Active Galactic Nuclei (AGNs) are highly localised, unusually luminous regions at galactic centres. The extreme luminosity originates from an accretion disc surrounding a supermassive black hole, rather than the galaxy’s stellar population itself [1]. Several kinds of such object have been identified including the Seyfert galaxies and Quasars. They offer a challenge to low resolution spectroscopy in so far as they are faint with large redshifts, requiring sufficient signal for clear interpretation from know emission line profiles.

HB93 1821+643 (Hewitt & Burbidge catalogue, 1993; 7C 1821+6419 in the 7^th Cambridge catalogue) is a Seyfert galaxy with V-magnitude 14.2. It is further classified as type 1 Seyfert, by dint of having a wide range of emission line widths (in contrast to the more uniform widths of type 2).

The spectrum shows relative flux against wavelength measured in the usual way with normal calibration procedures. Three lines characteristic of Seyfert type 2 galaxies [2][3] are shown which have been placed by redshifting their positions from rest according the published value, z = 0.297; the rest-frame wavelengths are shown against the line label. The sharpest of these is a so-called ‘forbidden’ line of type OIII with rest wavelength 5006.84 Angstroms. While shifted through nearly 1500 Angstroms, it is very closely aligned with the observed emission line. Conversely, measuring the position of the [OIII] line the spectrum gives a good estimation of the redshift (identical to published value, to three decimal places).

The photons from this galaxy impinging on my camera had a light-travel time of 3.42 billion years – spanning roughly the duration of the existence of life on Earth.

[1] Keel, Bill. 2002. Bill Keel’s WWW Gallery - Active Galaxies and Quasars. https://pages.astronomy.ua.edu/keel/agn/.

[2] Peña-Herazo, H. A., F. Massaro, V. Chavushyan, N. Masetti, A. Paggi, and A. Capetti. 2022. Turin-SyCAT: A Multifrequency Catalog of Seyfert Galaxies. Astronomy & Astrophysics 659 (March): A32.

[3] Walker, Richard. 2017. Spectral Atlas for Amateur Astronomers: A Guide to the Spectra of Astronomical Objects and Terrestrial Light Sources. Large type. Cambridge University Press.