MPG Press Release           

Europe's space telescope ISO reveals the energy source of luminous galaxies

ISO was launched in November 1995 and is the first infrared observatory in space that provides spectrometers, a camera and a photometer. ISO's decisive advantage is its free access to infrared light that is usually blocked by the earth's atmosphere, as well as its high sensitivity. ISO's telescope and instruments are surrounded by a large tank of superfluid helium cooling them to a few degrees above absolute zero. At this temperature, the disturbing infrared emission from the telescope itself is completely suppressed. Astronomers at the Max-Planck-Institut für extraterrestische Physik in Garching, Germany, together with dutch scientists in Groningen and Utrecht, have built one of the ISO instruments, the Short Wavelength Spectrometer SWS, and are now using it to study infrared galaxies.

It has been known for some time that the ULIRGs result from collisions and merging of galaxies. For years, astronomers have hotly debated how this could trigger the huge luminosity. Some argued that the concentration of gas in the merging galaxies causes a dramatic increase in the rate of formation of hot, young stars, the resulting 'starburst' being the power source of the ULIRG. Others thought that, like in distant quasars, a central black hole is 'fed' with large amounts of matter, causing it to shine unusually bright.

The major difficulty in solving this puzzle are the huge amounts of dust hiding the centers of these galaxies from our direct view. Dust absorbs light and re-emits it as infrared radiation. Thereby dust is the origin of the huge infrared luminosity, but it also prevents us from seeing the centers of these galaxies in visible light. All that could be observed until recently is a thin layer at the surface of the galaxy we see in visible light, and the infrared reradiation by warm dust, which however tells very little about how the dust is actually heated.

With the ISO-SWS, it is now possible to observe at wavelengths where the absorption is much less severe, penetrating the dust clouds enshrouding the centers of galaxies, and to look for characteristic signatures of the ultimate energy source. Here, it is crucial that SWS can obtain spectra, that means give precise wavelength information for the detected radiation. That such spectra provide characteristic signatures is seenin galaxies with a well known energy source: for example, only galaxies powered by a central black hole show strong emission of highly ionized Neon and Oxygen (Fig._2).

New light is now shed on Arp 220, the brightest and closest of the enigmatic ULIRGs, by the ISO-SWS observations the Garching scientists have obtained (Fig._1). Among the spectral lines detected for the first time are those of molecular hydrogen, which reveal that the fierce radiation heats much of the dense molecular gas to surprisingly high temperatures. That the most massive and hot stars are able to heat the surfaces of surrounding dense gas clouds to temperatures as high as one thousand degrees was shown by the Garching scientists in great detail through other ISO observations of sources within our own Galaxy.

The biggest excitement however was created by the search for the specific signatures of the power source: while lines typical for starbursts, such as singly ionized Neon [NeII], are clearly seen, there is no trace of the signposts for a central black hole powerhouse, like triply ionized Oxygen [OIV] or four times ionized Neon [NeV]. "This is the first time we can prove that most if not all of the luminosity in one of the ultraluminous infrared galaxies comes from star formation", says Professor Genzel from the Garching ISO team. "To understand how and for how long such vigorous star formation can occur in these galaxies is now one of the most interesting questions in astrophysics", he adds.

A careful analysis of the observations showed again why the astronomers had to wait for ISO to solve the starburst/black hole puzzle: Even at the favourable wavelengths accessible to ISO-SWS, dust absorption still somewhat weakens the radiation from Arp 220. This explains why visible light observations here were indeed useless: only one in ten billion visible photons can penetrate the dust clouds.

More ULIRGs are now being observed with ISO and their spectra searched for signs of starbursts and black holes. While most seem to be dusty starforming galaxies similar to Arp 220, there is also one case so far of a dust enshrouded quasar where a black hole is the intrinsic power source.

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Figure 1 (0.2 MB)

In the ultraluminous infrared galaxy Arp 220, spectra taken with ESA's Infrared Space Observatory ISO have revealed the presence of emission lines indicating vigorous star formation. In contrast, emission lines like [OIV] that would indicate powering by a central black hole are absent. ISO observations can penetrate the thick clouds of absorbing dust seen in the visible light image (insert) and reveal the true nature of Arp 220.

Figure 2:

Figure 2 (0.3 MB)

ISO-SWS spectra of a galaxy known to be powered by star formation (M82, top panel) and another one known to be powered by a central black hole (Circinus, bottom panel) demonstrate that emission lines from highly ionized atoms (marked red in the Circinus spectrum) are a specific signature for an active nucleus containing a black hole.