Faint Stars in a Faint Galaxy: I. Ultra Deep Photometry of the Bo\"otes I Ultra Faint Dwarf Galaxy

TL;DR

This paper presents ultra-deep Hubble Space Telescope imaging of the Boötes I ultra faint dwarf galaxy, analyzing its stellar population and luminosity function to understand its low-mass stars and initial mass function.

Contribution

It provides the deepest photometric data for Boötes I and compares two photometry methods, enhancing understanding of its stellar content and low-mass initial mass function.

Findings

Luminosity function reaches 50% completeness at magnitude 27.4 in F814W.

Probable member stars are consistent with known metallicity and isochrones.

Limiting stellar mass detected is less than 0.3 solar masses.

Abstract

We present an analysis of new extremely deep images of the resolved stellar population of the Bo\"otes I ultra faint dwarf spheroidal galaxy. These new data were taken with the Hubble Space Telescope, using the Advanced Camera for Surveys (Wide Field Camera) and Wide Field Camera 3 (UVIS), with filters F606W and F814W (essentially V and I), as part of a program to derive the low-mass stellar initial mass function in this galaxy. We compare and contrast two approaches to obtaining the stellar photometry, namely ePSF and DAOPHOT. We identify likely members of Bo\"otes I based on the location of each star on the color-magnitude diagram, obtained with the DAOPHOT photometry from the ACS/WFC data. The probable members lie close to stellar isochrones that were chosen to encompass the known metallicity distribution derived from spectroscopic data of brighter radial-velocity member stars and are consistent with the main-sequence turnoff. The resulting luminosity function of the Bo\"otes I galaxy has a 50% completeness limit of 27.4 in F814W and 28.2 in F606W (Vega magnitude system), which corresponds to a limiting stellar mass of $\le 0.3 M_\odot$.