Appearance change events observed in the galaxy NGC 5273

V-band (green) and g-band (lime) ASAS-SN light curves of NGC 5273. Credit: Neustadt et al, 2022

The astronomers analyzed new and archival multi-wavelength observations of the active galaxy NGC 5273. As a result, they detected so-called ‘appearance change’ events in this galaxy’s active galactic nucleus. The discovery is reported in an article published Nov. 7 on

An AGN is a compact region in the center of a galaxy, brighter than the light of the surrounding galaxy. They are very energetic due to the presence of a black hole or star formation activity at the center of the galaxy.

Astronomers generally divide AGNs into two groups based on emission line characteristics. Type 1 AGN show both wide and narrow emission lines, while type 2 AGN have only narrow emission lines. However, observations have revealed that some AGNs switch between different spectral types; therefore, they were dubbed AGN with appearance change (CL).

At a distance of about 53.8 million light-years, NGC 5273 is a Syefert galaxy hosting a low-luminosity variable AGN. The active galactic nucleus of NGC 5273 is relatively faint compared to other AGNs. Previous observations of this galaxy have revealed that it began to steadily increase in brightness in late 2021, peaking in 2022.

Now, a team of astronomers led by Jack MM Neustadt of the Ohio State University in Columbus, Ohio, reports the detection of new AGN appearance-changing behavior in NGC 5273. The discovery is the result of a detailed analysis of a large set of infrared, optical, ultraviolet and X-ray data acquired by various spacecraft and ground-based telescopes.

The study by Neustadt’s team found that at least one aspect change event occurred in NGC 5273. the active galactic nucleus of NGC 5273, one of the few AGNs known to change its appearance in the infrared.

The data show that the change of aspect event in NGC 5273 coincides in time with an AGN flare that occurred in 2014 and with evidence of historical variability before 2000. These results suggest that the other flares in this galaxy may also have been events temporary changes in appearance from a reference type 1.8/1.9 to type 1.

Overall, the study found NGC 5273’s AGN to vary by factors of 2 to 10 in the infrared to X-ray flux, with short flares in 2002, 2014, 2016, and one long flare starting in late 2021.

Summarizing the results, the researchers concluded that the aspect change events in NGC 5273 are likely due to changes in how the broad line region (BLR) processes continuous emission, as evidenced by the Eddington ratio change, by Balmer decrement and emerging broad near-infrared emission lines.

“Changes in wind due to changes in λEdd could then lead to changes in the BLR, although it is unclear how this would specifically affect the Balmer Decrement or NIR broad lines,” the authors explained.

More information:
JMM Neustadt et al, Once is an Instance, Twice is a Hobby: Multiple Optical and Near-Infrared Changing-Look Events in NGC 5273, arXiv (2022). DOI: 10.48550/arxiv.2211.03801

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Citation: Observed appearance change events in galaxy NGC 5273 (2022, November 17) retrieved November 17, 2022 from

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