MAMBO Deep Field Surveys

Blank-field surveys with the SCUBA and MAMBO bolometer arrays were able to resolve most of the COBE submm/mm background into individual sources, revealing a previously unknown population of intermediate- and high-redshift starbursts. These are thought to be spheroidal galaxies in their primary formation phase and are of paramount interest for understanding galaxy formation. The current status of our efforts to study this mm background population can be summarized as follows:

Figure: Upper row and lower left: The three MAMBO deep fields, not including the wide field data from 2001/2002. Noise levels are typically 0.5 to 0.7 mJy in the field centers, rising toward the edges due to low exposure. Dimensions are arcmin, flux level mJy. Lower right: Extending the deep MAMBO survey with shallow imaging in the flanking fields of Abell 2125 and the Lockman Hole, we detected three extremely bright sources in the Abell 2125 field, which is displayed here as a signal/noise map. These objects are clearly identified with low redshift (∼0.3, 0.29, 1.38), radio-loud and X-ray-bright QSOs. They appear to belong to a population different from the fainter MAMBO and SCUBA background sources which are radio- and X-ray-quiet, higher-redshift, optically obscured starbursts.

Figure: Cumulative number counts of SCUBA and MAMBO background sources. The lines are model ``predictions'' (Voss 2002). The highest flux MAMBO counts are based on the brightest sources in the Abell 2125 field. They clearly deviate from the extrapolated starburst population seen at lower flux levels. Excluding the three brightest sources, the counts show a turnover (upper limit at 7 mJy).

Figure: Simulated 1.2mm map of one square degree, based on observed number counts, the estimated redshift distribution, cosmological simulations of dark matter evolution, and a simple assumption on the baryon density bias. Greyscale ranges from 0 to 5 mJy. The typical size of the deepest MAMBO or SCUBA survey fields (10 arcmin) is indicated. Since the luminous mm background sources are either QSOs or extremely intense starbursts (>1000Msunyr-1), they constitute good tracers of peaks in the large-scale cosmic density distribution and are thereby ideal targets for searches of high-redshift galaxy clusters (from Voss 2002).

• We have mapped three fields (Fig. 1) at 1.2 mm to noise levels of 0.5-0.7 mJy over a total of ∼500 arcmin². We recently extended the Lockman Hole and Abell 2125 fields to an area of about 500 arcmin² each to shallower depth of 1.5 mJy in order to measure the number density and the properties of the brightest background sources. Three unusually bright sources were found in the Abell 2125 field (Fig. 1) which might reveal the presence of a previously unknown low-redshift, mm-bright QSO population.
• We obtained deep optical and near-IR images of our fields, which show no significant correlation between the mm sources and optical or near-IR sources. This makes spectroscopic redshift determinations possible only in very few cases but places constraints on the nature and redshift of these objects (Dannerbauer et al. 2002).
• Our deep (rms 5 to 13 μJy) 20 cm VLA images of the three fields show that nearly all of the strongly significant sources can be identified with weak radio sources. The mm-to-radio flux ratio permits an estimate of their redshifts of typically between 1 and 4.
• We have reobserved five of the brighter MAMBO sources with the Plateau de Bure interferometer in order to more accurately determine the source positions and to try to identify optical/NIR counterparts. The MAMBO sources were confirmed and in three cases a faint (K=20.5 to 22.5 mag) possible near-infrared counterpart was identified.
• We have compared our MAMBO and VLA maps of the Lockman hole with deep XMM images, and found some correspondence between the X-ray and radio sources, between the mm- and radio-sources, but not between the X-ray and mm sources. This first large-area comparison between the mm and X-rays confirms earlier SCUBA results that the X-ray background sources, which are intermediate redshift AGN, do not contribute significantly to the mm background.
• In collaboration with a British group (Eales, Ivison, Dunne) we have reobserved 23 MAMBO sources with SCUBA and detected 19 of them at 850 μm, and two at 450 μm. Optical spectroscopy lacks, but the submm-to-mm spectral slopes yield a very rough estimate of the source redshifts, although the redshift is degenerate with the dust temperature which cannot be determined independently. We have identified several sources which may lie at very high (z>5) redshifts.
• We have refined models which describe the evolution of the galaxy luminosity function, given observational constraints from the mm and submm source number counts (Fig. 2), the estimated source redshift distribution, and the integrated COBE background brightness (Voss 2002).
• To study spatial clustering we simulated large scale 1.2 mm maps (Fig. 3) for comparison with our observations.

⇒ Bertoldi et al. (2000); Carilli et al. (2001c, 2002b); Dannerbauer et al. (2002); Voss (2002); Eales et al. (2002)