X-ray observations of sub-mm LABOCA galaxies in the eCDFS

Abstract

We explore the X-ray properties of the 126 sub-mm galaxies (SMGs) of the LABOCA survey in the CDFS and the eCDFS regions. SMGs are believed to experience massive episodes of star formation. Our goal is to examine whether star formation coexists with AGN activity, determine the fraction of highly obscured AGN, and finally to obtain an idea of the dominant power mechanism in these sources. Using Spitzer and radio arcsecond positions for the SMGs, we find 14 sources with significant X-ray detections. For most of these there are only photometric redshifts available, with their median redshift at ∼2.3. Taking only the CDFS area into account that has the deepest X-ray observations, we estimate an X-ray AGN fraction of <26 ± 9% among SMGs. The X-ray spectral properties of the majority of the X-ray AGN that are associated with SMGs are consistent with high obscuration, >10 23 cm −2 , but there is no unambiguous evidence of Compton-thick sources. Detailed spectral energy distribution fittings show that the bulk of total IR luminosity originates in star forming processes, although a torus component is usually present. Finally, stacking analysis of the X-ray undetected SMGs reveals a signal in the soft (0.5-2 keV) and marginally in the hard (2-5 keV) X-ray bands. The hardness ratio of the stacked signal is relatively soft (−0.40 ± 0.10) corresponding to Γ ∼ 1.6. This argues against a high fraction of Compton-thick sources among the X-ray undetected SMGs.

Dusty star-forming galaxies at high redshift

“…The SMGs not hosting AGN were consistent with X-ray emission from star-formation and HMXBs. Further X-ray analysis of SMGs estimate AGN fractions of 20-29(±7)% (Laird et al, 2010), <26±9% (Georgantopoulos et al, 2011), 14-28% and 17( +16 −6 )% . The Wang et al (2013a) work is of particular note since its sample is the ∼100 ALMA-confirmed 870µm-selected sources in CDFS with unambiguous counterparts, enabling effectively more precise measurements in both X-ray and FIR.…”

“…The X-ray properties of DSFGs have been studied in detail over the past decade, in particular for 850-870µm-selected SMGs (Fabian et al, 2000;Alexander et al, 2005a,b;Pope et al, 2006;Laird et al, 2010;Lutz et al, 2010;Georgantopoulos et al, 2011;Gilli et al, 2011;Hill & Shanks, 2011;Bielby et al, 2012;Johnson et al, 2013;Wang et al, 2013b). Unfortunately, all of these studies are limited by small number statistics (N sources < 100) cause by very time-intensive, deep X-ray observations which are required to disentangle X-ray emission dominated by AGN Figure 27: A summary of X-ray observations of ∼850µm-selected SMGs.…”

The Coevolution of Supermassive Black Holes and Massive Galaxies at High Redshift

“…In particular instances of quite large gravitational amplification we can even get estimates of the BH mass. These observations should be complemented by X-ray observations, which for magnification 10 will allow us to get a detailed view of the nuclear emission even for highly obscured and quite low luminosity objects (Matt et al 2004;Georgantopoulos et al 2011). The predicted intrinsic X-ray luminosity for typical lensed galaxies exceeds L X 10 42 erg s −1 ; the magnification by factors 10 makes them easily detectable in pointed observations up to z 3 with Chandra and the next generation of X-ray instruments.…”

“…The FIR/submm selection elicits galaxies with bolometric luminosity associated to star formation larger than the AGN bolometric luminosity (L FIR /L AGN 1; see Georgantopoulos et al 2011;Laird et al 2010;Rafferty et al 2011;). The few objects that exhibit a ratio L FIR /L AGN ∼ 1 have a large X-ray power L X 3 × 10 44 erg s −1 and large luminosity in the Hα line (Alexander et al 2008).…”

“…• nuclear activity, by exploiting the follow-up in X rays of galaxies with large SFR mainly selected at FIR/sub-mm wavelengths or in the K−band (e.g., Borys et al 2005;Alexander et al 2005Alexander et al , 2008Laird et al 2010;Symeonidis et al 2010;Xue et al 2010;Georgantopoulos et al 2011;Carrera et al 2011;Melbourne et al 2011;Rafferty et al 2011;Mullaney et al 2012a;) and of galaxies starforming at a lower rate hence more easily selected at UV wavelengths via the stacking technique (e.g., Fiore et al 2012b;Treister et al 2011;Willott 2011;Basu-Zych et al 2013);…”