WHAT'S NEW SINCE PUBLICATION:
Galaxies and Cosmology

Hubble Deep Field.
[Courtesy of Robert Williams and the Hubble Deep Field Team (STScI) and NASA.]

• Observations made with the Hubble Space Telescope suggest that there may be very significant amounts of hydrogen in intergalactic space.  The discovery could account for close to one-half of the baryonic matter in the universe.  The results were published  by Todd Tripp, Blair Savage, and Edward Jenkins in The Astrophysical Journal Letters, 534, L1, May 1, 2000.  The results were also summarized in a press release from the Space Telescope Science Institute.
  
• Results from a balloon flight of the Boomerang experiment over Antarctica in 1998 have revealed that the universe appears to be extremely close to being flat.  These data support the inflationary model extension of the Big Bang theory.  The results were reported in Nature, 404, 955, 2000 (April 27).
  
• Observations of very distant supernovae made in 1998 suggest that the universe may actually be accelerating rather than slowing down!  If the observations and their interpretations turn out to be correct, this would imply a large amount of energy populates empty space, providing a negative pressure pushing on the fabric of spacetime.  This would also suggest a non-zero cosmological constant.  See e.g. Science News, December 19/26, 1998 and Science News, February 12, 2000.
  
• Past estimates of the distance to the Large Magellanic Cloud have ranged from 135,000 ly to 180,000 ly.  A study of the distance based on eclipsing binary star systems suggests a distance of 149,000 ly plus/minus 3 to 4 percent, which is about 10 percent smaller than the "canonical value" of 163,000 ly.  Edward Fitzpatrick, Edward Guinan, Ignasi Ribas, and colleagues announced their results at the June 2000 meeting of the American Astronomical Society.  (See Sky and Telescope, September 2000.)
  
• It appears that the Milky Way's high-velocity clouds are massive gas clouds "raining" down on the plane of the Galaxy.  If true, these clouds would play a major role in the chemical evolution of the Galaxy, according to Bart P. Wakker, et al., in an article in Nature, 402, 388, November 15 1999.  These clouds could help resolve the long-standing "G-dwarf" problem.  The origin of the clouds remains unknown, but could be the result of gases left over from the formation of the Local Group, or they could be gas from stars stripped from dwarf galaxies.
  
• Final (?) results of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale have been announced.  Based on calibrating the distances to 25 galaxies using Cepheid variable stars, the HST Key Project team have determined the value of the Hubble constant H₀ (see Section 25.2, p. 1112) to be 70 km/s/Mpc +/- 7 km/s/Mpc (an accuracy of 10%).  The distance calibrations using Cepheids have allowed a half dozen secondary distance indicators to be calibrated, thus extending the depth of the survey samples.  For more information, see http://www.ipac.caltech.edu/H0kp/H0KeyProj.html.   (The results were presented at the Centennial meeting of the American Astronomical Society, May 30 - June 3, 1999.)

• Results of the new Hubble constant determination mentioned above implies a Hubble time of H₀^-1 of 14.0 +/- 1.4 Gyr.  If the age of the globular clusters is 13 +/- 2.5 Gyr (95% confidence level), then the Einstein-de Sitter model of the universe is ruled out (see Section 25.2, pp. 1113-1114).  This seems to imply that the Big Bang cosmology gives consistent answers for the age of the universe and its oldest stars.

• Astronomers continue to push the observational limits.  The current record holder for the most distant galaxy observed has a redshift of z = 6.68, announced in Nature (April 15, 1999; Vol 398). 
  
  As of April 19, 2000, the most distant quasar reported is at z = 5.8.
  
  The previous record holder (z = 5.54) was discussed in the May 2, 1998 issue of Science News, and will be described in detail in an upcoming issue of Astrophysical Journal Letters.  Other previous record holders include 0140+326RD1   with a redshift of z=5.34 (announced in September 1997 and published in The Astrophysical Journal Letters, May 10, 1998) , and CL1358+6 with z=4.92 (observed by the Hubble Space Telescope on January 13, 1996; the results were released on July 30, 1997).  (Interestingly, CL1358+6 is located behind the galaxy CL1358+62, and appears elongated due to gravitational lensing.)  The work currently being carried out leading the detection of z > 5 galaxies can only be carried out using the Keck I and Keck II observatories.

