Unveiling the Reddest Galaxies in the Universe with VIDEO and Spitzer

Abstract

We used data from the Visible and Infrared Survey Telescope for Astronomy Deep Extragalactic Observations (VIDEO) Survey and from the Spitzer Extragalactic Representative Survey (SERVS) to generate a sample of Extremely Red Galaxies (ERGs) with colours H - [4:5] > 4, by matching H-band (with a bandpass centred on 1:65mm) near-infrared data, and 4:5mm midinfrared data. Using optical data from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) and a pre-release of data from the European Southern Observatory Spitzer Imaging Extragalactic Survey (ESIS), extra wavelength bands covering each of the sources in our sample was assembled to increase the wavelengths available. Through the use of Le Phare, a photometric redshift, spectral energy distribution, fitting code, we calculated the probabilities of many different models across a range of redshifts (z) and extinctions (AV ) for these sources, along with what type of model, be it QSO or Galaxy, was the best fit for them. The sample constructed was ~ 3:75 magnitudes brighter than the ERGs previously explored in previous work by Caputi et al. (2012) and were at lower redshifts (0:4 < z < 3:0). The sample was also found to be primarily modelled by QSO templates. Using data from the Herschel Multi-tiered Extragalactic Survey (HerMES), a far-infrared survey, and assuming a specific dust template model, we converted the 250mm fluxes to a total infrared luminosity, and then used a standard calibration to convert this to the Star Formation Rate (SFR) for each source. These SFRs were very variable, with one source at ~ 0 M_yr-1 and one even greater than 1000 M_yr-1. We used all of this information to determine the probabilities of each possibility, and their reliability. We also discussed the best fit solutions and if we can trust them, including the importance of the optical data for deriving reliable source properties. We then related our sample of sources to similar ERGs in the literature before finishing with a simulation calculation to estimate how many galaxies with our chosen colour we would expect to see in the area of sky investigated. It was found that current semi-analytical models such as Henriques’ work, based on the Millenium Simulation (Henriques et al., 2009), predicts an ERG density ~ 54 times less than that found in this work. Finally we discuss a potential for follow up work, including different SED fitting techniques, an extra sample of templates, and spectroscopic follow-up.