2004ApJ...600..106W

Tracking the star formation rate to high redshifts requires knowledge of the contribution from both optically visible and obscured sources. The dusty, optically obscured galaxies can be located by X-ray and infrared surveys. To establish criteria for selecting such sources based only on X-ray and infrared surveys, we determine the ratio of infrared to X-ray brightness that would be observed by SIRTF and Chandra for objects with the same spectral shapes as nearby starbursts if seen at high redshift. The parameter infrared/X-ray (IR/X) is defined as IR/X=(flux density observed in the SIRTF Multiband Imaging Photometer 24 µm filter in mJy)/(total flux observed within 0.5-2.0 keV in units of 10^–16 ergs/s/cm2). Based on observations of NGC 4038/39 (``The Antennae''), NGC 3690+IC 694 (Arp 299 or Mrk 171), M82, and Arp 220, nine starburst regions are compared using mid-infrared spectra taken by the Infrared Space Observatory and X-ray spectra obtained with Chandra. The IR/X are determined as they would appear for 1<z<3. The mean IR/X over this redshift range is 1.3 and is not a significant function of redshift or luminosity, indicating that SIRTF surveys reaching 0.4 mJy at 24 µm should detect the same starbursts as deep Chandra X-Ray Observatory surveys detect at a flux of 0.3x10^–16 ergs/s/cm2. The lower bound of IR/X for starbursts is about 0.2, suggesting that objects with IR/X smaller than this have an active galactic nucleus (AGN) X-ray component in addition to the starburst. Values of IR/X for the obscured AGNs within NGC 1068, the Circinus galaxy, and NGC 6240 are also determined for comparison, although interpretation is complicated by the circumnuclear starbursts in these galaxies. Any sources found in surveys having IR/X>4 would not match any of the objects considered.