52 research outputs found
Disentangling the Galaxy at low Galactic latitudes
We have used the field stars from the open cluster survey BOCCE, to study
three low-latitude fields imaged with the Canada-France-Hawaii telescope
(CFHT), with the aim of better understanding the Galactic structure in those
directions. Due to the deep and accurate photometry in these fields, they
provide a powerful discriminant among Galactic structure models. In the present
paper we discuss if a canonical star count model, expressed in terms of thin
and thick disc radial scales, thick disc normalization and reddening
distribution, can explain the observed CMDs. Disc and thick disc are described
with double exponentials, the spheroid is represented with a De Vaucouleurs
density law. In order to assess the fit quality of a particular set of
parameters, the colour distribution and luminosity function of synthetic
photometry is compared to that of target stars selected from the blue sequence
of the observed colour-magnitude diagrams. Through a Kolmogorov-Smirnov test we
find that the classical decomposition halo-thin/thick disc is sufficient to
reproduce the observations--no additional population is strictly necessary. In
terms of solutions common to all three fields, we have found a thick disc scale
length that is equal to (or slightly longer than) the thin disc scale.Comment: Accepted for publication on MNRA
Star Formation in the Milky Way and Nearby Galaxies
We review progress over the past decade in observations of large-scale star
formation, with a focus on the interface between extragalactic and Galactic
studies. Methods of measuring gas contents and star formation rates are
discussed, and updated prescriptions for calculating star formation rates are
provided. We review relations between star formation and gas on scales ranging
from entire galaxies to individual molecular clouds.Comment: 55 pages, 15 figures, in press for Annual Reviews of Astronomy and
Astrophysics; Updated with corrected equation 5, improved references, and
other minor change
The Milky Way: An Exceptionally Quiet Galaxy; Implications for the formation of spiral galaxies
[Abridged]We compare both the Milky Way and M31 galaxies to local external
disk galaxies within the same mass range, using their relative locations in the
planes formed by V_flat versus M_K, j_disk, and the average Fe abundance of
stars in the galaxy outskirts. We find, for all relationships, that the MW is
systematically offset by ~ 1 sigma, showing a significant deficiency in stellar
mass, in angular momentum, in disk radius and [Fe/H] in the stars in its
outskirts at a given V_flat. On the basis of their location in the M_K, V_flat,
and R_d volume, the fraction of spirals like the MW is 7+/-1%, while M31
appears to be a "typical'' spiral. Our Galaxy appears to have escaped any
significant merger over the last ~10 Gyrs which may explain why it is deficient
by a factor 2 to 3 in stellar mass, angular momentum and outskirts metallicity
and then, unrepresentative of the typical spiral. As with M31, most local
spirals show evidence for a history shaped mainly by relatively recent merging.
We conclude that the standard scenario of secular evolution is generally unable
to reproduce the properties of most (if not all) spiral galaxies. However, the
so-called "spiral rebuilding'' scenario proposed by Hammer et al. 2005 is
consistent with the properties of both distant galaxies and of their
descendants - the local spirals.Comment: 14 pages, 6 figures, to appear in Ap
The Haunted Halos of Andromeda and Triangulum: A panorama of galaxy formation in action
We present a deep photometric survey of M31, conducted with the CFHT and INT,
covering the inner 50 kpc of the galaxy, the Southern quadrant out to 150 kpc,
and extending to M33. This is the first systematic panoramic study of this very
outermost region of galaxies. We detect several streams and other large-scale
structures, and two new dwarf galaxies: And XV and XVI. The discovery of
substructure on the minor axis, together with the fact that the light profile
between 0.5 < R < 1.3 follows the exponential ``extended disk'', is
particularly important in shedding light on the mixed and sometimes conflicting
results reported in previous studies. Underlying the substructures lies a
faint, metal-poor, smooth and extremely extended halo, reaching out to at least
150 kpc. The smooth halo component in M31 has a profile that can be fit with a
Hernquist model of immense scale radius ~55 kpc, almost a factor of 4 larger
than theoretical predictions. Alternatively a power-law with exponent -1.91 +/-
0.11 can be fit to the profile. The total luminosity of this structure is
similar to that of the halo of the Milky Way. This vast, smooth, underlying
halo is reminiscent of a classical monolithic model and completely unexpected
from modern galaxy formation models. M33 is also found to have an extended
metal-poor halo component, which can be fit with a Hernquist model also of
scale radius ~55 kpc. These extended slowly-decreasing halos will provide a
challenge and strong constraints for further modeling. [Abridged]Comment: 38 pages, 55 figures, submitted to ApJ. High resolution version
available at http://palantir.u-strasbg.fr/~ibata/pdf/0704.1318.pdf Please do
not bother with the astro-ph version - the figures are devoid of informatio
Search for low-mass PMS companions around X-ray selected late B stars
We have observed 49 X-ray detected bright late B-type dwarfs to search for
close low-mass pre-main sequence (PMS) companions using the European Southern
Observatory's ADONIS (Adaptive Optics Near Infrared System) instrument. We
announce the discovery of 21 new companions in 9 binaries, 5 triple, 4
quadruple system and 1 system consisting of five stars. The detected new
companions have K magnitudes between 6.5 mag and 17.3 mag. and angular
separations ranging from 0.2 and 14.1 arcsec (18-2358 AU).Comment: Accepted for publication in Astronomy and Astrophysic
The Extreme Outer Regions of Disk Galaxies: I. Chemical Abundances of HII Regions
We present the first results of an ongoing project to investigate the
present-day chemical abundances of the extreme outer parts of galactic disks,
as probed by the emission line spectra of a new sample of HII regions. The
galaxies studied here, NGC628, NGC1058 and NGC6946, are all late-type spiral
galaxies, characterized by larger than average HI-to-optical sizes. Our deep
Halpha images have revealed the existence of recent massive star formation,
traced by HII regions, out to, and beyond, two optical radii in these galaxies
(defined by the B-band 25th magnitude isophote). Optical spectra of these
newly-discovered HII regions are used to investigate their densities,
ionization parameters, extinctions and in particular their oxygen and nitrogen
abundances. Our measurements reveal gas-phase abundances of O/H~10-15% of the
solar value, and N/O~20-25% of the solar value, at radii of 1.5-2 R25. Clear
evidence also exists for diminished dust extinction (Av~0-0.2) at large radii.
