6,678 research outputs found
Quantitative Morphology of Moderate Redshift Galaxies : How Many Peculiars are There ?
The advent of the Hubble Space Telescope (HST) has provided images of
galaxies at moderate and high redshifts and changed the scope of galaxy
morphologies considerably. It is evident that the Hubble Sequence requires
modifications in order to incorporate all the various morphologies one
encounters at such redshifts. We investigate and compare different approaches
to quantifying peculiar galaxy morphologies on images obtained from the Medium
Deep Survey (MDS) and other surveys using the Wide Field Planetary Camera 2
(WFPC2) on board the HST, in the I band (filter F814W). We define criteria for
peculiarity and put them to use on a sample of 978 galaxies, classifying them
by eye as either normal or peculiar. Based on our criteria and on concepts
borrowed from digital image processing we design a set of four purely
morphological parameters, which comprise the overall texture (or
``blobbiness'') of the image; the distortion of isophotes; the filling-factor
of isophotes; and the skeleta of detected structures. We also examine the
parameters suggested by Abraham et al. (1995). An artificial neural network
(ANN) is trained to distinguish between normal and peculiar galaxies. While the
majority of peculiar galaxies are disk-dominated, we also find evidence for a
significant population of bulge-dominated peculiars. Consequently, peculiar
galaxies do not all form a ``natural'' continuation of the Hubble sequence
beyond the late spirals and the irregulars. The trained neural network is
applied to a second, larger sample of 1999 WFPC2 images and its probabilistic
capabilities are used to estimate the frequency of peculiar galaxies at
moderate redshifts as .Comment: 32 pages, latex and 9 figures, Ap. J., accepte
Spectroscopic Survey of {\gamma} Doradus Stars I. Comprehensive atmospheric parameters and abundance analysis of {\gamma} Doradus stars
We present a spectroscopic survey of known and candidate \,Doradus
stars. The high-resolution, high signal-to-noise spectra of 52 objects were
collected by five different spectrographs. The spectral classification,
atmospheric parameters (\teff, , ), and chemical
composition of the stars were derived. The stellar spectral and luminosity
classes were found between G0-A7 and IV-V, respectively. The initial values for
\teff\ and \logg\ were determined from the photometric indices and spectral
energy distribution. Those parameters were improved by the analysis of hydrogen
lines. The final values of \teff, \logg\ and were derived from the iron
lines analysis. The \teff\ values were found between 6000\,K and 7900\,K, while
\logg\,values range from 3.8 to 4.5\,dex. Chemical abundances and
values were derived by the spectrum synthesis method. The values were
found between 5 and 240\,km\,s. The chemical abundance pattern of
\,Doradus stars were compared with the pattern of non-pulsating stars.
It turned out that there is no significant difference in abundance patterns
between these two groups. Additionally, the relations between the atmospheric
parameters and the pulsation quantities were checked. A strong correlation
between the and the pulsation periods of \,Doradus variables
was obtained. The accurate positions of the analysed stars in the H-R diagram
have been shown. Most of our objects are located inside or close to the blue
edge of the theoretical instability strip of \,Doradus.Comment: 18 pages, 13 figure
Evolution of the Color-Magnitude Relation in High-Redshift Clusters: Blue Early-Type Galaxies and Red Pairs in RDCS J0910+5422
The color-magnitude relation has been determined for the RDCS J0910+5422
cluster of galaxies at redshift z = 1.106. Cluster members were selected from
HST ACS images, combined with ground--based near--IR imaging and optical
spectroscopy. The observed early--type color--magnitude relation (CMR) in
(i_775 -z_850) versus z_850 shows intrinsic scatters in color of 0.042 +/-
0.010 mag and 0.044 +/- 0.020 mag for ellipticals and S0s, respectively. From
the scatter about the CMR, a mean luminosity--weighted age t > 3.3 Gyr (z > 3)
is derived for the elliptical galaxies.
Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in
(i_775 - z_850) by 0.07 +/- 0.02 mag, with respect to the ellipticals. The
ellipticity distribution as a function of color indicates that the face-on S0s
in this particular cluster have likely been classified as elliptical. Thus, if
anything, the offset in color between the elliptical and S0 populations may be
even more significant.
The color offset between S0 and E corresponds to an age difference of ~1 Gyr,
for a single-burst solar metallicity model. A solar metallicity model with an
exponential decay in star formation will reproduce the offset for an age of 3.5
Gyr, i.e. the S0s have evolved gradually from star forming progenitors.
The early--type population in this cluster appears to be still forming. The
blue early-type disk galaxies in RDCS J0910+5422 likely represent the direct
progenitors of the more evolved S0s that follow the same red sequence as
ellipticals in other clusters.
Thirteen red galaxy pairs are observed and the galaxies associated in pairs
constitute ~40% of the CMR galaxies in this cluster.Comment: ApJ, in pres
Early-type galaxies with core collapse supernovae
It is widely accepted that the progenitors of core collapse SNe are young
massive stars and therefore their host galaxies are mostly spiral or irregular
galaxies dominated by a young stellar population. Surprisingly, among
morphologically classified hosts of core collapse SNe, we find 22 cases where
the host has been classified as an Elliptical or S0 galaxy. To clarify this
apparent contradiction, we carry out a detailed morphological study and an
extensive literature search for additional information on the sample objects.
Our results are as follows: 1. Of 22 "early type" objects, 17 are in fact
misclassified spiral galaxies, one is a misclassified irregular, and one is a
misclassified ring galaxy. 2. Of the 3 objects maintaining the early type
classification, one (NGC2768) is a suspected merger remnant, another (NGC4589)
is definitely a merger, and the third (NGC2274) is in close interaction. The
presence of some amount of young stellar population in these galaxies is
therefore not unexpected. These results confirm the presence of a limited, but
significant, number of core collapse SNe in galaxies generally classified of
early type. In all cases, anyway, there are independent indicators of the
presence in host galaxies of recent star formation due to merging or
gravitational interaction.Comment: 12 pages, 1 figure, 1 table, accepted for publication in A&
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