282 research outputs found
On the limits of measuring the bulge and disk properties of local and high-redshift massive galaxies
A considerable fraction of the massive quiescent galaxies at \emph{z}
2, which are known to be much more compact than galaxies of
comparable mass today, appear to have a disk. How well can we measure the bulge
and disk properties of these systems? We simulate two-component model galaxies
in order to systematically quantify the effects of non-homology in structures
and the methods employed. We employ empirical scaling relations to produce
realistic-looking local galaxies with a uniform and wide range of
bulge-to-total ratios (), and then rescale them to mimic the
signal-to-noise ratios and sizes of observed galaxies at \emph{z} 2.
This provides the most complete set of simulations to date for which we can
examine the robustness of two-component decomposition of compact disk galaxies
at different . We confirm that the size of these massive, compact galaxies
can be measured robustly using a single S\'{e}rsic fit. We can measure
accurately without imposing any constraints on the light profile shape of the
bulge, but, due to the small angular sizes of bulges at high redshift, their
detailed properties can only be recovered for galaxies with \gax\ 0.2.
The disk component, by contrast, can be measured with little difficulty
How Robust Are the Size Measurements of High-redshift Compact Galaxies?
Massive quiescent galaxies at are apparently much more compact
than galaxies of comparable mass today. How robust are these size measurements?
We perform comprehensive simulations to determine possible biases and
uncertainties in fitting single-component light distributions to real galaxies.
In particular, we examine the robustness of the measurements of the luminosity,
size, and other structural parameters. We devise simulations with increasing
realism to systematically disentangle effects due to the technique
(specifically using GALFIT) and the intrinsic structures of the galaxies. By
accurately capturing the detailed substructures of nearby elliptical galaxies
and then rescaling their sizes and signal-to-noise to mimic galaxies at
different redshifts, we confirm that the massive quiescent galaxies at are significantly more compact intrinsically than their local
counterparts. Their observed compactness is not a result of missing faint outer
light due to systematic errors in modeling. In fact, we find that fitting
multi-component galaxies with a single S\'ersic profile, the procedure most
commonly adopted in the literature, biases the inferred sizes higher by up to
10% - 20%, which accentuates the amount of size evolution required. If the sky
estimation has been done robustly and the model for the point-spread function
is fairly accurate, GALFIT can retrieve the properties of single-component
galaxies over a wide range of signal-to-noise ratios without introducing any
systematic errors.Comment: 18 pages, 11 figures, 8 tables; Accepted for publication in Ap
Stellar Photometric Structures of the Host Galaxies of Nearby Type 1 Active Galactic Nuclei
We present detailed image analysis of rest-frame optical images of 235
low-redshift ( 0.35) type 1 active galactic nuclei (AGNs) observed with
the Hubble Space Telescope. The high-resolution images enable us to perform
rigorous two-dimensional image modeling to decouple the luminous central point
source from the host galaxy, which, when warranted, is further decomposed into
its principal structural components (bulge, bar, and disk). In many cases, care
must be taken to account for structural complexities such as spiral arms, tidal
features, and overlapping or interacting companion galaxies. We employ Fourier
modes to characterize the degree of asymmetry of the light distribution of the
stars, as a quantitative measure of morphological distortion due to
interactions or mergers. We examine the dependence of the physical parameters
of the host galaxies on the properties of the AGNs, namely radio-loudness and
the width of the broad emission lines. In accordance with previous studies,
narrow-line (H FWHM km~s) type 1 AGNs, in contrast to
their broad-line (H FWHM km~s) counterparts, are
preferentially hosted in later type, lower luminosity galaxies, which have a
higher incidence of pseudo-bulges, are more frequently barred, and are less
morphologically disturbed. This suggests narrow-line type 1 AGNs experienced a
more quiescent evolutionary history driven primarily by internal secular
evolution instead of external dynamical perturbations. The fraction of AGN
