the order parameters Phi_s=s_{max}/M, i.e., the
size of the largest connected component divided by the number of edges, or
Phi_k=k_{max}/M, the largest degree in the network divided by the number of
edges. If, for example the energy is chosen to be E=-s_{max}, the observed
transition is analogous to the percolation phase transition of random graphs.
For this choice of the energy, the phase-diagram in the [,T] plane is
constructed. Single vertex energies of the form
E=sum_i f(k_i), where k_i is the degree of vertex i, are also studied.
Depending on the form of f(k_i), first order and continuous phase transitions
can be observed. In case of f(k_i)=-(k_i+c)ln(k_i), the transition is
continuous, and at the critical temperature scale-free graphs can be recovered.Comment: 12 pages, 12 figures, minor changes, added a new refernce, to appear
in PR
A search for hot massive extrasolar planets around nearby young stars with the adaptive optics system NACO
We report on a survey devoted for the search of exo-planets around young and
nearby stars carried out with NACO at the VLT. The detection limit for 28 among
the best available targets vs. the angular separation from the star is
presented. The non-detection of any planetary mass companion in our survey is
used to derive, for the first time, the frequency of the upper limit of the
projected separation planet-stars. In particular, we find that in 50 % of
cases, no 5 MJ planet (or more massive) has been detected at projected
separations larger than 14 AU and no 10 MJ planet (or more massive) has been
detected at projected separations larger than 8.5 AU. In 100 % of cases, these
values increase to 36 AU and 65 AU respectively. The excellent sensitivity
reached by our study leads to a much lower upper limit of the projected
planet-star separation compared with previous studies. For example, for the
Beta Pictoris group, (~ 12 Myr), we did not detect any 10 MJ planet at
distances larger than 15 AU. A previous study carried out with 4 m class
telescopes put an upper limit for 10 MJ planets at ~ 60 AU. For our closest
target (V 2306 Oph - d = 4.3 pc) it is shown that it would be possible to
detect a 10 MJ planet at a minimum projected separation from the star of 1 AU
and a 5 MJ planet at a minimum projected separation of 3.7 AU. Our results are
discussed with respect to mechanisms explaining planet formation and migration
and to forthcoming observational strategies and future planet finder
observations from the ground.Comment: 35 pages, 10 figures, pdf file, ApJ, accepted, see also
http://www.mpia.de/homes/masciad
Bandgaps in the propagation and scattering of surface water waves over cylindrical steps
Here we investigate the propagation and scattering of surface water waves by
arrays of bottom-mounted cylindrical steps. Both periodic and random
arrangements of the steps are considered. The wave transmission through the
arrays is computed using the multiple scattering method based upon a recently
derived formulation. For the periodic case, the results are compared to the
band structure calculation. We demonstrate that complete band gaps can be
obtained in such a system. Furthermore, we show that the randomization of the
location of the steps can significantly reduce the transmission of water waves.
Comparison with other systems is also discussed.Comment: 4 pages, 3 figure
Interplay of disorder and nonlinearity in Klein-Gordon models: Immobile kinks
We consider Klein-Gordon models with a -correlated spatial disorder.
We show that the properties of immobile kinks exhibit strong dependence on the
assumptions as to their statistical distribution over the minima of the
effective random potential. Namely, there exists a crossover from monotonically
increasing (when a kink occupies the deepest potential well) to the
non-monotonic (at equiprobable distribution of kinks over the potential minima)
dependence of the average kink width as a function of the disorder intensity.
We show also that the same crossover may take place with changing size of the
system.Comment: 7 pages, 4 figure
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