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Constraining the Effects of Metallicity on the X-ray Binary Luminosity Function in NGC 300

Presentation #425.08 in the session Compact Object Binaries.

Published onJun 29, 2022
Constraining the Effects of Metallicity on the X-ray Binary Luminosity Function in NGC 300

We present an analysis of the X-ray binary (XRB) population of the nearby galaxy NGC 300. Using a new Chandra/ACIS-I observation in conjunction with archival Chandra observations, and Hubble ACS and WFC, GALEX FUV, and Spitzer 24 micron imaging, we identify 72 XRB candidates in NGC 300. The XRB population likely contains a significant fraction of high mass-XRBs; we therefore measure the star formation rate (SFR) in the immediate vicinity (~100 pc) of each XRB and determine the most probable metallicity of each system based on the measured metallicity gradient across the star-forming disk. The XRBs are then sorted into subsamples based on (1) galactocentric distance or (2) local SFR, and X-ray luminosity functions down to ~1036 erg s-1 are calculated and modeled with power laws. We find a pronounced increase in the integrated X-ray luminosity LX per unit SFR with decreasing metallicity that is significantly significantly steeper than reported in prior studies. We use the star formation history of NGC 300 to predict the number of compact objects that exist in NGC 300 as a function of galactocentric radius, and find that the observed number of XRBs represent ~5% of the total compact object population within the inner ~4.5 kpc. The outer, lower metallicity portion of the disk is dominated by younger stellar populations, and the predicted number of XRBs represents ~30% of the total compact object population in the outer disk. We therefore conclude that the steep intragalactic LX-SFR-Z relationship observed in NGC 300 is driven primarily by stellar age effects, with a weaker dependence on metallicity.

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