1,691 research outputs found
On the Continuous Formation of Field Spheroidal Galaxies in Hierarchical Models of Structure Formation
We re-examine the assembly history of field spheroidals as a potentially
powerful discriminant of galaxy formation models. Whereas monolithic collapse
and hierarchical, merger-driven, models suggest radically different histories
for these galaxies, neither the theoretical predictions nor the observational
data for field galaxies have been sufficiently reliable for precise conclusions
to be drawn. A major difficulty in interpreting the observations, reviewed
here, concerns the taxonomic definition of spheroidals in merger-based models.
Using quantitative measures of recent star formation activity drawn from the
internal properties of a sample of distant field galaxies in the Hubble Deep
Fields, we undertake a new analysis to assess the continuous formation of
spheroidal galaxies. Whereas abundances and redshift distributions of modelled
spheroidals are fairly insensitive to their formation path, we demonstrate that
the distribution and amount of blue light arising from recent mergers provides
a more sensitive approach. With the limited resolved data currently available,
the rate of mass assembly implied by the observed colour inhomogeneities is
compared to that expected in popular Lambda-dominated cold dark matter models
of structure formation. These models produce as many highly inhomogeneous
spheroidals as observed, but underpredict the proportion of homogeneous,
passive objects. We conclude that colour inhomogeneities, particularly when
combined with spectroscopic diagnostics for large, representative samples of
field spheroidals, will be a more valuable test of their physical assembly
history than basic source counts and redshift distributions. Securing such data
should be a high priority for the Advanced Camera for Surveys on Hubble Space
Telescope.Comment: 14 pages, 7 figures, submitted to MNRA
The nature of the SDSS galaxies in various classes based on morphology, colour and spectral features -- I. Optical properties
We present a comprehensive study of the nature of the SDSS galaxies divided
into various classes based on their morphology, colour, and spectral features.
The SDSS galaxies are classified into early-type and late-type; red and blue;
passive, HII, Seyfert, and LINER, returning a total of 16 fine classes of
galaxies. We examine the luminosity dependence of seven physical parameters of
galaxies in each class. We find that more than half of red early-type galaxies
(REGs) have star formation or AGN activity, and that these active REGs have
smaller axis ratio and bluer outside compared to the passive REGs. Blue
early-type galaxies (BEGs) show structural features similar to those of REGs,
but their centres are bluer than REGs. HII BEGs are found to have bluer centres
than passive BEGs, but HII REGs have bluer outside than passive REGs.
Bulge-dominated late-type galaxies have red colours. Passive red late-types are
similar to REGs in several aspects. Most blue late-type galaxies (BLGs) have
forming stars, but a small fraction of BLGs do not show evidence for current
star formation activity. Differences of other physical parameters among
different classes are inspected, and their implication on galaxy evolution is
discussed.Comment: 16 pages, 10 tables, 16 figures; accepted for publication in MNRA
Witnessing the active assembly phase of massive galaxies since z = 1
We present an analysis of ~60 000 massive (stellar mass M_star > 10^{11}
M_sun) galaxies out to z = 1 drawn from 55.2 deg2 of the United Kingdom
Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) and the Sloan
Digital Sky Survey (SDSS) II Supernova Survey. This is by far the largest
survey of massive galaxies with robust mass estimates, based on infrared
(K-band) photometry, reaching to the Universe at about half its present age. We
find that the most massive (M_star > 10^{11.5} M_sun) galaxies have experienced
rapid growth in number since z = 1, while the number densities of the less
massive systems show rather mild evolution. Such a hierarchical trend of
evolution is consistent with the predictions of the current semi-analytic
galaxy formation model based on Lambda CDM theory. While the majority of
massive galaxies are red-sequence populations, we find that a considerable
fraction of galaxies are blue star-forming galaxies. The blue fraction is
smaller in more massive systems and decreases toward the local Universe,
leaving the red, most massive galaxies at low redshifts, which would support
the idea of active 'bottom-up' formation of these populations during 0 < z < 1.Comment: Accepted for publication in MNRAS; replaced with revised version
(minor changes in results and wordings); MNRAS online early version availabl
Automated Morphological Classification of SDSS Red Sequence Galaxies
(abridged) In the last decade, the advent of enormous galaxy surveys has
motivated the development of automated morphological classification schemes to
deal with large data volumes. Existing automated schemes can successfully
distinguish between early and late type galaxies and identify merger
candidates, but are inadequate for studying detailed morphologies of red
sequence galaxies. To fill this need, we present a new automated classification
scheme that focuses on making finer distinctions between early types roughly
corresponding to Hubble types E, S0, and Sa. We visually classify a sample of
984 non-starforming SDSS galaxies with apparent sizes >14". We then develop an
automated method to closely reproduce the visual classifications, which both
provides a check on the visual results and makes it possible to extend
morphological analysis to much larger samples. We visually classify the
galaxies into three bulge classes (BC) by the shape of the light profile in the
outer regions: discs have sharp edges and bulges do not, while some galaxies
are intermediate. We separately identify galaxies with features: spiral arms,
bars, clumps, rings, and dust. We find general agreement between BC and the
bulge fraction B/T measured by the galaxy modeling package GIM2D, but many
visual discs have B/T>0.5. Three additional automated parameters -- smoothness,
axis ratio, and concentration -- can identify many of these high-B/T discs to
yield automated classifications that agree ~70% with the visual classifications
(>90% within one BC). Both methods are used to study the bulge vs. disc
frequency as a function of four measures of galaxy 'size': luminosity, stellar
