Email: ie{at}astro{dot}caltech{dot}edu

Office: 256 Cahill

1200 E California Blvd
Pasadena, CA 91125
MC 249-17

Phone: (626)-395-6857


Detailed elemental abundances from low resolution stellar spectroscopy

(Figure from Escala, Kirby, Gilbert, et al. 2018b, in prep)

Studying the M31 stellar halo is the body of my thesis work. Stellar halos provide a record of the earliest stages of a galaxy's formation as well as the mass growth of later epochs. Measurements of [Fe/H] and [α/Fe] can probe the minor merging history of a galaxy, providing a direct way to test the hierarchical assembly paradigm. The stellar halo and tidal streams of M31 provide an essential contrast to the same structures around the Milky Way. While measurements of [Fe/H] and [α/Fe] have been made in the Milky Way, little is known about the detailed chemical abundances of the M31 system. To make progress with existing telescopes, I have applied spectral synthesis to low-resolution spectroscopy across a wide spectral range (from blue to NIR wavelengths). We have obtained deep, low-resolution spectra of red giants in the tidal streams and stellar halo of M31, resulting in higher signal-to-noise per spectral resolution element. By applying my technique to red giant branch (RGB) stars in Milky Way globular clusters with existing measurements from higher-resolution spectroscopy, I have demonstrated that my technique reproduces previous measurements derived from higher resolution spectra over a more limited spectral range (red and NIR wavelengths). We aim to apply this method to RGB stars in the stellar halo of M31 to obtain measurements of [Fe/H] and [α/Fe] of sufficient quality to construct quantitative models of galactic chemical evolution in the M31 system.

Modeling chemical abundance distributions of Local Group dwarf galaxies

(Figures from Escala et al. 2018)

As a junior graduate student at Caltech, I investigated the stellar metallicity distribution functions (MDFs), including iron and alpha element abundances, in dwarf galaxies from the Feedback in Realistic Environments (FIRE) project. I examined both isolated dwarf galaxies and those that are satellites of a Milky Way mass galaxy. In particular, I studied the effects of a sub-grid turbulent model for the diffusion of metals in gas. Compared to simulations without diffusion, we observe a narrowing of both the metallicity distribution function (left) and abundance ratio distributions, which results from individual particles being driven toward the average metallicity. This effect results in better agreement with observations of Local Group dwarf galaxies (right), e.g., in terms of the metallicity distribution function shape and width. We also find agreement between the small intrinsic scatter in the alpha-to-iron ratio in simulations and observations of dwarf galaxies. This implies that the interstellar medium in dwarf galaxies is well-mixed at nearly all cosmic times. Lastly, via comparison between isolated and satellite dwarf galaxies, I found that the similarly between their chemical abundance distributions suggests that environmental effects play a minor role compared to internal chemical evolution in the FIRE simulations.

Determining the physical characteristics of a set of unusually magnetically active brown dwarfs

See Table 4 and Section 6.2 of Kao et al. 2016. Figure from Escala et al., in prep.

As an undergraduate, I worked on developing a semi-empirical method to determine the physical parameters of a set of late L-type and T-type brown dwarfs from their low-resolution, near-infrared spectra. I calibrated empirical H20-J and K-H spectral indicides, which disentangle effective temperature and surface gravity, to spectral indicies generated from brown dwarf atmospheric models, using the benchmark brown dwarfs Gliese 570D (T7.5) and HN Peg B (T2.5) as fiducials. We improved on prior work by Burgasser et al. by including a probabilistic treatment of spectral indicies and integrating the uncertainties in the adopted benchmark parameteres and the measured indicies via Monte Carlo methods. I compared the results to those from atmospheric model fitting, and checked that the results were consistent between calibrations. The set of brown dwarfs exhibits unusual magnetic activity, thus their parameters may provide insight into dynamo theory in the substellar objects regime.