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 [S II] doublet to subtract the contribution of the narrow emission lines to the H alpha+[N II] complex and to isolate the BLR emission. Significant residuals that suggest a BLR, are present. However, the results change substantially when the [O I] doublet is used. Furthermore, the spectra are also reproduced well by just including a wing in the narrow H alpha and [N II] lines, thus not requiring the presence of a BLR. We conclude that the 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 10 objects, 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 clustered between similar to 500 and 2000 Schwarzschild radii (i.e., similar to 5-100 light days). These values are similar to 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 found BLR in objects with Eddington ratios as low as L-bol/L-Edd similar to 10(-5), with the faintest BLR having a luminosity of similar to 10(38) erg s(-1). This contrasts with theoretical models that predict the BLR disappearance at low luminosity. 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. We suggest that BLR clouds cannot coexist with the hot inner region and that they only form in the correspondence with a thin accretion disk.