arXiv:astro-ph/0606676v1 28 Jun 2006
Accepted ApJL, 2006 June 26
Preprint typeset using LATEX style emulateapj v. 04/03/99
SPITZER 70µm SOURCE COUNTS IN GOODS-NORTH
D. T. Frayer1, M. T. Huynh1, R. Chary1, M. Dickinson2, D. Elbaz3, D. Fadda1, J. A.
Surace1, H. I. Teplitz1, L. Yan1, B. Mobasher4
Accepted ApJL, 2006 June 26
We present ultra-deep Spitzer 70µm observations of GOODS-North (Great Observatories Origins Deep
Survey). For the first time, the turn-over in the 70µm Euclidean-normalized differential source counts
is observed. We derive source counts down to a flux density of 1.2mJy. From the measured source
counts and fluctuation analysis, we estimate a power-law approximation of the faint 70µm source counts
of dN/dS ∝ S−1.6, consistent with that observed for the faint 24µm sources. An extrapolation of
the 70µm source counts to zero flux density implies a total extragalactic background light (EBL) of
7.4 ± 1.9nW m−2sr−1. The source counts above 1.2mJy account for about 60% of the estimated EBL.
From fluctuation analysis, we derive a photometric confusion level of σc= 0.30 ± 0.15mJy (q = 5) for
the Spitzer 70µm band.
Subject headings: cosmology: observations — galaxies: evolution — galaxies: high-redshift — infrared:
Deep 24µm observations (Chary et al. 2004; Papovich
et al. 2004; Fadda et al. 2006) have demonstrated the
ability of the Multiband Imaging Photometer for Spitzer
(MIPS, Rieke et al. 2004) to study the mid-infrared (mid-
IR) properties of high-redshift galaxies (Yan et al. 2004;
Le Floc’h et al. 2004, 2005; P´ erez-Gonz´ alez et al. 2005;
Daddi et al. 2005; Papovich et al. 2006; Caputi et al.
2006). The interpretation of the 24µm data are compli-
cated by the presence of strong emission and absorption
features (e.g., Armus et al. 2004) redshifted into the 24µm
band. Observations at longer wavelengths, such as 70µm
which is closer to the far-infrared peak of the spectral en-
ergy distribution (SED) and is away from the strong mid-
IR features, are crucial for constraining the infrared lumi-
nosities and star-formation rates.
The previous deep Guaranteed Time Observer (GTO)
surveys did not achieve sufficient sensitivity at 70µm to de-
tect distant luminous infrared galaxies (LIRGs; 1011L⊙∼<
Lir∼< 1012L⊙), without stacking 70µm data for a large
number of 24µm-selected sources (Dole et al. 2006). Much
deeper observations are needed at 70µm to individually de-
tect the z ∼ 1 LIRGs that account for the majority of the
extragalactic background light (Elbaz et al. 2002; Lagache
et al. 2004). In this letter, we present initial results for
the deepest 70µm survey taken to date with Spitzer.
The ultra-deep 70µm observations of the northern field
of the Great Observatories Origins Deep Survey (GOODS-
N) were carried out in Cycle-1 of the General Observer
(GO) program (Spitzer program 3325). The survey covers
the central 10′× 10′of GOODS-N to a depth of 10.6ks.
The data were taken using small-field photometry mode
with 10MIPS-second data collection events (DCEs). The
field was observed with an 8 position cluster map for each
astronomical observational request (AOR). The observa-
tions were repeated with 12 AORs taking 34.5 hours of
observatory time in total. The mapping order and dither
positions of the cluster positions within the AORs were
varied to provide uniform coverage and data quality across
the field. The data were embargoed until after the GTO
proprietary period and were released to our team in 2005
August. In addition to the GO data, we used the MIPS
GTO data of GOODS-N (Spitzer program 81, Dole et al.
2004a). The GTO data were taken in slow scan mode with
one degree scan legs and have an integration time of 600s
at 70µm, covering an area of 0.6deg2.
3. DATA REDUCTION
The raw data were downloaded from the Spitzer Science
Center (SSC) archive and were processed from scratch us-
ing the offline Germanium Reprocessing Tools (GeRT, S13
version 1.0). The instrumental artifacts in the basic cal-
ibration data (BCDs) were removed adopting the filter-
ing techniques used for the reduction of the extragalac-
tic First Look Survey (xFLS, Frayer et al. 2006). The
BCD pipeline processing and filtering procedures were op-
timized for these deep photometry data. We adopted the
updated S13 calibration, which assumes an absolute flux
calibration factor based on stellar SEDs of 702MJysr−1
per MIPS-70 data unit.We then multiplied the data
by the color correction factor of 1.09 to place the data
on a constant νfν scale, which is also the appropriate
color correction (within 2%) for a wide range of possi-
ble galaxy SEDs (see SSC web pages for calibration and
color-correction details). In comparison, the calibration
correction adopted here is 3.4% larger than the calibra-
tion adopted for the xFLS analysis (Frayer et al. 2006).
1Spitzer Science Center, California Institute of Technology 220–06, Pasadena, CA 91125; email@example.com.
2National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85726.
3DSM/DAPNIA/Service d’Astrophysique, CEA/SACLAY, 91191 Gif-sur-Yvette Cedex, France.
4Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218.
6 Frayer et al.
GOODS-N 70µm Source Counts
Note.—The first 6 rows are for the ultra-deep field (0.0277deg2), the next 5 rows are for the intermediate field
(0.0462deg2), and the remaining rows are for GTO-deep field (0.614deg2). The corrected counts are equal to the observed
counts multiplied by the reliability value and divided by the completeness correction. The uncertainties for the Euclidean-
normalized differential counts [(dN/dS)S2.5] include the Poissonian noise, the uncertainties associated with the reliability,
completeness, and flux biasing corrections, and the absolute calibration uncertainty of 10%.