X‐Ray Properties of Lyman Break Galaxies in the Hubble Deep Field–North Region

We describe the X-ray properties of a large sample of $z\sim3$ Lyman BreakGalaxies (LBGs) in the region of the Hubble Deep Field North, derived from the1 Ms public Chandra observation. Of our sample of 148 LBGs, four are detectedindividually. This immediately gives a measure of the bright AGN fraction inthese galaxies of $\sim 3$~per cent, which is in agreement with that derivedfrom the UV spectra. The X-ray color of the detected sources indicates thatthey are probably moderately obscured. Stacking of the remainder shows asignificant detection ($6\sigma$) with an average luminosity of $3.5 \times10^{41}$~erg s$^{-1}$ per galaxy in the rest frame 2-10 keV band. We have alsostudied a comparison sample of 95 z$\sim 1$ ``Balmer Break'' galaxies. Eight ofthese are detected directly, with at least two clear AGN based on their highX-ray luminosity and very hard X-ray spectra respectively. The remainder are ofrelatively low luminosity ($<10^{42}$~erg s$^{-1}$), and the X-rays could arisefrom either AGN or rapid star-formation. The X-ray colors and evidence fromother wavebands favor the latter interpretation. Excluding the clear AGN, wededuce a mean X-ray luminosity of $6.6 \times 10^{40}$~erg s$^{-1}$, a factor$\sim 5$ lower than the LBGs. The average ratio of the UV and X-rayluminosities of these starforming galaxies $L_{\rm UV}/L_{\rm X}$, however, isapproximately the same at $z = 1$ as it is at $z = 3$. This scaling impliesthat the X-ray emission follows the current star formation rate, as measured bythe UV luminosity. We use our results to constrain the star formation rate at$z\sim 3$ from an X-ray perspective (truncated).Comment: 33 pages, 5 figures, Accepted for Publication in Ap