BL Lac host galaxies: how to systematically characterise them in optical-NIR spectroscopy

Abstract

Host galaxies of Active Galactic Nuclei give crucial information on the interaction between accreting Supermassive Black Holes and their surroundings, and on their common evolution. Their study in the case of aligned jetted AGN - BL Lacertae objects in particular - is complicated by the non-thermal jet component, whose bright and multi-frequency emission easily dominates over the whole electromagnetic spectrum. BL Lac host galaxies have thus been sparsely studied, and their elliptical nature is currently a hypothesis supported by few observations. With the broad aim of a systematic analysis of these sources, and in light of the many optical and NIR spectroscopic facilities that are now available, we implement an easily applicable method to determine whether a BL Lac is hosted in an elliptical or spiral galaxy. Building on the only systematic study currently available on BL Lac hosts, we worked on a sample of realistic BL Lac synthetic spectra. We analysed them and characterized their statistics using QSFit, a publicly available spectroscopy software. If BL Lac host galaxies were both elliptical and spiral, our method would be able to discriminate between them, provided that BL Lac jets are fainter than $L_γ\sim10^{46}$erg/s. Just two runs of QSFit for each BL Lac spectrum would return a single parameter, that would allow for a first broad distinction between the two classes. We finally discuss the two galaxy types that introduce some uncertainty in their classification, that might lead to possible classification biases.

Figures (7)

• Figure 1: Top: phenomenological SEDs for the 5 $\gamma$–ray luminosity bins derived for BL Lacs by ghisellini17, as labelled. Bottom: spiral (solid lines) and elliptical (dotted lines) galaxy templates from Polletta_2007 used to model the host galaxy emission in the simulated spectra. The grey shaded area in both panels represents the working range of the QSFit software, between $1215$ Å and $7300$ Å (rest frame).
• Figure 2: Top: synthetic spectrum with PL and host galaxy components comparable, with host-to-jet ratio of the order of unity as labelled. Middle, Bottom: QSFit spectral analysis performed on the spectrum of the top panel, respectively with Ell5 and Sb. In this example, QSFit recognises the correct family to which the host galaxy belongs: the $\chi^2_{red}$ obtained with the elliptical template Ell5 is better than the one obtained with spiral Sb.
• Figure 3: Top: synthetic spectrum with PL and host galaxy components comparable, with host-to-jet ratio of the order of unity as labelled. Middle, Bottom: QSFit spectral analysis performed on the spectrum of the top panel, respectively with Ell5 and Sb. Here the noise in the bluer region of the spectrum doesn't allow QSFit to distinguish the correct type of the host galaxy, as we can see from the $\chi^2_{red}$ of analyses.
• Figure 4: Top: synthetic spectrum with dominant host galaxy component, with a large host-to-jet ratio, as labelled. Middle, Bottom: QSFit spectral analysis performed on the spectrum of the top panel, respectively with Ell5 and Sb.
• Figure 5: Top: the plot shows the ratio of the statistics $R$ obtained from the two analyses performed by QSFit with the two recipes. The different colors indicate the R-band host-to-jet ratio ($L_R^{host}/L_R^{BLLac}$) as shown with the colourbar, the markers shapes refer to the input galaxy templates of the spectra. The linear fit in red was obtained by fitting the points of the spectra that obtained null host galaxy normalisation from both QSFit recipes, i.e. those with dominant jet and thus same statistics for both galaxy templates of the QSFit analyses. The inset of the plot shows the zoom of the region around $R=1$, the purple dashed lines indicates the upper ($\alpha$) and the lower ($\beta$) limits of the region of uncertainty. Bottom: the sketch shows how we can use the previous plot to discriminate between spiral and elliptical host galaxies in our BL Lac synthetic sample. The confusion region is the area where we cannot distinguish whether the host galaxy is elliptical or spiral.