Radio Galaxy Zoo: Optical and infrared colors of radio host galaxies


Working notes on the optical/infrared properties of the consensus host galaxies from Radio Galaxy Zoo.

LR’s recent emails and main points

SDSS \((u-r)\) colors of the RGZ hosts are consistent with expectations for giant elliptical galaxies. The majority of the \(\sim3200\) galaxies lie along the red sequence, with a small amount of scatter at higher redshifts. Earlier results showing them all over the map were likely due to a combination of galaxies near the telescope observable limit and/or \(u\)-band dropouts.

The small fraction of RGZ hosts in the blue cloud are interesting and potentially worthy of individual follow-up; they might either be genuine spirals or blue ellipticals with recent amounts of star formation, possibly merger induced.

Optical color-magnitude diagram of SDSS counterparts to Radio Galaxy Zoo hosts, split as a function of redshift. Dashed line shows the fit to the red sequence at \(z=0.10\); neither the slope nor offset of the red sequence for RGZ hosts evolves significantly out to at least \(z=0.40\).

Jarrett et al. (2011) and Stern et al. (2012) describe redder wise colors for AGN – referenced in Banfield et al. (2014). Can someone check this out? Are we pointing out anything new with red IR ellipticals?

The redshift distribution of the optical hosts is reasonably close to the Main Galaxy Sample in SDSS, with a median \(z\) between \(0.2\) and \(0.3\).