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Register a new userA 1200-micron MAMBO survey of the GOODS-N field: a significant population of submillimetre drop-out galaxies
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We present a 1200-micron image of the Great Observatories Origin Deep Survey North (GOODS-N) field, obtained with the Max Planck Millimeter Bolometer array (MAMBO) on the IRAM 30-m telescope. The survey covers a contiguous area of 287 square arcmin to a near-uniform noise level of ~0.7mJy/beam. After Bayesian flux deboosting, a total of 30 sources are recovered (>=3.5sigma). An optimal combination of our 1200-micron data and an existing 850-micron image from the Submillimetre Common-User Bolometer Array (SCUBA) yielded 33 sources (>=4sigma). We combine our GOODS-N sample with those obtained in the Lockman Hole and ELAIS-N2 fields (Scott et al. 2002; Greve et al. 2004) in order to explore the degree of overlap between 1200-micron- and 850-micron-selected galaxies (hereafter SMGs), finding no significant difference between their 850-micron to 1200-micron flux density distributions. However, a noise-weighted stacking analysis yields a significant detection of the 1200-micron-blank SCUBA sources, whereas no significant 850-micron signal is found for the 850-micron-blank MAMBO sources. The hypothesis that the 850/1200-micron flux density distribution of SCUBA sources is also representative of the MAMBO population is rejected at the ~4sigma level, via Monte Carlo simulations. Therefore, although the populations overlap, galaxies selected at 850 and 1200micron are different, and there is compelling evidence for a significant 1200-micron-detected population which is not recovered at 850micron. These are submm drop-outs (SDOs), with S_850/S_1200 = 0.7-1.7, requiring very cold dust or unusual spectral energy distributions (T_d ~ 10K; beta ~ 1), unless SDOs reside beyond the redshift range observed for radio-identified SMGs, i.e. at z > 4.
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How does the submm galaxy population detected by SCUBA fit into galaxy evolution? How do these rare starbursting systems, which contribute significantly to high redshift star-formation, relate to other galaxy populations? Deep radio observations have been most useful for studying these systems, but still leave a significant fraction of the population unidentified. Now with the deep IRAC and MIPS images from the GOODS Spitzer Legacy program and a re-analysis of the deep radio data, we are able to identify counterparts for a large fraction of SCUBA galaxies in GOODS-N. All of these counterparts are detected by Spitzer. Given the vast multi-wavelength data in this field, we can study the spectral energy distributions (SEDs) of these systems and determine what is fueling their intense infrared luminosities. A rest-frame composite optical-to-radio SED for all spectroscopically identified submm sources shows that the average SCUBA galaxy is consistent with models of ultraluminous starburst galaxies, although cooler than those observed locally. Because of this, the submm flux alone consistently overestimates IR when using spectral templates which obey the local ULIRG temperature-luminosity relation. The wide range of 24/850 micron flux ratios as a function of redshift indicates the presence of strong mid-IR features, to be confirmed with deep IRS spectroscopy. The IRAC colours of the submm systems provide useful redshift constraints, since, at these redshifts, IRAC samples the stellar bump. The Spitzer photometry of this large sample of submm galaxies has allowed us to put constraints on many of the outstanding issues in submm astronomy.
[abridged] We derive photometric redshifts from 17-band optical to mid-IR photometry of 74 robust counterparts to 68 of the 126 submillimetre galaxies (SMGs) selected at 870um by LABOCA observations in the ECDFS. The median photometric redshift of identified SMGs is z=2.2pm0.1, the interquartile range is z=1.8-2.7 and we identify 10 (~15%) high-redshift (z>3) SMGs. We derive a simple redshift estimator for SMGs based on the 3.6 and 8um fluxes, which is accurate to Delta_z~0.4 for SMGs at z<4. A statistical analysis of sources around unidentified SMGs identifies a population of likely counterparts with a redshift distribution peaking at z=2.5pm0.3, which likely comprises ~60% of the unidentified SMGs. This confirms that the bulk of the undetected SMGs are coeval with those detected in the radio/mid-IR. We conclude that ~15% of all the SMGs are below the flux limits of our survey and lie at z>3 and hence ~30% of all SMGs have z>3. We estimate that the full S_870um>4mJy SMG population has a median redshift of 2.5pm0.6. In contrast to previous suggestions we find no significant correlation between S_870um and redshift. The median stellar mass of the SMGs derived from SED fitting is (9.2pm0.9)x10^10Msun and the interquartile range is (4.7-14)x10^10Msun, although we caution that uncertainty in the star-formation histories results in a factor of ~5 uncertainty in these stellar masses. The median characteristic dust temperature of SMGs is 35.9pm1.4K and the interquartile range is 28.5-43.3K. The infrared luminosity function shows that SMGs at z=2-3 typically have higher far-IR luminosities and luminosity density than those at z=1-2. This is mirrored in the evolution of the star-formation rate density (SFRD) for SMGs which peaks at z~2. The maximum contribution of bright SMGs to the global SFRD (~5% for SMGs with S_870um>4mJy; ~50% for SMGs with S_870um>1mJy) also occurs at z~2.
