25 research outputs found
The Butcher-Oemler Effect in Abell 2317
This paper presents deep narrow band photometry of the cluster A2317
(z=0.211) carried out using KPNO 4 m and Steward 2.3 m telescopes. Using rest
frame Stromgren photometry, it is determined that A2317 has an unusually high
fraction of blue galaxies (the Butcher-Oemler effect) for its redshift
(f_B=0.35). We demonstrate that the ratio of blue to red galaxies has a strong
dependence on absolute magnitude such that blue galaxies dominate the top of
the luminosity function. Spectrophotometric classification shows that a
majority of the red galaxies are E/S0's, with a small number of reddened
starburst galaxies. Butcher-Oemler galaxies are shown to be galaxies with star
formation rates typical of late-type spirals and irregular. Starburst systems
were typically found to be on the lower end of the cluster luminosity function.
In addition, blue galaxies are preferentially found in the outer edges of the
cluster, whereas the red galaxies are concentrated in the cluster core.Comment: 23 pages including 1 table and 6 figures, AASTeX v4.0. Accepted by
Ap.J. Data, referee report and response are avaliable from
http://zebu.uoregon.edu/~j
Cluster Populations in A115 and A2283
This paper presents four color narrow-band photometry of clusters A115
() and A2283 () in order to follow the star formation history
of various galaxy types. Although located at similar redshifts, the two
clusters display very different fractions of blue galaxies (i.e. the
Butcher-Oemler effect, for A115, for A2283). A system
of photometric classification is applied to the cluster members that divides
the cluster population into four classes based on their recent levels of star
formation. It is shown that the blue population of each cluster is primarily
composed of normal starforming (SFR < 1 M_{\sun} yrs) galaxies at the
high luminosity end, but with an increasing contribution from a dwarf starburst
population below . This dwarf starburst population appears to be
the same population of low mass galaxies identified in recent HST imaging (Koo
et al 1997), possible progenitors to present-day cluster dwarf ellipticals,
irregulars and BCD's. Deviations in the color-magnitude relationship for the
red galaxies in each cluster suggest that a population of blue S0's is evolving
into present-day S0 colors at this epoch. The radial distribution of the blue
population supports the prediction of galaxy harassment mechanisms for tidally
induced star formation operating on an infalling set of gas-rich galaxies.Comment: 28 pages including 2 tables and 9 figures, AASTeX v4.0. Accepted by
Ap.J. Data, referee report and response are avaliable from
http://zebu.uoregon.edu/~j
The Age of Ellipticals and the Color-Magnitude Relation
Using new narrowband color observations of early-type galaxies in clusters,
we reconstruct the color-magnitude relation (CMR) with a higher degree of
accuracy than previous work. We then use the spectroscopically determined ages
and metallicities from three samples (Trager et al 2008, Thomas et al 2005,
Gallazzi et al 2006), combined with multi-metallicity SED models, to compare
predicted colors for galaxies with young ages (less than 8 Gyr) with the known
CMR. We find that the CMR cannot by reproduced by the spectroscopically
determined ages and metallicities in any of the samples despite the high
internal accuracies to the spectroscopic indices. In contrast, using only the
index to determine [Fe/H], and assuming a mean age of 12 Gyr for a
galaxy's stellar population, we derive colors that exactly match not only the
color zeropoint of the CMR but also its slope. We consider the source of young
age estimates, the H index, and examine the conflict between red
continuum colors and large H values in galaxy spectra. We conclude that
our current understanding of stellar populations is insufficient to correctly
interpret H values and that the sum of our galaxy observations supports
an old and monolithic scenario of galaxy formation. This result has a
devastating impact on every study that has used the H index to calculate
galaxy age, as the use of the H versus MgFe diagram will result in
incorrectly deduced young ages.Comment: 29 pages, 9 figures. This version deviates from the version accepted
by Ap
Age and Metallicity Estimation of Globular Clusters from Stromgren Photometry
We present a new technique for the determination of age and metallicity in
composite stellar populations using Stromgren filters. Using principal
component (PC) analysis on multi-color models, we isolate the range of values
necessary to uniquely determine age and metallicity effects. The technique
presented herein can only be applied to old (tau > 3 Gyrs) stellar systems
composed of simple stellar populations, such as globular clusters and
elliptical galaxies. Calibration using new photometry of 40 globular clusters
with spectroscopic [Fe/H] values and main sequence fitted ages links the PC
values to the Str\"omgren colors for an accuracy of 0.2 dex in metallicity and
0.5 Gyrs in age.Comment: 24 pages AAS LaTeX, 9 figures, accepted for publication in PAS
The Ages of Dwarf Ellipticals
We present narrow band photometry of 91 dwarf ellipticals in the Coma and
Fornax clusters taken through the Stromgren (uvby) filter system. Dividing the
