MAMBO 1.2 mm observations of BzK -selected star-forming galaxies at z~2

1. MAMBO 1.2 mm observations of BzK-selected star-forming galaxies at z~2 H. Dannerbauer (MPIA), E. Daddi (NOAO), M. Onodera (NAOJ), X. Kong (NAOJ), H. Röttgering (Leiden), N. Arimoto (NAOJ), M. Brusa (MPE), A. Cimatti (INAF - Arcreti), J. Kurk (MPIA), M.D. Lehnert (MPE), M. Mignoli (INAF - Bologna), A. Renzini (INAF-Padova) I) Introduction Recently, Daddi et al. (2004, ApJ, 617, 746) presented a technique based on optical/near-IR photometry in the B, z and K bands that allows to obtain virtually complete samples of galaxies at redshift 1.4 < z < 2.5 at least to KVega < 20, see Fig. 1. A BzK sample includes all galaxies down to K-luminosity (i.e. stellar mass) and hence this is the only sample that can address questions like: what fraction of the galaxies at z=2 down to some stellar mass limit are luminous starbursts; what is the fraction of time spent as a starburst. Submm/mm selected galaxies (SMGs; see for a review Blain et al. 2002, Physics Report, 369, 111) in many cases fulfill the BzK criteria, as many of them lie at z~2 (Chapman et al. 2005, ApJ, 622, 772), and one expects that the most active star-forming BzK (hereafter sBzK) galaxies could actually be SMGs. The ensemble of sBzK galaxies with K<20 have been detected with SCUBA with a flux of ~0.8 mJy at 850 μm (5σ; Daddi et al. 2005, ApJL, 631, L13). Fig.1 --- BzK plot of K-selected galaxies. This figure shows the BzK properties of K-selected sources with K<20 from the K20/GOODS region (from Daddi et al. 2004). Roughly 95% of the K20 objects are spectroscopically identified and all the star-forming galaxies with z>1.4 are found to lie in the BzK >-0.2 region. II) Observations and Results With the major aim to investigate the relation between optically-selected and far-IR selected galaxies at high redshifts, and the formation processes of massive galaxies, we have started a project of follow-up observations of sBzK galaxies in the millimeter with the 117-element Max-Planck Millimeter Bolometer (MAMBO; Kreysa et al. 1998, Proc. SPIE, 3357, 319). In the winter semester 2004/05, we performed MAMBO 1.2 mm observations of five K-band luminous sBzK-preselected vigorous starburst galaxies at z~2. Two of these were detected at more than 99.5% confidence levels, with 1.2 mm fluxes around 1.5 mJy (3.2-3.6σ level). This translates in star-formation rates (SFRs) of about ≈ 500-1500 Msun yr-1, see Fig. 2. The two galaxies detected with MAMBO were the ones with the highest SFRs estimated from the rest-frame UV (SFRs ≥ 500 Msun yr-1). Although based on a small sample of galaxies, the consequences of this pilot study (Dannerbauer et al. 2006, ApJL, 637, L5), appear very relevant for constraining the role of obscured star-formation at high-redshifts and the link between optical selected vigorous starbursts at z~2 and the SMG population. The two MAMBO detections were also confirmed at 24 µm with the Multiband Imaging Photometer on Spitzer (MIPS). OBJ2742 is detected with a flux S24µm≈ 70±26µJy and OBJ2426 is brighter with a flux S24µm≈200±50 µJy. The 24 µm/1.2 mm flux ratios for our two sBzK galaxies are fully consistent with the range reported by Ivison et al. (2004, ApJS, 154, 124) for MAMBO galaxies, and they are also consistent with SCUBA sources (Frayer et al. 2004, ApJS, 154, 137; Egami et al. 2004, ApJS, 154, 130). Fig. 2 --- B-band (top) and z-band (bottom) Suprime-Cam Subaru images for the fields around the two MAMBO detections (dashed circle): OBJ2742 and OBJ2426. The beam size of MAMBO (10.7“ FWHM, big circle) is fully contained in the images. Both sources show an irregular, possibly merger-like morphology. North is up, and east is to the left. Fig. 3 --- The two panels in this figure emphasize the different observable properties of optically selected SMGs (sBzKs) versus far-IR selected SMGs (SCUBA and MAMBO selected galaxies, local ULIRGs). (Left Panel) SFRIR vs. SFRUV for the five sBzKs observed by MAMBO (red open square), and for the typical sBzKs from the GOODS-N field (filled square; Daddi et al. 2005). For comparison, we show the same quantities for local ULIRGs (open triangles; Goldader et al. 2002, ApJ, 568, 651; Trentham et al. 1999, AJ, 117, 2152), and the median for radio-identified SMGs from the SCUBA redshift survey (open diamond; Smail et al. 2004, ApJ, 616, 71; Chapman et al. 2005). The dashed lines represent different ratios of SFRIR to SFRUV from 1 to 100. (Right Panel)K-band magnitude (Vega) vs. 1.2 mm flux for our two sBzKs detected by MAMBO (red open squares), and for the typical sBzK from the GOODS-N field (filled square). For comparison, we show the same quantiies for radio-identified MAMBO galaxies from the NTT Deep Field (crosses; Dannerbauer et al. 2004, ApJ, 606, 664), and SMGs with z≥1.5 from the SCUBA redshift survey (open diamonds; we have estimated fluxes at 1.2 mm by scaling the 850 μm fluxes by a factor of 2.5). The dashed lines represent different fν ratios of 1.2 mm to K-band flux from 10 to 2000. The far-IR- and UV-derived SFRs of the detected sBzK galaxies agree reasonably well. This is in stark contrast with local ULIRGs and high-z SMGs for which the UV is reported to underestimate SFRs by factors of 10-100 (Fig. 3, left panel), but similar to the average sBzK-ULIRG galaxy at z~2 (Fig.1 in Daddi et al. 2005). Also the two sBzK galaxies detected at 1.2 mm are brighter in K than the typical near-IR-counterparts of MAMBO and SCUBA selected sources, despite being fainter in the mm/submm, implying a significantly different K-band to submm/mm flux ratio (Fig. 3, right panel). These differences can be reasonably understood by assuming that the far-IR emitting region of most far-IR selected galaxies are observed behind a thick dust screen, nearly opaque to the optical and near-IR radiation, as opposed to optically selected galaxies that tend to allow transmission, or measurable attenuation, of most of their light from young stars. This suggests a scenario in which z~2 galaxies, after their rapid (sub)mm brightest phase opaque to optical/UV light, evolve into a longer lasting phase of K-band bright and massive objects. We conclude by suggesting that a follow-up study of the most UV-active sBzK galaxies could be a promising alternative for finding submillimeter/millimeter sources, possibly complementary to the radio preselection method presented by Chapman et al. (2001, ApJ, 548, L147) for a similar redshift range z~1.5–2.8 or the Spitzer-MIPS preselection (Lutz et al. 2005, ApJL, 632, L13).