A follow-up survey using the Submillimetre High-Angular Resolution Camera (SHARC-II) at 350 microns has been carried out to map the regions around several 850 micron-selected sources from the Submillimetre HAlf Degree Extragalactic Survey (SHADES). These observations probe the infrared luminosities and hence star-formation rates in the largest existing, most robust sample of submillimetre galaxies (SMGs). We measure 350 micron flux densities for 24 850 micron sources, seven of which are detected at >2.5-sigma within a 10 arcsec search radius of the 850 micron positions. When results from the literature are included the total number of 350 micron flux density constraints of SHADES SMGs is 31, with 15 detections. We fit a modified blackbody to the far-infrared (FIR) photometry of each SMG, and confirm that typical SMGs are dust-rich (Mdust~9x10^8 Msun), luminous (Lfir~2x10^12 Lsun), star-forming galaxies with intrinsic dust temperatures of ~35 K and star-formation rates of ~400 Msun/yr. We have measured the temperature distribution of SMGs and find that the underlying distribution is slightly broader than implied by the error bars, and that most SMGs are at 28 K with a few hotter. We also place new constraints on the 350 micron source counts, N350(>25mJy)~200-500 deg^-2.
We present a study of the cosmic infrared background, which is a measure of the dust obscured activity in all galaxies in the Universe. We venture to isolate the galaxies responsible for the background at 1mm; with spectroscopic and photometric redshifts we constrain the redshift distribution of these galaxies. We create a deep 1.16mm map (sigma ~ 0.5mJy) by combining the AzTEC 1.1mm and MAMBO 1.2mm datasets in GOODS-N. This combined map contains 41 secure detections, 13 of which are new. By averaging the 1.16mm flux densities of individually undetected galaxies with 24um flux densities > 25uJy, we resolve 31--45 per cent of the 1.16mm background. Repeating our analysis on the SCUBA 850um map, we resolve a higher percentage (40--64 per cent) of the 850um background. A majority of the background resolved (attributed to individual galaxies) at both wavelengths comes from galaxies at z > 1.3. If the ratio of the resolved submillimeter to millimeter background is applied to a reasonable scenario for the origins of the unresolved submillimeter background, 60--88 per cent of the total 1.16mm background comes from galaxies at z > 1.3.
We use ultradeep 20 cm data from the Karl G. Jansky Very Large Array and 850 micron data from SCUBA-2 and the Submillimeter Array of an 124 arcmin^2 region of the Chandra Deep Field-north to analyze the high radio power (P_20cm>10^31 erg s^-1 Hz^-1) population. We find that 20 (42+/-9%) of the spectroscopically identified z>0.8 sources have consistent star formation rates (SFRs) inferred from both submillimeter and radio observations, while the remaining sources have lower (mostly undetected) submillimeter fluxes, suggesting that active galactic nucleus (AGN) activity dominates the radio power in these sources. We develop a classification scheme based on the ratio of submillimeter flux to radio power versus radio power and find that it agrees with AGN and star-forming galaxy classifications from Very Long Baseline Interferometry. Our results provide support for an extremely rapid drop in the number of high SFR galaxies above about a thousand solar masses per year (Kroupa initial mass function) and for the locally determined relation between X-ray luminosity and radio power for star-forming galaxies applying at high redshifts and high radio powers. We measure far-infrared (FIR) luminosities and find that some AGNs lie on the FIR-radio correlation, while others scatter below. The AGNs that lie on the correlation appear to do so based on their emission from the AGN torus. We measure a median radio size of 1.0+/-0.3 arcsecond for the star-forming galaxies. The radio sizes of the star-forming galaxies are generally larger than those of the AGNs.
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