Speaker
Description
In the local Universe, it is observed that the X-ray luminosity ($L_X$) of HMXB populations is correlated with the host galaxy’s star formation rate (SFR). Recent X-ray studies of high-redshift galaxies find a positive evolution of this correlation with redshift. This trend is attributed to the formation of more luminous HMXBs in lower metallicity ($Z$) environments, as predicted by binary population synthesis models. While there is observational evidence that HMXB populations in nearby low-$Z$ dwarf galaxies have enhanced $L_X$/SFR, the correlation between $L_X$, SFR, and $Z$ is poorly constrained and, due to the difficulty of obtaining $Z$ measurements at high redshift, it has not yet been proven that the redshift evolution of $L_X$/SFR is driven by the $Z$-dependence of HMXBs. Better understanding how HMXB $L_X$ varies with $Z$ and redshift will constrain: (1) whether HMXBs in low-$Z$ environments can be progenitors of the heavy BH binaries discovered by gravitational wave observatories, (2) the contribution of HMXBs to the X-ray heating and reionization of gas in the early Universe, and (3) estimates of HMXB contamination to X-ray searches for low-luminosity AGN and intermediate mass black holes.
We present the results of an X-ray stacking study of star-forming galaxies at $z$~2, whose goal is to test the connection between the redshift evolution and $Z$-dependence of HMXBs. Our galaxy sample is selected from the MOSDEF survey, which obtained rest-frame optical spectra of ~1500 galaxies at $z$~2 in the CANDELS fields. Stacking Chandra data from these fields, we confirm that the $L_X$/SFR of galaxies at $z$~2 is enhanced relative to $z$=0. Splitting our sample into different $Z$ bins, we find a statistically significant decrease of $L_X$/SFR with $Z$ that is in good agreement with theoretical predictions and with the $L_X$-SFR-$Z$ relation in the local Universe. Thus, this study provides the first direct evidence that the $z$-evolution of the HMXB $L_X$/SFR is indeed driven by metallicity. We also present preliminary results from X-ray stacking studies at lower redshifts, which will better constrain the $L_X$-SFR-$Z$ relation, enabling more detailed comparisons with theoretical models.
| Affiliation | Center for Astrophysics | Harvard & Smithsonian |
|---|---|
| Topic | Compact and diffuse sources in galaxies and in the Galactic Center |
