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Register a new userPredictions for Cosmological Infrared Surveys from Space with the Multiband Imaging Photometer for SIRTF (MIPS)
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H. Dole University ofn Arizona
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We make predictions for the cosmological surveys to be conducted by MIPS/SIRTF at 24, 70 and 160 microns, for the GTO and the legacy programs, using the latest knowledge of the instrument. In addition to detector and cirrus confusion noise, we discuss in detail the derivation of the confusion noise due to extragalactic sources, that depends strongly on the shape of the source counts at a given wavelength and on the telescope and detector pixel sizes. We show that it is wise in general to compare the classical photometric criterion and the so called source density criterion to predict the confusion levels. We obtain, using the model of Lagache, Dole, & Puget (2002) limiting fluxes of 50 microJy, 3.2 mJy and 36 mJy at 24, 70 and 160 microns (resp.). We compute the redshift distributions of the detected sources at each wavelength, and show that they extend up to z ~ 2.7 at 24 microns and up to z ~ 2.5 at 70 and 160 microns, leading to resolve at most 69, 54 and 24% of the Cosmic Infrared Background (CIB) at 24, 70 and 160 microns (resp.). We estimate which galaxy populations will be used to derive the luminosity function evolution with redshift. We also give the redshift distributions of the unresolved sources in the FIR range, that dominates the fluctuations of the CIB, and a predicted power spectrum showing the feasibility of fluctuations (due to Poissonian & clustered source distributions) measurements. The main conclusion is that MIPS (and SIRTF in general) cosmological surveys will greatly improve our understanding of galaxy evolution by giving data with unprecedented accuracy in the mid and far infrared range.
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We make predictions for the cosmological surveys to be conducted by the future Herschel mission operating in the far-infrared. The far-infrared bands match the peak of the CIRB, the brightest background of astrophysical origin. Therefore, surveys in these bands will provide essential information on the evolutionary properties of Luminous and Ultra-Luminous Infrared Galaxies (LIGs and ULIGs), starburst and normal galaxies. Our predictions are based on a new phenomenological model obtained from the 15-micron luminosity function of galaxies and AGN, fitting all the ISOCAM observables (source counts and redshift distributions) and also the recently published Spitzer source counts in the 24-micron band. We discuss the confusion noise due to extragalactic sources, depending strongly on the shape of the source counts and on the telescope parameters. We derive the fraction of the CIRB expected to be resolved by Herschel in the different wavebands and we discuss extragalactic surveys that could be carried on by Herschel for different scientific puropouses (i.e. ultra-deep, deep and shallow).
Granato et al(2004) have elaborated a physically grounded model exploiting the mutual feedback between star-forming spheroidal galaxies and the active nuclei growing in their cores to overcome, within the hierarchical clustering scenario for galaxy formation, one of the main challenges facing such scenario, the fact that massive spheroidal galaxies appear to have formed earlier and faster than predicted by previous models. Adopting the choice by Granato et al for the parameters governing the history of the SF,of chemical abundances and of the gas and dust content of galaxies, we are left with only two parameters affecting the time and mass dependent SED of spheroidal galaxies. After complementing the model with a simple description of evolutionary properties of starburst, normal late-type galaxies and AGNs we have successfully compared the model with a broad variety of observational data, deep K-band, ISO, IRAS, SCUBA, radio counts, the corresponding redshift distributions, the IR background spectrum, and also with data for EROs. We also present detailed predictions for the GOODS and SWIRE surveys with the Spitzer Space Telescope. We find that the GOODS deep survey at 24$mu$m and the SWIRE surveys at 70 and 160$mu$m are likely to be severely confusion limited. The GOODS surveys in the IRAC channels are expected to resolve most of the background, to explore the full passive evolution phase of spheroidal galaxies and most of their active star-forming phase, detecting galaxies up to zsimeq 4. A substantial number of high z star-forming spheroidal galaxies should also be detected by the 24mum SWIRE and GOODS surveys, while the 70 and 160mum will be particularly useful to study the evolution of such galaxies in the range 1 lsim z lsim 2.[abridged]
We describe the absolute calibration of the Multiband Imaging Photometer for Spitzer (MIPS) 160 micron channel. After the on-orbit discovery of a near-IR ghost image that dominates the signal for sources hotter than about 2000 K, we adopted a strategy utilizing asteroids to transfer the absolute calibrations of the MIPS 24 and 70 micron channels to the 160 micron channel. Near-simultaneous observations at all three wavelengths are taken, and photometry at the two shorter wavelengths is fit using the Standard Thermal Model. The 160 micron flux density is predicted from those fits and compared with the observed 160 micron signal to derive the conversion from instrumental units to surface brightness. The calibration factor we derive is 41.7 MJy/sr/MIPS160 (MIPS160 being the instrumental units). The scatter in the individual measurements of the calibration factor, as well as an assesment of the external uncertainties inherent in the calibration, lead us to adopt an uncertainty of 5.0 MJy/sr/MIPS160 (12%) for the absolute uncertainty on the 160 micron flux density of a particular source as determined from a single measurement. For sources brighter than about 2 Jy, non-linearity in the response of the 160 micron detectors produces an under-estimate of the flux density: for objects as bright as 4 Jy, measured flux densities are likely to be ~20% too low. This calibration has been checked against that of ISO (using ULIRGS) and IRAS (using IRAS-derived diameters), and is consistent with those at the 5% level.
The absolute calibration and characterization of the Multiband Imaging Photometer for Spitzer (MIPS) 70 micron coarse- and fine-scale imaging modes are presented based on over 2.5 years of observations. Accurate photometry (especially for faint sources) requires two simple processing steps beyond the standard data reduction to remove long-term detector transients. Point spread function (PSF) fitting photometry is found to give more accurate flux densities than aperture photometry. Based on the PSF fitting photometry, the calibration factor shows no strong trend with flux density, background, spectral type, exposure time, or time since anneals. The coarse-scale calibration sample includes observations of stars with flux densities from 22 mJy to 17 Jy, on backgrounds from 4 to 26 MJy sr^-1, and with spectral types from B to M. The coarse-scale calibration is 702 +/- 35 MJy sr^-1 MIPS70^-1 (5% uncertainty) and is based on measurements of 66 stars. The instrumental units of the MIPS 70 micron coarse- and fine-scale imaging modes are called MIPS70 and MIPS70F, respectively. The photometric repeatability is calculated to be 4.5% from two stars measured during every MIPS campaign and includes variations on all time scales probed. The preliminary fine-scale calibration factor is 2894 +/- 294 MJy sr^-1 MIPS70F^-1 (10% uncertainty) based on 10 stars. The uncertainty in the coarse- and fine-scale calibration factors are dominated by the 4.5% photometric repeatability and the small sample size, respectively. The 5-sigma, 500 s sensitivity of the coarse-scale observations is 6-8 mJy. This work shows that the MIPS 70 micron array produces accurate, well calibrated photometry and validates the MIPS 70 micron operating strategy, especially the use of frequent stimulator flashes to track the changing responsivities of the Ge:Ga detectors.
The SIRTF InfraRed Spectrograph (IRS) is faced with many of the same calibration challenges that were experienced in the ISO SWS calibration program, owing to similar wavelength coverage and overlapping spectral resolutions of the two instruments. Although the IRS is up to ~300 times more sensitive and without moving parts, imposing unique calibration challenges on their own, an overlap in photometric sensitivities of the high-resolution modules with the SWS grating sections allows lessons, resources, and certain techniques from the SWS calibration programs to be exploited. We explain where these apply in an overview of the IRS photometric calibration planning.
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