SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES): Confusion-limited Submillimeter Galaxy Number Counts at 450 μm

Zhen-Kai Gao^1,2*, Chen-Fatt Lim^1,3, Yu-Hsuan Hwang^1,4, Wei-Hao Wang¹, Chian-Chou Chen¹, Chorng-Yuan Hwang²

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
²Graduate Institute of Astronomy, National Central University, Taoyuan, Taiwan
³Graduate Institute of Astrophysics, National Taiwan University, Taipei, Taiwan
⁴Department of Physics, National Taiwan University, Taipei, Taiwan

* Presenter:Zhen-Kai Gao, email:zkgao@asiaa.sinica.edu.tw

To fully understand the galaxy formation and evolution requires information on the dust-obscured part of the cosmic star formation. We present the results from the SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES). As a JCMT large program, STUDIES aims to reach the confusion limit at 450 μm in both the COSMOS and SXDS fields. By now, we have finished our 300-hour observations in the COSMOS field, and ~27% of the allocated time has been acquired in the SXDS field. Combining the archival data, we obtain a confusion-limited 450 μm map of the COSMOS field, which covers ≃ 450 arcmin² in area and achieves a noise level at the map center of ~0.6 mJy. Before the completion of the observations, we had still done many works with the preliminary data. We utilized machine learning techniques to extend a small sample of 164 submillimeter galaxies (SMGs) to 6182 SMG candidates (in 1.6 deg²) with the help of the COSMOS2015 catalog to study the clustering and dark matter halo masses of SMGs. The inferred dark matter halo masses are ≃ (2.0 ± 0.5) × 10¹³ h^-1 M_☉, which are consistent with being the progenitors of the present-day massive elliptical galaxies. We also examined dusty star-forming galaxies as contaminants among the NUV-r-J selected quiescent galaxies (QGs). The contamination rate is ~10%, and the contaminants can be removed from the QG candidates by 24 μm, submillimeter, or 3GHz detections. Finally, we present the deepest 450 μm galaxy number counts in the COSMOS field derived from our confusion-limited map. We detected 479 sources at 3.5σ with flux densities ranging from ~2 to ~40 mJy. We derived the raw number counts from the directly extracted sources and performed Monte Carlo simulations to recover the intrinsic number counts. Compared to the direct number counts in the literature, our counts extend to flux densities that are ~40% fainter. We find that the trend of the faint-end counts is still consistent with no faint-end drop-off or flatten-out. This suggests that there remain abundant faint 450 μm sources unresolved by current SCUBA-2 imaging. To understand the possible evolution of this population, we also compare our counts with those derived from models. The results show reasonable consistency from the faint end to ~20 mJy, but a steeper turn than the model predictions at the bright end. We also compare the integrated surface brightness down to 2.0 mJy of our counts with that measured by COBE. We find that our confusion-limited SCUBA-2 image could directly resolve 50% of the 450 μm COBE extragalactic background light.

Keywords: galaxies, submillimeter, evolution, high-redshift