2,568 research outputs found
NGC 2419 does not challenge MOND, Part 2
I argue that, despite repeated claims of Ibata et al., the globular cluster
NGC 2419 does not pose a problem for modified Newtonian dynamics (MOND). I
present a new polytropic model with a running polytropic index. This model
provides an improved representation of the radial distribution of surface
brightness while maintaining a reasonable fit to the velocity dispersion
profile. Although it may be argued that the differences with these observations
remain large compared to the reported random errors, there are several
undetectable systematic effects which render a formal likelihood analysis
irrelevant. I comment generally upon these effects and upon the intrinsic
limitations of pressure supported objects as tests of gravity.Comment: 3 page, 2 figure
Galactic halo stellar structures in the Triangulum-Andromeda region
This letter reports on the Galactic stellar structures that appear in the
foreground of our Canada-France-Hawaii-Telecopse/MegaCam survey of the halo of
the Andromeda galaxy. We recover the main sequence and main sequence turn-off
of the Triangulum-Andromeda structure recently found by Majewski and
collaborators at a heliocentric distance of ~20 kpc. The survey also reveals
another less populated main sequence at fainter magnitudes that could
correspond to a more distant stellar structure at ~28 kpc. Both structures are
smoothly distributed over the ~76 sq. deg. covered by the survey although the
closer one shows an increase in density by a factor of ~2 towards the
North-West. The discovery of a stellar structure behind the
Triangulum-Andromeda structure that itself appears behind the low-latitude
stream that surrounds the Galactic disk gives further evidence that the inner
halo of the Milky Way is of a spatially clumpy nature.Comment: accepted for publication in ApJL, 4 pages, 4 figures. Significant
changes including a larger dataset and a more thorough discussio
A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way
We present an analysis of the asymmetries in the population of Galactic
M-giant stars present in the 2MASS All Sky catalogue. Several large-scale
asymmetries are detected, the most significant of which is a strong
elliptical-shaped stellar over-density, close to the Galactic plane at (l=240,
b=-8), in the constellation of Canis Major. A small grouping of globular
clusters (NGC 1851, NGC 1904, NGC 2298, and NGC 2808), coincident in position
and radial velocity, surround this structure, as do a number of open clusters.
The population of M-giant stars in this over-density is similar in number to
that in the core of the Sagittarius dwarf galaxy. We argue that this object is
the likely dwarf galaxy progenitor of the ring-like structure that has recently
been found at the edge of the Galactic disk. A numerical study of the tidal
disruption of an accreted dwarf galaxy is presented. The simulated debris fits
well the extant position, distance and velocity information on the ``Galactic
Ring'', as well as that of the M-giant over-densities, suggesting that all
these structures are the consequence of a single accretion event. The disrupted
dwarf galaxy stream orbits close to the Galactic Plane, with a pericentre at
approximately the Solar circle, an orbital eccentricity similar to that of
stars in the Galactic thick disk, as well as a vertical scale height similar to
that of the thick disk. This finding strongly suggests that the Canis Major
dwarf galaxy is a building block of the Galactic thick disk, that the thick
disk is continually growing, even up to the present time, and that thick disk
globular clusters were accreted onto the Milky Way from dwarf galaxies in
co-planar orbits.Comment: 13 pages, 18 figures (2 in colour), accepted for publication in MNRA
The flaring HI disk of the nearby spiral galaxy NGC 2683
New deep VLA D array HI observations of the highly inclined nearby spiral
galaxy NGC 2683 are presented. Archival C array data were processed and added
to the new observations. To investigate the 3D structure of the atomic gas
disk, we made different 3D models for which we produced model HI data cubes.
The main ingredients of our best-fit model are (i) a thin disk inclined by 80
degrees; (ii) a crude approximation of a spiral and/or bar structure by an
elliptical surface density distribution of the gas disk; (iii) a slight warp in
inclination; (iv) an exponential flare; and (v) a low surface-density gas ring.
