Galaxy formation and evolution

Abstract:

The deprojection of the surface brightness distribution of an axisymmetric galaxy does not have a unique solution unless the galaxy is viewed precisely edge-on. I present an algorithm that finds the full range of smooth axisymmetric density distributions consistent with a given surface brightness distribution and inclination angle, and use it to investigate the effects of this non-uniqueness on the line-of-sight velocity profiles (VPs) of two-integral models of both real and toy discy galaxies viewed at a range of inclination angles. Photometrically invisible face-on discs leave very clear signatures in the minor-axis VPs of the models (Gauss-Hermite coefficients h4 ≳ 0.1), provided the disc-to-bulge ratio is greater than about 3 per cent. I discuss the implications of these hitherto neglected discs for dynamical modelling.

Abstract:

A self-consistent analysis of near-UV, HST/FOC images of the elliptical galaxy NGC 4552 is used to show that its central spike has brightened by a factor similar to 4.5 between 1991 and 1993, and has decreased its luminositv by a factor similar to 2.0 between 1993 and 1996. A strong UV continuum over the energy distribution of the underlying galaxy is concurrently revealed shortward of lambda similar to 3200 Angstrom by our FOS spectra extending from the near-UV to red wavelengths. Nuclear emission-line profiles of both permitted and forbidden lines are best modelled with a combination of broad and narrow components, with FWHM of similar to 3000 km s(-1) and similar to 700 km s(-1), respectively. Current diagnostics based on the emission line intensity ratios definitely places the spike among AGNs, just at the border between Seyferts and LINERs. This evidence argues for the variable central spike being produced by a modest accretion event onto a central massive black hole (BH), with the accreted material having possibly being stripped from a star in a close fly-by with the BH. In this regard, one has to look at NGC 4552 as the faintest known AGN.