• A gamma ray burst was identified optically just 22 seconds after the detection by the Compton Gamma Ray Observatory!   Details about this optical detection of GRB990123 (discovered January 23, 1999) can be found at http://astro.caltech.edu/~jsb/GRB/index.html.   If the object is located at z > 1.6, and if the radiation was isotropic, then the implied energy release is 1.9 times the rest-mass energy of the Sun!   This would imply more energy released than is contained in the rest-mass energy of a typical neutron star (of mass 1.4 solar masses).  One possible explanation may be that the energy is actually beamed rather than emitted isotropically (see http://astro.caltech.edu/~jsb/Papers/grb990123_nature.pdf).

• Neutrino oscillations have been verified at the Super-Kamiokande laboratory in Japan and were announced June 5, 1998 at the Neutrino '98 Conference in Takayama, Japan.  These results imply a small but non-zero rest mass for neutrinos (estimated to be 0.07 +/- 0.04 eV).  This discovery could have profound implications for the solution of the solar neutrino problem (discussed in Section 11.1), the mechanisms for supernova production, dark matter, and the predominance of matter over anti-matter in the universe.   For more information see http://www.phys.hawaii.edu/~superk.

• On December 14, 1997, a gamma-ray burst was detected that appears to have released 100 times more energy than a supernova explosion.  On May 5, 1998 a radio burst was detected from a different part of the sky that also suggests a supernova explosion with 100 times more energy than is typical.  Dubbed "hypernovae," these events might represent the link between supernovae and gamma-ray bursts.  It may be that typical supernovae yield neutron stars, while hypernovae produce black holes.  For more information, see Science News, May 9, 1998 and May 23, 1998.

• In a combined effort involving the Hubble Space Telescope and the Keck Observatory, astronomers announced on May 6, 1998, that a gamma-ray burst has been detected on December 14, 1997 that represents the most energetic event ever detected in the universe.  The gamma-ray burst (GRB 971214) appears to have originated in a galaxy having a redshift of z = 3.4.  This implies that the energy output would be hundreds of times greater than previously theorized.  Within a few seconds, the burst appears to have released more energy than hundreds of supernovae combined.  Details of the findings can be found in the May 7, 1998 issue of Nature.
  
  The first-ever optical counterpart of a gamma-ray burst was detected by the New Technology Telescope and the Keck Observatory. The burst was detected by several space-based, high-energy observatories on February 28, 1997 and observed at visible wavelengths from the ground during the following week. The Hubble Space Telescope was used to track the fading fireball over the next month. 
  
  A nice summary of the debate over the nature and distance to the gamma-ray bursters can be found in the December 1995 issue of the Publications of the Astronomical Society of the Pacific (Vol. 107). In there you will find a number of articles based on the 75th anniversary astronomical debate between Bohdan Paczynski and Donald Q. Lamb, held at the Smithsonian's Natural History Museum on April 20, 1995. For background on the debate see Section 25.4 of Modern Astrophysics.
  
  Just over two months later, a second gamma-ray burst has been identified with another optical counterpart. GRB970508 was detected on May 8, 1997 by the BeppoSAX Satellite (an x-ray telescope operated by the Italian Space Agency and the Netherlands Agency for Aerospace Programs). The coordinates of the source (RA = 6^h 53^m 49.43^s, Dec = +79^o 16' 19.6", Equinox J2000) were relayed to other astronomers within hours of the detection. Optical telescopes then detected a faint variable source with a redshift of z = 0.8, consistent with galaxies and quasars. If this observation is confirmed, it would indicate that gamma-ray bursters are indeed extragalactic after all.

• New results from high-redshift studies of galaxies seem to suggest that not only does the universe appear to be open, but that it's expansion may actually be accelerating! (see Modern Astrophysics, pp. 1257-1258, 1313).  If this scenario proves to be the case, this would suggest that a repulsive force is at work on a cosmic scale in our universe, perhaps due to energy hidden in the vacuum.  It may turn out that Einstein's cosmological constant wasn't a blunder after all  (Physics News Update, March 4, 1998.)!  However, not all research teams are in agreement at the present time.