sample by dwarf morphology into nucleated (dEN) and non-nucleated (dE) dwarfs
reveals two distinct populations of early-type systems based on integrated
colors. The class of dEN galaxies are redder in their continuum colors as
compared to bright cluster ellipticals and dE type dwarfs, and their position
in multi-color diagrams can only be explained by an older mean age for their
underlying stellar populations. By comparison with the narrow band photometry
of the M87 globular cluster system (Jordan et al. 2002), we find that dENs are
a higher metallicity continuation of the old, metal-poor color sequence of
galactic globulars and the blue population of M87 globulars. Bright ellipticals
and dE dwarfs, on the other hand, follow the color sequence of the metal-rich,
red population of M87 globulars. A comparison to SED models, convolved to a
simple metallicity model, finds that dENs and blue globulars are 3 to 4 Gyrs
older than cluster ellipticals and 5 Gyrs older than dE type galaxies. The
implication is that globulars and dEN galaxies are primordial and have
metallicities set by external constraints such as the enrichment of their
formation clouds. Bright ellipticals and dE galaxies have metallicities and
ages that suggest an extended phase of initial star formation to produce a
younger mean age, even if their formation epoch is similar to that of dENs and
blue globulars, and an internally driven chemical evolutionary history.Comment: 13 pages AAS LaTeX, 6 figures, accepted for publication in A
Ages and Metallicities of Cluster Galaxies in a779 Using Modified Strömgren Photometry
In the quest for the formation and evolution of galaxy clusters, Rakos and co-workers introduced a spectrophotometric method using modified Strömgren photometry, but with the considerable debate toward the project's abilities, we re-introduce the system by testing for the repeatability of the modified Strömgren colors and compare them with the Strömgren colors, and check for the reproducibility of the ages and metallicities (using the Principle Component Analysis (PCA) technique and the GALEV models) for the six common galaxies in all three A779 data sets. As a result, a fair agreement between two filter systems was found to produce similar colors (with a precision of 0.09 mag in (uz - vz), 0.02 mag in (bz - yz), and 0.03 mag in (vz - vz)) and the generated ages and metallicities are also similar (with an uncertainty of 0.36 Gyr and 0.04 dex from PCA and 0.44 Gyr and 0.2 dex using the GALEV models). We infer that the technique is able to relieve the age-metallicity degeneracy by separating the age effects from the metallicity effects, but it is still unable to completely eliminate it.We further extend this paper to re-study the evolution of galaxies in the low mass, dynamically poor A779 cluster (as it was not elaborately analyzed by Rakos and co-workers in their previous work) by correlating the luminosity (mass), density, and radial distance with the estimated age, metallicity, and the star formation history. Our results distinctly show the bimodality of the young, low-mass, metal-poor population with a mean age of 6.7 Gyr (± 0.5 Gyr) and the old, high-mass, metal-rich galaxies with a mean age of 9 Gyr (± 0.5 Gyr). The method also observes the color evolution of the blue cluster galaxies to red (Butcher-Oemler phenomenon), and the downsizing phenomenon. Our analysis shows that modified Strömgren photometry is very well suited for studying low- and intermediate-z clusters, as it is capable of observing deeper with better spatial resolution at spectroscopic redshift limits, and the narrow-band filters estimate the age and metallicity with fewer uncertainties compared to other methods that study stellar population scenarios
The Color-Magnitude Relation in Coma: Clues to the Age and Metallicity of Cluster Populations
We have observed three fields of the Coma cluster of galaxies with a narrow
band (modified Stromgren) filter system. Observed galaxies include 31 in the
vicinity of NGC 4889, 48 near NGC 4874, and 60 near NGC 4839 complete to
M_5500=-18 in all three subclusters. Spectrophotometric classification finds
all three subclusters of Coma to be dominated by red, E type (ellipticals/S0's)
galaxies with a mean blue fraction, f_B, of 0.10. The blue fraction increases
to fainter luminosities, possible remnants of dwarf starburst population or the
effects of dynamical friction removing bright, blue galaxies from the cluster
population by mergers. We find the color-magnitude (CM) relation to be well
defined and linear over the range of M_5500=-13 to -22. After calibration to
multi-metallicity models, bright ellipticals are found to have luminosity
weighted mean [Fe/H] values between -0.5 and +0.5, whereas low luminosity
ellipticals have [Fe/H] values ranging from -2 to solar. The lack of CM
relation in our continuum color suggests that a systematic age effect cancels
the metallicity effects in this bandpass. This is confirmed with our age index
which finds a weak correlation between luminosity and mean stellar age in
ellipticals such that the stellar populations of bright ellipticals are 2 to 3
Gyrs younger than low luminosity ellipticals.Comment: 26 pages AAS LaTeX, 6 figures, accepted for publication in A