Diffuse Ultraviolet Emission in Galaxies

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1 National Aeronautics and Space Administration Diffuse Ultraviolet Emission in Galaxies Anne Pellerin and Martin J. Meyer Taken from: Produced by NASA Goddard Space Flight Center and Space Telescope Science Institute. The full contents of this book include more Hubble science articles, an overview of the telescope, and more. The complete volume and its component sections are available for download online at:

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3 Diffuse Ultraviolet Emission in Galaxies Anne Pellerin and Martin J. Meyer In the arms and bars of spiral galaxies we see bright nebular regions knotty areas rich in gas and dust where stars are forming. Star formation is a fundamental process of the universe, and a key to our understanding of many topics, ranging from the evolution of galaxies to the origins of life. Because of its complexity, we need many types of observations to investigate various aspects of star formation and evolution. Recently, we gained new insights by studying archival exposures of the spiral galaxy NGC 1313, taken by Hubble s Advanced Camera for Surveys (ACS) in Our initial goal was to find the source of unexplained ultraviolet (UV) emission, diffusely distributed in NGC 1313 and other spiral galaxies, which had been observed in the 1990s. We discovered that the likely source is a particular population of apparently isolated, hot, massive, young stars. To an astronomer, a star is massive if it is initially more massive than about 10 times the mass of the Sun. The most massive stars, which are more rare, seem to start life around solar masses (M sun ), but they lose a significant amount of their initial mass in energetic stellar winds. Meanwhile, the smallest stars are about 1/10 M sun. Over this range, the basic properties of stars lifespan, temperature, and luminosity vary enormously. At the low-mass end, the stars are cool, red, and dim, and could live many hundreds of billion years. At the high-mass end, the stars are hot, blue, and bright, and live only a few million years (Myr) or so. Remarkably, over this whole wide range, astronomers can estimate the spectrum of light from a star the variation of its brightness with wavelength from estimates of the star s initial mass, approximate composition, and age. Indeed, today the existence and proven reliability of such computations of stellar spectra are a mainstay for interpreting astronomical observations, including our study of the stars in NGC We used such models twice in the case of the ACS exposures of NGC First, we estimated the intrinsic properties of each resolved star from its observed colors brightness in blue, green, and red filters. Second, we estimated the amount The barred spiral galaxy NGC 1313 is 50,000 light-years across and lies 14 million light-years away in the southern constellation of Reticulum. 69

4 of UV radiation each star produces. We focused on the hottest, most massive stars, which are the only ones capable of producing large amounts of UV radiation. We divided these stars into two groups, using their traditional names: O-type stars, with initial masses >20 M sun and lifespans < 5 Myr, and B-type stars of 8 20 M sun, which live 5 25 Myr. Then we went back to the ACS exposures to investigate the locations of the O and B stars with respect to the bright knots of gas and dust where stars are forming. When a giant cloud contracts under the force of gravity or external compression, it becomes a nursery of newborn stars, which form in collapsing pockets of the cloud. When not obscured by surrounding dust, the spectacular appearance of such nurseries like the Orion Nebula in our own galaxy, or the bright knots in the arms and bar of NGC 1313 is due to the O and B stars, which are typically few in number, but of overwhelming effect. The intense UV emission of these hot stars breaks molecules into atoms and ions, excites nebular emissions, and drives powerful winds, which can cut open and reshape the cloud. If O stars are present, they dominate events, propelling high-energy winds of ionized atoms that mark the spectrum of a star-forming region with a unique signature (see sidebar). In the 1990s, when Hubble s Faint Object Camera (FOC) a first-generation instrument supplied by the European Space Agency obtained UV images of star-forming galaxies, it had been expected that star-forming regions would produce most of the UV emission. It was a surprise to find that typically 80% of the UV emission from those galaxies is diffuse that is, homogeneously distributed outside any obvious star-forming knots. However, unlike the more recent ACS instrument, the FOC observations had insufficient spatial resolution and sensitivity to detect individual stars in the galaxies. To explain the origin of the diffuse UV emission, scientists proposed that the light could originate from star-forming regions, but then be scattered by the widely distributed dust and gas in the host galaxy. This theory was partly disproved, however, by subsequent spectroscopic studies using Hubble. These observations found that the UV emission from star-forming regions includes the signature of O-star winds. They found, however, that this signature is lacking in the diffuse emission, which indicates that at least some UV emission must come from some source other than star-forming regions. 70

