CV in PDF
Cluster Science with ASTRO-H (Hitomi)
MIT News: Dead X-ray satellite reveals "quiet" center of massive galaxy cluster
Miller, Simionescu, & Bautz. 2016 'Simulating
Astro-H Observations of Sloshing Gas Motions in the Cores of Galaxy
Clusters', ApJ, 821, 62
et al. 2014 'ASTRO-H White Paper: Clusters of
Galaxies and Related Science', arXiv:1412.1176
al. 2014 'ASTRO-H White Paper: New Spectral
List of all ASTRO-H White Papers
In-Flight Calibration of High-Energy Astrophysics
The International Astronomical
Consortium for High-Energy Calibration (IACHEC)
Suzaku X-ray Imaging Spectrometer
The Outskirts of Clusters of Galaxies
2014 PhD Thesis, University of Maryland
al. 2012 AIP Conference Proceedings, 1427, 13
Fossil Groups of Galaxies: Their Nature &
al. 2016 poster presented at April 2016 AAS
High-Energy Astrophysics Divison (HEAD) Meeting, Naples, FL
al. 2012 'Finding Fossil Groups: Optical
Identification and X-Ray Confirmation', ApJ, 747, 94
Supernovae and Superbubbles: Enrichment and Heating of
the Local ISM
Miller, Tsunemi, Bautz et al. 2008 PASJ, 60S, 95
Figure 1. ROSAT 3/4 keV map of the Galactic plane. The Suzaku pointing
toward the North Polar Spur is identified.
The interstellar medium (ISM) of our Galaxy consists of a number of phases
with different temperatures and chemical abundances, regulated by the
processes of star formation and evolution. Hot, X-ray emitting gas of 1-3
million K is observed in bubbles and superbubbles throughout the Galaxy.
Such features are observed in the Local Hot Bubble, evolved supernova
remnants such as the Cygnus Loop, and the North Polar Spur, a region of
enhanced soft X-ray and radio emission projected above the plane of the
Galaxy (see Figure 1). The North Polar Spur (NPS) X-ray emission is
thought to arise from reheating of a shell swept out by the continuous
stellar winds and supernovae from the Scorpius-Centaurus OB association, at
a distance of about 200 pc from the Sun.
Working with Prof. Hiroshi Tsunemi (Osaka University) and members of the
Suzaku Science Working Group, I have analyzed Suzaku
observations of the NPS to constrain the plasma conditions along this line
of sight. The good low-energy effective area and spectral resolution of
the X-ray Imaging Spectrometers (XIS) allow us to detect NVII (from
N+6) and CVI (from C+5) emission for the first time.
We have discovered an excess of nitrogen in the hot NPS plasma compared to
the solar value, indicating enrichment from evolved AGB stars. Due to the
time necessary for these stars to evolve, they are unlikely to exist in the
Sco-Cen association; instead the stellar activity is reheating
previously-enriched material in the ISM. This result supports other recent
studies that hint at enhanced AGB enrichment in the local Galaxy.
This research was performed as part of a short-term postdoctoral fellowship
from the Japanese Society for the Promotion of Science (JSPS). I lived in
Japan for six months from September 2005 through March 2006.
The Neutral Gaseous Halos of Milky Way-type Galaxies:
Galaxy Accretion vs. Galactic Fountain
Miller, Bregman & Wakker 2009 ApJ, 692, 470
Miller & Bregman 2005 Extra-planar Gas, ASP Conf. Series, Vol. 331, ed. R. Braun
Figure 2. Position-velocity slice of the HI emission in M 83,
centered on the location of an HVC candidate. Emission between 500-600
km/s (heliocentric) is due to the normal HI disk. The HVC is indicated
by the crosshairs, and it has a deviation velocity of +70 km/s.
