The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies

Abstract

LINER nuclei, which are present in many nearby galactic bulges, may bethe manifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whetherthe compact UV nuclear sources present in many LINERs are clusters ofmassive stars, rather than being directly related to the accretionprocess. We have used the Hubble Space Telescope to monitor the UVvariability of a sample of 17 galaxies with LINER nuclei and compactnuclear UV sources. Fifteen of the 17 galaxies were observed more thanonce, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlatedvariations are seen in two independent bands (F250W and F330W).Comparison to previous UV measurements indicates, for many objects,long-term variations by factors of a few over decade timescales.Variability is detected in LINERs with and without detected compactradio cores, in LINERs that have broad Hα wings detected in theiroptical spectra (``LINER 1s''), and in those that do not (``LINER 2s'').This variability demonstrates the existence of a nonstellar component inthe UV continuum of all types and sets a lower limit to the luminosityof this component. Interestingly, all the LINERs that have detectedradio cores have variable UV nuclei, as one would expect from bona fideactive galactic nuclei. We note a trend in the UV color (F250W/F330W)with spectral type-LINER 1s tend to be bluer than LINER 2s. This trendmay indicate a link between the shape of the nonstellar continuum andthe presence or the visibility of a broad-line region. In one target,the poststarburst galaxy NGC 4736, we detect variability in a previouslynoted UV source that is offset by 2.5" (~60 pc in projection) from thenucleus. This may be the nearest example of a binary active nucleus andof the process leading to black hole merging.Based on observations with the Hubble Space Telescope, which is operatedby AURA, Inc., under NASA contract NAS 5-26555.