First detection of cold dust in the northern shell of NGC 5128 (Centaurus A)

Abstract

Deep far-infrared (FIR) imaging data obtained with ISOPHOT at 90 µm, 150 µm, and 200 µm detected the thermal emission from cold dust in the northern shell region of NGC 5128 (Centaurus A), where previously neutral hydrogen and molecular gas has been found. A somewhat extended FIR emission region is present in both the 150 µm and 200 µm map, while only an upper flux limit could be derived from the 90 µm data. The FIR spectral energy distribution can be reconciled with a modified blackbody spectrum with very cold dust color temperatures and emissivity indices in the range 13 K < T_(Dust) < 15.5 K and 2 > β > 1, respectively, where the data favor the low temperature end. A representative value for the associated dust mass is M_(Dust) ≈ 7 x 10^4 M_⊙, which together with the HI gas mass gives a gas-to-dust ratio of ≈ 300, close the average values of normal inactive spiral galaxies. This value, in conjunction with the atomic to molecular gas mass ratio typical for a spiral galaxy, indicates that the interstellar medium (ISM) from the inner part of a captured disk galaxy is likely the origin of the outlying gas and dust. These observations are in agreement with recent theoretical considerations that in galaxy interactions leading to stellar shell structures the less dissipative clumpy component of the ISM from the captured galaxy can lead to gaseous shells. Alternatively, the outlying gas and dust could be a rotating ring structure resulting from an interaction or even late infall of tidal material of a merger in the distant past. With all three components (atomic gas, molecular gas, dust) of the ISM present in the northern shell region, local star formation may account for the chains of young blue stars surrounding the region to the east and north. The dust cloud may also be involved in the disruption of the large scale radio jet before entering the brighter region of the northern radio lobe.

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