Infrared and millimetre-wavelength evidence for cold accretion within a z=2.83 Lyman α blob

Abstract

This paper discusses infrared and millimetre-wavelength observations of a Lyman α blob (LAB) discovered by Smith & Jarvis, a candidate for ionization by the cold accretion scenario discussed in Fardal et al. and Dijkstra et al.We have observed the counterpart galaxy at infrared wavelengths in deep observations with the Spitzer Space Telescope using the IRAC 3.6, 4.5, 5.8 and 8.0 μm and MIPS 24 μm bands, as well as using the Max-Planck Millimeter Bolometer (MAMBO-2) at a wavelength of 1.2mm with the IRAM 30 m telescope. These observations probe the ≳ 95 kpc Lyman α halo for the presence of obscured active galactic nucleus (AGN) components or the presence of a violent period of star formation invoked by other models of ionization for these mysterious objects. 24 μm observations suggest that an obscured AGN would be insufficiently luminous to ionize the halo, and that the star formation rate within the halo may be as low as <140M⊙ yr^−1 depending on the model spectral energy distribution (SED) used. This is reinforced by our observations at 1.2mm using MAMBO-2, which yield an upper limit of star formation rate <550M⊙ yr^−1 from our non-detection to a 3σ flux limit of 0.86 mJy beam^−1. Finding no evidence for either AGN or extensive star formation, we conclude that this halo is ionized by a cold accretion process. We derive model SEDs for the host galaxy, and use the Bruzual & Charlot and Maraston libraries to show that the galaxy is well described by composite stellar populations of total mass 3.42 ± 0.13 × 10^11 or 4.35 ± 0.16 × 10^11M⊙ depending on the model SEDs used.