Evidence for Black Hole Growth in Local Analogs to Lyman Break Galaxies

TL;DR

This study uses X-ray observations of local Lyman Break Analogs to find evidence of active galactic nuclei, suggesting black hole growth in galaxies similar to distant star-forming galaxies, with implications for understanding early universe black hole evolution.

Contribution

It provides new X-ray evidence for AGN presence in local LBAs, highlighting their potential as laboratories for black hole growth studies in the early universe.

Findings

X-ray luminosities indicate AGN activity in some LBAs.

X-ray to far-IR ratios are higher than pure starbursts.

Evidence suggests obscured AGN with starburst-dominated bolometric output.

Abstract

We have used XMM-Newton to observe six Lyman Break Analogs (LBAs): members of the rare population of local galaxies that have properties that are very similar to distant Lyman Break Galaxies. Our six targets were specifically selected because they have optical emission-line properties that are intermediate between starbursts and Type 2 (obscured) AGN. Our new X-ray data provide an important diagnostic of the presence of an AGN. We find X-ray luminosities of order 10^{42} erg/s and ratios of X-ray to far-IR luminosities that are higher than values in pure starburst galaxies by factors ranging from ~ 3 to 30. This strongly suggests the presence of an AGN in at least some of the galaxies. The ratios of the luminosities of the hard (2-10 keV) X-ray to [O III]\lambda 5007 emission-line are low by about an order-of-magnitude compared to Type 1 AGN, but are consistent with the broad range seen in Type 2 AGN. Either the AGN hard X-rays are significantly obscured or the [O III] emission is dominated by the starburst. We searched for an iron emission line at ~ 6.4 keV, which is a key feature of obscured AGN, but only detected emission at the ~ 2\sigma level. Finally, we find that the ratios of the mid-infrared (24\mu m) continuum to [O III]\lambda 5007 luminosities in these LBAs are higher than the values for Type 2 AGN by an average of 0.8 dex. Combining all these clues, we conclude that an AGN is likely to be present, but that the bolometric luminosity is produced primarily by an intense starburst. If these black holes are radiating at the Eddington limit, their masses would lie in the range of 10^5 to 10^6 M_{sun}. These objects may offer ideal local laboratories to investigate the processes by which black holes grew in the early universe.