1. • IMF2.png
Testing the Universality of the Stellar IMF Using Chandra
Dave Coulter1, Bret Lehmer2, Rafael Eufrasio2
1. Portland State University, Department of Physics, PO Box 751, Portland, OR 97207
2. X-ray Astrophysics Laboratory, NASA Goddard Space Flight Center/Johns Hopkins UniversityContext
Methods
Results
Peacock et al. (2014)Low σ => low mass
Conroy & van
Dokkum (2012)
NGC 4621
NGC 524
Low Mass
High Mass
Peacock et al. (2014)
Galaxy Sample: Early-Type, Low Mass Ellipticals Computing LuminositiesEvidence for a Variable IMF?
• Clues to IMF shape: FeH traces dwarf star
populations => steep IMF for high mass ellipticals
• Quandary: Low mass ellipticals still well described by
Kroupa (MW) IMF
• We construct synthetic spectra using ratio of
soft and hard counts
• Converting photon counts first to flux, we then
calculate luminosities
• We combine data from all sources to create
luminosity functions
Conroy&vanDokkum(2012)
• Local galaxies ≈ 150-200 Mly
• Low mass selected (σ ≈ 100 km s-1) to
fill in the gaps in Peacock et al. 2014
• Age and metallicity comparable to
Peacock survey group
• Green ellipses denote galaxy
boundaries from IR 2MASS survey
• We use IR data to compute galaxy K-
band luminosity (2µm)
• Small circles denote X-ray sources
• Cyan circles have an X-ray luminosity ≥
1038 ergs s-1
Soft band
0.5 – 2.0 keV
Hard band
2.0 – 7.0 keV
NGC 4387
NGC 4458
NGC 4550
NGC 4551
NGC 7457
NGC 4339
normalizedcountss-1keV-1
Energy (keV)
Previous Work Initial Results and Future Work
• Model blends between
Kroupa (MW) IMF and
steep IMF => predicts low
mass galaxies to have more
X-ray binaries per mass
• Region our low mass galaxy
points will fill
• Model blends between
Kroupa (MW) IMF and
shallow IMF => predicts
low mass galaxies to have
less X-ray binaries per mass
• Significantly expanded catalog of measured galaxies
• Current work has created some constraints on model
fitting
• Several models were generated and will be tested for
best fit
AcknowledgmentsReferences
Cappellari, M et al., 2012,
Nature, 484 - 485
Peacock, M, et al., 2014,
ApJ, 784, 70
van Dokkum, P. G., &
Conroy, C. 2012, ApJ, 760,
71
van Dokkum, P. G., &
Conroy, C. 2010, van
Dokkum, Nature, 468, 940
Special thanks to Bret
Lehmer and Rafael Eufrasio,
without whose help I would
still be reading Ciao
documentation.
Also many thanks to USRA
for generously funding this
reseach, and NASA GSFC for
providing this summer 2015
opportunity.
Ensemble point
(avg of new points)
Newly measured
galaxies
Original Peacock points
• Initial findings suggest an invariant
IMF may be plausible
Building Models & Providing Constraints
LinkedIn
Contact
davecoulter@gmail.com
X-Ray Binary Stars & Chandra A Mystery…
• Its shape describes the relative
proportion of low mass to high mass
stars.
• Assumed to be universal
• An X-ray binary is a star system
with a compact object (black
hole or neutron star), and a
companion star
• As the companion is accreted
onto the compact object,
gravitational potential energy
is converted to X-ray light.
• Chandra observes from
High Earth Orbit
• Our observations use a
0.5 – 7.0 keV bandpass
with a 0.5” resolution
• Systematic increase
in M/L as a function
of galaxy mass
disagrees with
models… Why?
• Dark matter?
Rejected by
Cappellari et al.
(2012)
• Variable IMF? M/L
can be explained by
both steep and
shallow IMF
Pearson Education, Inc.
What is the IMF?
• For every burst of star formation, the
initial mass function (IMF) describes
the distribution of stellar masses
Increasing mass
IncreasingMasstoLight
(MW)
Conroy & van Dokkum (2010)
Fender & Belloni; Science Magazine (2012)
NASA/CXC/NGST