This unaltered story was originally published by U.S. NASA Space Agency.
URL: https://www.nasa.gov
Content is in public domain.

------------



II: A Magnetically Driven Flow in the Starburst Ring of NGC 1097

['Enrique Lopez-Rodriguez', 'Kavli Institute For Particle Astrophysics', 'Cosmology', 'Kipac', 'Stanford University', 'Stanford', 'Ca', 'Usa', 'Elopezrodriguez Stanford.Edu', 'Http']

Date: 2022-03

Kinematic studies using several gas tracers (e.g., CO, HCN) of the interstellar medium (ISM) in galaxies have shown streaming motions from the bar toward the active nuclei associated with spiral structures. These results are commonly interpreted as gas inflows fueling the active nuclei from kpc scales through the starburst ring (Kohno et al. 2003; Fathi et al. 2006; Prieto et al. 2019). In a hydrodynamical (HD) framework, the gas flow along the galactic bar suffers a large deflection angle driven by the gravitational potential of the bar. As gas is dissipative, it is shocked and loses angular momentum, which creates a radial component. The gas flow then transitions from the gas lane to a new orbit, producing a ring and/or central spiral structures around the nucleus of the galaxy (Athanassoula 1992a, 1992b; Piner et al. 1995). The gas can subsequently collapse, increase the density within the ring, and form a starburst ring (i.e., Athanassoula 1992a, 1992b; Sormani et al. 2015). High-density regions that correspond with sharp changes in gas velocities and temperature are typically identified as a shock driven by the galactic bar. The gas is shocked at the orbits crossing from the galactic bar to the ring. Hereafter, we refer to these dense regions as "contact regions." Note that this is a label considered in this manuscript to identify features of the same kinematic system produced by the bar potential. Although HD models can reproduce the kinematics of galactic bars and formation of starburst rings, these models have difficulty reproducing the gas inflows toward the nucleus of the galaxy. Thin bars with high axial ratios and without nuclear rings are required to reproduce the observed gas inflows (Piner et al. 1995).

Magnetic fields (B-fields) have been found to be strong in the dust lanes and nuclear rings of barred galaxies (Beck et al. 1999, 2002, 2005), where magnetic forces can dominate the gas flows (Beck et al. 1999, 2005). These results suggest that the B-fields are dynamically important along the bar and nuclear rings of barred galaxies. Galactic bars generate shearing gas flows that stretch and amplify the B-field. A galactic bar provides a non-axisymmetric perturbation of the gravitational potential in a galaxy. Chiba & Lesch (1994) argued that regular B-fields may be enhanced by velocity gradients, and Moss et al. (1998) showed that dynamos can be affected by the presence of a galactic bar. Non-axisymmetric perturbations result in a B-field that rotates in a bar with high dynamo modes (m = 1 or 2), where the resulting B-field may be a composition of ring-like and spiral structures toward the galaxy center (Moss et al. 1998). For an axisymmetric potential, the galactic dynamo predicts an azimuthal B-field mode of m = 0 (i.e., Chiba & Lesch 1994; Moss et al. 1998). In general, the non-axisymmetric gas flows in bars interact with B-fields and the magnetic stress removes angular momentum from the gas at the shocks. The dominant B-fields then deflect the gas flow from the galactic bar to a new orbit, producing a central ring and/or central spiral toward the nucleus. In this scenario, the deflection of the gas flow is driven by the transition between a compressed B-field in the shocks to a magnetohydrodynamic (MHD) dynamo toward the nucleus. Two-dimensional MHD simulations (Kim & Stone 2012) have shown that for magnetized models of barred galaxies, features such as shock waves at ∼1 kpc from the central black hole, MHD dynamos, and magnetic arms are indicative of the B-fields dominating the gas flows toward the central black hole. The gas flows probably follow the B-field, which feeds the black hole with matter from the host galaxy. MHD models can predict the observed gas inflow toward the central black hole with the combination of a bar and a ring. Thus, characterization of the observed B-field morphology (i.e., B-field modes) and direction at the location of these shocks (i.e., contact regions) provides the keys for understanding the gas flows toward the active nuclei from the galactic bar.