mass, velocity dispersion, and radius. All size indicators show a fall in disc
fraction and a rise in bulge fraction among larger galaxies.Comment: 24 pages, 20 figures, MNRAS accepte
Probing the Nature of High-z Short GRB 090426 with Its Early Optical and X-ray Afterglows
GRB 090426 is a short duration burst detected by Swift ( s
in the observer frame, and s in the burst frame at
). Its host galaxy properties and some -ray related
correlations are analogous to those seen in long duration GRBs, which are
believed to be of a massive-star origin (so-called Type II GRBs). We present
the results of its early optical observations with the 0.8-m TNT telescope at
Xinglong observatory, and the 1-m LOAO telescope at Mt. Lemmon Optical
Astronomy Observatory in Arizona. Our well-sampled optical afterglow lightcurve
covers from seconds to seconds post the GRB trigger. It
shows two shallow decay episodes that are likely due to energy injection, which
end at seconds and seconds, respectively. The decay
slopes post the injection phases are consistent with each other (). The X-ray afterglow lightcurve appears to trace the optical, although
the second energy injection phase was missed due to visibility constraints
introduced by the {\em Swift} orbit. The X-ray spectral index is without temporal evolution. Its decay slope is consistent with the
prediction of the forward shock model. Both X-ray and optical emission is
consistent with being in the same spectral regime above the cooling frequency
(). The fact that is below the optical band from the very early
epoch of the observation provides a constraint on the burst environment, which
is similar to that seen in classical long duration GRBs. We therefore suggest
that death of a massive star is the possible progenitor of this short burst.Comment: 7 pages, 1 figures, 2 tables, revised version, MNRAS, in pres
Galaxy evolution by color-log(n) type since redshift unity in the Hubble Ultra Deep Field
We explore the use of the color-log(n) plane (where n is the global Sersic
index) as a tool for subdividing the high redshift galaxy population in a
physically-motivated manner. Using a series of volume-limited samples out to
z=1.5 in the Hubble Ultra Deep Field (UDF) we confirm the correlation between
color-log(n) plane position and visual morphology observed locally and in other
high redshift studies in the color and/or structure domain. Via comparison to a
low redshift sample from the Millennium Galaxy Catalogue we quantify evolution
by color-log(n) type, accounting separately for the specific selection and
measurement biases against each. Specifically, we measure decreases in B-band
surface brightness of 1.57 +/- 0.22 mag/sq.arcsec and 1.65 +/- 0.22
mag/sq.arcsec for `blue, diffuse' and `red, compact' galaxies respectively
between redshift unity and the present day.Comment: 12 pages, 6 figures, to be published in A&A (accepted 29/10/08
Discourse or dialogue? Habermas, the Bakhtin Circle, and the question of concrete utterances
This is the author's accepted manuscript. The final publication is available at Springer via the link below.This article argues that the Bakhtin Circle presents a more realistic theory of concrete dialogue than the theory of discourse elaborated by Habermas. The Bakhtin Circle places speech within the âconcrete whole utteranceâ and by this phrase they mean that the study of everyday language should be analyzed through the mediations of historical social systems such as capitalism. These mediations are also characterized by a determinate set of contradictionsâthe capital-labor contradiction in capitalism, for exampleâthat are reproduced in unique ways in more concrete forms of life (the state, education, religion, culture, and so on). Utterances always dialectically refract these processes and as such are internal concrete moments, or concrete social forms, of them. Moreover, new and unrepeatable dialogic events arise in these concrete social forms in order to overcome and understand the constant dialectical flux of social life. But this theory of dialogue is different from that expounded by Habermas, who tends to explore speech acts by reproducing a dualism between repeatable and universal âabstractâ discursive processes (commonly known as the ideal speech situation) and empirical uses of discourse. These critical points against Habermas are developed by focusing on six main areas: sentences and utterances; the lifeworld and background language; active versus passive understandings of language; validity claims; obligation and relevance in language; and dialectical universalism
Evolution in the Disks and Bulges of Group Galaxies since z=0.4
We present quantitative morphology measurements of a sample of optically
selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope
(HST) Advanced Camera for Surveys (ACS) and the GIM2D surface
brightness--fitting software package. The group sample is derived from the
Canadian Network for Observational Cosmology Field Redshift survey (CNOC2) and
follow-up Magellan spectroscopy. We compare these measurements to a similarly
selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z <
0.12. We find that, at both epochs, the group and field fractional bulge
luminosity (B/T) distributions differ significantly, with the dominant
difference being a deficit of disk--dominated (B/T < 0.2) galaxies in the group
samples. At fixed luminosity, z=0.4 groups have ~ 5.5 +/- 2 % fewer
disk--dominated galaxies than the field, while by z=0.1 this difference has
increased to ~ 19 +/- 6 %. Despite the morphological evolution we see no
evidence that the group environment is actively perturbing or otherwise
affecting the entire existing disk population. At both redshifts, the disks of
group galaxies have similar scaling relations and show similar median
asymmetries as the disks of field galaxies. We do find evidence that the
fraction of highly asymmetric, bulge--dominated galaxies is 6 +/- 3 % higher in
groups than in the field, suggesting there may be enhanced merging in group
environments. We replicate our group samples at z=0.4 and z=0 using the
semi-analytic galaxy catalogues of Bower et al (2006). This model accurately
reproduces the B/T distributions of the group and field at z=0.1. However, the
model does not reproduce our finding that the deficit of disks in groups has
increased significantly since z=0.4.Comment: Accepted for publication in MNRAS. 20 pages, 17 figure
E/S0 Galaxies on the Blue Color-Stellar Mass Sequence at z=0: Fading Mergers or Future Spirals?
