The Hubble Survey of GRB and Supernova Host...

The Hubble Survey of GRB and Supernova Host Environments

Andrew Fruchter

Taken from: Hubble 2007: Science Year in Review

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 com-plete volume and its component sections are available for download online at:

www.hubblesite.org/hubble_discoveries/science_year_in_review

National Aeronautics and Space Administration

Hubble 2007: Science Year in Review

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An artist’s rendition of the twin beams of matter and energy that blast their way out of a collapsing star and that will be seen by Earth observ-ers as a gamma-ray burst. (Image credit: NASA E/PO, Sonoma State University, Aurore Simonnet)

The Hubble Survey of GRB and Supernova Host Environments

Andrew Fruchter

Gamma-ray bursts (GRBs) are the most powerful explosions after the Big Bang. From their discovery over three decades ago,

the tangled tale of GRBs has had the makings of a good mystery story: Cold War intrigue, extreme phenomena, sudden death

(of stars), eclectic clues, and circumstantial evidence, as well as advanced instrumentation operating at its limit. Recently,

Hubble has added a new subplot by discovering a relationship between GRBs and the amount of “metals” in their hosts. (To

astronomers, metals are chemical elements heavier than hydrogen and helium.) This new relationship offers insights into

the death throes of massive stars, and into the chemical evolution of the universe.

Gamma rays are photons—units of light—with over 100,000 times more energy than photons of visible light. Gamma rays

are associated with the most energetic phenomena in nature, like nuclear reactions and the radical accelerations near black

holes. GRBs—sporadic bursts of gamma rays observed in space—were first discovered in the 1960s by satellites designed

to detect violations of the nuclear test-ban treaty. When the discovery was declassified in 1973—after ruling out the pos-

sibility that GRBs were manmade—no one knew whether the sources of the gamma rays were located nearby, perhaps even

in the Solar System, or far beyond our Milky Way galaxy. In 1997, BeppoSAX, a Dutch–Italian high-energy observatory in

space, solved this part of the riddle by matching GRBs with suddenly appearing and quickly fading sources of visible light

in galaxies at great distances.

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Hubble pictures of the visible counterpart and host galaxy of GRB 990123, detected by BeppoSAX on January 23, 1999. From left to right, these pictures were taken 2.5 weeks, 2 months, and 1 year after the gamma-ray burst. Over this time span, the visible counterpart faded by more than a factor of 1 million. The disturbed shape of the host suggests merging galaxies. This is the brightest gamma-ray burst yet seen. Had it occurred in one of the Magellanic Clouds, the nearest neighboring galaxies of the Milky Way, it would have appeared 100 times brighter than the full Moon, and easily visible in the daylight.

Most GRBs emit gamma rays for only a few seconds to a few minutes, and then are gone forever. Their brevity indicates

they originate in small regions, only about the diameter of a large star, which is roughly how far light can travel in a minute.

When this characteristic size was first calculated, it spawned an immediate conundrum. Astronomers realized that if all

those gamma rays were packed in a volume so small, the density of light would be so enormous that collisions between the

photons themselves would convert the spectrum of the explosion from one dominated by gamma rays into one like that of

a very hot star. As a result, GRBs today are thought to originate from cataclysms whose volumes expand at over 99.9% the

speed of light, not from slower stellar explosions.

The energies involved in GRBs are extreme by any measure. The visible counterpart of the brightest GRB yet, GRB 990123,

spied within a minute of the burst itself, could be seen through a pair of binoculars, even though it was located 10 billion

light-years away. That is 73% of the time and distance back to the Big Bang! If the explosion had been isotropic—directed

equally in all directions—the emitted energy in gamma rays alone would equal the mass of the Sun converted directly

to light by Einstein’s famous formula E = mc 2. (Energy [E] released equals converted mass [m] times the speed of light

[c] squared.)

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GRBs are probably not isotropic, however, but directional—like a spotlight in the theater. If so, only a minor percentage of

such explosions would be observed as GRBs—those viewed from select angles. Although the high-energy radiation from a

GRB fades rapidly, it often continues to emit in the optical, radio, and even x-ray wavelength ranges for days to months after

the original outburst. Astronomers think this afterglow is cause by the impact of the explosive outflow on the surrounding

interstellar medium.

As the afterglow fades, the remaining light has the signature of a core-collapse supernova (CCSN). CCSNs occur when a

massive star exhausts its nuclear fuel, and losing the supporting pressure of light pushing outward from the core, collapses

catastrophically under the force of gravity.

In all cases where observers could determine the type of supernova underlying the GRB, they have found a Type Ic supernova.

This relatively rare type of CCSN is distinguished by a lack of hydrogen and helium features in the spectrum. The progenitor

stars most likely lost their outer layers of hydrogen and helium in strong stellar winds, possibly in eruptions. In fact, it may

be the absence of these outer layers that permits the burst of gamma rays to penetrate all the way through the star.

