Hubble Space Telescope Probes a Galaxy with an Active Nucleus
4 October 1990The NASA/ESA Hubble Space Telescope has provided an unprecedented, detailed view of highly energetic events in the core of a galaxy 30 million light-years away. The observations are a first step in HST's search for super-massive black holes at the nuclei of active galaxies.
Holland Ford (Johns Hopkins University and Space Telescope Science Institute), Ian Evans, Anne Kinney (Space Telescope Science Institute), Lee Armus and Saul Caganoff (Johns Hopkins University) used the HST's Wide Field and Planetary Camera (WF/PC) to look into the core of the galaxy NGC 1068. The resulting data will be used to support follow-on observations by the HST's Faint Object Spectrograph (FOS) Investigation Definition Team headed by Richard Harms (Applied Research Corporation). The FOS observation will analyze light from deep within the nucleus to help astronomers better understand the dynamics of the "engine" which powers the galaxy's unusual activity.
Located at a distance roughly two thirds of the way to the great Virgo clusters of galaxies, NGC 1068 looks like a normal barred spiral galaxy. However, since the year 1909, the core of the galaxy has been known to be the source of unusual activity, which is made evident by the presence of extremely hot (ionized), fast moving clouds of gas in the vicinity of the galaxy's nucleus. Similar galactic fireworks have been detected at the heart of other galaxies as well, and they are collectively referred to as Active Galactic Nuclei (AGN).
The core of NGC 1068 is approximately 1/50 the entire luminosity of our galaxy, the Milky Way. Other AGNs emit as much energy as twenty Milky Way galaxies put together (equal to the combined light of more than one trillion stars like our Sun). Because the brightness of an AGN varies on time scales of weeks, astronomers know that the diameter of the region producing this enormous luminosity is no more than a few tens of light days across (50 to 100 times the size of the Solar System).
Thermonuclear reactions in stars cannot generate such a prodigious amount of energy. The favored mechanism is a massive black hole, where energy is released as matter plunges into the hole's deep "gravity-well". A black hole with a mass equivalent to approximately 100 million stars like our Sun would be required to account for the nuclear activity in NGC 1068.
"Because NGC 1068 is very bright and relatively nearby, HST images and spectra should enable us to determine the exact location of the black hole, and determine the effects of jets, winds, and ionizing radiation from the black hole on the central 100 parsecs [300 light years] of the surrounding galaxy," says Ford.
"The WF/PC images resolve the center of the galaxy and reveal far more detail than has been seen in any pictures previously taken from the ground," says Ford. Several gaseous clouds, some as small as 10 light-years across, are clearly resolved in the central 150 light-years. Three of the clouds correspond exactly with the brightest radio sources previously observed near the core.
Though NGC 1068's "central engine" is not directly visible, its effect is clearly evident on numerous gaseous clouds in the vicinity of the nucleus. The clouds are glowing as a result of radiation beamed out of the hidden "central engine". One of the clouds is also possibly being compressed by a jet of plasma ejected from the nucleus.
The WF/PC images show that the spatial distribution of the gas clouds, even on the smallest scale, appear to trace a cone of ionizing radiation, which is beamed out from the central engine. These images confirm an earlier theory that NGC 1068's suspected black hole is shrouded in a dense ring of dust. Energy released from the nucleus escapes through the ring's "donut hole" to produce a brilliant "searchlight" of radiation.
NGC 1068 was observed by the WF/PC in the Planetary Camera mode through two filters; the first was a narrow-band filter which transmits star light and the light emitted by doubly ionized oxygen, while the second was a medium-band filter which passes light from stars while excluding light from ionized gas. The researchers subtracted the two pictures to obtain an image, which only shows the clouds of ionized gas near the nucleus. The images were then improved through image reconstruction techniques.
The Faint Object Spectrograph Team will now use this remarkable new view of NGC 1068 to target specific clouds that may be reflecting light from the vicinity of the suspected black hole.