We identify a population of morphologically defined E/S0 galaxies lying on
the blue sequence at the present epoch. Using three samples, we analyze
blue-sequence E/S0s with stellar masses >10^8 Msun, arguing that individual
objects may be evolving either up toward the red sequence or down into the blue
sequence. Blue-sequence E/S0 galaxies become more common with decreasing
stellar mass, comprising <2% of E/S0s near the "shutdown mass" M_s ~ 1-2 x
10^11 Msun, increasing to >5% near the "bimodality mass" M_b ~ 3 x 10^10 Msun,
and sharply rising to >20-30% below the "threshold mass" M_t ~ 4-6 x 10^9 Msun.
The strong emergence of blue-sequence E/S0s below M_t coincides with a
previously reported global increase in mean atomic gas fractions below M_t for
galaxies of all types on both sequences, suggesting that the availability of
cold gas may be basic to blue-sequence E/S0s' existence. Environmental analysis
reveals that many sub-M_b blue-sequence E/S0s reside in low to intermediate
density environments. In mass-radius and mass-sigma scaling relations,
blue-sequence E/S0s are more similar to red-sequence E/S0s than to late-type
galaxies, but they represent a transitional class. While some of them,
especially in the high-mass range from M_b to M_s, resemble major-merger
remnants that will likely fade onto the red sequence, most blue-sequence E/S0s
below M_b show signs of disk and/or pseudobulge building, which may be enhanced
by companion interactions. We argue that sub-M_b blue-sequence E/S0s occupy a
"sweet spot" in stellar mass and concentration, with both abundant gas and
optimally efficient star formation, which may enable the formation of large
spiral disks. [abridged]Comment: AJ, submitted, revised, 21 pages with 15 figures (one in two parts,
one color); full resolution version available at
http://www.physics.unc.edu/~sheila/kgb.pd
Unveiling the oldest and most massive galaxies at very high redshift
(Abridged) This work explores the existence of high redshift massive galaxies
unveiled with Spitzer+IRAC, but missed by conventional selection techniques
based on optical and near-infrared observations. To this end, we use the
multi-wavelength imaging data available for the GOODS-South field, and select a
flux-limited sample from the IRAC 3.6um image to m(AB)<23.26. We confine our
study to the galaxies undetected by the optical HST+ACS imaging and close to
the detection limit of the K-band image (K>23.5 AB). Our selection unveiled 20
galaxies on which we performed a detailed photometric analysis. For each
galaxy, we built an SED based on optical-to-8um photometry to estimate the
photo-z and to derive the main galaxies physical properties. The majority of
the sample sources show degenerate/bimodal solutions for the photometric
redshifts (Abridged). These can either be heavily dust-enshrouded (Av~2-4)
starbursts at 210^12 Lsun, or massive
post-starburst galaxies in the redshift interval 4<z<9 with stellar masses of
10^11 Msun. One galaxy, the only source in our sample with both an X-ray and a
24um detection, might be an extremely massive object at z~8 detected during a
post-starburst phase with concomitant QSO activity (although a lower-z solution
is not excluded). Our investigation of Spitzer-selected galaxies with very red
SEDs and completely undetected in the optical reveals a potential population of
massive galaxies at z>4 which appear to include significant AGN emissions.
These sources may be the oldest stellar systems at z~4. These, previously
unrecognized, optically obscured objects might provide an important
contribution to the massive-end (M>10^11 sun) of the high-z stellar mass
function and they would almost double it (Abridged).Comment: Accepted for publication in Astronomy and Astrophysic
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