Research

• Curriculum Vitae
  CV in PDF
  
  
• Cluster Science with ASTRO-H (Hitomi)
  MIT News: Dead X-ray satellite reveals "quiet" center of massive galaxy cluster
  ZuHone, Miller, Simionescu, & Bautz. 2016   'Simulating Astro-H Observations of Sloshing Gas Motions in the Cores of Galaxy Clusters', ApJ, 821, 62
  Kitayama et al. 2014   'ASTRO-H White Paper: Clusters of Galaxies and Related Science', arXiv:1412.1176
  Smith et al. 2014   'ASTRO-H White Paper: New Spectral Features', arXiv:1412.1172
  List of all ASTRO-H White Papers
  
  
• In-Flight Calibration of High-Energy Astrophysics Instrumentation
  The International Astronomical Consortium for High-Energy Calibration (IACHEC)
  Suzaku X-ray Imaging Spectrometer
  
  
• The Outskirts of Clusters of Galaxies
  George, J. 2014   PhD Thesis, University of Maryland
  Miller et al. 2012   AIP Conference Proceedings, 1427, 13
  
  
• Fossil Groups of Galaxies: Their Nature & Evolution
  Miller et al. 2016   poster presented at April 2016 AAS High-Energy Astrophysics Divison (HEAD) Meeting, Naples, FL
  Miller et 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
  Press Release
  Thesis

  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. Diffuse 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 halo gas.

  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 and are 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 kpc.

  These results garnered attention for a press release at the January 2004 meeting of the American Astronomical Society. Full details of the work are published in 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 Picture
  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 gas.

  
  
• Probing Nearby Systems with Quasars Absorption Lines
  Miller, 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
  Miller, 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 intra-cluster medium.

  
  
• 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 this galaxy.