602 research outputs found
Modeling Star counts in the Monoceros stream and the Galactic anti-centre
There is a continued debate as to the form of the outer disc of the Milky Way
galaxy, which has important implications for its formation. Stars are known to
exist at a galacto-centric distance of at least 20 kpc. However, there is much
debate as to whether these stars can be explained as being part of the disc or
whether another extra galactic structure, the so called Monoceros ring/stream,
is required. To examine the outer disc of the Galaxy toward the anti-centre to
determine whether the star counts can be explained by the thin and thick discs
alone. Using Sloan star counts and extracting the late F and early G dwarfs it
is possible to directly determine the density of stars out to a galacto-centric
distance of about 25 kpc. These are then compared with a simple flared disc
model. A flared disc model is shown to reproduce the counts along the line of
sights examined, if the thick disc does not have a sharp cut off. The flare
starts at a Galacto-centric radius of 16 kpc and has a scale length of
4.5+/-1.5 kpc. Whilst the interpretation of the counts in terms of a
ring/stream cannot be definitely discounted, it does not appear to be
necessary, at least along the lines of sight examined towards the anti centre.Comment: 11 pages, 4 figures, accepted to be published in A&
Reaffirming the connection between the Galactic stellar warp and the Canis Major overdensity
We perform a critical re-analysis and discussion of recent results presented
in the literature which interpret the CMa overdensity as the signature of an
accreting dwarf galaxy or a new substructure within the Galaxy. Several issues
are addressed. We show that arguments against the ``warp'' interpretation are
based on an erroneous perception of the Milky Way. There is nothing anomalous
with colour--magnitude diagrams on opposite sides of the average warp mid-plane
being different. We witnessed the rise and fall of the blue plume population,
first attributed to young stars in a disrupting dwarf galaxy and now discarded
as a normal disc population. Similarly, there is nothing anomalous in the outer
thin+thick disc metallicities being low (-1<[Fe/H]<-0.5), and spiral arms (as
part of the thin disc) should, and do, warp. Most importantly, we show
unambiguously that, contrary to previous claims, the warp produces a stellar
overdensity that is distance-compatible with that observed in CMa.The CMa
over-density remains fully accounted for in a first order approach by Galactic
models without new substructures. Given the intrinsic uncertainties (concerning
the properties of the warp, flare and disc cutoff, the role of extinction and
degeneracy), minor deviations with respect to these models are not enough to
support the hypothesis of an accreted dwarf galaxy or new substructure within
the Milky Way disc.Comment: A&A Letter, accepted, 4 pages, 3 figure
Galactic Warp in the overdensity of the Canis Major Region
Bellazzini et al. (2006b) claim that Lopez-Corredoira et al.'s (2002) warp
model is totally unable to reproduce the Canis Major structure in the red clump
stars. However, slight variations in the azimuth of the Lopez-Corredoira et al.
(2002) warp model, justified by the uncertainties in the parameter as well as
the local irregularities with respect to the average model, derive a result
much closer to the observations of the overdensity south/north. The bump of red
clump stars with m_K=13-13.5 around l=241 deg., b=-8.5 deg. and the depth of
the Canis Major structure are also explainable in terms of the warp with an
appropriate extrapolation of constant height between galactocentric radii of 13
and 16 kpc, as observed roughly in the southern warp, instead of a
monotonically increasing height like the northern warp; and the observed
velocity distribution of stars cannot exclude the warp possibility. A warp
model is therefore still a possible explanation of the Canis Major overdensity,
and the hypothesis of the existence of a dwarf galaxy is unnecessary, although
still a possibility too.Comment: 6 pages, 3 figures, accepted to be published in MNRA
Against the Tide. A Critical Review by Scientists of How Physics and Astronomy Get Done
Nobody should have a monopoly of the truth in this universe. The censorship and suppression of challenging ideas against the tide of mainstream research, the blacklisting of scientists, for instance, is neither the best way to do and filter science, nor to promote progress in the human knowledge. The removal of good and novel ideas from the scientific stage is very detrimental to the pursuit of the truth. There are instances in which a mere unqualified belief can occasionally be converted into a generally accepted scientific theory through the screening action of refereed literature and meetings planned by the scientific organizing committees and through the distribution of funds controlled by "club opinions". It leads to unitary paradigms and unitary thinking not necessarily associated to the unique truth. This is the topic of this book: to critically analyze the problems of the official (and sometimes illicit) mechanisms under which current science (physics and astronomy in particular) is being administered and filtered today, along with the onerous consequences these mechanisms have on all of us.\ud
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The authors, all of them professional researchers, reveal a pessimistic view of the miseries of the actual system, while a glimmer of hope remains in the "leitmotiv" claim towards the freedom in doing research and attaining an acceptable level of ethics in science
