61 research outputs found
Near infrared star counts as a test of Galactic bar structure
We present survey data in the narrow-band L filter (nbL), taken at UKIRT, for
a total area of 277 square arcmin, roughly equally divided between four regions
at zero Galactic latitude and longitudes +-4.3 deg and +-2.3 deg. The 80%
completeness level for these observations is at roughly magnitude 11.0. This
magnitude limit, owing to the low coefficient for interstellar extinction at
this wavelength (A(nbL)=0.047 A(V)), allows us to observe bulge giants. We
match the nbL-magnitudes with DENIS survey K magnitudes, and find 95 per cent
of nbL sources are matched to K sources. Constructing colour-magnitude
diagrams, we deredden the magnitudes and find evidence for a longitude
dependent asymmetry in the source counts. We find that there are ~15% and ~5%
more sources at the negative longitude than at the corresponding positive
longitude, for the fields at +-4.3 deg and +-2.3 deg respectively. This is
compared with the predictions of some Galactic bar models. We find an asymmetry
in the expected sense, which favours gas dynamical models and the recent
deconvolution of surface photometry data (Binney et al. 1991; Binney, Gerhard &
Spergel 1997), over earlier treatments of photometric data (e.g. Dwek et al.
1995).Comment: 10 pages, Latex, MNRAS accepte
The Inner Galaxy resolved at IJK using DENIS data
We present the analysis of three colour optical/near-infrared images, in IJK,
taken for the DENIS project. The region considered covers 17.4 square deg and
lies within |l|<5 deg, |b|<1.5 deg. The adopted methods for deriving photometry
and astrometry in these crowded images, together with an analysis of the
deficiencies nevertheless remaining, are presented. The numbers of objects
extracted in I,J and K are 748000, 851000 and 659000 respectively, to magnitude
limits of 17,15 and 13. 80% completeness levels typically fall at magnitudes
16, 13 and 10 respectively, fainter by about 2 magnitudes than the usual DENIS
limits due to the crowded nature of these fields. A simple model to describe
the disk contribution to the number counts is constructed, and parameters for
the dust layer derived. We find that a formal fit of parameters for the dust
plane, from these data in limited directions, gives a scalelength and
scaleheight of 3.4+-1.0 kpc and 40+-5 pc respectively, and a solar position
14.0+-2.5 pc below the plane. This latter value is likely to be affected by
localised dust asymmetries. We convolve a detailed model of the systematic and
random errors in the photometry with a simple model of the Galactic disk and
dust distribution, to simulate expected colour-magnitude diagrams. These are in
good agreement with the observed diagrams, allowing us to isolate those stars
from the inner disk and bulge. After correcting for local dust-induced
asymmetries, we find evidence for longitude-dependent asymmetries in the
distant J and K sources, consistent with the general predictions of some
Galactic bar models. We consider complementary L-band observations in a second
paper.Comment: 14 pages, 33 figures, LaTeX, MNRAS accepte
The merging history of the Milky Way
The age distribution, and chemical elemental abundances, of stars in the halo of the Milky Way provide constraints on theories of galaxy formation. As one specific example, the accretion of satellite galaxies similar to the present retinue of dwarf spheroidals (dSphs) would provide an observable metal-poor, intermediate-age population. This paper presents a quantitative assessment of any contribution made by such stars to the stellar halo. The bulk of the stellar populations in the halo show a well-defined turn-off, at B-V ~ 0.4, implying that the vast majority of the stars are old. The fraction of stars which lie blueward of this well-defined turn-off, with metallicities similar to that of the present dSphs, is used in this paper to place limits on the importance of the recent accretion of such systems. Very few (~ -1.5 dex). Direct comparison of this statistic with the colour distribution of the turnoff stars in the Carina dwarf allows us to derive an upper limit on the number of mergers of such satellite galaxies into the halo of the Milky Way. This upper limit is ~ 40 Carina-like galaxies. The higher metallicity data constrain satellite galaxies like the Fornax dwarf; only <~ 5 of these could have been accreted within the last < ~ 10 Gyr. We note that the low star-formation rates inferred for dSphs predict distinctive elemental abundance signatures; future data for field halo stars, including candidate younger stars, will provide a further robust test of accretion models
The Photo-Evaporation of Dwarf Galaxies During Reionization
During the period of reionization the Universe was filled with a cosmological
background of ionizing radiation. By that time a significant fraction of the
cosmic gas had already been incorporated into collapsed galactic halos with
virial temperatures below about 10000 K that were unable to cool efficiently.
