Environment and Properties of Submillimeter Galaxies

Abstract

Extra-galactic surveys with large submillimeter bolometer cameras discovered a population of dust obscured starbursting galaxies at high redshifts. These submillimeter galaxies (SMGs) are crucial in the stellar build up of the Universe, and their abundance and epoch of formation place important constraints in models of galaxy evolution.<br /> Using the Max-Planck Millimeter Bolometer Array (MAMBO) on the IRAM 30 m telescope, we mapped the central 20'x20' of the COSMOS field at 1.2 mm to an rms level of 1 mJy per 11'' beam. We found 15 significant (S/N>4) millimeter sources, 11 of which have a reliable 1.4 GHz Very Large Array (VLA) radio counterpart. Ten more lower significance millimeter sources were identified based on a radio association. We used 24 um data to complement the radio identifications. Using a stacking analysis on the MAMBO map, we estimated that radio (S_nu>45 uJy), 24 um (S_nu>550 uJy) and millimeter sources (S_nu>4 mJy) contribute with about 12% of the total cosmic background radiation at 1.2 mm. <br /> About 70% of these SMGs could be classified as high-redshift star-forming galaxies based on the (B-z) and (z-K) colors and ~35% have mid-infrared (IR) colors consistent with the presence of a powerful active galactic nuclei (AGN). We find a median radio/millimeter and optical/IR photometric redshifts of 2.3 and 2.6, respectively, and a median stellar mass of 10^{11.4+/-0.4} solar masses, in agreement with previous studies of SMGs. This suggests that these galaxies are among the most massive at high-redshift. <br /> We studied the environment of our MAMBO sources by constructing projected density maps of low-redshift and high-redshift K-band selected galaxies. We found that four MAMBO sources are located at the center of strong overdensities of high-redshift galaxies. These associations are statistically likely to be real. The photometric redshifts of the galaxies in these structures are consistent with the ones of their associated SMGs, all of them being in the range z=1.5-2.0. They do not appear to be forming a red-sequence in the color-magnitude diagrams, but the SMGs are among the brightest K-band selected objects in these overdense galaxy groups. The overdensities are compact in size, typically with a diameter of ~20'', or ~170 kpc at z=1.5-2.0. This provides evidence supporting that SMGs are the progenitors of local spheroidals.<br /> Using the Plateau-du-Bureau Interferometer (PdBI), we obtained 1.3 mm continuum maps centered at the position of three significant MAMBO sources. Two of them lack significant counterparts (S/N>4) in the radio maps (J100007+021149 and J100026+021529). The other MAMBO source (J100008+02130) is embedded in a strong overdensity of high-redshift galaxies. Each of the former two MAMBO sources could be identified with single S/N>3.5 PdBI 1.3 mm sources. They could be associated with ~3 sigma radio peaks and faint optical counterparts. Their optical/IR and radio/millimeter photometric redshifts indicate that these sources are at z>3. This suggests that an important fraction of the SMGs is located at redshifts higher than the standard median value of 2.3 estimated from spectroscopic surveys of radio-selected SMGs. The MAMBO source in the strong overdensity appears to be associated with more than two PdBI 1.3 mm sources, however only one of them showed a radio counterpart. The detection of multiple millimeter components supports the picture that obscured galaxies do appear in galaxy groups.<br /> Using the IRAM 30 m telescope, we detected four carbon monoxide (CO) emission lines in one of our brightest MAMBO sources (J100038+020822). This source is a bright type-1 AGN at z=1.8275. Large velocity gradient modeling of the CO line intensities yields molecular gas (H_2) densities in the range 10^{3.5-4.0} cm^{-3} and kinetic temperatures between 50 K and 200 K. We find a dust mass of 1.2x10^9 solar masses, and a cold and hot dust temperatures of 42+/-5 K and 160+/-25 K, respectively. A comparison of its multi-wavelength SED and morphology indicates that this object shares properties of both starburst and AGN, being likely in the transition between a starbursting SMG to a QSO.