Fundación Galileo Galilei - INAF Telescopio Nazionale Galileo 28°45'14.4N 17°53'20.6W 2387.2m A.S.L.

Students from Florence University use data taken at Telescopio Nazionale Galileo to discover a changing-look active nucleus

In April 2019 a group of undergraduate students from the University of Florence (Italy) had the unique opportunity to perform observations at Telescopio Nazionale Galileo (TNG). They collected images and spectra of spiral galaxies with the DOLORES (Device Optimized for the LOw RESolution) instrument operating it from the TNG control room, under the supervision of TNG staff and their professors. The objects to be observed were carefully selected by the students themselves, but they did not know that one of the galaxies, NGC 4156, was hiding a secret. X-ray observations from space had hinted that the nucleus of NGC 4156 might be active, but no traces of such activity had ever been detected in the optical spectra, so that the nature of the nucleus of NGC 4156 remained unclear. Actually, the galaxy had been included in the students' target list just because it could be observed simultaneously with NGC 4151, a galaxy known to host a powerful active nucleus. Once the spectrum of NGC 4156 was reduced, broad hydrogen lines and a bright continuum emission in the blue part of the spectrum (the unambiguous signatures of an active galactic nucleus) unexpectedly showed up. NGC 4156, previously considered a rather "dull" and uninteresting galaxy, revealed itself as hosting a so-called "changing-look" active nucleus.

Not so many objects of this kind are known, and the mechanism underlying the change of their spectral features is still unclear. A Director's Discretionary Time was granted to the Florence group to perform additional observations of NGC 4156 in May 2022. The collected data confirmed the presence of the broad hydrogen lines but the bright blue continuum was no longer detected. Therefore, NGC 4156 seems now to be going back to its previous state. The 2022 spectra also allowed to measure the mass of the black hole powering the active nucleus of NGC 4156. The mass turned out to be 120 millions of solar masses, 30 times as that of the black hole at the center of our galaxy.

The 2019 students' observing campaign was part of an ongoing collaboration between the TNG and the Department of Physics and Astronomy of the University of Florence, started in 2017, aiming at offering students enrolled in the "Physics and astrophysics" (Laurea) and "Physical and astrophysical sciences" (Master) programs and attending the "Complementi di astronomia" course a complete experience of how observational research in astrophysics is carried out at the professional level.

Comparison of the SDSS 2004 spectrum (red) with the
        spectra acquired at the TNG

Comparison of the SDSS 2004 spectrum (red) with the spectra acquired at the TNG. The TNG/LR-B data observed in 2019 and 2022 are represented by blue and black lines, respectively; while the TNG/VHR-R 2022 spectrum by a green line.

The group of students in the control room of the TNG
        during the 2019

The group of students in the control room of the TNG during the 2019 observing campaign.

The results of this study can be found at the following link.

Useful links:
TNG and the University of Florence
Studenti scienziati osservano l’evoluzione di una galassia lontana