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D. Weedman, V. Charmandaris (Cornell U.), A. Zezas (CfA)
Tracking the star formation rate to high redshifts and distinguishing starbursts from AGN requires knowledge of obscured sources. Dusty, optically-obscured galaxies can be located by 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 like nearby starbursts if seen at high redshifts, defining IR/X = (flux density in SIRTF MIPS 24 micron filter in mJy)/(total Chandra 0.5-2.0 keV flux in units of E-16 ergs per sec per sq cm). Regions used are within: NGC 4038+4039 (the ``Antennae"), NGC 3690+IC 694 (Mkn 171 or Arp 299), M82, and Arp 220. The starburst systems studied have adequate Chandra and ISO spectral imaging data that identical spatial regions can be used to determine IR/X. We determine IR/X of 9 starburst regions as they would appear for 1 < z < 3. The SIRTF MIPS 24 micron filter bandpass was folded into the ISO rest-frame spectrum to determine the rest-frame effective wavelength and corresponding flux density for different redshifts. The Chandra X-ray flux was determined within rest-frame energy bands corresponding to observer's frame 0.5-2.0 keV for different redshifts, after fitting spectra to ACIS observations.
For the starbursts in this sample, the mean IR/X over 1 < z < 3 is about 1.3, and this ratio is not a significant function of redshift or luminosity. Wide-field SIRTF surveys will reach 24 micron sensitivities of 0.4 mJy, so a limiting SIRTF starburst source should have a typical Chandra soft band flux of 0.3E-16. We find a lower bound of IR/X for starbursts at about 0.3, implying that a SIRTF 0.4 mJy source brighter than 1.3E-16 in the Chandra soft band can be confidently classified as having some luminosity arising from an obscured AGN rather than being purely a starburst.
We acknowledge NASA for supporting this research through the Chandra X-ray Center (NAS 8-39073) and the SIRTF Infrared Spectrograph team (JPL 960803).
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.