645 research outputs found
The role of E+A and post-starburst galaxies – I. Models and model results
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.14416.xDifferent compositions of galaxy types in the field in comparison to galaxy clusters as described by the morphology–density relation in the local universe are interpreted as a result of transformation processes from late- to early-type galaxies. This interpretation is supported by the Butcher–Oemler effect. We investigate E+A galaxies as an intermediate state between late-type galaxies in low-density environments and early-type galaxies in high-density environment to constrain the possible transformation processes. For this purpose, we model a grid of post-starburst galaxies by inducing a burst and/or a halting of star formation on the normal evolution of spiral galaxies with our galaxy evolution code galev. From our models, we find that the common E+A criteria exclude a significant number of post-starburst galaxies, and propose that comparing their spectral energy distributions leads to a more sufficient method to investigate post-starburst galaxies. We predict that a higher number of E+A galaxies in the early universe cannot be ascribed solely to a higher number of starburst, but is a result of a lower metallicity and a higher burst strength due to more gas content of the galaxies in the early universe. We find that even galaxies with a normal evolution without a starburst have an Hδ-strong phase at early galaxy ages.Peer reviewe
Quantitative Estimates of Environmental Effects on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies
A simple model is constructed to evaluate the change of star formation rate
of a disk galaxy due to environmental effects in clusters of galaxies. Three
effects, (1) tidal force from the potential well of the cluster, (2) increase
of external pressure when the galaxy plows into the intracluster medium, (3)
high-speed encounters between galaxies, are investigated. General analysis
indicates that the star formation rate increases significantly when the
pressure of molecular clouds rises above in yr. The tidal force from the potential well of the cluster increases
pressures of molecular clouds in a disk galaxy infalling towards the cluster
center. Before the galaxy reaches the cluster center, the star formation rate
reaches a maximum. The peak is three to four times larger than the initial
value. If this is the main mechanism of the Butcher-Oemler effect, blue
galaxies are expected to be located within kpc from the center of
the cluster. However this prediction is inconsistent with the recent
observations. The increase of external pressure when the galaxy plows into the
intracluster medium does not change star formation rate of a disk galaxy
significantly. The velocity perturbation induced by a single high-speed
encounter between galaxies is too small to affect star formation rate of a disk
galaxy, while successive high-speed encounters (galaxy harassment) trigger star
formation activity because of the accumulation of gas in the galaxy center.
Therefore, the galaxy harassment remains as the candidate for a mechanism of
the Butcher-Oemler effect.Comment: 12 pages, 13 figures. To be published in Ap
HST observations of star clusters in NGC 1023: Evidence for three cluster populations?
Using HST images we have carried out a study of cluster populations in the
nearby S0 galaxy NGC 1023. In two WFPC2 pointings we have identified 221
cluster candidates. The small distance (~9 Mpc) combined with deep F555W and
F814W images allows us to reach about two magnitudes below the expected
turn-over of the globular cluster luminosity function. NGC 1023 appears to
contain at least three identifiable cluster populations: the brighter clusters
show a clearly bimodal color distribution with peaks at V-I = 0.92 and at V-I =
1.15 and in addition there are a number of fainter, more extended objects with
predominantly red colors. Among the brighter clusters, we find that the blue
clusters have somewhat larger sizes than the red ones with mean effective radii
of R(eff) ~ 2 and R(eff) ~ 1.7 pc, respectively. These clusters have luminosity
functions (LFs) and sizes consistent with what is observed for globular
clusters in other galaxies. Fitting Gaussians to the LFs of the blue and red
compact clusters we find turn-over magnitudes of M(TO,blue)=-7.58 and
M(TO,red)=-7.37 in V and dispersions of sigma(V,blue)=1.12 and
sigma(V,red)=0.97. The fainter, more extended clusters have effective radii up
to R(eff) ~ 10-15 pc and their LF appears to rise at least down to M(V) ~ -6,
few of them being brighter than M(V) = -7. We suggest that these fainter
objects may have a formation history distinct from that of the brighter GCs.Comment: 27 pages, 13 figures. Accepted for publication in the Astronomical
Journa
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