Whatever their type, CCSNs are harbingers of ongoing star formation. Because their massive progenitor stars live a short

time—only a few million to a few tens of million years—they are unable to travel far from their birth environment, which

is shared with myriad longer-lived stars. (Less massive stars burn their fuel more slowly and live longer than the massive

progenitors of CCSNs. Our own Sun has a lifetime of about 10 billion years.)

Realizing that GRBs are caused by the deaths of massive stars, we wondered if they are distributed in and among galaxies

like massive stars in general, or whether specific locations are favored. The main purpose of the Hubble survey of GRB and

supernova host environments was to address this issue, and the results were a surprise.

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The Hubble Survey

Our team collected Hubble pictures of 42 GRB hosts and 16 CCSN hosts to compare and contrast the environments. Hubble

is well suited to this investigation because of its unique ability to resolve the host galaxies, which are mostly small, distant,

and faint.

With a single exception—the spiral galaxy that is the host of GRB 990705—we found that the GRB hosts are small, low-

mass, irregular galaxies. Such galaxies generally have low metallicity.

By contrast, about half of the CCSN hosts have a well-defined and highly organized spiral structure. Typically, these hosts

are several times more massive than the GRB hosts, and have a much higher metallicity. Even when the GRB sample is

restricted to the same range of distances as the CCSNs (less than about 8 billion light-years), only 1 in 20 GRB hosts is a

spiral. That is a factor-of-10 reduction from the CCSN host fraction.

Lower-mass galaxies probably have lower metallicity because the metal-enriched ejecta of supernovas is blown straight out

of the galaxy. The gravity of more massive galaxies, on the other hand, will restrain supernova outflows and eventually pull

the material back into the interstellar medium, where it can be reprocessed into new stars, resulting in even more metals.

Independent of the exact theoretical reasons for our results, the observations directly imply that large spiral galaxies like our

own Milky Way rarely produce GRBs. Yet those hoping for a spectacular display may not have to look too far. Our neighbor

galaxies, the Magellanic Clouds, appear to be typical of GRB hosts, and best of all, they are at a convenient—but safe—

distance to watch the fireworks if a GRB occurs.

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A mosaic of 42 hosts of gamma-ray bursts hosts taken by Hubble. The positions of these GRBs were all sufficiently accurate to clearly identify a host. In those cases where the position was also certain enough to distinguish where on the host it occurred, that location is shown by either a green circle or a cross hair. Where the location was known to about a pixel or better, a cross hair was used. In the other cases, the size of the green circle indicates the positional error. Each square in the mosaic is 3.75 arcseconds across.

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A mosaic of the hosts of 16 core-collapse supernovas discovered by the Great Observatories Origins Deep Survey. The positions of all the core-collapse supernovas are marked, and because they were all discovered by Hubble, the positions are known to subpixel accuracy. The squares in this mosaic are twice the linear size on the sky of the squares in the previous figure. In contrast to the gamma-ray burst hosts, which are largely irregular galaxies, about one-half of the core-collapse supernova hosts are spirals.

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Metallicity, Type 1C Supernovas, and GRBs

The “metallicity” of an astronomical object or environment indicates the abundance of such heavy elements as nitrogen, oxygen, and

carbon. At the time of the Big Bang, all normal matter was created solely in the form of hydrogen and helium. All heavy elements any-

where in the universe were manufactured after the Big Bang, inside stars. Stellar explosions and outflows disperse metals into space,

where they can be swept up in newly forming stars. All the materials of Earth, for example—including atoms essential to life—were

created inside earlier generations of stars.

The role of heavy elements in the death of stars has nothing to do with their weight, and everything to do with their interaction with light.

The atomic structures of hydrogen and helium—the schemes of their internal energy levels—are relatively simple. As a result, their

spectra are simple, because they include only the few “lines”—energies or wavelengths absorbed or emitted—which correspond to

transitions between the sparse internal energy levels; therefore, a gas of hydrogen and helium is nearly transparent. By contrast, a gas

of metals interacts much more strongly with light—and therefore has higher opacity—because the internal atomic structures of heavier

elements are much more complex, resulting in spectra with many more lines.

It stands to reason that the largest stellar explosions should accompany the deaths of the most massive stars. It is less obvious that the

character of the explosion—and the possibility of a GRB—should depend on metallicity, as the Hubble survey has found.

The most likely mechanism is the outward pressure of light on the atmosphere of the star. For massive stars of high metallicity, this

pressure is high, and can blow away a large fraction of the star’s mass. In addition, mass loss reduces the rotation rate of the star, and

rapid rotation may be important for creating the beams that produce GRBs. Low-metallicity stars will lose less mass and rotational speed

because of outflows, because their atmospheres are more transparent and therefore, experience less pressure from below.

Andy Fruchter was born outside New York City and grew up in northern Virginia. He did his undergraduate

work at Harvard University and then went to Cambridge University on a Marshall Scholarship. Concerned

that if he stayed in high-energy physics he would end up making bombs, he studied biophysics at Cam-

bridge. Wanting to pursue more fundamental physics, however, he continued on to Princeton University,

where he worked on millisecond pulsars with Joseph Taylor. While things do blow up in astrophysics, Andy

figures at least no one (so far as we know) is at fault.

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