hosts showing merger signatures is larger for more luminous sources. Radio-loud
AGNs generally preferentially live in earlier type (bulge-dominated), more
massive hosts, although a minority of them appears to contain a significant
disk component. We do not find convincing evidence for enhanced merger
signatures in the radio-loud population.Comment: Published in ApJ
The Carnegie-Irvine Galaxy Survey. III. The Three-Component Structure of Nearby Elliptical Galaxies
Motivated by recent developments in our understanding of the formation and
evolution of massive galaxies, we explore the detailed photometric structure of
a representative sample of 94 bright, nearby elliptical galaxies, using
high-quality optical images from the Carnegie-Irvine Galaxy Survey. The sample
spans a range of environments and stellar masses, from M* = 10^{10.2} to
10^{12.0} solar mass. We exploit the unique capabilities of two-dimensional
image decomposition to explore the possibility that local elliptical galaxies
may contain photometrically distinct substructure that can shed light on their
evolutionary history. Compared with the traditional one-dimensional approach,
these two-dimensional models are capable of consistently recovering the surface
brightness distribution and the systematic radial variation of geometric
information at the same time. Contrary to conventional perception, we find that
the global light distribution of the majority (>75%) of elliptical galaxies is
not well described by a single Sersic function. Instead, we propose that local
elliptical galaxies generically contain three subcomponents: a compact (R_e < 1
kpc) inner component with luminosity fraction f ~ 0.1-0.15; an
intermediate-scale (R_e ~ 2.5 kpc) middle component with f ~ 0.2-0.25; and a
dominant (f = 0.6), extended (R_e ~ 10 kpc) outer envelope. All subcomponents
have average Sersic indices n ~ 1-2, significantly lower than the values
typically obtained from single-component fits. The individual subcomponents
follow well-defined photometric scaling relations and the stellar mass-size
relation. We discuss the physical nature of the substructures and their
implications for the formation of massive elliptical galaxies.Comment: To appear in The Astrophysical Journal; 36 pages, 2 tables, 38
figures; For the full resolution version, see:
http://users.obs.carnegiescience.edu/shuang/PaperIII.pdf ; For the atlas of
all selected models, see
http://users.obs.carnegiescience.edu/shuang/AppendixE.pd
The Carnegie-Irvine Galaxy Survey. IV. A Method to Determine the Average Mass Ratio of Mergers That Built Massive Elliptical Galaxies
Many recent observations and numerical simulations suggest that nearby
massive, early-type galaxies were formed through a "two-phase" process. In the
proposed second phase, the extended stellar envelope was accumulated through
many dry mergers. However, details of the past merger history of present-day
ellipticals, such as the typical merger mass ratio, are difficult to constrain
observationally. Within the context and assumptions of the two-phase formation
scenario, we propose a straightforward method, using photometric data alone, to
estimate the average mass ratio of mergers that contributed to the build-up of
massive elliptical galaxies. We study a sample of nearby massive elliptical
galaxies selected from the Carnegie-Irvine Galaxy Survey, using two-dimensional
analysis to decompose their light distribution into an inner, denser component
plus an extended, outer envelope, each having a different optical color. The
combination of these two substructures accurately recovers the negative color
gradient exhibited by the galaxy as whole. The color difference between the two
components ( ~ 0.10 mag; ~ 0.14 mag), based on the
slope of the M_stellar-color relation for nearby early-type galaxies, can be
translated into an estimate of the average mass ratio of the mergers. The rough
estimate, 1:5 to 1:10, is consistent with the expectation of the two-phase
formation scenario, suggesting that minor mergers were largely responsible for
building up to the outer stellar envelope of present-day massive ellipticals.
With the help of accurate photometry, large sample size, and more choices of
colors promised by ongoing and future surveys, the approach proposed here can
reveal more insights into the growth of massive galaxies during the last few
Gyr.Comment: Accepted by ApJ; 20 pages, 11 figures, 1 table; The high resolution
figures and the full table can be downloaded from here:
https://github.com/dr-guangtou/cgs_colorgra
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