A New Method to Separate Star‐forming from AGN Galaxies at Intermediate Redshift: The Submillijansky Radio Population in the VLA‐COSMOS Survey

Abstract

We explore the properties of the submillijansky radio population at 20 cm by applying a newly developed optical color-based method to separate star forming (SF) from AGN galaxies at intermediate redshifts (z 1.3). Although optical rest-frame colors are used, our separation method is shown to be efficient, and not biased against dusty starburst galaxies. This classification method has been calibrated and tested on a local radio selected optical sample. Given accurate multi-band photometry and redshifts, it carries the potential to be generally applicable to any galaxy sample where SF and AGN galaxies are the two dominant populations.In order to quantify the properties of the submillijansky radio population, we have analyzed ∼ 2, 400 radio sources, detected at 20 cm in the VLA-COSMOS survey. 90% of these have submillijansky flux densities. We classify the objects into 1) star candidates, 2) quasi stellar objects, 3) AGN, 4) SF, and 5) high redshift (z > 1.3) galaxies. We find, for the composition of the submillijansky radio population, that SF galaxies are not the dominant population at submillijansky flux levels, as previously often assumed, but that they make up an approximately constant fraction of 30 − 40% in the flux density range of ∼ 50 µJy to 0.7 mJy. In summary, based on the entire VLA-COSMOS radio population at 20 cm, we find that the radio population at these flux densities is a mixture of roughly 30 − 40% of SF and 50 − 60% of AGN galaxies, with a minor contribution (∼ 10%) of QSOs.

IDENTIFICATION OF A COMPLETE 160 μm FLUX-LIMITED SAMPLE OF INFRARED GALAXIES IN THEISOLOCKMAN HOLE 1 deg2DEEP FIELDS: SOURCE PROPERTIES AND EVIDENCE FOR STRONG EVOLUTION IN THE FIR LUMINOSITY FUNCTION FOR ULIRGs

“…In an attempt to determine the spectral type of the 12 galaxies in our sample for which we lack optical spectra, we make use of a photometric (rest-frame color based) method extensively studied in Smolčić et al (2008). Smolčić et al (2006) have shown that principal component rest-frame colors (P1 and P2 hereafter) drawn from the modified Strömgren photometric system (3500-5800 Å; Odell et al 2002) essentially trace the position of a galaxy in the BPT diagram (Baldwin et al 1981;Kewley et al 2001Kewley et al , 2006Kauffmann et al 2003).…”

“…Smolčić et al (2006) have shown that principal component rest-frame colors (P1 and P2 hereafter) drawn from the modified Strömgren photometric system (3500-5800 Å; Odell et al 2002) essentially trace the position of a galaxy in the BPT diagram (Baldwin et al 1981;Kewley et al 2001Kewley et al , 2006Kauffmann et al 2003). Hence, they can be used as efficient tracers of galaxy type, such as low-luminosity AGN (Seyfert and LINER), star forming, and composite galaxies (see Smolčić et al 2006Smolčić et al , 2008 for more details). Here, we extend the method developed by Smolčić et al (2008), that utilizes only the P1 color to disentangle star forming from AGN galaxies, to a probabilistic approach that uses both, P1 and P2 colors, and we adapt it to an IR-(rather than radio-) selected sample.…”

“…Hence, they can be used as efficient tracers of galaxy type, such as low-luminosity AGN (Seyfert and LINER), star forming, and composite galaxies (see Smolčić et al 2006Smolčić et al , 2008 for more details). Here, we extend the method developed by Smolčić et al (2008), that utilizes only the P1 color to disentangle star forming from AGN galaxies, to a probabilistic approach that uses both, P1 and P2 colors, and we adapt it to an IR-(rather than radio-) selected sample. Figure 11.…”

“…Although the fraction of galaxies with H ii-region-like spectral types decreases at higher infrared luminosity (similar to what is observed for nearby galaxies selected at 60 μm), the number statistics in this high luminosity bin are too low to draw conclusions about the fractions of different spectral types. In an attempt to provide additional information on the spectral types of our high infrared luminosity sources, we have employed a new technique developed from studies of SDSS galaxies (Smolčić et al 2008) that maps UV/optical continuum colors onto the spectral line diagnostic diagram. This method is described in the Appendix, where the SDSS photometry for all of our MIPS 160 μm sources is used to derive "P1,P2" photometric spectral types for each source, following the prescription given by Smolčić et al (2008).…”

“…In an attempt to provide additional information on the spectral types of our high infrared luminosity sources, we have employed a new technique developed from studies of SDSS galaxies (Smolčić et al 2008) that maps UV/optical continuum colors onto the spectral line diagnostic diagram. This method is described in the Appendix, where the SDSS photometry for all of our MIPS 160 μm sources is used to derive "P1,P2" photometric spectral types for each source, following the prescription given by Smolčić et al (2008). These results both confirm the large H ii-region-like fraction among the lower luminosity infrared sources and show that composite and AGN spectral types appear to increase among the highest luminosity sources.…”