• Recent results suggest that the Milky Way's high-velocity clouds (see p. 925 of Modern Astrophysics) may be leftover clouds that were part of the formation of the Local Group of galaxies.   If this suggestion by Leo Blitz is correct, then these hydrogen-rich clouds would have been more common in the early universe, and may be related to the clouds that produce quasar absorption lines.  As galaxies such as our Milky Way consume these clouds, new star formation may be triggered.  These results are described in the October 1997 issue of Sky and Telescope magazine, and they have been submitted for publication in The Astrophysical Journal.

• Certain results from the Hipparcos astrometry satellite are proving very interesting. For instance, according to Feast and Whitelock (see also Feast and Catchpole, 1997, MNRAS, 286, L1), the Hipparcos data seems to suggest that Cepheids are more luminous than previously thought. If this proves to be the case, the distance ladder may need to be revised upward. According to the Hipparcos data, the universe may be some 10 percent larger than previously thought and the oldest stars may be no more than 11 billion years old, consistent with a universe that is 12 billion years old. On the other hand, analysis of RR Lyrae parallax data seems to reach a different conclusion, particularly with regard to the distance modulus of the LMC (see e.g., Fernley, et al.).  Apparently the question regarding the age of the universe is still under debate.

• In 1997, the Compton Gamma-Ray Observatory detected a gamma-ray halo (Astronomy Picture of the Day) surrounding the core of the Milky Way Galaxy.  Several explanations have been suggested for the unexpected halo:  (1)  cosmic-ray particles from supernovae may be colliding with low-energy photons; (2)  there may be a halo of neutron stars that were also produced by supernovae; and (3) exotic subatomic particles associated with the Galaxy's dark-matter halo may be producing the emission.   For further information, see the observatory's press release.

• Do all luminous quasars reside in luminous galaxies? The answer to the question addressed in Section 26.2 (pp. 1193-1194) of Modern Astrophysics appears to be yes. Using the Hubble Space Telescope, Bahcall, Kirhokos, Saxe, and Schneider report in The Astrophysical Journal, 479, 642, 1997 that "the images presented in this paper show that more than half of the entire sample of 20 quasars has obvious hosts and that there is solid evidence that most, if not all, of the remaining quasars also have host galaxies."

• The proper motions of stars have been measured near the center of the Milky Way, providing further support for the existence of a supermassive black hole (or potentially many smaller black holes) of approximately 2.6 million solar masses within 0.1 ly of the center (see Section 22.4). Reinhard Genzel and Andreas Eckart (Max Planck Institute) made the observations over five years using the New Technology Telescope (NTT). The results were published in the October 3, 1996 issue of Nature.  (See also Physics Today, March 1998, p. 21.)

• The Hubble Space Telescope has detected evidence of "pre-galactic blobs" (galactic building blocks), supporting the idea that galactic evolution was largely a "bottom-up" process (see Section 24.2). Eighteen gigantic star clusters, all located roughly 11 billion light-years from Earth, appear to be sufficiently close together that they will eventually merge to form a larger galaxy (image). The observation also supports the notion that cold dark matter may have played a predominant role in galaxy formation (Section 28.1), although some hot dark matter may also be required to produce large-scale structure in the universe. Further information can be found in the September 5, 1996 issue of Nature. [Images courtesy of Rogier Windhorst and Sam Pascarelle (Arizona State University) and NASA.]

• A spectacular example of a gravitational lens (Section 26.4) is shown here. Multiple distorted images of a distant young blue galaxy are formed by the cluster of galaxies in the foreground, cluster 0024+1654. The image was obtained by the Hubble Space Telescope and released to the public in April 1996. [Courtesy of W. N. Colley and E. Turner (Princeton University), J. A. Tyson (Bell Labs, Lucent Technologies), and NASA.]

• The image at the top of the page is of a small region of Ursa Major. The Hubble Deep Field was obtained by WF/PC 2 using data obtained between December 18 and 28, 1995. The entire mosaic measures about 1 arcmin across and contains at least 1500 galaxies, many only about 30th magnitude. Analysis of the data should provide important information about galaxy evolution and the early universe. There are many good reviews of the science coming from the Hubble Deep Field, such as "Probing the Faintest Galaxies", Ferguson, Henry C., Williams, Robert E., and Cowie, Lennox L, Physics Today, April 1997, pp. 24-30.  (Note:  Plans are currently underway for another Hubble Deep Field observing campaign, this time in the southern hemisphere.)

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Last modified September 12, 2005.