Chandra X-Ray Image of the Elliptical Galaxy NGC 720

NASA Press Release for NGC 720

See Articles in The Boston Globe and

David A. Buote

Experimental Astrophysics
X-Ray Astronomy


Academic History

Professor Buote received the A.B. Degree in Physics from UC Berkeley (1990), and the S.M. (1992) and Ph.D. (1995) Degrees in Physics from MIT. After a brief postdoctoral study at MIT, he moved to England to assume a Research Associate position at the Institute of Astronomy, Cambridge University. In 1998 he was awarded a Chandra Fellowship which he held at Lick Observatory, UC Santa Cruz. He joined the faculty at UC Irvine in January, 2001.

Research Summary

My research uses X-ray observations as a probe of galaxy formation, a topic that encompasses a wide range of key unresolved astrophysical problems such as the nature and distribution of dark matter in the universe and the ejection of metals into the intergalactic medium. X-ray observations offer a unique window to address these and other aspects of galaxy formation. Of particular relevance to my current research is that X-ray observations provide the only means to study the diffuse hot gas (T = 10-100 MK) in elliptical galaxies, groups, and clusters of galaxies, which usually extends to larger radii and, in the case of clusters, contains even more baryonic mass than do the stars. With data being provided by two milestone satellites, the Chandra X-ray Observatory and the X-ray Multi-Mirror Mission (XMM), the field of X-ray astronomy has entered an exciting period of discovery. The focus of my current research is to use the outstanding data from these satellites in conjunction with data from other wavebands to build a unified picture of how galaxies and systems of galaxies assemble and evolve into the structures that we see today.

Dark Matter

We have used data from the Chandra Observatory to place strong constraints on the dark matter (DM) on both galaxy and cluster scales. By analyzing the shape and orientation of the X-ray image of the elliptical galaxy NGC 720 we showed that the image contours are too elongated to be explained by the gravitational field generated by the observed stars. Therefore, there must be an extended, massive DM halo that is similarly elongated to, or slightly more so, than the stellar image -- a conclusion reinforced by the observed offset in the position angles of the major axes of the X-ray and stellar image contours. This geometric evidence for DM also cannot be explained by general modified gravity theories such as MOND which instead attribute signatures of DM to a violation of Newton's Law of Gravitation. This evidence for DM is apparently the first dynamical tracer in any stellar system that has successfully distinguished DM from MOND. The inferred elongated shape of the DM halo in NGC 720 also rules out the spherical halos predicted by the models that assume the DM interacts with itself via a force other than gravity. Yet the measured elongation of the DM is still weaker than the disk-like DM distributions predicted by models that assume the DM consists of cold molecular material.

Chandra X-Ray Image of the Galaxy Cluster A2029

NASA Press Release for A2029

See Articles in , Knight Ridder News Service , and San Jose Mercury News

Using another high resolution Chandra observation we have analyzed the deprojected temperature and density profile of the hot X-ray emitting gas in the massive galaxy cluster A2029 to measure the radial mass profile deep down into its core. The Chandra image reveals this cluster to have a highly regular morphology on scales down to less than 1% of its virial radius, making it unusually well-suited for the study of the mass profile on the smallest scales in a galaxy cluster. We find the radial mass profile to be highly consistent with the (NFW) prediction of the standard Cold Dark Matter model and inconsistent with the shallow density profiles predicted for the self-interacting dark matter model.

Metals and Star Formation

The metals present in the hot gas of groups and clusters of galaxies provide a fossil record of the star formation history of these systems. We have completed initial analyses of the XMM data of two of the brightest galaxy groups, NGC 1399 and NGC 5044. In both cases we obtained strong constraints on the Fe and Si abundances as a function of radius: both abundances take values between 1-2 solar within 30-50 kpc radii of these systems and decrease to values 0.3-0.5 solar out to the largest radii probed (~100 kpc). In particular, the super-solar Fe abundances obtained with XMM confirm that the very sub-solar values obtained in the central regions of these and other groups with data from previous satellites arose from an Fe Bias caused by neglecting the non-isothermal distribution of the hot gas. These Fe and Si abundances measured with XMM imply that approximately 80% of the Fe mass within r ~50 kpc originates from Type Ia supernovae (SNIa). This SNIa fraction is similar to that inferred for the Sun and therefore suggests a stellar initial mass function similar to the Milky Way.

Galaxy Cluster Dynamics & Radio Halos

The hot gas in a galaxy cluster provides a vital window to its current dynamical state through measurements of temperature variations and image morphology. By quantifying cluster morphologies with power ratios (essentially ratios of multipole moments), I have recently provided the first quantitative comparison of the dynamical states of clusters possessing radio halos. A correlation between the 1.4 GHz power of the radio halo (or relic) and the magnitude of the dipole power ratio was discovered such that these quantities are (essentially) directly proportional to each other. This correlation not only confirms previous circumstantial evidence relating the presence of radio halos to mergers but, more importantly, establishes for the first time a quantitative relationship between the ``strength'' of radio halos and relics and the ``strength'' of mergers; i.e., the strongest radio halos appear only in those clusters currently experiencing the largest departures from a virialized state. The radio power / dynamical state correlation supports the idea that shocks in the X-ray gas generated by mergers of subclusters accelerate (or re-accelerate) the relativistic particles responsible for the radio emission.


List of Refereed Publications and Some Conference Proceedings

List of Preprints at