Spectroscopic FIR mapping of the disk and galactic wind of M 82 with Herschel-PACS

Context. We present maps of the main cooling lines of the neutral atomic gas ([OI] at 63 and 145 μm and [CII] at 158 μm) and in the [OIII] 88 μm line of the starburst galaxy M82, carried out with the PACS spectrometer on board the Herschel satellite. Aims. Our aim is to study the nature of the neutral atomic gas of M82 and to compare this gas with the molecular and ionized gas in the M82 disk and outflow. Methods. By applying PDR modeling we were able to derive maps of the main ISM physical parameters, including the optical depth (τ_[CII]), at unprecedented spatial resolution (300 pc). Results. We can clearly kinematically separate the disk from the outflow in all lines. The τ_[CII] is less than 1 everywhere, is lower in the outflow than in the disk, and within the disk is lower in the starburst region. The [CII] and [OI] distributions are consistent with PDR emission both in the disk and in the outflow. Surprisingly, in the outflow, the atomic and the ionized gas traced by the [OIII] line both have a deprojected velocity of 75 km s ^-1, very similar to the average velocity of the outflowing cold molecular gas (100 km s^-1) and several times smaller than the outflowing material detected in Hα (600 km s^-1). This suggests that the cold molecular and neutral atomic gas and the ionized gas traced by the [OIII] 88 μm line are dynamically coupled to each other but decoupled from the Hα emitting gas. Conclusions. We propose a scenario where cold clouds from the disk are entrained into the outflow by the winds where they likely evaporate, surviving as small, fairly dense cloudlets (n_H 500-1000 cm^-3, G₀ 500-1000, T_gas300 K). We show that the UV photons provided by the starburst are sufficient to excite the PDR shells around the molecular cores and probably also the ionized gas that flows at the same PDR velocity. The mass of the neutral atomic component in the outflow is ≥2-8 × 10⁷ M to be compared with that of the molecular component (3:3 × 10⁸ M) and of the Hα emitting gas (5:8 × 10⁶ M). The mass loading factor, M_Outflow/SFR, of the molecular plus neutral atomic gas in the outflow is 2. Energy and momentum driven outflow models can explain the data equally well, if all the outflowing gas components are taken into account.