The HST view of the broad line region in low luminosity AGN
  We analyze the properties of the broad line region (BLR) in low luminosity
AGN by using HST/STIS spectra. We consider a sample of 24 nearby galaxies in
which the presence of a BLR has been reported from their Palomar ground-based
spectra. Following a widely used strategy, we used the [SII] doublet to
subtract the contribution of the narrow emission lines to the H-alpha+[NII]
complex and to isolate the BLR emission. Significant residuals that suggest a
BLR, are present. However, the results change substantially when the [OI]
doublet is used. Furthermore, the spectra are also reproduced well by just
including a wing in the narrow H-alpha and [NII] lines, thus not requiring the
presence of a BLR. We conclude that complex structure of the narrow line region
(NLR) is not captured with this approach and that it does not lead to general
robust constraints on the properties of the BLR in these low luminosity AGN.
Nonetheless, the existence of a BLR is firmly established in 5 Seyferts, and 5
LINERs. However, the measured BLR fluxes and widths in the 5 LINERs differ
substantially with respect to the ground-based data. The BLR sizes in LINERs,
which are estimated by using the virial formula from the line widths and the
black hole mass, are about 1 order of magnitude greater than the extrapolation
to low luminosities of the relation between the BLR radius and AGN luminosity
observed in more powerful active nuclei. We ascribe the larger BLR radius to
the lower accretion rate in LINERs when compared to the Seyfert, which causes
the formation of an inner region dominated by an advection-dominated accretion
flow (ADAF). The estimated BLR sizes in LINERs are comparable to the radius
where the transition between the ADAF and the standard thin disk occurs due to
disk evaporation.