While the interstellar medium of galaxies has been studied for decades, our
understanding of the diffuse gas surrounding galaxies remains in its
infancy. Spiral galaxies like the Milky Way appear to be embedded in a
halo of hot (T ~ 106 K), X-ray and UV-emitting gas.
ionized gas (DIG) is seen in edge-on spiral galaxies, with ionized hydrogen
emission extending up to 5 kpc from the galaxy plane.
Cold neutral gas is observed to be
falling onto our own Milky Way galaxy in the form of high-velocity clouds
(HVCs), systems of neutral hydrogen that are moving at speeds between
80-300 km/s relative to the Local Standard of Rest.
Recent results have hinted at an interaction between the HVCs and the hot
Despite the recent slew of observational data, debate continues about the
origin of the various phases of gas. While the DIG material is likely
produced within the disk,
the hot X-ray halo has been
explained as material either injected by supernova-driven superbubbles in
the disk or accreted from the hot intergalactic medium that pervades our
Local Group of galaxies. The nature of the
HVCs is likewise controversial, with scenarios such as a supernova-driven
Galactic fountain and satellite accretion placing them
within 5-100 kpc, while a third model places them inside dark matter
mini-halos at Local Group distances of about 1 Mpc.
These mini-halos, if confirmed, would constitute a dynamically important
fraction of the mass in the local universe.
For my Ph.D. dissertation project, I worked with Joel Bregman (U. Michigan)
and Bart Wakker (U. Wisconsin) to search the galaxies M 83 and M 51 in
neutral hydrogen (HI) emission for HVC analogs.
These galaxies are similar in morphology to the Milky Way.
The deep observations are 5--10 times more sensitive than previous
studies, and they reveal a layer of HI
gas that rotates more slowly than the kinematically cold HI disk.
Similar structures have been seen in a handful of other spiral galaxies
consistent with the predictions of the galactic fountain model.
Using an automated detection technique, we discovered a number of
discrete HI clouds, likely HVC
analogs, in both galaxies.
An example is shown in Figure 2, which shows a
position-velocity plot displaying a clump of HI emission at a projected
velocity 70 km/s different from that of the HI disk.
This work represents the first systematic search for small-scale anomalous
gas within other galaxies, and the results indicate that both the galactic
fountain and satellite accretion models are necessary to explain all the
detected HVC analogs. Extrapolation to the Milky Way, assuming that the
HVC analogs are drawn from a similar sample as the Milky Way ensemble,
indicates that the typical distance to Galactic HVCs is less than about 25
These results garnered attention for a press release at the
January 2004 meeting of the American Astronomical Society. Full details of the work are
Miller et al. (2009),
and my dissertation is available
online. We have obtained new
data that are undergoing analysis, and this will be a continuing project
over the next several years.
Baryons in Cosmic Filaments
Miller, Dupke & Bregman 2006 Proceedings of the 2004 STScI Symposium, ed. M. Livio & S. Casertano
Bregman, Dupke & Miller 2004 ApJ, 614, 31
Most of the baryons in the local universe are believed to be in a warm to
hot phase (105-107 K) and lie in moderate overdensity
regions, such as filaments connecting rich clusters of galaxies.
. This is the Warm-Hot Intergalactic Medium
(WHIM), often referred to as the ``cosmic web'' of baryons. Renato Dupke
(U. Michigan), Joel Bregman and I are working on a project to search for
Lyman series and O+5 (OVI) absorption against quasars projected
behind filaments, which we have mapped between supercluster members
(z < 0.05 for the supercluster sample).
Initial results from HST UV spectroscopy show Ly alpha absorption from three
filaments against four background quasars. While
interesting, large numbers of sightlines are necessary to understand
systematic effects and constrain the density of the WHIM. Additional
quasars have been observed in recent cycles of HST and FUSE, and these
data are being analyzed. We have also identified a sample of quasar
spectra available in the HST and FUSE data archives, and have begun
analysis of these data. This project is the most comprehensive attempt to
date to detect cosmic filaments, and results will place constraints on the
hot baryon phase model.