Probing the Canis Major stellar over-density as due to the Galactic warp
Proper-motion, star counts and photometric catalog simulations are used to
explain the detected stellar over-density in the region of Canis Major (CMa),
claimed to be the core of a disrupted dwarf galaxy (Martin et al. 2004,
Bellazzini et al. 2003), as due to the Galactic warp and flare in the external
disk. We compare the kinematics of CMa M-giant selected sample with surrounding
Galactic disk stars in the UCAC2 catalog and find no peculiar proper motion
signature: CMa stars mimic thick disk kinematics. Moreover, when taking into
account the Galactic warp and flare of the disk, 2MASS star count profiles
reproduce the CMa stellar over-density. This star count analysis is confirmed
by direct comparison with synthetic color-magnitude diagrams simulated with the
Besancon models (Robin et al. 2003) that include the warp and flare of the
disk. The presented evidence casts doubt on the identification of the CMa
over-density as the core of a disrupted Milky Way satellite. This however does
not make clear the origin of over-densities responsible for the ring structure
in the anticenter direction of the Galactic halo (Newberg et al. 2002; Yanny et
al. 2003; Zaggia et al. 2004, in preparation).Comment: Accepted for publication in A&A Letters, 4 page
The Milky Way's external disc constrained by 2MASS star counts
Context. Thanks to recent large scale surveys in the near infrared such as
2MASS, the galactic plane that most suffers from extinction is revealed and its
overall structure can be studied. Aims. This work aims at constraining the
structure of the Milky Way external disc as seen in 2MASS data, and in
particular the warp. Methods. We use the Two Micron All Sky Survey (hereafter
2MASS) along with the Stellar Population Synthesis Model of the Galaxy,
developed in Besancon, to constrain the external disc parameters such as its
scale length, its cutoff radius, and the slope of the warp. In order to
properly interpret the observations, the simulated stars are reddened using a
three dimensional extinction map. The shape of the stellar warp is then
compared with previous results and with similar structures in gas and dust.
Results. We find new constraints on the stellar disc, which is shown to be
asymmetrical, similar to observations of HI. The positive longitude side is
found to be easily modelled with a S shape warp but with a slope significantly
smaller than the slope seen in the HI warp. At negative longitudes, the disc
presents peculiarities which are not well reproduced by any simple model.
Finally, comparing with the warp seen in the dust, it seems to follow a slope
intermediate between the gas and the stars.Comment: 9 pages. Accepted for publication in Astronomy and Astrophysic
Chemical abundance patterns in the inner Galaxy: the Scutum Red Supergiant Clusters
The location of the Scutum Red-Supergiant (RSG) clusters at the end of the
Galactic Bar makes them an excellent probe of the Galaxy's secular evolution;
while the clusters themselves are ideal testbeds in which to study the
predictions of stellar evolutionary theory. To this end, we present a study of
the RSGs' surface abundances using a combination of high-resolution H-band
spectroscopy and spectral synthesis analysis. We provide abundance measurements
for elements C, O, Si, Mg, Ti, and Fe. We find that the surface abundances of
the stars studied are consistent with CNO burning and deep, rotationally
enhanced mixing. The average a/Fe ratios of the clusters are solar, consistent
with a thin-disk population. However, we find significantly sub-solar Fe/H
ratios for each cluster, a result which strongly contradicts a simple
extrapolation of the Galactic metallicity gradient to lower Galacto-centric
distances. We suggest that a simple one-dimensional parameterization of the
Galaxy's abundance patterns is insufficient at low Galactocentric distances, as
large azimuthal variations may be present. Indeed, we show that the abundances
of O, Si and Mg are consistent with independent measurements of objects in
similar locations in the Galaxy. In combining our results with other data in
the literature, we present evidence for large-scale (~kpc) azimuthal variations
in abundances at Galacto-centric distances of 3-5kpc. While we cannot rule-out
that this observed behaviour is due to systematic offsets between different
measurement techniques, we do find evidence for similar behaviour in a study of
the barred-spiral galaxy NGC4736 which uses homogeneous methodology. We suggest
that these azimuthal abundance variations could result from the intense but
patchy star formation driven by the potential of the central bar.Comment: 13 pages, 9 figures, accepted to Ap
Why the Canis Major overdensity is not due to the Warp: analysis of its radial profile and velocities
In response to criticism by Momany et al. (2004), that the
recently-identified Canis Major (CMa) overdensity could be simply explained by
the Galactic warp, we present proof of the existence of a stellar population in
the direction of CMa that cannot be explained by known Galactic components. By
analyzing the radial distribution of counts of M-giant stars in this direction,
we show that the Momany et al. (2004) warp model overestimates the number of
stars in the Northern hemisphere, hence hiding the CMa feature in the South.