NGC 1097 (D = 19.1 Mpc, 1'' = 92.6 pc; Willick et al. 1997) is typically classified as a barred spiral (SBb), which contains a low-luminosity active nucleus surrounded by a circumnuclear starburst ring of ∼2 kpc in diameter (Hummel et al. 1987; Gerin et al. 1988). An inner bar at ∼ 28° is found within the starburst ring (Quillen et al. 1995; Prieto et al. 2005). Herschel images show that the active nucleus does not contribute to the total far-infrared (FIR) emission in the central 2 kpc (Sandstrom et al. 2010), in contrast with other nearby active galaxies (i.e., Cygnus A and NGC 1068; Lopez-Rodriguez et al. 2018b, 2018a). The thermal emission from the starburst ring contributes up to 60% of the total flux at 100 μm within the central 2 kpc. The starburst ring is embedded in an outer bar of ∼20 kpc in diameter at an angle of 148° and two spiral arms at larger scales (see Figure 5 in Quillen et al. 1995). The dust lanes have low star formation rates and low opacity (Quillen et al. 1995).

The equipartition B-field strength is estimated to be ∼60 μG in the starburst ring of NGC 1097 (Beck et al. 1999, 2005). The B-fields in the starburst ring spiral down toward the active nucleus at an angle of ∼30° (Figure 1), which is spatially coincident with the inner bar at ∼28° (Quillen et al. 1995; Prieto et al. 2005). At larger scales, the gas streams follow the outer bar and then twist to follow the spiral arms at scales of several tens of kpc. The fact that the B-fields follow the spiral arms and the circumnuclear ring indicates the action of a large-scale galactic dynamo, which may be enhancing the B-field strength in this galaxy due to differential rotation. Beck et al. (1999) suggested that magnetic stress may be an efficient mechanism to fuel the central active nucleus in NGC 1097. Further analysis of the thermal and nonthermal emission using radio polarimetric observations have shown that most of the star formation efficiency of the clouds in the starburst ring drops with increasing the B-field strength (Tabatabaei et al. 2018). The energy balance in the ISM of the staburst ring shows that the magnetic energy is in close equipartition with the turbulent kinetic energy. Both energies are a factor of ten higher than the thermal energy. These results imply that the starburst ring is magnetically critical, where the clouds are supported against the gravitational collapse. This results in inefficient high-mass star formation. Indeed, the starburst ring has a slightly lower star formation rate, ∼2 M ⊙ yr −1 (Hsieh et al. 2011), than the typical 3–11 M ⊙ yr −1 in circumnuclear starbursts in barred galaxies (Jogee et al. 2005).

Figure 1. B-field orientation and direction in the central 1 kpc starburst ring of NGC 1097. Hubble Space Telescope WFC3/F438W ultraviolet image (color scale), B-field orientation at 3.5 cm (background streamlines), B-field direction at 3.5 cm (black and white streamlines), and B-field orientation at 89 μm (yellow lines) are shown. This figure illustrates the main results from this work. Download figure: Standard image High-resolution image

NGC 1097 offers one of the best laboratories for studying the B-field in the dense ISM of a barred galaxy. Our goal is to characterize the morphology of the B-field inferred through magnetically aligned dust grains in the dense ISM of the central kpc of NGC 1097. By comparing the FIR polarimetric observations with the radio polarimetric observations and the kinematics of the molecular gas, we characterize the dust polarized emission and gas flows across the starburst ring.

We describe in Section 2 the specifics of our observations. Section 3 shows the analysis of our observations with radio and molecular gas observations. The decomposition of the B-field morphology at 89 μm and radio observations, and the estimation of the B-field direction are shown in Section 4. Our discussions are described in Section 5 and our main conclusions are summarized in Section 6.

[END]

[1] Url: https://iopscience.iop.org/article/10.3847/1538-4357/ac2e01/meta

Published by NASA.Gov

via Magical.Fish Gopher News Feeds:
gopher://magical.fish/1/feeds/news/nasa/

You are viewing proxied material from magical.fish. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.