Butcher-Oemler Effect
Encyclopedia
The Butcher-Oemler Effect is a scientific hypothesis
suggesting the cores of galaxy clusters at intermediate redshift
(z ~ 0.3) contain a larger fraction of blue galaxies than do the cores of low redshift clusters. The claim was first put forward by Harvey Butcher
and Augustus Oemler in a 1978 Astrophysical Journal paper.
The original Butcher-Oemler paper presents photometry
of two clusters of galaxies, Cl 0024+1654 at z = 0.39 and 3C 295
at z = 0.46. These clusters are not atypical in their morphology or richness. They are both rich and centrally concentrated, rather like the nearby Coma Cluster. The surprising conclusion is that in the cores of these distant clusters more blue galaxies are observed than are observed in the cores of nearby clusters of similar richness and morphology.
Cluster galaxy "blueness" may, under certain circumstances, be used as an indicator of ongoing star formation
. Astronomers have identified three spectral classes in which a significant set of blue galaxy cluster members may be categorized. The first is objects undergoing vigorous star formation with very blue colors and spectra showing emission-filled H-delta
lines. Secondly, post-starburst galaxies are also observed. These are cluster members showing similarly blue colors as starburst galaxies only with moderate to strong H-delta absorption. Third are cluster members showing broad and/or high excitation line spectra, often found in active galactic nuclei. The Butcher-Oemler observations suggest at intermediate redshift a higher rate of star formation may be observed in a fraction of the galaxies in the cores of rich clusters than in the cores of rich clusters at low redshift.
The 1978 Butcher-Oemler paper sparked considerable debate. The ensuing series of Butcher-Oemler papers spans six years concluding with The Evolution of Galaxies in Clusters. V. A Study of Populations since z ~ 0.5. This paper presents photometry of 33 clusters of galaxies with redshifts varying from 0.003, the Virgo Cluster
, to 0.54, the cluster Cl 0016+16. This study bolsters the conclusion put forward in their original 1978 paper, that there has been "strong, recent evolution of galaxies in clusters".
The Butcher-Oemler conclusion generated numerous investigations of the cores of rich clusters at intermediate redshift (0.3 ≤ z ≤ 0.9): Couch and Newell (1984), like Butcher and Oemler, acquired broadband photometry of such environments. Couch et al. (1983), Ellis et al. (1985), MacLaren, Ellis
and Couch (1988), Aragon-Salamanca, Ellis and Sharples (1991) and Aragon-Salamanca et al. (1993) imaged clusters with redshifts between 0.5 ≤ z ≤ 0.9 in both optical
and infrared
bands. Spectroscopic
observations were conducted by Dressler and Gunn
(1982), Lavery and Henry (1985) and Couch and Sharples (1987). Studies of the morphologies of the galaxies in the cores of these clusters were undertaken using the Hubble Space Telescope
by Couch et al. (1994) and Dressler et al. (1994). The outcome of these investigations is that the Butcher-Oemler effect is widespread in rich clusters at z > 0.2 and is due to vigorous episodes of star formation in a subset of the cluster members.
The effect does appear to be confined to rich clusters, at least as rich or richer than the Virgo cluster. Allington-Smith et al. (1993) observed galaxies in small groups out to a redshift of 0.5 and found no relation between the fraction of blue members and group richness; groups of all richnesses were observed to have similar high fractions of blue galaxies. Colless et al. (1990, 1993) have confirmed that better than 95% of blue field galaxies
brighter
than bJ = 22.5 are at redshifts less than z = 0.5, verifying at the 90% confidence level models predicting no luminosity evolution of the field galaxy population since z = 1. Using the Hubble Space Telescope, Dressler et al. (1994) observed the cluster CL 0930+4713 at 0.41 and Couch et al. (1994) observed AC 114 at z = 0.31 and Abell 370 at z = 0.37. These authors independently arrived at the conclusion that every observation of a merger
between cluster members, both of which showed blue colors, had the spectroscopic signature of a starburst or post-starburst object. It is plausible therefore, that the Butcher-Oemler effect may be partially the result of galaxy-galaxy mergers.
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
suggesting the cores of galaxy clusters at intermediate redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
(z ~ 0.3) contain a larger fraction of blue galaxies than do the cores of low redshift clusters. The claim was first put forward by Harvey Butcher
Harvey Raymond Butcher
Harvey Raymond Butcher III is an astronomer who has made significant contributions in observational astronomy and instrumentation which have advanced understanding of the formation of stars and of the universe. He received a B.Sc...
and Augustus Oemler in a 1978 Astrophysical Journal paper.
