From Astrobites : “Zooming In on an Active Galactic Nucleus Outflow with Webb” 

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From Astrobites

Sarah Bodansky
11.29.22

Title: GOALS-Webb: Resolving the Circumnuclear Gas Dynamics in NGC 7469 in the Mid-Infrared
Authors: Vivian U et al.
First Author’s Institution: University of California-Irvine
Status: Published in ApJL

As supermassive black holes accrete matter, they often like to blow off some steam in the form of outflows. Supermassive black holes are thought to power active galactic nuclei, which are often obscured by dust. Astronomers are interested in how active galactic nucleus outflows impact a galaxy’s interstellar medium and to what extent outflows could trigger or halt star formation in the interstellar medium. Since active galactic nuclei are often dusty, obscuration has made it difficult to study outflows — at least, until JWST came onto the scene.

Today’s authors inspect NGC 7469, a nearby galaxy uniquely suited for studying the relationship between active galactic nucleus outflows and star formation. NGC 7469 contains a Seyfert nucleus surrounded by a ring with active star formation, and previous observations show evidence of outflows. With new spectroscopy from JWST, the authors take a detailed look at how the gas and dust of NGC 7469 are affected by outflows.

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Active galactic nucleus. U Arizona

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Hubble Space Telescope image of NGC 7469 (upper right) and IC 5283 (lower left). [NASA, ESA, Aaron S. Evans (UVA, NRAO, State University of New York at Stony Brook), Hubble Heritage–ESA/Hubble Collaboration]

Hunting for Outflows with Spectroscopy

With mid-infrared integral field spectroscopy from Webbs’s Mid-InfraRed Instrument (MIRI), the authors use several emission lines to study where outflows occur and how they interact with the interstellar medium.

In Figure 1, a map of the flux for [Fe II], H2, and [Ar II] emission lines reveals that the H2 flux, from molecular gas, is mostly concentrated around the nucleus, while [Fe II] and [Ar II], forbidden lines emitted from ionized gas, are brightest in the ring of NGC 7469.

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Figure 1: Regions of the ring and the nucleus of NGC 7469 are bright in [Fe II] and [Ar II] while H2 is mainly bright in the nucleus. [Adapted from U et al. 2022]

Additional features emerge in the spectra from nine regions arrayed in a 3×3 grid around the nucleus of NGC 7469 (shown in Figure 2). One such feature is [Mg V]. Producing this line requires a lot of energy, and it’s quite bright. What’s more, in the region to the east of NGC 7469’s center, the [Mg V] peak is noticeably shifted to shorter wavelengths (blueshifted) relative to its central region. This blueshifted emission indicates that matter in this region of the galaxy is moving toward us — in other words, there is an outflow of gas in the eastern region of NGC 7469. The outflow only appears to occur in the eastern region, although the authors note that matter could be moving away from us in the western region, but the redshifted component is too weak to be detected.

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Figure 2: Top left: A grid of the regions where spectra were taken on top of an image of NGC 7469, with the different regions labeled according to their direction relative to the center. Bottom: The spectra from all nine regions, with spectral lines labeled. Top right: The spectra zoomed in around the [Mg V] emission line. The spectrum taken in the region east of the center is shown in turquoise, and its peak is blueshifted relative to the spectrum from the central region. [U et al. 2022]

How Do Outflows Affect the Interstellar Medium?
Plot of the brightness ratio of H2 and polycyclic aromatic hydrocarbon emission as as function of the H2 luminosity density

Figure 3: The ratio of the brightness of an H2 emission line to the brightness of a PAH emission line at 6.2 microns is plotted as a function of the density of brightness in H2 for nine regions around the center of NGC 7469. [Adapted from U et al. 2022]
The spectra of NGC 7469 also show lines caused by polycyclic aromatic hydrocarbons (PAHs), which are molecules that form part of the galaxy’s dust. Although emission from both PAHs and molecular gas is expected to be strong around star-forming regions, PAHs are ripped apart by active galactic nucleus outflows. The influence of outflows can be traced by taking the ratio of the brightness from H2 emission to the brightness from PAH emission (L(H2)/L(PAH)) — if this ratio is high, then the gas likely has experienced shocks due to an outflow. Figure 3 shows that the regions to the north and west of the center have the highest ratios of L(H2)/L(PAH) while the regions at the corners of the grid have the lowest ratios. Since the corner regions include the star-forming ring of NGC 7469, the emission from PAHs is expected to be high there, while the H2 emission is mostly concentrated in the nucleus. The authors propose that the high L(H2)/L(PAH) ratio in the north and west is the result of shocks in these regions powered by the active galactic nucleus outflow seen through [Mg V].

With new Webb data, today’s authors took a high-resolution view of the gas and dust around NGC 7469’s nucleus and found that an active galactic nucleus outflow appears to interact with its interstellar medium. As high-resolution spectroscopy from JWST allows astronomers to study active galactic nuclei and their outflows in unprecedented detail, surely more will be discovered about the role of active galactic nuclei in regulating star formation.

See the full article here .

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct.


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