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Finding Massive Stars at the Edge of Galaxies

by Fabio Bresolin, IfA Astronomer

left M83 pinwheel galaxy. right, Orion Nebula

Left: The massive star regions of M83, the Southern Pinwheel, is a composite of GALEX and optical data, with UV-bright knots appearing in blue and the H II region in red. M83 is 15 million light-years away in the constellation Hydra (NASA/JPL-Caltech/MPIA). Right: The Great Orion Nebula, known for its star-forming region, is 1,500 light-years away and in the same spiral arm of our Galaxy as the Sun (NASA Hubble and ESA).

The Orion Nebula is arguably the most famous stellar nursery in the solar neighborhood. The origin of the nebula's glow lies in its prodigious output of ionizing radiation by a single hot star that is 40 times heavier than the Sun. This ionizing radiation strips electrons from hydrogen atoms, producing a region of ionized gas called an H II region. Yet, this beautiful and picturesque cloud of ionized gas is minute by extragalactic standards. We can easily observe giant nebulae outside our Milky Way galaxy. These H II regions trace the spiral arms of their host galaxies like beads on a string. Powered by hundreds of hot stars, some of which can be 100 times more massive than the Sun, these can be thousands of times more luminous than the Orion Nebula.

The spectral analysis of the radiation from these ionized gas clouds aids astronomers in unraveling the chemical makeup of the material that eventually collapses into new stars. When massive stars conclude their short existence after only a few million years, they explode as supernovae, injecting freshly synthesized metals (elements heavier than hydrogen and helium) into the interstellar medium, so that successive generations of stars contain progressively larger amounts of heavy elements. The study of the fundamental constituents of matter, such as carbon, oxygen, and nitrogen, in remote parts of the cosmos is therefore made possible largely by looking at sites where massive stars form. Measuring how the chemical composition changes with location in galaxies helps us to understand how these systems evolve. The exploration of the chemical evolution of the whole Universe is also achieved by observing distant H II regions and "starbursting" H II galaxies, which are undergoing an intense period of star formation.

Up until very recently, the chemical analysis of spiral galaxies had been limited to the bright disks visible in optical light. This is where virtually all of the luminous matter, in the form of stars and ionized gas, is located. However, spiral galaxies extend much farther out, as revealed a long time ago by radio wavelength observations of extended envelopes of neutral hydrogen (atoms of hydrogen that are not ionized, also called H I). More recently, large numbers of old stars have been found well beyond the traditional optical radii of a few nearby spirals. Is inconspicuous, low-level star formation still taking place there, or did it shut down sometime in the past? In the former case, we should be able to detect traces of star formation activity, either by direct observations of young massive stars, or indirectly via the surrounding gas that they ionize.

Five years ago, the presence of massive stars in the very outskirts of a few nearby spiral galaxies was confirmed by the NASA Galaxy Evolution Explorer (GALEX) satellite through the detection of the ultraviolet radiation that is copiously emitted by hot stars. We now know that one spiral galaxy out of three shows signs of ongoing, but very modest, star formation activity in its extended disk, which is organized in filamentary structures that extend well beyond its optical radius. In extreme cases, the ultraviolet bright young stars lie at distances from the galactic center of up to three to four times the radius of the optically luminous matter. This discovery showed that new stars can emerge in what seemed like desolate stretches of space between galaxies. The raw material of stars abounds, as shown by vast amounts of neutral hydrogen. The finding also suggested that in these remote regions some galaxies could be forming first-generation stars out of chemically pristine material. If so, the extended ultraviolet disks would be important because no one has yet observed first-generation stars.

A peculiarity of the H II regions associated with the star-forming sites at large distances from galactic centers is that their intrinsic luminosity is very small, typically one percent that of the giant regions located in the inner parts of the parent galaxy. This happens because each one hosts a mere handful of ionizing stars. Effectively, we are looking at Orion Nebula-size star-forming regions. The reason why larger H II regions do not occur in the outer disks of spiral galaxies is currently being debated.

With a team of collaborators in Cambridge, England (Emma Ryan-Weber, Robert Kennicutt, and Quinton Goddard), I have investigated the chemical composition of about 50 star-forming regions in the outskirts of the prototypical extended ultraviolet disk galaxy M83. A few years ago, I had measured the metallicity (essentially the amount of oxygen) of several H II regions in the inner, bright part of the same galaxy. The new study offered the opportunity to see in what way, if any, the chemical composition of the regions most remote from the center differs from those near the galaxy center.

The result surprised us. The outer H II regions are not as chemically pristine as we were expecting. A still poorly known mechanism, which might involve large-scale neutral gas flows, either from the more chemically evolved central regions, or perhaps originating in some past galaxy interaction, has both enhanced and homogenized the metal content of the extended disk of M83. We do not know if this is true for other galaxies. Recently, we have used the Subaru Telescope on Mauna Kea to verify whether another extended disk galaxy, NGC 4625, shows the same behavior found in M83. Right now, it seems that we must look elsewhere in our quest for a generation of extremely metal-poor massive stars.