The combination of our measurements of outer disk HII region abundances with
those for inner disk HII regions published in the literature is a powerful
probe of the shape of abundance gradients over unprecedented radial baselines.
Within the limits of the current dataset, the radial abundance variations are
consistent with single log-linear relationships, although the derived slopes
can often differ considerably from those found if only inner disk HII regions
are used to define the fit. Interestingly, both the mean level of enrichment
and the ratio of N/O measured in extreme outer galactic disks are similar to
those values measured in some high redshift damped Lyman-alpha absorbers,
suggesting that outer disks at the present epoch are relatively unevolved.
(abridged)Comment: 36 pages, 10 embedded postscript files, 3 jpeg files, 7 postscript
tables; accepted for publication in the Astronomical Journal (August issue
Starcounts Redivivus. IV. Density Laws Through Photometric Parallaxes
In an effort to more precisely define the spatial distribution of Galactic
field stars, we present an analysis of the photometric parallaxes of 70,000
stars covering nearly 15 square degrees in seven Kapteyn Selected Areas. We
address the affects of Malmquist Bias, subgiant/giant contamination,
metallicity and binary stars upon the derived density laws. The affect of
binary stars is the most significant. We find that while the disk-like
populations of the Milky Way are easily constrained in a simultaneous analysis
of all seven fields, no good simultaneous solution for the halo is found. We
have applied halo density laws taken from other studies and find that the
Besancon flattened power law halo model (c/a=0.6, r^-2.75) produces the best
fit to our data. With this halo, the thick disk has a scale height of 750 pc
with an 8.5% normalization to the old disk. The old disk scale height is
280-300 pc. Corrected for a binary fraction of 50%, these scale heights are 940
pc and 350-375 pc, respectively. Even with this model, there are systematic
discrepancies between the observed and predicted density distributions. Our
model produces density overpredictions in the inner Galaxy and density
underpredictions in the outer Galaxy. A possible solution is modeling the
stellar halo as a two-component system in which the halo has a flattened inner
distribution and a roughly spherical, but substructured outer distribution.
Further reconciliation could be provided by a flared thick disk, a structure
consistent with a merger origin for that population. (Abridged)Comment: 66 pages, accepted to Astrophysical journal, some figures compresse
The evolution of the Galactic metallicity gradient from high-resolution spectroscopy of open clusters
Open clusters offer a unique possibility to study the time evolution of the
radial metallicity gradients of several elements in our Galaxy, because they
span large intervals in age and Galactocentric distance, and both quantities
can be more accurately derived than for field stars. We re-address the issue of
the Galactic metallicity gradient and its time evolution by comparing the
empirical gradients traced by a sample of 45 open clusters with a chemical
evolution model of the Galaxy. At variance with previous similar studies, we
have collected from the literature only abundances derived from
high--resolution spectra. The clusters have distances kpc and ages
from Myr to 11 Gyr. We also consider the -elements Si, Ca,
Ti, and the iron-peak elements Cr and Ni. The data for iron-peak and
-elements indicate a steep metallicity gradient for R_GC<12R_{\rm GC}\lesssim 12$ kpc (that we associate with an
early phase of disk formation from the collapse of the halo) and in a slow
inflow of material per unit area in the outer regions at a constant rate with
time.Comment: 16 pages, 18 figures, A&A accepte
A search for late-type supergiants in the inner regions of the Milky Way
We present the results of a narrow-band infrared imaging survey of a narrow
strip (12' wide) around the galactic equator between 6 deg and 21 deg of
galactic longitude aimed at detecting field stars with strong CO absorption,
mainly late-type giants and supergiants. Our observations include follow-up low
resolution spectroscopy (R = 980) of 191 selected candidates in the H and K
bands. Most of these objects have photometric and spectroscopic characteristics
consistent with them being red giants, and some display broad, strong
absorption wings due to water vapor absorption between the H and K bands. We
also identify in our sample 18 good supergiant candidates characterized by
their lack of noticeable water absorption, strong CO bands in the H and K
windows, and HK_S photometry suggestive of high intrinsic luminosity and
extinctions reaching up to A_V ~40 mag. Another 9 additional candidates share
the same features except for weak H2O absorption, which is also observed among
some M supergiants in the solar neighbourhood. Interesting differences are
noticed when comparing our stars to a local sample of late-type giants and
supergiants, as well as to a sample of red giants in globular clusters of
moderately subsolar metallicity and to a sample of bulge stars. (...) We
propose that the systematic spectroscopic differences of our inner Galaxy stars
are due to their higher metallicities that cause deeper mixing in their
mantles, resulting in lower surface abundances of C and O and higher abundances
of CN, which contribute to the strength of the CaI and NaI features at low
resolution. Our results stress the limitations of using local stars as
templates for the study of composite cool stellar populations such as central
starbursts in galaxies (Abridged).Comment: 21 pages (including figures), A&A accepte
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