The original Butcher-Oemler paper presents photometry
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
of two clusters of galaxies, Cl 0024+1654 at z = 0.39 and 3C 295
3C 295
3C 295 is a narrow-line radio galaxy located in the constellation of Boötes. With a redshift of 0.464, it is approximately 5 billion light-years from Earth...
at z = 0.46. These clusters are not atypical in their morphology or richness. They are both rich and centrally concentrated, rather like the nearby Coma Cluster. The surprising conclusion is that in the cores of these distant clusters more blue galaxies are observed than are observed in the cores of nearby clusters of similar richness and morphology.
Cluster galaxy "blueness" may, under certain circumstances, be used as an indicator of ongoing star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
. Astronomers have identified three spectral classes in which a significant set of blue galaxy cluster members may be categorized. The first is objects undergoing vigorous star formation with very blue colors and spectra showing emission-filled H-delta
H-alpha
H-alpha is a specific red visible spectral line created by hydrogen with a wavelength of 656.28 nm, which occurs when a hydrogen electron falls from its third to second lowest energy level...
lines. Secondly, post-starburst galaxies are also observed. These are cluster members showing similarly blue colors as starburst galaxies only with moderate to strong H-delta absorption. Third are cluster members showing broad and/or high excitation line spectra, often found in active galactic nuclei. The Butcher-Oemler observations suggest at intermediate redshift a higher rate of star formation may be observed in a fraction of the galaxies in the cores of rich clusters than in the cores of rich clusters at low redshift.
The 1978 Butcher-Oemler paper sparked considerable debate. The ensuing series of Butcher-Oemler papers spans six years concluding with The Evolution of Galaxies in Clusters. V. A Study of Populations since z ~ 0.5. This paper presents photometry of 33 clusters of galaxies with redshifts varying from 0.003, the Virgo Cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...
, to 0.54, the cluster Cl 0016+16. This study bolsters the conclusion put forward in their original 1978 paper, that there has been "strong, recent evolution of galaxies in clusters".
The Butcher-Oemler conclusion generated numerous investigations of the cores of rich clusters at intermediate redshift (0.3 ≤ z ≤ 0.9): Couch and Newell (1984), like Butcher and Oemler, acquired broadband photometry of such environments. Couch et al. (1983), Ellis et al. (1985), MacLaren, Ellis
Richard Ellis (astronomer)
Richard Salisbury Ellis CBE FRS is the Steele Professor of Astronomy at the California Institute of Technology ....
and Couch (1988), Aragon-Salamanca, Ellis and Sharples (1991) and Aragon-Salamanca et al. (1993) imaged clusters with redshifts between 0.5 ≤ z ≤ 0.9 in both optical
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
bands. Spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
observations were conducted by Dressler and Gunn
James E. Gunn
James Edward Gunn is the Eugene Higgins Professor of Astronomy at Princeton University. Gunn's early theoretical work in astronomy has helped establish the current understanding of how galaxies form, and the properties of the space between galaxies...
(1982), Lavery and Henry (1985) and Couch and Sharples (1987). Studies of the morphologies of the galaxies in the cores of these clusters were undertaken using the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
by Couch et al. (1994) and Dressler et al. (1994). The outcome of these investigations is that the Butcher-Oemler effect is widespread in rich clusters at z > 0.2 and is due to vigorous episodes of star formation in a subset of the cluster members.
The effect does appear to be confined to rich clusters, at least as rich or richer than the Virgo cluster. Allington-Smith et al. (1993) observed galaxies in small groups out to a redshift of 0.5 and found no relation between the fraction of blue members and group richness; groups of all richnesses were observed to have similar high fractions of blue galaxies. Colless et al. (1990, 1993) have confirmed that better than 95% of blue field galaxies
Field galaxy
A field galaxy is a galaxy that does not belong to a larger cluster of galaxies, but is gravitationally alone. The vast majority of galaxies exist outside of clusters.Most low surface brightness galaxies are field galaxies....
brighter
Magnitude (astronomy)
Magnitude is the logarithmic measure of the brightness of an object, in astronomy, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.-Background:...
than bJ = 22.5 are at redshifts less than z = 0.5, verifying at the 90% confidence level models predicting no luminosity evolution of the field galaxy population since z = 1. Using the Hubble Space Telescope, Dressler et al. (1994) observed the cluster CL 0930+4713 at 0.41 and Couch et al. (1994) observed AC 114 at z = 0.31 and Abell 370 at z = 0.37. These authors independently arrived at the conclusion that every observation of a merger
Galaxy merger
Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies...
between cluster members, both of which showed blue colors, had the spectroscopic signature of a starburst or post-starburst object. It is plausible therefore, that the Butcher-Oemler effect may be partially the result of galaxy-galaxy mergers.