How is the present day structure of a galaxy connected to its past star formation history? We know that early type galaxies populate a relatively tight 2-dimensional plane in the 3-dimensional parameter space described by velocity dispersion, effective radius (R), and effective surface brightness (I). This is known as the Fundamental Plane. We ask the question: do early type galaxies also populate a multi-dimensional space in terms of their star formation histories, analogous to the Fundamental Plane? To address this, we study the stellar populations of galaxies in 3-D Fundamental Plane space and track variations as a function of velocity dispersion, R, and I. We find that early type galaxies do indeed form a 2-D family in stellar population space. However, rather than this 2-D space corresponding to the face-on Fundamental Plane, it in fact corresponds to slices through the thickness of the plane: stellar populations vary with velocity dispersion (as is well known), but vary also with surface brightness for fixed values of velocity dispersion and R. Thus, although the Fundamental Plane is quite tight, outlying galaxies which fall above or below the plane (i.e., at higher or lower I) appear to have experienced different star formation histories than the galaxies which fall on the midplane of the Fundamental Plane, such that lower surface brightness galaxies have experienced shorter durations of star formation, while higher surface brightness galaxies have experienced more prolonged star formation than corresponding galaxies on the midplane. For more information, keep an eye on astro-ph!

The properties of galaxies are often studied as a function of galaxy "size". Different size measurements (stellar mass, luminosity, stellar velocity dispersion) are used roughly interchangeably. However, due to correlated residuals, the stellar populations of galaxies depend in different ways on these various size measurements, leading to results between different studies that appear to be in conflict. By explicitly separating out the stellar population dependencies on velocity dispersion, luminosity (L), and color, we are able to show that velocity dispersion is a better predictor of a galaxy's past star formation history than is L. Stellar population age, metallicity, and enhancement in alpha-elements all correlate strongly with galaxy velocity dispersion. The residual variations in stellar population age and metallicity at fixed velocity dispersion are correlated with one another and with L in such a way as to strengthen the existing metallicity trends and counter-act the existing age trends, leading to a strong L-metallicity relation and a very weak L-age relation. These correlated residuals at fixed velocity dispersion have opposing effects on the galaxy color which tend to cancel out such that there is no color-magnitude relation at fixed velocity dispersion: the observed color-magnitude relation is entirely the result of color-velocity dispersion and magnitude-velocity dispersion relations. For more information, keep an eye on astro-ph!

Recent improvements in the modelling of stellar atmospheres has made it possible to determine the effects of non-Solar abundance patterns on stellar absorption-line spectra. Ricardo Schiavon has taken advantage of these abundance sensitivity calculations to construct stellar population models that include the effects of non-Solar abundances for the elements C, N, O, Fe, Mg, Ca, Na, Si, Cr, and Ti. These models, described in Schiavon (2007), make it possible to dial in your favorite abundance pattern and get out predicted absorption line strengths for a model stellar population. In practice, one most often wants to do the reverse: take an observed set of line strengths and determine the abundance pattern of the input galaxy. We have implemented a method that automates this process and made it publicly available in an IDL code called "EZ_Ages". The method is presented in Graves & Schiavon 2008, astro-ph/0803.1483, along with extensive tests to demonstrate that it works. To download the code and instructions for its use, click here.

A significant fraction of early type galaxies show emission lines characteristic of Low-Ionization Nebular Emission Regions, or LINERs. I used ~6000 spectra from the Sload Digital Sky Survey (SDSS), combining many individual spectra to achieve high signal-to-noise, in order to study the stellar populations of these LINERs, as compared to similar early type galaxies which lack emission. The LINERs have similar metallicities and element abundance ratios, but they are typically ~2 billion years younger than corresponding early type galaxies without emission. These ages represent a luminosity-weighted average of the stars in the galaxy, rather than a true age estimate, but they suggest that LINERs were forming stars up until more recently than their emission-free counterparts. If the LINER emission in these galaxies is powered by low-level accretion onto a black hole at the galaxy's center, this may indicate a link between the star formation history of a galaxy and the activity of its massive black hole. However, the LINER emission may be distributed in many of these galaxies rather than being concentrated around the galaxy center, suggesting that the emission may be powered by post-AGB stars. For more information, see Graves et al. 2007, ApJ, 671, 243.

My undergraduate senior thesis research used HST/ACS images and HST/STIS spectroscopy to put an upper limit on a point source remnant of the SN1987A progenitor. There is still no pulsar in the supernova remnant to date, and no visible point source. Our upper limit (L < 8 x 10^33 ergs/s) was tight enough that we were able to restrict possible accretion scenarios onto a compact remnant in the center of the supernova. Spherical accretion onto either a neutron star or a black hole appears to be incompatible with our limit. If an accretion disk exists in the remnant, it is limited to a low mass accretion rate (< 0.3 Eddington) at a small radius (r < 10^10 cm). For more information, see Graves et al. 2005, ApJ 629, 944.