442 research outputs found
The impact of spheroid stars for Macho microlensing surveys of the Andromeda Galaxy
The Andromeda Galaxy (M31) is an important test case for a number of
microlensing surveys looking for massive compact halo objects (Machos). A
long-standing theoretical prediction is that the high inclination of the M31
disk should induce an asymmetry in the spatial distribution of M31 Macho
events, whilst the distribution of variable stars and microlensing events in
the M31 disk should be symmetric. We examine the role of stars in the M31
visible spheroid as both lenses and sources to microlensing events. We compute
microlensing event number density maps and estimate pixel-lensing rates and
event durations for three-component models of M31 which are consistent with the
observed rotation curve, surface brightness profile and dynamical mass
estimates. Three extreme models are considered: a massive spheroid model; a
massive disk model; and a massive Macho halo model. An important consequence of
the spheroid is that, even if Machos are absent in M31, an asymmetric spatial
signature is still generally expected from stellar lensing alone. The relative
mass-to-light ratio of the spheroid and disk populations controls which of
these signatures dominates the overall stellar spatial distribution. We find
that the inclusion of the spheroid weakens the M31 Macho spatial asymmetry by
about 20-30% over a disk-only asymmetry for the models considered. We also find
for our models that Machos dominate over most of the far disk provided they
contribute at least ~25% of the halo dark matter density. The presence of the
spheroid also has beneficial consequences for M31 lensing surveys. The stellar
asymmetry is likely to be important in distinguishing between a spheroidal
Macho halo or a highly flattened halo or dark matter dominated disk, since
spatial asymmetries of opposing signs are expected in these cases. (Abridged)Comment: 10 pages. Accepted for publication in MNRA
The microlensing rate and distribution of free-floating planets towards the Galactic bulge
Ground-based optical microlensing surveys have provided tantalising, if
inconclusive, evidence for a significant population of free-floating planets
(FFPs). Both ground and space-based facilities are being used and developed
which will be able to probe the distrubution of FFPs with much better
sensitivity. It is vital also to develop a high-precision microlensing
simulation framework to evaluate the completeness of such surveys. We present
the first signal-to-noise limited calculations of the FFP microlensing rate
using the Besancon Galactic model. The microlensing distribution towards the
Galactic centre is simulated for wide-area ground-based optical surveys such as
OGLE or MOA, a wide-area ground-based near-IR survey, and a targeted
space-based near-IR survey which could be undertaken with Euclid or WFIRST. We
present a calculation framework for the computation of the optical and
near-infrared microlensing rate and optical depth for simulated stellar
catalogues which are signal-to-noise limited, and take account of extinction,
unresolved stellar background light and finite source size effects, which can
be significant for FFPs. We find that the global ground-based I-band yield over
a central 200 deg^2 region covering the Galactic centre ranges from 20
Earth-mass FFPs year^-1 up to 3,500 year^-1 for Jupiter FFPs in the limit of
100% detection efficiency, and almost an order of magnitude larger for a K-band
survey. For ground-based surveys we find that the inclusion of finite source
and the unresolved background reveals a mass-dependent variation in the spatial
distribution of FFPs. For a space-based H-band covering 2 deg^2, the yield
depends on the target field but maximizes close to the Galactic centre with
around 76 Earth through to 1,700 Jupiter FFPs year^-1. For near-IR space-based
surveys the spatial distribution of FFPs is found to be largely insensitive to
the FFP mass scale.Comment: 14 pages, submitted to A&A and accepte
Microlensing Halo Models with Abundant Brown Dwarfs
All previous attempts to understand the microlensing results towards the
Large Magellanic Cloud (LMC) have assumed homogeneous present day mass
functions (PDMFs) for the lensing populations. Here, we present an
investigation into the microlensing characteristics of haloes with spatially
varying PDMFs and anisotropic velocity dispersion tensors. One attractive
possibility -- suggested by baryonic dark cluster formation in pregalactic and
protogalactic cooling flows -- is that the inner halo is dominated by stellar
mass objects, whereas low mass brown dwarfs become more prevalent on moving
outwards. The contribution to the microlensing rate must be dominated by dark
