760 research outputs found
Flare in the Galactic stellar outer disc detected in SDSS-SEGUE data
Aims. We explore the outer Galactic disc up to a Galactocentric distance of
30 kpc to derive its parameters and measure the magnitude of its flare.
Methods. We obtained the 3D density of stars of type F8V-G5V with a colour
selection from extinction-corrected photometric data of the Sloan Digital Sky
Survey - Sloan Extension for Galactic Understanding and Exploration
(SDSS-SEGUE) over 1,400 deg^2 in off-plane low Galactic latitude regions and
fitted it to a model of flared thin+thick disc.
Results. The best-fit parameters are a thin-disc scale length of 2.0 kpc, a
thin-disc scale height at solar Galactocentric distance of 0.24 kpc, a
thick-disc scale length of 2.5 kpc, and a thick-disc scale height at solar
Galactocentric distance of 0.71 kpc. We derive a flaring in both discs that
causes the scale height of the average disc to be multiplied with respect to
the solar neighbourhood value by a factor of 3.3^{+2.2}_{-1.6} at R=15 kpc and
by a factor of 12^{+20}_{-7} at R=25 kpc.
Conclusions. The flare is quite prominent at large R and its presence
explains the apparent depletion of in-plane stars that are often confused with
a cut-off at R>15 kpc. Indeed, our Galactic disc does not present a truncation
or abrupt fall-off there, but the stars are spread in off-plane regions, even
at z of several kpc for R>20 kpc. Moreover, the smoothness of the observed
stellar distribution also suggests that there is a continuous structure and not
a combination of a Galactic disc plus some other substructure or extragalactic
component: the hypothesis to interpret the Monoceros ring in terms of a tidal
stream of a putative accreted dwarf galaxy is not only unnecessary because the
observed flare explains the overdensity in the Monoceros ring observed in SDSS
fields, but it appears to be inappropriate.Comment: Accepted for publication in A&
Complete Zeldovich approximation
We have developed a generalization of the Zeldovich approximation (ZA) that
is exact in a wide variety of situations, including plannar, spherical and
cilyndrical symmetries. We have shown that this generalization, that we call
complete Zeldovich approximation (CZA), is exact to second order at an
arbitrary point within any field. For gaussian fields, the third order error
have been obtained and shown to be very small. For statistical purposes, the
CZA leads to results exact to the third order.Comment: 11 pages+1 figure, accepted in ApJ Letter
Azimuthal dependence of the density distribution in outer galactic discs accreting intergalactic flows
AIMS. The amplitude and scaleheight of the Galactic gas disc density are not
axisymmetric against expectations in a self-gravity axisymmetric disc. However,
this lopsidedness can be explained in terms of intergalactic accretion flows,
which produce non-axisymmetric pressure on the disc. This mechanism could be
also responsible for the formation of a warp.
METHODS. We analytically derive the relationship between the disc density and
the self-gravity and external pressure.
RESULTS. The same scenario of accretion as we proposed years ago to explain
the formation of the warp explains the azimuthal dependence of the density and
its scaleheight, with minimum/maximum in the positions of maximum amplitude of
the warp (phi=95 deg. and 275 deg.), as expected from its pressure
distribution.Comment: 4 pages, accepted to be published in A&A-letter
Fluctuations of K-band galaxy counts
We measure the variance in the distribution of off-plane (|b|>20 deg.)
galaxies with m_K<13.5 from the 2MASS K-band survey in circles of diameter
between 0.344 deg. and 57.2 deg. The use of a near-infrared survey makes
negligible the contribution of Galactic extinction to these fluctuations. We
calculate these variances within the standard Lambda-CDM model assuming that
the sources are distributed like halos of the corresponding mass, and it
reproduces qualitatively the galaxy counts variance. Therefore, we test that
the counts can be basically explained in terms only of the large scale
structure. A second result of this paper is a new method to determine the two
point correlation function obtained by forcing agreement between model and
data. This method does not need the knowledge of the two-point angular
correlation function, allows an estimation of the errors (which are low with
this method), and can be used even with incomplete surveys.