5 When we plotted the O stars on the ACS picture of NGC 1313, we found they are generally located within star-forming regions, as expected. The B stars, however, are more widely distributed, with an impressive number located outside starforming regions. When we calculated the amount of UV light coming directly from the O and B stars, we found that 75 96% must come from these isolated B stars. This result is consistent with the earlier, direct, UV observations. It supports the hypothesis that diffuse UV emission within the disks of spiral galaxies originates in the light emitted by hot, young stars stars less massive and longer-lived than O stars, and located away from the stellar nurseries where O stars are typically found Pixel Index Pixel Index Pixel Index The differing distributions of O stars and B stars in NGC On the left, O stars are found near the bright knots of star formation (blue regions on the color image of NGC 1313 at right), because they cannot travel far in their very short lifetimes (< 5 Myr). At the center, B stars, which live longer (5 25 Myr), can travel far from current star-forming regions. This B-star distribution accounts for the diffuse UV emission in spiral galaxies. A cloud that collapses is initially gravitationally bound, which means that the stars it produces are also bound. Indeed, we can find many examples of gravitationally bound clusters of mature stars, which must be fossil remnants of collapsed clouds. What, therefore, do we make of this population of widely distributed, apparently isolated B stars that show no evidence of nebulosity? 71

6 When a Stellar Wind Becomes a Hurricane The hottest and most massive stars generate so much light that pressure from their photons pushes away the gaseous outer layers from the star in the form of powerful, dense winds. Astronomers can study this type of outflow because it produces a unique signature in a star s spectrum. The signature of this wind is called a P Cygni line profile, named after a blue star in the constellation Cygnus, located about 6,000 light-years from Earth. Of the estimated 200 billion stars in our Milky Way Galaxy, P Cygni is near the top in terms of its energy output. P Cygni s spectrum is quite unlike that of most stars. Typically, a stellar spectrum will feature numerous dark absorption lines, which are produced by atoms in its outer layers absorbing certain wavelengths of light emitted from below. A star s spectrum may also exhibit bright emission lines the signature of hot, glowing atoms surrounding the star. Both types of spectral lines appear at specific frequencies that are reproducible in the laboratory, and depend on the atoms that produce them. A star s spectrum will appear shifted from the corresponding laboratory-produced spectrum if the star is moving toward or away from the observer. This Doppler shift named after the Austrian physicist Christian Johann Doppler, who first described the physical principle in 1842 is a property of all wave phenomena. Approaching stars show wavelengths that are compressed (i.e., shorter in length, or bluer). The light waves of receding stars are seen stretched (i.e., longer in wavelength, and thus, redder). Star Cooler Gas top: Light passed through a prism is dispersed into its constituent parts. A gas cloud located between us and a star that is cooler than the star will absorb certain frequencies of the star s light and be detected as missing (dark) bands in its spectrum. This is known as an absorption line spectrum. bottom: Hot glowing gases emit light only at certain specific frequencies depending on the elemental composition of the gas. These appear as bright (emission) lines in what is known as an emission line spectrum. Like many stars, P Cygni s spectrum shows both absorption and emission lines. But what makes it strange is the fact that the absorption lines are shifted toward the blue end of the spectrum, meaning these atoms are moving toward Earth, while the emission lines are shifted toward the red, meaning those atoms are moving away from Earth. What could cause this unusual pattern? Empty space Hot Glowing Gas Canadian astronomer Carlyle Beals provided an explanation in papers published between 1929 and He realized that we are seeing spectral lines 72

