A Parametric Galactic Model toward the Galactic Bulge Based on Gaia and Microlensing Data. (arXiv:2104.03306v1 [astro-ph.GA])

A Parametric Galactic Model toward the Galactic Bulge Based on Gaia and Microlensing Data. (arXiv:2104.03306v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Koshimoto_N/0/1/0/all/0/1">Naoki Koshimoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Baba_J/0/1/0/all/0/1">Junichi Baba</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bennett_D/0/1/0/all/0/1">David P. Bennett</a>

We developed a parametric Galactic model toward the Galactic bulge by fitting
to spatial distributions of the Gaia DR2 disk velocity, VVV proper motion,
BRAVA radial velocity, OGLE-III red clump star count, and OGLE-IV star count
and microlens rate, optimized for use in microlensing studies. We include the
asymmetric drift of Galactic disk stars and the dependence of velocity
dispersion on Galactic location in kinematic model, which has been ignored in
most previous models used for microlensing studies. We show that our model
predicts a microlensing parameter distribution that is significantly different
from the one with a typically used model in the previous studies. Through our
modeling, we estimate various fundamental model parameters for our Galaxy,
including the initial mass function (IMF) in the inner Galaxy. Combined
constraints from star counts and the microlensing event timescale distribution
from the OGLE-IV survey, in addition to a prior on the bulge stellar mass,
enable us to successfully measure IMF slopes using a broken power law form over
a broad mass range, $alpha_{rm bd} = 0.22^{+0.20}_{-0.55}$ for $M < 0.08 ,
M_{odot}$, $alpha_{rm ms} = 1.16^{+0.08}_{-0.15}$ for $0.08 , M_{odot}
leq M < M_{rm br}$, and $alpha_{rm hm} = 2.32^{+0.14}_{-0.10}$ for $M geq
M_{rm br}$, as well as a break mass at $M_{rm br} = 0.90^{+0.05}_{-0.14} ,
M_{odot}$. This is significantly different from the Kroupa IMF for local
stars, but similar to the Zoccali IMF measured from a bulge luminosity
function. We also estimate the dark matter mass fraction in the bulge region of
$28 pm 7$% which could be larger than a previous estimate. Because our model
is purely parametric, it can be universally applied using the parameters
provided in this paper.

We developed a parametric Galactic model toward the Galactic bulge by fitting
to spatial distributions of the Gaia DR2 disk velocity, VVV proper motion,
BRAVA radial velocity, OGLE-III red clump star count, and OGLE-IV star count
and microlens rate, optimized for use in microlensing studies. We include the
asymmetric drift of Galactic disk stars and the dependence of velocity
dispersion on Galactic location in kinematic model, which has been ignored in
most previous models used for microlensing studies. We show that our model
predicts a microlensing parameter distribution that is significantly different
from the one with a typically used model in the previous studies. Through our
modeling, we estimate various fundamental model parameters for our Galaxy,
including the initial mass function (IMF) in the inner Galaxy. Combined
constraints from star counts and the microlensing event timescale distribution
from the OGLE-IV survey, in addition to a prior on the bulge stellar mass,
enable us to successfully measure IMF slopes using a broken power law form over
a broad mass range, $alpha_{rm bd} = 0.22^{+0.20}_{-0.55}$ for $M < 0.08 ,
M_{odot}$, $alpha_{rm ms} = 1.16^{+0.08}_{-0.15}$ for $0.08 , M_{odot}
leq M < M_{rm br}$, and $alpha_{rm hm} = 2.32^{+0.14}_{-0.10}$ for $M geq
M_{rm br}$, as well as a break mass at $M_{rm br} = 0.90^{+0.05}_{-0.14} ,
M_{odot}$. This is significantly different from the Kroupa IMF for local
stars, but similar to the Zoccali IMF measured from a bulge luminosity
function. We also estimate the dark matter mass fraction in the bulge region of
$28 pm 7$% which could be larger than a previous estimate. Because our model
is purely parametric, it can be universally applied using the parameters
provided in this paper.

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