We show that photoionization of this gas by the fresh cosmic UV background
boiled the gas out of the gravitational potential wells of its host halos. We
calculate the photoionization heating of gas inside spherically symmetric dark
matter halos, and assume that gas which is heated above its virial temperature
is expelled. In popular Cold Dark Matter models, the Press-Schechter halo
abundance implies that about 50-90% of the collapsed gas was evaporated at
reionization. The gas originated from halos below a threshold circular velocity
of 10-15 km/s. The resulting outflows from the dwarf galaxy population at
redshifts 5-10 affected the metallicity, thermal and hydrodynamic state of the
surrounding intergalactic medium. Our results suggest that stellar systems with
a velocity dispersion below about 10 km/s, such as globular clusters or the
dwarf spheroidal galaxies of the Local Group, did not form directly through
cosmological collapse at high redshifts.Comment: 29 pages, 7 PostScript figures, accepted for ApJ. Final version,
revised due to referee comments. Figures 6 & 7 have been corrected for a
small numerical erro
Further Evidence for a Merger Origin for the Thick Disk: Galactic Stars Along Lines-of-sight to Dwarf Spheroidal Galaxies
The history of the Milky Way Galaxy is written in the properties of its
stellar populations. Here we analyse stars observed as part of surveys of local
dwarf spheroidal galaxies, but which from their kinematics are highly probable
to be non-members. The selection function -- designed to target metal-poor
giants in the dwarf galaxies, at distances of ~100kpc -- includes F-M dwarfs in
the Milky Way, at distances of up to several kpc. Thestars whose motions are
analysed here lie in the cardinal directions of Galactic longitude l ~ 270 and
l ~ 90, where the radial velocity is sensitive to the orbital rotational
velocity. We demonstrate that the faint F/G stars contain a significant
population with V_phi ~ 100km/s, similar to that found by a targeted, but
limited in areal coverage, survey of thick-disk/halo stars by Gilmore, Wyse &
Norris (2002). This value of mean orbital rotation does not match either the
canonical thick disk or the stellar halo. We argue that this population,
detected at both l ~ 270 and l ~ 90, has the expected properties of `satellite
debris' in the thick-disk/halo interface, which we interpret as remnants of the
merger that heated a pre-existing thin disk to form the thick disk.Comment: Accepted, Astrophysical Journal Letter
Tracing Galaxy Formation with Stellar Halos I: Methods
If the favored hierarchical cosmological model is correct, then the Milky Way
system should have accreted ~100-200 luminous satellite galaxies in the past
\~12 Gyr. We model this process using a hybrid semi-analytic plus N-body
approach which distinguishes explicitly between the evolution of light and dark
matter in accreted satellites. This distinction is essential to our ability to
produce a realistic stellar halo, with mass and density profile much like that
of our own Galaxy, and a surviving satellite population that matches the
observed number counts and structural parameter distributions of the satellite
galaxies of the Milky Way. Our model stellar halos have density profiles which
typically drop off with radius faster than those of the dark matter. They are
assembled from the inside out, with the majority of mass (~80%) coming from the
\~15 most massive accretion events. The satellites that contribute to the
stellar halo have median accretion times of ~9 Gyr in the past, while surviving
satellite systems have median accretion times of ~5 Gyr in the past. This
implies that stars associated with the inner halo should be quite different
chemically from stars in surviving satellites and also from stars in the outer
halo or those liberated in recent disruption events. We briefly discuss the
expected spatial structure and phase space structure for halos formed in this
manner. Searches for this type of structure offer a direct test of whether