remnants (of about 0.5 solar masses) to recover the observed timescales of the
microlensing experiments. But, even though stellar remnants control the rate,
they do not dominate the mass of the baryonic halo, and so the well-known
enrichment and mass budget problems are much less severe. Using a simple ansatz
for the spatial variation of the PDMF, models are constructed in which the
contribution of brown dwarfs to the mass of the baryonic halo is 55 % and to
the total halo is 30 %. An unusual property of the models is that they predict
that the average timescale of events towards M31 is shorter than the average
timescale towards the LMC. This is because the longer line of sight towards M31
probes more of the far halo where brown dwarfs are the most common constituent.Comment: 17 pages, 1 figure, in press at The Astrophysical Journal (Letters
Reference image selection for difference imaging analysis
Difference image analysis (DIA) is an effective technique for obtaining
photometry in crowded fields, relative to a chosen reference image. As yet,
however, optimal reference image selection is an unsolved problem. We examine
how this selection depends on the combination of seeing, background and
detector pixel size. Our tests use a combination of simulated data and quality
indicators from DIA of well-sampled optical data and under-sampled
near-infrared data from the OGLE and VVV surveys, respectively. We search for a
figure-of-merit (FoM) which could be used to select reference images for each
survey. While we do not find a universally applicable FoM, survey-specific
measures indicate that the effect of spatial under-sampling may require a
change in strategy from the standard DIA approach, even though seeing remains
the primary criterion. We find that background is not an important criterion
for reference selection, at least for the dynamic range in the images we test.
For our analysis of VVV data in particular, we find that spatial under-sampling
is best handled by reversing the standard DIA procedure and convolving target
images to a better-sampled (poor seeing) reference image.Comment: 14 pages, 8 figures, 4 tables, accepted for publication in MNRA
Latest results from the POINT-AGAPE pixel-lensing survey of the Andromeda Galaxy
I report on recent results from the POINT-AGAPE pixel-lensing experiment,
which is engaged in a search towards the Andromeda galaxy (M31) for
gravitational microlensing signatures from massive compact halo objects
(Machos). An analysis of two years of data reveals over 360 light-curves
compatible with microlensing. The third year of data, currently being analysed,
will be crucial in determining how many of these candidates are long-period
variables rather than microlensing. Within the dataset we have isolated a
subset of four high signal-to-noise ratio, short duration events which are
compelling microlensing candidates. The properties and possible origins of
these events are discussed.Comment: 6 pages, 4 figures. Proceedings contribution for the 4th
International Workshop on "The Identification of Dark Matter", York,
September 2nd - 6th 200
Analysis of a Hubble Space Telescope Search for Red Dwarfs: Limits on Baryonic Matter in the Galactic Halo
We re-examine a deep {\it Hubble Space Telescope} pencil-beam search for red
dwarfs, stars just massive enough to burn Hydrogen. The authors of this search
(Bahcall, Flynn, Gould \& Kirhakos 1994) found that red dwarfs make up less
than 6\% of the galactic halo. First, we extrapolate this result to include
brown dwarfs, stars not quite massive enough to burn hydrogen; we assume a
mass function. Then the total mass of red dwarfs and brown dwarfs
is 18\% of the halo. This result is consistent with microlensing results
assuming a popular halo model. However, using new stellar models and parallax
observations of low mass, low metallicity stars, we obtain much tighter bounds
on low mass stars. We find the halo red dwarf density to be of the halo,
while our best estimate of this value is 0.14-0.37\%. Thus our estimate of the
halo mass density of red dwarfs drops to 16-40 times less than the reported
result of Bahcall et al (1994). For a mass function, this suggests
a total density of red dwarfs and brown dwarfs of 0.25-0.67\% of the
halo, \ie , (0.9-2.5)\times 10^9\msun out to 50 kpc. Such a low result would
conflict with microlensing estimates by the \macho\ group (Alcock \etal
1995a,b).Comment: 13 pages, 2 figures. Figure one only available via fax or snail-mail
To be published in ApJL. fig. 2 now available in postscript. Some minor
changes in dealing with disk forground. Some cosmetic changes. Updated
reference
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