The slope of NGC 2683's flare is comparable, but somewhat steeper than those of
other spiral galaxies. NGC 2683's maximum height of the flare is also
comparable to those of other galaxies. On the other hand, a saturation of the
flare is only observed in NGC 2683. Based on the comparison between the high
resolution model and observations, we exclude the existence of an extended
atomic gas halo around the optical and thin gas disk. Under the assumption of
vertical hydrostatic equilibrium we derive the vertical velocity dispersion of
the gas. The high turbulent velocity dispersion in the flare can be explained
by energy injection by (i) supernovae, (ii) magneto-rotational instabilities,
(iii) ISM stirring by dark matter substructure, or (iv) external gas accretion.
The existence of the complex large-scale warping and asymmetries favors
external gas accretion as one of the major energy sources that drives
turbulence in the outer gas disk. We propose a scenario where this external
accretion leads to turbulent adiabatic compression that enhances the turbulent
velocity dispersion and might quench star formation in the outer gas disk of
NGC 2683.Comment: accepted for publication in A&
Was SN1997ff at z~1.7 magnified by gravitational lensing?
The quest for the cosmological parameters has come to fruition with the
identification of a number of supernovae at a redshift of . Analyses of
the brightness of these standard candles reveal that the Universe is dominated
by a large cosmological constant. The recent identification of the
SN1997ff in the northern Hubble Deep Field has provided further evidence for
this cosmology. Here we examine the case for gravitational lensing of SN1997ff
due to the presence of galaxies lying along our line of sight. We find that,
while the alignment of SN1997ff with foreground masses was not favorable for it
to be multiply imaged and strongly magnified, two galaxies did lie close enough
to result in significant magnification: for the case where these
elliptical galaxies have velocity dispersion . Given the small
difference between supernova brightnesses in different cosmologies, detailed
modeling of the gravitational lensing properties of the intervening matter is
therefore required before the true cosmological significance of SN1997ff can be
deduced.Comment: 3 pages, 2 figures. Accepted for publication in MNRAS. Missing
reference adde
Sagittarius: The Nearest Dwarf Galaxy
We have discovered a new Galactic satellite galaxy in the constellation of
Sagittarius. The Sagittarius dwarf galaxy is the nearest galaxy known, subtends
an angle of degrees on the sky, lies at a distance of 24 \kpc from the
Sun, \sim 16 \kpc from the centre of the Milky Way. Itis comparable in size
and luminosity to the largest dwarf spheroidal, has a well populated red
horizontal branch with a blue HB extension; a substantial carbon star
population; and a strong intermediate age stellar component with evidence of a
metallicity spread. Isodensity maps show it to be markedly elongated along a
direction pointing towards the Galactic centre and suggest that it has been
tidally distorted. The close proximity to the Galactic centre, the
morphological appearance and the radial velocity of 140 km/s indicate that this
system must have undergone at most very few close orbital encounters with the
Milky Way. It is currently undergoing strong tidal disruption prior to being
integrated into the Galaxy. Probably all of the four globular clusters, M54,
Arp 2, Ter 7 and Ter 8, are associated with the Sagittarius dwarf galaxy, and
will probably share the fate of their progenitor.Comment: MNRAS in press, 22pp uuencoded PS file, 26 printed figures available
on request from [email protected]
Blue Horizontal Branch Stars in the Sagittarius dwarf spheroidal galaxy
We report on the recovery of a Blue Horizontal Branch (BHB) population
belonging to the Sagittarius Dwarf Spheroidal Galaxy (Sgr). The sequence is
clearly identified in the (V, V-I) Color Magnitude Diagram (CMD) obtained for
about 500,000 stars in the region of the globular cluster M~54. The BHB
morphology is similar to the analogous sequence in M~54, but it is
unambiguously associated with Sgr since {\it(i)} it is detected well outside
the main body of the cluster, up to more than 5 tidal radii from the cluster
center and {\it(ii)} the BHB stars follow the radial distribution of the other
stellar populations of Sgr. This finding finally demonstrates that the Sgr
galaxy hosts a significant (of the order of 10%) old and metal-poor
stellar population ([Fe/H]\ltsima -1.3; age \gtsima 10 Gyr), similar to that of
its oldest clusters (M~54, Ter~8). We also show that the Sgr BHB sequence found
here is the counterpart of the analogous feature observed by Newberg et al.
(2002) in the Sgr Stream, in a field more than 80\degr away from the center
of the galaxy.Comment: 12 pages, 4 figures. Accepted for publication in ApJ Letter
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