Jet-Disturbed Molecular Gas Near the Seyfert 2 Nucleus in M51

1. OVRO CO(2-1) Image Spatial Resolution: 0.8”x1.2” Scoville et al. 1998 ApJ, 493, L63 Kohno et al. 1996 ApJ, 461, L29 S. Beckwith (STScI), Hubble Heritage Team (STScI/AURA), ESA, NASA CO(2-1) contour overlaid on VLA 6cm continuum image 34 pc VLA: Crane & van der Hulst 1992, AJ, 103, 1146 [OIII] image overlaid on VLA 3.6cm continuum contour CO(2-1) velocity Black dots: [OIII] velocity & its dispersion Bradley et al. 2004, ApJ, 603, 463 HST 439nm Image (Archival Data) CO(2-1) Image Jet-Disturbed Molecular Gas Near the Seyfert 2 Nucleus in M51 Satoki Matsushita, Sebastien Muller, & Jeremy Lim Abstract We carried out CO(2-1) & CO(1-0) line observations of the nuclear region of Seyfert 2 galaxy M51 with the new A configuration of the IRAM Plateau de Bure Interferometer (PdBI), yielding a spatial resolution lower than 15 pc. The images show no clear evidence of the previously suggested putative circumnuclear disk around the nucleus, but show two separate features located on the eastern and western sides of the nucleus. The western feature shows an elongated structure along the radio jet and a good velocity correspondence with optical emission lines associated with the jet, suggesting that this feature is a jet-entrained gas. The eastern feature is elongated nearly east-west, and a velocity gradient appears with the opposite sense of that previously inferred for the putative disk. Possible explanations for the observed gas distribution & kinematics are that a rotating gas disk disturbed by the jet, streaming gas, or a ring with another smaller counter- or Keplarian-rotating gas disk inside. Introduction M51 is one of the nearest Seyfert galaxies (7.1 Mpc; Takats & Vinko 2006) with type 2 AGN. A pair of radio jets emanates from the nucleus, and narrow line regions (NLRs) are associated with the jet (e.g., Bradley et al. 2004). Interferometric images in molecular gas show blueshifted emission on the eastern side of the nucleus, and redshifted gas on the western side (right figures). This shift is almost perpendicular to the jet axis, and the estimated column density is consistent with that estimated from X-ray absorption toward the nucleus, suggesting that the molecular gas can be a rotating disk and play an important role in obscuring the AGN. However, SMA CO(3-2) observations suggest a velocity gradient along the jet in addition to that perpendicular to the jet (Matsushita et al. 2004), which imply more complicated features than a simple disk structure. We therefore performed sub-arcsecond resolution CO(2-1) & CO(1-0) imaging observations of the center of M51 with IRAM PdBI to study the distribution and kinematics of the molecular gas around the AGN in more detail. Molecular Gas Distribution & Kinematics Left images show CO(2-1) distribution near the Seyfert 2 nucleus. The synthesized beam sizes reached to 0.40” x 0.31” (14 pc x 11 pc). The overall distribution and kinematics are consistent with past observations, if we degrade our image to lower angular resolution. Our images, however, show more complicated structures and kinematics, and no clear evidence of the previous suggested circumnuclear disk/torus. Molecular gas on the western side of the nucleus is elongated along the radio jet, and the velocity gradient is also parallel to the jet. The molecular gas velocity matches well with the velocity of the [OIII] emission from the NLRs (see figures below), suggesting that the molecular gas is entrained by the jet. In addition, some of the material in NLRs may be supplied from molecular gas close to the AGN. Column Density toward the Seyfert 2 Nucleus Molecular gas column density estimated from CO(2-1) indicates 6.2 x 1021 cm-2, but that estimated from X-ray observations is 5.6 x 1024 cm-2 (Fukazawa et al. 2001), which has 3 orders of magnitude difference. This discrepancy can be explained by (1) dilution effect, if obscuring material locates much closer to the nucleus or in a very thin disk, or (2) obscuring material that cannot be seen in CO(2-1). It maybe possible to see in higher-J lines (e.g., CO J=3-2) or denser gas tracers (e.g., HCN). Does Molecular Gas Torus/Disk exist around the M51 Seyfert 2 Nucleus? From our data set, it is not clear whether there is a molecular gas disk/torus around the Seyfert 2 nucleus of M51 with the spatial scale of 10 pc or larger. So what are these molecular gas structures we observed? One possibility is that these are the remnants of a circumnuclear disk, but disturbed by the radio jet. Other possibility is that there is a ring with a radius of 30-40 pc, and another smaller counter- or Keplarian-rotating disk inside. This rather complicated structure is supported by the Hubble Space Telescope (HST) images of the nuclear region of M51 (left images), which show X-shape dark lanes in front of the nucleus. It is also possible that these structures are totally independent, and the western gas is jet-entrained structure, and the eastern gas can be streaming gas toward the nucleus.