136 research outputs found
On the Correlation Between Metallicity and the X-Shaped Morphology of the Milky Way Bulge
We demonstrate that failure to properly account for stellar evolution can
bias results in determinations of the spatial morphology of Galactic bulge
stars, focusing on the question of whether or not the X-shape is more
pronounced among the more metal-rich stars than among the metal-poor stars. We
argue that this trend, a result recently claimed by three separate groups, may
have been overestimated as it is relatively easier to detect a bimodality in
the distance distribution function at higher metallicities. This is due to
three factors. First, the intrinsic colour of red clump and red giant stars
vary with metallicity, at the level d(V-I)_{RC}/d\[M/H] ~ 0.25 mag dex^{-1},
and thus the ratio of red clump to red giant stars within a spectroscopic
sample will depend on the photometric selection of any investigation. Second,
the duration of ascent of the red giant branch goes down and the red clump
lifetime goes up as metallicity increases, which has the effect of increasing
the ratio of red clump to red giant stars by as much as ~33% over the range of
the bulge metallicity-distribution function. Finally, over the same metallicity
interval, the effective number of red giant branch bump stars is predicted to
increase by ~200%, and their presence becomes degenerate with the observational
parameters of the two red clumps, creating an illusory increase in
signal-to-noise for a second peak in the distance modulus distribution.Comment: 6 pages, 2 figures, accepted for publication in MNRA
Strong Evidence that the Galactic Bulge is Shining in Gamma Rays
There is growing evidence that the Galactic Center Excess identified in the
-LAT gamma-ray data arises from a population of faint
astrophysical sources. We provide compelling supporting evidence by showing
that the morphology of the excess traces the stellar over-density of the
Galactic bulge. By adopting a template of the bulge stars obtained from a
triaxial 3D fit to the diffuse near-infrared emission, we show that it is
detected at high significance. The significance deteriorates when either the
position or the orientation of the template is artificially shifted, supporting
the correlation of the gamma-ray data with the Galactic bulge. In deriving
these results, we have used more sophisticated templates at low-latitudes for
the bubbles compared to previous work and the
three-dimensional Inverse Compton (IC) maps recently released by the team. Our results provide strong constraints on Millisecond Pulsar
(MSP) formation scenarios proposed to explain the excess. We find that an
scenario, in which some of the relevant binaries
are and the rest are formed , is
preferred over a primordial-only formation scenario at confidence
level. Our detailed morphological analysis also disfavors models of the
disrupted globular clusters scenario that predict a spherically symmetric
distribution of MSPs in the Galactic bulge. For the first time, we report
evidence of a high energy tail in the nuclear bulge spectrum that could be the
result of IC emission from electrons and positrons injected by a population of
MSPs and star formation activity from the same site.Comment: 21 pages, 13 figures, V2: Minor changes to match submitted version,
V3: matches JCAP published versio
The Mira-based distance to the Galactic centre
Mira variables are useful distance indicators, due to their high luminosities
and well-defined period-luminosity relation. We select 1863 Miras from SAAO and
MACHO observations to examine their use as distance estimators in the Milky
Way. We measure a distance to the Galactic centre of kpc,
which is in good agreement with other literature values. The uncertainty has
two components of 0.2 kpc each: the first is from our analysis and
predominantly due to interstellar extinction, the second is due to zero-point
uncertainties extrinsic to our investigation, such as the distance to the Large
Magellanic Cloud (LMC). In an attempt to improve existing period-luminosity
calibrations, we use theoretical models of Miras to determine the dependence of
the period-luminosity relation on age, metallicity, and helium abundance, under
the assumption that Miras trace the bulk stellar population. We find that at a
fixed period of , changes in the predicted magnitudes can
be approximated by , and these coefficients are
nearly independent of period. The expected overestimate in the Galactic centre
distance from using an LMC-calibrated relation is 0.3 kpc. This
prediction is not validated by our analysis; a few possible reasons are
discussed. We separately show that while the predicted color-color diagrams of
solar-neighbourhood Miras work well in the near-infrared, though there are
offsets from the model predictions in the optical and mid-infrared.Comment: Accepted for publication in The Astrophysical Journal. 16 pages, 8
figures, 6 table
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