102,888 research outputs found
Angular momentum evolution of bulge stars in disc galaxies in NIHAO
We study the origin of bulge stars and their angular momentum (AM) evolution
in 10 spiral galaxies with baryonic masses above M in the
NIHAO galaxy formation simulations. The simulated galaxies are in good
agreement with observations of the relation between specific AM and mass of the
baryonic component and the stellar bulge-to-total ratio (). We divide the
star particles at into disc and bulge components using a hybrid
photometric/kinematic decomposition method that identifies all central mass
above an exponential disc profile as the `bulge'. By tracking the bulge star
particles back in time, we find that on average 95\% of the bulge stars formed
{\it in situ}, 3\% formed {\it ex situ} in satellites of the same halo, and
only 2\% formed {\it ex situ} in external galaxies. The evolution of the AM
distribution of the bulge stars paints an interesting picture: the higher the
final ratio, the more the specific AM remains preserved during the bulge
formation. In all cases, bulge stars migrate significantly towards the central
region, reducing their average galactocentric radius by roughly a factor 2,
independently of the final value. However, in the higher
() objects, the velocity of the bulge stars increases and the AM of
the bulge is almost conserved, whereas at lower values, the velocity of
the bulge stars decreases and the AM of bulge reduces. The correlation between
the evolution of the AM and suggests that bulge and disc formation are
closely linked and cannot be treated as independent processes.Comment: 17 pages, 16 Figures, 1 table; accepted for publication in MNRA
Chemical Composition of Faint (I~21 mag) Microlensed Bulge Dwarf OGLE-2007-BLG-514S
We present a high-resolution spectrum of a microlensed G dwarf in the
Galactic bulge with spectroscopic temperature T_eff = 5600 +/- 180 K. This I~21
mag star was magnified by a factor ranging from 1160 to 1300 at the time of
observation. Its high metallicity ([Fe/H] = 0.33 +/- 0.15) places this star at
the upper end of the bulge giant metallicity distribution. Using a K-S test, we
find a 1.6% probability that the published microlensed bulge dwarfs share an
underlying distribution with bulge giants, properly accounting for a radial
bulge metallicity gradient. We obtain abundance measurements for 15 elements
and perform a rigorous error analysis that includes covariances between
parameters. This star, like bulge giants with the same metallicity, shows no
alpha enhancement. It confirms the chemical abundance trends observed in
previously analyzed bulge dwarfs. At supersolar metallicities, we observe a
discrepancy between bulge giant and bulge dwarf Na abundances.Comment: 13 pages, 8 figures, 5 tables, submitted to Ap
Dynamical evolution of a bulge in an N-body model of the Milky Way
The detailed dynamical structure of the bulge in the Milky Way is currently
under debate. Although kinematics of the bulge stars can be well reproduced by
a boxy-bulge, the possible existence of a small embedded classical bulge can
not be ruled out. We study the dynamical evolution of a small classical bulge
in a model of the Milky Way using a self-consistent high resolution N-body
simulation. Detailed kinematics and dynamical properties of such a bulge are
presented.Comment: 2 pages, 2 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand Bornand (April 17-22, 2011), C. Reyle, A.
Robin, M. Schultheis (eds.
Growth of galactic bulges by mergers. II. Low-density satellites
Satellite accretion events have been invoked for mimicking the internal
secular evolutionary processes of bulge growth. However, N-body simulations of
satellite accretions have paid little attention to the evolution of bulge
photometric parameters, to the processes driving this evolution, and to the
consistency of this evolution with observations. We want to investigate whether
satellite accretions indeed drive the growth of bulges, and whether they are
consistent with global scaling relations of bulges and discs. We perform N-body
models of the accretion of satellites onto disc galaxies. A Tully-Fisher (M
\propto V_{rot}^ {alpha_TF}) scaling between primary and satellite ensures that
density ratios, critical to the outcome of the accretion, are realistic. We
carry out a full structural, kinematic and dynamical analysis of the evolution
of the bulge mass, bulge central concentration, and bulge-to-disc scaling
relations. The remnants of the accretion have bulge-disc structure. Both the
bulge-to-disc ratio (B/D) and the Sersic index (n) of the remnant bulge
increase as a result of the accretion, with moderate final bulge Sersic
indices: n = 1.0 to 1.9. Bulge growth occurs no matter the fate of the
secondary, which fully disrupts for alpha_TF=3 and partially survives to the
remnant center for alpha_TF = 3.5 or 4. Global structural parameters evolve
following trends similar to observations. We show that the dominant mechanism
for bulge growth is the inward flow of material from the disc to the bulge
region during the satellite decay. The models confirm that the growth of the
bulge out of disc material, a central ingredient of secular evolution models,
may be triggered externally through satellite accretion.Comment: Accepted for publication in A&A, 20 pages, 11 figures. Figs. 1 and 2
are low resolution ones: high-resolution versions available under request to
the author
YOUNG STARS IN AN OLD BULGE: A NATURAL OUTCOME OF INTERNAL EVOLUTION IN THE MILKY WAY
The center of our disk galaxy, the Milky Way, is dominated by a boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar photometry have concluded that the bulge stars are exclusively old. The perceived lack of young stars in the bulge strongly constrains its likely formation scenarios, providing evidence that the bulge is a unique population that formed early and separately from the disk. However, recent studies of individual bulge stars using the microlensing technique have reported that they span a range of ages, emphasizing that the bulge may not be a monolithic structure. In this Letter we demonstrate that the presence of young stars that are located predominantly nearer to the plane is expected for a bulge that has formed from the disk via dynamical instabilities. Using an N-body+ smoothed particle hydrodynamics simulation of a disk galaxy forming out of gas cooling inside a dark matter halo and forming stars, we find a qualitative agreement between our model and the observations of younger metal-rich stars in the bulge. We are also able to partially resolve the apparent contradiction in the literature between results that argue for a purely old bulge population and those that show a population comprised of a range in ages; the key is where to look
Reconciling the Galactic Bulge Turnoff Age Discrepancy with Enhanced Helium Enrichment
We show that the factor 2 discrepancy between spectroscopic and
photometric age determinations of the Galactic bulge main-sequence turnoff can
be naturally explained by positing an elevated helium enrichment for the bulge
relative to that assumed by standard isochrones. We obtain an upper bound on
the helium enrichment parameter of the bulge
given the requirement that
the spectroscopic and photometric ages be consistent and the limiting condition
of instantaneous star formation. The corresponding mean age for the bulge is
Gyr. We discuss phenomenological evidence that the
bulge may have had a chemical evolution that is distinct from the solar
neighborhood in this manner, and we make several testable predictions. Should
this emerging picture of the bulge as helium-enhanced hold, it will require the
development of new isochrones, new model atmospheres, and modified analysis and
cosmological interpretation of the integrated light of other bulges and
elliptical galaxies.Comment: 14 pages, 4 figures. Modified following referee report, subsequently
published in ApJ Letters, For a brief video summarizing the research results,
please see see the Ohio
State Astronomy Youtube channel.</a
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