The use of a better model of the warp has little influence on the morphology of
the overdensity and clearly displays an excess of stars grouped at a distance
of D=7.2\pm 0.3 kpc. To lend further support to the existence of a population
that does not belong to the Galactic disc, we present radial velocities of
M-giant stars in the centre of the CMa structure that were obtained with the
2dF spectrograph at the AAT. The extra population shows a radial velocity of
vr=109\pm4 km/s, which is significantly higher than the typical velocity of the
disc at the distance of CMa. This population also has a low dispersion (13\pm4
km/s). The Canis Major overdensity is therefore highly unlikely to be due to
the Galactic warp, adding weight to the hypothesis that we are observing a
disrupting dwarf galaxy or its remnants. This leads to questions on what part
of CMa was previously identified as the Warp and how to possibly disentangle
the two structures.Comment: 5 pages, 4 figures, in press at MNRAS. Revised version with new data
and discussio
Dark matter in the Milky Way, II. the HI gas distribution as a tracer of the gravitational potential
Context. Gas within a galaxy is forced to establish pressure balance against
gravitational forces. The shape of an unperturbed gaseous disk can be used to
constrain dark matter models. Aims. We derive the 3-D HI volume density
distribution for the Milky Way out to a galactocentric radius of 40 kpc and a
height of 20 kpc to constrain the Galactic mass distribution. Methods. We used
the Leiden/Argentine/Bonn all sky 21-cm line survey. The transformation from
brightness temperatures to densities depends on the rotation curve. We explored
several models, reflecting different dark matter distributions. Each of these
models was set up to solve the combined Poisson-Boltzmann equation in a
self-consistent way and optimized to reproduce the observed flaring. Results.
Besides a massive extended halo of M ~ 1.8 10^{12} Msun, we find a
self-gravitating dark matter disk with M=2 to 3 10^{11} Msun, including a dark
matter ring at 13 < R < 18.5 kpc with M = 2.2 to 2.8 10^{10} Msun. The
existence of the ring was previously postulated from EGRET data and coincides
with a giant stellar structure that surrounds the Galaxy. The resulting Milky
Way rotation curve is flat up to R~27 kpc and slowly decreases outwards. The
\hi gas layer is strongly flaring. The HWHM scale height is 60 pc at R = 4 kpc
and increases to ~2700R \la 40$ kpc can be successfully modeled by a self-gravitating
isothermal disk raises the question of whether this massive disk may have been
caused by similar merger events in the past.Comment: 19 pages, 21 figures, accepted for publication by A&
86 GHz SiO maser survey of late-type stars in the Inner Galaxy. I. Observational data
We present 86 GHz (v = 1, J = 2 -1) SiO maser line observations with the IRAM
30-m telescope of a sample of 441 late-type stars in the Inner Galaxy (-4 degr
< l < +30 degr). These stars were selected on basis of their infrared
magnitudes and colours from the ISOGAL and MSX catalogues. SiO maser emission
was detected in 271 sources, and their line-of-sight velocities indicate that
the stars are located in the Inner Galaxy. These new detections double the
number of line-of-sight velocities available from previous SiO and OH maser
observations in the area covered by our survey and are, together with other
samples of e.g. OH/IR stars, useful for kinematic studies of the central parts
of the Galaxy.Comment: 15 pages, 12 figures, accepted by A&A Journa
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