The nuclear spectrum of M 87

Abstract

The nuclear spectrum of M 87 covering the Lyα-Hα wavelength range was obtained with the HST Faint Object Spectrograph (FOS) trough a 0″.21 aperture. Contrary to some previous claims, a single power law (F _v ∼ v ^−α) can not reproduce the observed continuum shape and at least a broken power law is required for a good fit (α=1.75 and 1.41 shortward and longward of the break at ∼4500 Å). We detect a set of broad (FWHM ∼ 400 km s⁻¹) absorption lines arising in the gas associated with M 87. These are only lines from neutral and very low ionization species blueshifted by ∼150 km s⁻¹ relative to the M 87 systemic velocity, indicating a net gas outflow and turbulence. The excitation sensitive emission line ratios suggest that shocks may be the dominant energy supplier.

The nuclear source in M 87 is significantly variable. From the FOS target acquisition data, we have established that the flux from the optical nucleus varies by a factor ∼2 on time scales of ∼2.5 months and by as much as 25% over 3 weeks, and remains unchanged (≤ 2.5%) on time scales of ∼ 1 day. These time scales limit the physical size of the emitting region to a few hundred gravitational radii. The variability, combined with other observed spectral properties, strongly suggest that M 87 is intrinsically of BL Lac type but is viewed at an angle too large to reveal the classical BL Lac properties.