The Dust Temperature of the “ Dusty ” Radio Galaxy MG 1019 + 0535 : Evidence for an Outflow

Radio galaxies characteristically have strong Lyα emission lines. However, a few have Lyα equivalent widths that are substantially weaker in relation to other emission lines. One in particular was studied by Dey et al. (1995), MG 1019+0535 (z = 2.765). We report on our reduction of Infrared Space Observatory (ISO) data in the 160 µm-band for this galaxy. We also compile information on two other high redshift active galaxies with weak Lyα lines, the radio galaxy TXS 0211–122, and the AGN–starburst galaxy F 10214+4724, to provide a small weak-Lyα line sample. IRAS plus ISO data show that F 10214+4724 has a temperature 89 ± 12 K. TXS 0211–122 was not detected in either the submillimeter or microwave. Submillimeter measurements of MG 1019+0535 (Cimatti et al., 1998) were suggestive of a dust temperature in the range 35 K ≤ T d ≤ 80 K. However our 2-σ upper limit on the flux at 160 µm shows that T d ∼ < 32 K. An energy argument based on observations which constrain the total optical extinction strongly suggests that the dust temperature must be even lower; T d ∼ < 20 K. We find the contrast between the high-temperature dust in the active starburst galaxy and the low-temperature dust in the evolved, albeit active galaxy, is consistent with an expanding cloud of dust. We find that the temperature range can be reconciled if we are seeing MG 1019+0535 at a post-starburst age of ≈ 500 − 700 Myr, with the bulk of its dust cloud at a galactocentric distance R d ∼ > 300 kpc.