Verification of the Elliptical Galaxy Cooling Flow
Bregman, Otte, Miller & Irwin 2006 ApJ, 642, 759
Bregman, Fabian, Miller & Irwin 2006 ApJ, 642, 746
Bregman, Miller, Athey & Irwin 2005 ApJ, 635, 1031
Bregman, Miller & Irwin 2001 ApJ Letters, 553, L125
Many elliptical galaxies emit strongly in the X-ray in processes thought to
arise from 5-10 million K gas. The X-ray brightest ones actually radiate
energy so strongly that the gas should lose all of its thermal energy (i.e.
"cool") in much less than the age of the universe. A reasonable result is
that the gas should lose its buoyancy and flow towards the center of the
galaxy in a what is termed a "cooling flow." The cooling flow is expected
to be a steady-state, since gas coming off evolved stars and being accreted
by the galaxy will replenish the hot gas that has cooled. There is
contradictory evidence for and against this cooling flow model.
To test the model, Joel Bregman (U. Michigan), Jimmy Irwin (U. Michigan)
and I proposed observations to detect OVI (O+5) emission from a
sample of 23 galaxies. OVI is produced by gas near 300,000 K, so if the
hot gas is indeed cooling, it should pass through this temperature regime
and we should detect the OVI emission. We detect OVI in four galaxies, and
it seems to be at about the level one would expect if the same amount of
gas was cooling through 300,000 K as we see cooling at higher temperatures.
Three galaxies which should show OVI emission (i.e. which have strong X-ray
emission and thus should have strong cooling flows) do not, and place
strict upper limits on the amount of cooling material. Something in these
galaxies is dirupting the cooling process, possibly supernovae heating the
Probing Nearby Systems with Quasars Absorption
Knezek & Bregman 1999 ApJ Letters, 510, 95
Quasars make ideal background sources to probe the intervening diffuse
medium, as they are point-like, distant, UV and optically bright, and
remarkably free of spectral features. We have used quasars to probe the
ISM of elliptical galaxies, many of which contain hot X-ray gas but were
long thought to be free of cool gas. In collaboration with Joel Bregman,
Sara Ellison (U. Victoria), and Michael Murphy (IoA, Cambridge), I have
obtained HST, FUSE, and VLT/UVES spectra of a bright quasar projected
behind the outer part of the lenticular galaxy NGC 4203. We see
multi-phase absorption from HI, NaI, CaII, SiII, OVI, and many other
species, and the velocity structure hints at either an overlying
high-velocity cloud or a diffuse, absorbing halo. The absorption
properties are similar to more distant Lyman alpha absorbers, and its
proximity allows us to identify the site of absorption and draw conclusions
about quasar absorption line systems in general.
A serendipitous quasar sightline through the halo of the edge-on galaxy NGC
891 shows strong resonance absorption from cool gas, despite the projected
height of 5 kpc from the plane. The ISM in this galaxy has been
well-studied, and our observations of cool, metal-enriched gas will further
our understanding of the extra-planar medium of spiral galaxies.
Cool Gas in Clusters of Galaxies
Bregman & Knezek 2002 ApJ, 569, 134
Working with Joel Bregman (U. Michigan), I have used spectra of background
quasars to search for cool absorbing gas in rich clusters of galaxies. The
lack of observed absorption places limits on the mass loss from cluster
member galaxies and the lifetime of cool gas before it is heated by the hot
NGC 891 Surface Brightness Study - Senior Honors Project (Oberlin College)
Thesis (PDF, 112 pages, 1.7 MB)
Morrison, Miller, Harding, Stinebring & Boroson 1997 AJ, 113, 2061
My Senior Honors Project involved a faint optical surface-brightness study
of the edge-on spiral galaxy NGC 891, under the watchful eyes of Dan
Stinebring (Oberlin College) and Heather Morrison (Case Western Reserve
University). Among other things, we were interested in determining the
presence and nature of a thick disk and faint luminous halo associated with
Last updated: Wed Jul 6 09:13:06 EDT 2016