Using this method we get xi(z=0, r<10 h^{-1}Mpc)=(29.8+/-0.3)
(r/h^{-1}Mpc)^{-1.79+/-0.02}, which is the first measure of the amplitude of xi
in the local Universe for the K-band. It is more or less in agreement with
those obtained through red optical filters selected samples, but it is larger
than the amplitude obtained for blue optical filters selected samples.Comment: 7 pages, 5 figures, accepted to be published in A&
Generation of galactic disc warps due to intergalactic accretion flows onto the disc
A new method is developed to calculate the amplitude of the galactic warps
generated by a torque due to external forces. This takes into account that the
warp is produced as a reorientation of the different rings which constitute the
disc in order to compensate the differential precession generated by the
external force, yielding a uniform asymptotic precession for all rings.
Application of this method to gravitational tidal forces in the Milky Way due
to the Magellanic Clouds leads to a very low amplitude of the warp. If the
force were due to an extragalactic magnetic field, its intensity would have to
be very high, to generate the observed warps. An alternative hypothesis is
explored: the accretion of the intergalactic medium over the disk. A cup-shaped
distortion is expected, due to the transmission of the linear momentum; but,
this effect is small and the predominant effect turns out to be the
transmission of angular momentum, i.e. a torque giving an integral-sign shape
warp. The torque produced by a flow of velocity ~100 km/s and baryon density
\~10^{-25} kg/m^3 is enough to generate the observed warps and this mechanism
offers quite a plausible explanation. First, because this order of accretion
rate is inferred from other processes observed in the Galaxy, notably its
chemical evolution. The inferred rate of infall of matter, ~1 solar-mass/yr, to
the Galactic disc that this theory predicts agrees with the quantitative
predictions of this chemical evolution resolving key issues, notably the
G-dwarf problem. Second, because the required density of the intergalactic
medium is within the range of values compatible with observation. By this
mechanism, we can explain the warp phenomenon in terms of intergalactic
accretion flows onto the disk of the galaxy.Comment: 18 pages, 11 figures, accepted to be published in A&
Tracing the long bar with red-clump giants
Over the last decade a series of results have lent support to the hypothesis
of the existence of a long thin bar in the Milky Way with a half-length of 4.5
kpc and a position angle of around 45 deg. This is apparently a very different
structure from the triaxial bulge of the Galaxy.
In this paper, we analyse the stellar distribution in the inner 4 kpc of the
Galaxy to see if there is clear evidence for two triaxial or barlike
structures, or whether there is only one. By using the red-clump population as
a tracer of the structure of the inner Galaxy we determine the apparent
morphology of the inner Galaxy. Star counts from 2MASS are used to provide
additional support for this analysis.
We show that there are two very different large-scale triaxial structures
coexisting in the inner Galaxy: a long thin stellar bar constrained to the
Galactic plane (|b|<2 deg) with a position angle of 43.1 +- 1.8 deg, and a
distinct triaxial bulge that extends to at least |b|<7.5 deg with a position
angle of 12.6 +- 3.2 deg. The scale height of the bar source distribution is
around 100 pc, whereas for the bulge the value of this parameter is five times
larger.Comment: 16 pages, 35 figures, accepted for publication in A&
Inversion of stellar statistics equation for the Galactic Bulge
A method based on Lucy (1974, AJ 79, 745) iterative algorithm is developed to
invert the equation of stellar statistics for the Galactic bulge and is then
applied to the K-band star counts from the Two-Micron Galactic Survey in a
number of off-plane regions (10 deg.>|b|>2 deg., |l|<15 deg.).
The top end of the K-band luminosity function is derived and the morphology
of the stellar density function is fitted to triaxial ellipsoids, assuming a
non-variable luminosity function within the bulge. The results, which have
already been outlined by Lopez-Corredoira et al.(1997, MNRAS 292, L15), are
shown in this paper with a full explanation of the steps of the inversion: the
luminosity function shows a sharp decrease brighter than M_K=-8.0 mag when
compared with the disc population; the bulge fits triaxial ellipsoids with the
major axis in the Galactic plane at an angle with the line of sight to the
Galactic centre of 12 deg. in the first quadrant; the axial ratios are
1:0.54:0.33, and the distance of the Sun from the centre of the triaxial
ellipsoid is 7860 pc. The major-minor axial ratio of the ellipsoids is found
not to be constant. However, the interpretation of this is controversial. An
eccentricity of the true density-ellipsoid gradient and a population gradient
are two possible explanations.
The best fit for the stellar density, for 1300 pc<t<3000 pc, are calculated
for both cases, assuming an ellipsoidal distribution with constant axial
ratios, and when K_z is allowed to vary. From these, the total number of bulge
stars is ~ 3 10^{10} or ~ 4 10^{10}, respectively.Comment: 19 pages, 23 figures, accepted in MNRA
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