Download Diffuse Ultraviolet Emission in Galaxies

National Aeronautics and Space Administration
Diffuse Ultraviolet Emission
in Galaxies
Anne Pellerin and Martin J. Meyer
Taken from:
Hubble 2007: Science Year in Review
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Telescope Science Institute.
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Hubble 2007: Science Year in Review
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 2003. 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 30–40 solar masses (Msun), but they lose a significant amount of
their initial mass in energetic stellar winds. Meanwhile, the smallest stars are about 1/10 Msun. 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 1313.
We used such models twice in the case of the ACS exposures of NGC 1313. 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.
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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 Msun and lifespans < 5 Myr, and “B-type” stars of 8–20 Msun, 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.
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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
4000
3000
2000
1000
1000
2000
Pixel Index
3000
4000
1000
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Pixel Index
The differing distributions of O stars and B stars in NGC 1313. 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?
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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).
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
Star
toward the blue end of the spectrum, meaning these
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
Empty space
appear as bright (emission) lines in what is known as an emission line spectrum.
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?
Canadian astronomer Carlyle Beals provided an
Hot Glowing Gas
explanation in papers published between 1929 and
1934. He realized that we are seeing spectral lines
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Hydrogen Absorption Spectrum
Absorption Line Spectrum
Arrows indicate rapidly
outflowing gas from the star
10
B
A
Intensity
0.9
0.8
Star
0.7
C
Observer
0.6
B
A
0.5
450
500
550
600
650
Wavelength (nm)
700
750
800
Intensity
0.4
400
Emission line
Hydrogen Emission Spectrum
Emission
0.6
Absorption
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.
+
=
P Cygni Profile
C
w0
Absorption line
0.4
Wavelength
0.3
0.2
0.1
0
400
450
500
550
600
650
700
Wavelength (nm)
750
800
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
Normalized Flux
Emission Line Spectrum
Intensity
0.5
A B
w0
w0
1.5
PV
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
CIII
1
0.5
0
1100
1120
directions. When seen from Earth, the wind in front of the star produces
blueshifted absorption lines, because this material is blowing toward
PV
1140
1160
Wavelength (Å)
1180
Hot, rapidly outflowing gas from the star produces a symmetrically broadened emission line around a particular wavelength (W0) 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.
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).
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.
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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.
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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.
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