The FLS extragalactic component
D. Ancillary Data
Optical, near-infrared, and radio surveys provide essential support for the
extragalactic FLS. The principal requirement is for optical
identifications, the first step to understanding FLS sources. Deep (e.g.,
R ~ 25.5 mag) I- or R-band images covering the full 5 sq.deg. survey area
should contain the counterparts of most FLS sources, but we note that
accurate (~1 arcsec rms) positions are needed for making reliable optical
identifications of such faint infrared objects. The KPNO 4 m telescope is
equipped with a large-format camera capable of obtaining such data in a few
nights of observing. Most of the detected MIPS sources will have flux
densities near the 5*rms detection limit, and their rms positional
uncertainties owing to noise alone will be about 10% of the half-intensity
beamwidths, or 2 arcsec at 70 microns and 5 arcsec at 160 microns. Such
sources may first have to be identified with mid-infrared or radio sources
having more accurate positions before being optically identified.
The workshop also recommends covering the extragalactic FLS area with deep
(K ~ 18 mag) near-IR images which will be useful for distinguishing stars
from galaxies and extending the proposed IRAC spectrophotometry. Again,
these data are well within existing capabilities at KPNO, or of an enhanced
version of the Two-Micron All-Sky Survey (2MASS) project.
Multicolor (three or more bands) of relatively shallow optical images would
be useful for redshift estimates and classification of low-redshift
galaxies. The SDSS (Sloan Digital Sky Survey) or the KPNO 0.9 meter
telescopes would be capable of obtaining such data.
Radio images are scientifically valuable because most far-infrared sources
obey the tight FIR/radio flux correlation. Most radio sources fainter than
1 mJy at 1.4 GHz appear to be powered by starbursts and not AGN, so they
probably obey the FIR/radio correlation. Thus deep radio surveys and the
extragalactic FLS will find similar source populations, and deep radio
surveys can provide pre-launch images that will be similar to the FLS
images. The resulting radio catalogs can be used to select and identify
most FLS sources for further study before launch. FIR/radio flux ratios can
later be used to distinguish objects containing radio-loud AGN from
"normal" galaxies. The radio observations should be made with 5 arcsec
FWHM resolution (1) to yield sub-arcsec positions for even the faintest
detectable sources and (2) to sort out confusion in the larger (20 arcsec
at 70 microns, 47 arcsec at 160 microns) MIPS beams.
As a practical matter, only the VLA B-configuration at 1.4 GHz can produce
the necessary sky coverage, sensitivity, and resolution. The effective VLA
field-of-view is half the beam solid angle, or about 1/7 square deg at 1.4
GHz; thus 35 fields are needed to cover 5 square deg. The 1.4 GHz 5*sigma
sensitivies needed to match the FLS shallow survey are listed in the
following table:
Radio sensitivities needed to match the shallow FLS
---------------------------------------------------
Wavelength Sensitivity 1.4 GHz sensitivity
(microns) (mJy) (microJy)
24 1.3 90
70 4.5 70
160 27 80
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These numbers are based on the following assumptions:
-
In the source frame, (S(60 microns) / S(1.4 GHz)) ~ 140
-
Radio sources have spectral indices -0.7
-
Infrared sources have the average spectra of IRAS Bright Galaxy Sample sources
-
The average redshift of the fainter sources is (z) ~ 1
The VLA can reach the required sensitivity (~15 microJy/beam rms noise) in
about 10 hours per field (~350 hours total). In the B configuration, the
rms confusion is only about 1 microJy/beam and can be neglected. Fields
must be mosaiced in filled hexagonal patterns (seven fields minimum) for
uniform sensitivity on the sky, so FLS strips narrower than about 1 deg in
either dimension cannot be covered efficiently. Finally, strong sources
(1.4 GHz flux densities> several hundred mJy) and low declinations (< +40
deg) must be avoided lest the VLA images be limited by dynamic range, not
noise. The VLA will be in the B configuration during the first quarter of
2001.
Go back to Workshop Report or
FLS history page.
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