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4.9 Backgrounds

ISOPHOT was designed to measure the absolute sky brightness. As a consequence, all routine observations included emission from the astronomical background in the 2.5-240$\mu $m wavelength range. Most ISOPHOT observations involved discrete astronomical sources for which the background emission had to be subtracted. The presence of background emission contribute to

The PHT Instrument Dedicated Team together with the PHT Consortium have undertaken programmes to investigate the astronomical backgrounds. In the following sections the most important background emission components, the zodiacal and diffuse galactic emission, are discussed. In addition, confusion due to discrete galaxies is mentioned. Other components like the scattered light from zodiacal dust and the cosmic far-infrared background (Lagache & Puget 2000, [30]) are expected to give a minor contribution in most cases. More information can be found in the ISO Handbook Volume I, [20].

4.9.1 Confusion due to diffuse galactic emission

Confusion due to diffuse galactic emission or `cirrus confusion' became apparent from the far-infrared maps produced by IRAS. The diffuse galactic emission component peaks around 170$\mu $m and can be approximated by a $\lambda^{-2}$ modified blackbody of 17K. Analysis of the cirrus structure in IRAS maps (Gautier et al. 1992, [10]), Helou&Beichman 1990, [17]) showed that the cirrus confusion noise ${N_{cc}}$ can be expressed:

N_{cc} \approx 1.08~(\frac{\lambda}{100})^{2.5}
\langle B_{\nu}(\lambda) \rangle ^{1.5}~~~~[{\rm mJy}],
\end{displaymath} (4.8)

where $\lambda$ is wavelength in $\mu $m and $\langle B_{\nu}(\lambda)\rangle$ is the mean surface brightness in MJysr$^{-1}$. For example, for a field at 200$\mu $m and with a mean diffuse galactic emission of 5MJysr$^{-1}$, the cirrus confusion noise is $N_{cc}\approx$70mJy. Thus, two adjacent (point-)source free sky positions which are 1 beam apart (about 100$''$ at 200$\mu $m) can differ 70mJy per beam ($1~\sigma$) due to cirrus structure.

This result has been confirmed with ISOPHOT (Herbstmeier et al. 1998, [18] and references therein, Lagache & Puget 2000, [30]) at ISOPHOT wavelengths longward of 100$\mu $m.

4.9.2 Confusion by galaxies

Photometry of a target in the presence of discrete background galaxies can be hampered by faint background galaxies which may lie in the beam but are not detected individually, or by the presence of a identifiable bright source close to the target.

Faint source confusion can affect photometric studies of individual sources and dominates as long as the cumulative galaxy source counts $N(F_{\nu},\nu)$ rises more steeply than $F_{\nu}^{-1}$ with decreasing $F_{\nu}$. Cosmological source count models predict the turnover at $F_{\nu}<$0.1mJy. Below this flux density, the bright ource confusion limits should be included. Assuming the in-orbit point source flux limits (after 128s integration time and S/N = 10) in the most sensitive PHT filters at long wavelengths, the prediced level of galaxy confusion noise is given in Table 4.10.

Table 4.10: Galaxy confusion noise $N_{gc}$ for the tabulated point source flux limits with 128 s integration time and S/N = 10.
Det. $\lambda_{ref}$ Aperture $F_{\nu}$ $N_{gc}$
  [$\mu $m] [$''$] [mJy] [mJy]
P3 60 180 270 220
P3 100 180 240 280
C100 90 - 90 40
C200 170 - 200 120
C200 200 - 940 200

For on-target integration times longer than 128 s the noise level decreases but then galaxy confusion noise starts to become the main source of uncertainty: in the worst case 280 mJy (for the P3 detector, filter, $180''$ aperture). The galaxy confusion is on most regions in the sky below the cirrus confusion noise. For ISOPHOT, galaxy confusion becomes only important if the mean background level due to diffuse galactic emission is $ \langle B_{\nu}(\lambda) \rangle <$ 5MJysr$^{-1}$

Bright source confusion determines the maximum amount of extragalactic sources which can be counted in deep surveys of the darkest regions on the sky at high galactic latitudes. Assuming an Euclidian universe the confusion limit in sources per solid angle $n_q$ can be derived from Oliver 2001, [45]:

$\displaystyle \Omega_{eff} = 0.18\pi (1.2\frac{\lambda}{D})^2~~~~~{\rm [sr]},$     (4.9)
$\displaystyle n_{q} = \frac{1}{3q^2\Omega_{eff}}~~~~~{\rm [sr^{-1}]},$     (4.10)

where $D=0.6$ m is the diameter of the telescope and $q$ is the detection level above the noise in multiples of $\sigma$. For $q$=5 and $\lambda$=170$\mu $m, $n_5=\,2.04{\times}10^{-5}$ sr$^{-1}$ = 62 point sources per square degree.

Analysis of the infrared emission observed by COBE showed the presence of a new background component, the cosmic far-infrared background (CFIRB) which originates from far-infrared galaxies. Using deep ISOPHOT observations at 170$\mu $m in a region with low cirrus emission Lagache & Puget 2000, ([30]) isolated spatial fluctuations in the CFIRB. The CFIRB fluctuations are best described by a white noise power spectrum $P_{CFIRB}=7400~{\rm Jy^2\,sr^{-1}}$ corresponding to rms fluctuations around 0.07MJysr$^{-1}$ at 170$\mu $m.

4.9.3 Zodiacal light

For many types of ISOPHOT observations knowledge of the emission contribution from the zodiacal dust cloud is necessary. Data from COBE can be used but have to be adapted to the higher angular resolution observations of ISOPHOT. Absolute photometric observations in a number of ISOPHOT bands at several ecliptic longitudes/latitudes were obtained to establish the zodiacal emission distribution as seen by ISO.

COBE did not cover wavelengths between 5 and 12$\mu $m, in the range where the brightness of the zodiacal light rises very steeply. The measurements obtained with PHT-S, PHT-P and ISOCAM suggest that the zodiacal emission spectrum can be well approximated by blackbodies of  260-290K, depending on the Solar elongation and the ecliptic latitude. A method for subtracting the zodiacal emission in PHT-S spectra using the COBE data is presented in Ábrahám et al. 1997, [1]). ISOCAM CVF and ISOPHOT-S measurements have demonstrated that the spectrum is featureless between 5 and 16$\mu $m down to about 5% of the total brightness (Ábrahám et al. 1998, [2], Blommaert, Boulanger & Okumura 2001, [4]).

Ábrahám et al. 1997, [1] have searched for arcminute structure in the zodiacal emission at low, intermediate and high ecliptic latitudes. No structures or fluctuations were found at a level higher than 0.2% of the total brightness. At low ecliptic latitudes ( $\beta<15^\circ$) the zodiacal emission includes the dust bands and cometary tails (Ábrahám et al. 1998, [2]).

next up previous contents index
Next: 4.10 Global Instrument Data Up: 4. Instrumental Characteristics Previous: 4.8 Instrumental Polarisation
ISO Handbook Volume IV (PHT), Version 2.0.1, SAI/1999-069/Dc