7 Absorption Line Spectrum Hydrogen Absorption Spectrum B A Arrows indicate rapidly outflowing gas from the star Intensity B Star C A Observer Emission Line Spectrum Seen here are two ways of showing the same spectra. On the left are pictures of the dispersed light, and on the right are corresponding plots of intensity vs. wavelength. If the elemental composition of the gas producing the absorption and emission lines is the same (as shown), the identical frequencies will be absorbed or emitted. Intensity Wavelength (nm) Hydrogen Emission Spectrum Wavelength (nm) produced by P Cygni itself and from an expanding wind of hot, dense gas emanating from the star. The wind blows out from the star in all directions. When seen from Earth, the wind in front of the star produces blueshifted absorption lines, because this material is blowing toward Earth. These appear as troughs in plots of the star s brightness versus wavelength (see figure). Simultaneously, glowing gases that are moving away from the star in all directions produce broadened emission lines because some are redshifted, and others blueshifted. The combination of the blueshifted absorption and symmetrically-broadened emission lines gives rise to P Cygni s unusual spectrum (see figure). Intensity Normalized Flux Emission Absorption Wavelength Emission line P Cygni Profile w 0 A B + = w 0 C Absorption line w PV PV CIII Wavelength (Å) Hot, rapidly outflowing gas from the star produces a symmetrically broadened emission line around a particular wavelength (W 0 ) as some light is blueshifted (moving toward; see A) and some, redshifted (moving away; see B) from the observer. A second component of the light curve is produced by identical gas that lies in front of the star along the line of sight to the observer, and because it is cooler than the star, produces an absorption line (see C). When combined, the two effects create the characteristic spectral signature known as a P Cygni profile, an example of which is seen below. Lines associated with ionized phosphor (Pv) and carbon (Ciii) are identified in this particular spectrum. As described in this chapter, the diffuse ultraviolet emission from spiral galaxies such as NGC 1313 lacks P Cygni spectral lines. This absence indicates that hot, massive (type O) stars with strong stellar winds are not producing this light. 73

8 One possibility is that these B stars are indeed located in clusters, but the other stars are too dim for Hubble to detect. After all, NGC 1313 is 13.7 million light-years away, and stars there appear about 10,000 times fainter than similar stars located in our galaxy. Furthermore, Hubble can only resolve those features in NGC 1313 that are greater than about half the size of the Orion Nebula, which is a nursery for thousands of stars. While we cannot yet prove that the widely distributed B stars are indeed as isolated as they appear, that interpretation is the simplest. Indeed, the evidence is consistent with a theory that star-forming regions are typically short-lived and suffer violent ends. Such appears to be the case in the Orion Nebula, where intense stellar winds and the catastrophic explosions of O stars are blowing large quantities of dust and gas completely out of the young cluster. Because this dust and gas still constitutes most of the cluster s mass, the gravity binding the stars can weaken or disappear. In this case, the young stars dissociate and disconnect from remnants of the cloud. We suggest that the widely distributed B stars in NGC 1313 illustrate the aftermath of this scenario. All the evidence seems consistent with the theory of infant mortality, including the amount of spreading of the B stars from the spiral arms and bars, which seems roughly equal to a typical random stellar speed times a typical lifespan of a B star. If our Milky Way galaxy were observed from afar, we think that some diffuse UV emission would be detected. Although it is difficult to quantify from our position within the galaxy, we know for sure that a significant number of B stars are present both isolated and in small clusters and that they are producing UV emission. The Hubble archive now contains 17 years of astronomical observations. As illustrated by our research, archival data will continue to be useful for new purposes well beyond the research program for which they were obtained, and well beyond the end of the Hubble mission. Our results are testimony to the lasting uniqueness and excellence of Hubble data. 74

9 Anne Pellerin was born in Victoriaville, Quebec, Canada, and was educated at Université Laval in Quebec City. Anne is a postdoctoral fellow at the Space Telescope Science Institute. Her research interests are related to young stellar populations, particularly the most massive and hottest stars that dominate the light from starforming regions. She works with data from Hubble and the Far Ultraviolet Spectroscopic Explorer. Martin Meyer is a research fellow at the University of Western Australia and an honorary fellow at the University of Melbourne, his hometown. He has a particular interest in star formation and the dust and gas content of galaxies. He has worked with data from Hubble, the Spitzer Space Telescope, and the Parkes Radio Telescope. 75