By Mark Durré
[email protected]
Type 1 active galactic nuclei (AGN) show broad spectral lines, with characteristic velocities of several thousand kilometres per second. These are thought to come from a region (BLR – the broad-line region) of gas clouds close to the central black hole, but how these are distributed and orbit around the black hole is not well understood. A sub-class of these AGN, called narrow-line Seyfert 1 galaxies (NLSy1), show lower velocities of the order of 1000 km/s. There are two lines of thought about how these lower velocities come about; is it because the black hole is small and therefore gas orbits at lower velocity, or is it because the gas is in a disk and we are looking at it nearly-face on, so we see lower velocities?
To probe these objects, we can use infrared spectroscopy and compare it with the optical spectrum. Infrared (IR) wavelengths have the advantage of being able to penetrate further into obscuring dust, thus getting closer to the black hole. We used the SofI (Son of ISAAC) instrument on the New Technology Telescope at ESO’s La Silla Observatory to give us long-slit spectra from 1495 to 2518 nm (H– and K-bands). The galaxies were chosen from catalogues of NLSy1 optical spectra in the southern hemisphere (Chen et al., 2018, Schmidt et al., 2018). In three nights observing in Visitor Mode, we took IR spectra of 55 galaxies with redshifts from 0.01 to 0.5; of these we could measure the Paschen α emission line (at 1875 nm) for 49 galaxies and compare them with the catalogue spectra Hβ line emission.
One of the questions to be answered was: what are the line shapes for emission from AGNs? Conventionally, Type-1 AGN “permitted” emission lines (e.g., the hydrogen and helium lines) are fitted with both a broad and narrow Gaussian curve (for both the broad and narrow line regions). On the contrary, we could fit the hydrogen lines with a single Lorentzian function. This is preferred on theoretical grounds for a rotating disk with turbulence (see e.g., Goad et al., 2012), as well as by good old Occam’s razor! Any narrow-line region emission is totally overwhelmed by the BLR emission. The plot below shows an example of this comparison fit for the galaxy 6dF J2025574-482226, with just as good a fit but much less complexity.
When we compared the line widths and fluxes from both the Pa α and the Hβ line emission we get different results than we might expect; in general, the Pa α lines are somewhat narrower and a bit stronger than in normal astrophysical conditions. We can deduce, in the extreme environment around the AGN, a scenario where an intermixture of dusty and dust-free clouds (or alternatively a structured atmosphere) differentially absorbs the line radiation of the BLR, due to dust absorption and hydrogen bound-free absorption. The picture below illustrates this; we have differential Hβ/Pa α absorption either by clouds intercepting the line of sight (shown on the left), or by an atmosphere above the accretion disc/BLR with different densities and central concentrations of dust and/or hydrogen bound-free absorption (as on the right). The black disc is the accretion disc and the flattened BLR.
To find out more you can view our paper on arXiv or at Durré, M., & Mould, J. 2022, MNRAS, 509, 2377.
