Low-Luminosity Active Galactic Nuclei: Are They UV-Faint and Radio Loud?

Low-luminosity AGNs are perceived to be radio loud and devoid of a ``big blue bump'', indicating a transition from a radiatively efficient, geometrically thin, accretion disc in high-luminosity AGNs, to a geometrically thick, radiatively inefficient accretion flow at low luminosities and accretion rates. I revisit the issue of the spectral energy distributions (SEDs) of low-luminosity AGNs using recently published, high-angular-resolution data at radio, UV, and X-ray wavelengths, for a sample of 13 nearby galaxies with LINER nuclei. I show that, contrary to common wisdom, low-luminosity AGNs have significant nonstellar UV flux, and UV/X-ray luminosity ratios similar, on average, to those of Seyfert 1 nuclei ~10^4 times more luminous. The alpha_ox index that quantifies this ratio is in the range between -0.8 to -1.4, and is below the extrapolation to low luminosities of the relation between alpha_ ox and UV luminosity observed at higher luminosities. In terms of radio loudness, most of the LINERs are indeed radio loud (or sometimes even ``super radio loud'') based on their radio/UV luminosity ratios, when compared to the most luminous quasars. However, the entire distribution of radio loudness has been shown to shift to higher radio/UV ratios at low AGN luminosities. In the context of this global shift, some LINERs (the majority) can be considered radio quiet, and some (from among those with black hole masses >~10^8.5 M_sun) are radio loud. The SEDs of low-luminosity (~10^40 erg/s) AGNs are thus quite similar to those of Seyferts up to luminosities of ~10^44erg/s, and there is no evidence for a sharp change in the SEDs at the lowest luminosities. Thin AGN accretion discs may therefore persist at low accretion rates, in analogy to some recent findings for Galactic stellar-mass accreting black holes.Comment: MNRAS, in pres

A Spectroscopic Method to Measure Macho Proper Motions

A Massive Compact Halo Object (Macho) that lenses a background star will magnify different parts of the rotating stellar disk by varying amounts. The differential magnification will cause a shift in the centroid of the star's spectral lines during the lensing event. The shift is proportional to the ratio of the stellar radius to the projected separation of the Macho from the star. It therefore provides a direct measure of the Einstein ring radius, and so also a measure of the Macho's proper motion (angular speed). This measurement can remove some of the degeneracy between mass, distance to the lens, and transverse velocity that exists in the interpretation of results from ongoing microlensing experiments, and is an independent test of the lensing nature of the event. We show that using the high precision attainable by stellar radial velocity measurements, it is possible to measure proper motions for $\sim 10\%$ of Machos that lens A-stars in the Large Magellanic Cloud (LMC), i.e.\ $\sim 7\%$ of the type of relatively high-magnification events that have been reported to date. If this proper-motion measurement were combined with a parallax measurement of the ``reduced velocity'', then the Macho mass, distance, speed, and direction could each be separately determined. The shift can be measured for $\sim 20\%$ of the A-star events generated by Machos in the dark halo of the LMC. This in turn would provide a measurement of the fraction of LMC vs. Galactic Macho events.Comment: 16 pages, LaTex, two PostScript figures, OSU-TA-1/9