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Feeding and feedback in the active nucleus of Mrk 1157 probed with the Gemini Near-Infrared Integral-Field Spectrograph


  • Rogemar A. Riffel,

    Corresponding author
    1. Departamento de Física, Centro de Ciências Naturais e Exatas, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
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  • Thaisa Storchi-Bergmann

    1. Instituto de Física, Universidade Federal do Rio Grande do Sul, CP 15051, Porto Alegre 91501-970, RS, Brazil
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We have mapped the stellar and gaseous kinematics, as well as the emission-line flux distributions and ratios, from the inner ≈450 pc radius of the Seyfert 2 galaxy Mrk 1157, using two-dimensional near-infrared J- and Kl-band spectra obtained with the Gemini Near-Infrared Integral-Field Spectrograph instrument at a spatial resolution of ≈35 pc and velocity resolution of ≈40 km s−1.

The stellar velocity field shows a rotation pattern, with a discrete S-shaped zero velocity curve – a signature of a nuclear bar. The presence of a bar is also supported by the residual map between the observed rotation field and a model of circular orbits in a Plummer potential. The stellar velocity dispersion (σ*) map presents a partial ring of low-σ* values (50–60 km s−1) at 250 pc from the nucleus surrounded by higher σ* values from the galaxy bulge. We propose that this ring has origin in kinematically colder regions with recent star formation. The stellar velocity dispersion of the bulge (100 km s−1) implies in a black hole mass of MBH= 8.3+3.2− 2.2× 106 M.

Emission-line flux distributions are most extended along the position angle PA = 27°/153°, reaching at least 450 pc from the nucleus and following the orientation observed in previous optical emission-line [O iii] imaging and radio jets. The molecular hydrogen gas has an excitation temperature Texc≈ 2300 K and its emission is dominated by thermal processes, mainly due to X-ray heating by the active nucleus, with a possible small contribution from shocks produced by the radio jet. The [Fe ii] excitation has a larger contribution from shocks produced by the radio jet, as evidenced by the line-ratio maps and velocity dispersion maps, which show spatial correlation with the radio structures. The coronal lines are resolved, extending up to ≈150 pc and are also slightly more extended along PA = 27°/153°.

The gaseous kinematics show two components: one due to the gas located in the galaxy plane, in similar rotation to that of the stars, and another in outflow, which is oriented close to the plane of the sky, thus extending to high latitudes, as the galaxy plane is inclined by ≈45° relative to the plane of the sky. The gas rotating in the plane dominates the H2 and Paβ emission, while the gas in outflow is observed predominantly in [Fe ii] emission. The [Fe ii] emission is originated in gas being pushed by the radio jet, which destroys dust grains, releasing Fe. From the outflow velocities and implied geometry, we estimate a mass-outflow rate of inline image for the ionized gas and a kinetic power for the outflow of inline image.

The distinct flux distributions and kinematics of the H2- and [Fe ii]-emitting gas, with the former more restricted to the plane of the galaxy and the latter tracing the outflows related to radio jets, are a common characteristic of six Seyfert galaxies (ESO 428-G14, NGC 4051, 7582 and 4151, Mrk 1066, and now Mrk 1157) we have studied so far using similar two-dimensional observations and other two (Circinus and NGC 2110) using long-slit observations. We conclude that the H2 emission surrounding the nucleus in the galaxy plane is a tracer of the gas feeding to the active nucleus, while the [Fe ii] emission is a tracer of its feedback.