HUDF-JD2: Mid-infrared Evidence for a z~2 Luminous Infrared Galaxy

TL;DR

This paper presents mid-infrared observations of the galaxy JD2, revealing it is likely a dusty starburst galaxy at z~2 with high star formation, challenging previous high-redshift interpretations based solely on optical/near-infrared data.

Contribution

The study provides new mid-infrared photometry that supports a lower redshift (z~2) for JD2, contrasting earlier high-redshift estimates, and highlights the importance of mid-infrared data in galaxy redshift determination.

Findings

Mid-infrared flux rise indicates a z~1.7 luminous infrared galaxy.

The galaxy has a bolometric luminosity of (2-6)×10^{11} L_sun.

Optical/near-infrared data alone favor z>6, but mid-infrared data supports z~2.

Abstract

The Hubble Ultra Deep Field source JD2 presented in Mobasher et al. (2005) is an unusual galaxy that is very faint at all wavelengths shortward of 1.1 micron. Photometric redshift fits to data at 0.4 to 8 microns yield a significant probability that it is an extremely massive galaxy at z~6.5. In this paper we present new photometry at 16 microns and 22 micron from Spitzer Infrared Spectrograph (IRS) peak-up imaging of the Great Observatories Origins Deep Survey (GOODS) fields. We find that the spectral energy distribution shows a factor of ~4 rise in flux density between the 16 micron and 22 micron bandpass which is most likely due to the entrance of polycyclic aromatic hydrocarbon emission features into the 22 micron and 24 micron passbands. The flux ratio between these bandpasses can be best fit by a z~1.7 luminous infrared galaxy with a bolometric luminosity of (2-6)\times10^{11} L_sun corresponding to a star-formation rate of 80 M_sun/yr. The predicted flux density values at other longer wavelengths are below the detection limits of current instrumentation but such sources could potentially be detected in lensed submillimeter surveys. Re-evaluation of the optical/near-infrared photometry continues to favor z>6 photometric redshift solutions, but we argue that the consistency of the multiwavelength parameters of this galaxy with other dusty starbursts favor the z~2 mid-infrared photometric redshift. The data presented here provide evidence that optically undetected near-infrared sources which are detected at 24 microns are most likely dusty, starburst galaxies at a redshift of z~2 with stellar masses >10^{10} M_sun.