K band SINFONI spectra of two z ~ 5 submillimeter galaxy systems : upper limits to the unobscured star formation from [O II] optical emission line searches

Abstract

We present deep SINFONI K-band integral field spectra of two submillimeter galaxy systems (SMG): BR 1202-0725 and J1000+0234, at z = 4:69 and 4:55; respectively. Spectra extracted for each object in the two systems do not show any signature of the [O ii] 3726, 29 Ã…emission-lines, placing upper flux limits of 3:9 and 2:5 10ð€€€18 erg sð€€€1 cmð€€€2 for BR 1202-0725 and J1000+0234, respectively. Using the relation between the star formation rate (SFR) and the luminosity of the [O ii] doublet, we estimate unobscured SFR upper limits of 10ð€€€15 M yrð€€€1 and 30ð€€€40 M yrð€€€1 for the objects of the two systems, respectively. For the SMGs, these values are at least two orders of magnitude lower than those derived from SED and IR luminosities. The di erences on the SFR values would correspond to internal extinction of, at least, 3.4ð€€€4.9 and 2.1ð€€€3.6 mag in the visual for BR 1202-0725 and J1000+0234 SMGs, respectively. The upper limit for the [O ii]-derived SFR in one of the LAEs (Ly 2) in the BR1202-0725 system is at least one order of magnitude lower than the previous SFR derived from infrared tracers, while both estimates are in good agreement for Ly 1. The lower limits to the internal extinction in these two Lyman-alpha emitters are 0:6 mag and 1:3 mag, respectively. No evidence for [O ii] emission associated with Ly 1 is identified in our data, implying that residuals of the K-band sky emission lines after subtraction in mediumband imaging data could provide the adequate flux.