cosmology is indeed hierarchical on small scales.Comment: 22 pages, 16 figures, submitted to Ap
Young Globular Clusters and Dwarf Spheroidals
Most of the globular clusters in the main body of the Galactic halo were
formed almost simultaneously. However, globular cluster formation in dwarf
spheroidal galaxies appears to have extended over a significant fraction of a
Hubble time. This suggests that the factors which suppressed late-time
formation of globulars in the main body of the Galactic halo were not operative
in dwarf spheroidal galaxies. Possibly the presence of significant numbers of
``young'' globulars at R_{GC} > 15 kpc can be accounted for by the assumption
that many of these objects were formed in Sagittarius-like (but not
Fornax-like) dwarf spheroidal galaxies, that were subsequently destroyed by
Galactic tidal forces. It would be of interest to search for low-luminosity
remnants of parental dwarf spheroidals around the ``young'' globulars Eridanus,
Palomar 1, 3, 14, and Terzan 7. Furthermore multi-color photometry could be
used to search for the remnants of the super-associations, within which outer
halo globular clusters originally formed. Such envelopes are expected to have
been tidally stripped from globulars in the inner halo.Comment: 18 pages, with 2 figures, in LaTeX format; to appear in the
Astrophysical Journal in February 200
Discovery of another peculiar radial distribution of Blue Stragglers in Globular Clusters: The case of 47 Tuc
We have used high resolution WFPC2-HST and wide field ground-based
observations to construct a catalog of blue straggler stars (BSS) in the
globular cluster 47 Tuc spanning the entire radial extent of the cluster.
The BSS distribution is highly peaked in the cluster center, rapidly
decreases at intermediate radii, and finally rises again at larger radii. The
observed distribution closely resembles that discovered in M3 by Ferraro et al
(1993,1997). To date, complete BSS surveys covering the full radial extent (HST
in the center and wide field CCD ground based observations of the exterior)
have been performed for only these two clusters. Both show a bimodal radial
distribution, despite their different dynamical properties. BSS surveys
covering the full spatial extent of more globular clusters are clearly required
to determine how common bimodality is and what its consequence is for theories
of BSS formation and cluster dynamics.Comment: 21 pages, 6 figures, accepted for pubblication in Ap
Galactic Indigestion: Numerical Simulations of the Milky Way's Closest Neighbor
Are dwarf spheroidal galaxies dark matter dominated? We present N-body
simulations of the interaction between the Milky Way and its closest companion,
the Sagittarius dwarf spheroidal galaxy, constrained by new kinematic, distance
and surface density observations detailed in a companion paper. It is shown
that there is no possible self-consistent solution to the present existence of
the Sagittarius dwarf if its distribution of luminous matter traces the
underlying distribution of mass. The luminous component of the dwarf galaxy
must therefore be shielded within a small dark matter halo. Though at present
we are unable to construct a fully self-consistent model that includes both the
stellar and dark matter components, it is shown numerically that it is possible
that a pure dark matter model, approximating the dark matter halo deduced for
the Sagittarius dwarf from analytical arguments, may indeed survive the
Galactic tides.
The orbit of the Sagittarius dwarf galaxy around the Milky Way is considered,
taking into account the perturbative effects of the Magellanic Clouds. It is
shown that at the present time, the orbital period must be short, ~ 0.7 Gyr;
the initial orbital period for a 10^9 Solar mass model will have been ~ 1 Gyr.
It is found that a close encounter with the Magellanic Clouds may have occured,
though the chances of such an interaction affecting the orbit of the
Sagittarius dwarf galaxy is negligible.Comment: To appear in Ap
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