The outflow of gas from the Centaurus A circumnuclear disk. Atomic spectral line maps from Herschel/PACS and APEX

Abstract

The physical state of the gas in the central 500 pc of NGC 5128 (the radio galaxy Centaurus A), was investigated using the fine-structure lines of carbon [CI], [CII]; oxygen [OI], [OIII], and nitrogen [NII], [NIII] as well as the ¹²CO(4-3) molecular line. The circumnuclear disk (CND) is traced by emission from dust and the neutral gas ([CI] and ¹²CO). A gas outflow with a line-of-sight velocity of 60 km s^-1 is evident in both lines. The [CI] emission from the CND is unusually strong with respect to that from CO. The center of the CND (R < 90 pc) is bright in [OI], [OIII], and [CII]; [OI] λ63 μm emission dominates that of [CII] even though it is absorbed with optical depths τ = 1.0-1.5. The outflow is well-traced by the [NII] and [NIII] lines and also seen in the [CII] and [OIII] lines that peak in the center. Ionized gas densities are highest in the CND (about 100 cm^-3) and low everywhere else. Neutral gas densities range from 4000 cm^-3 (outflow, extended thin disk ETD) to 20 000 cm^-3 (CND). The CND radiation field (G_o ≈ 4) is weak compared to the ETD starburst field (G_o ≈ 40). The outflow has a much stronger radiation field (G_o = 130). The total mass of all the CND gas is 9.1 ± 0.9×10⁷M_⊙ but the mass of the outflowing gas is only 15-30% of that. The outflow most likely originates from the shock-dominated CND cavity surrounding the central black hole. With a factor of three uncertainty, the mass outflow rate is ≈ 2 M_⊙ yr^-1, a thousand times higher than the accretion rate of the black hole. Without replenishment, the CND will be depleted in 15-120 million years. However, the outflow velocity is well below the escape velocity.