46 research outputs found
Optical-Near Infrared Color Gradients of Elliptical Galaxies and Their Environmental Dependence
We have studied the environmental effect on optical-NIR color gradients of
273 nearby elliptical galaxies. Color gradient is a good tool to study the
evolutionary history of elliptical galaxies, since the steepness of the color
gradient reflects merging history of early types. When an elliptical galaxy
goes through many merging events, the color gradient can be get less steep or
reversed due to mixing of stars. One simple way to measure color gradient is to
compare half-light radii in different bands. We have compared the optical and
near infrared half-light radii of 273 early-type galaxies from Pahre(1999). Not
surprisingly, we find that r(V)s (half-light radii measured in V-band)
are in general larger than r(K)s (half-light radii measured in K-band).
However, when divided into different environments, we find that elliptical
galaxies in the denser environment have gentler color gradients than those in
the less dense environment. Our finding suggests that elliptical galaxies in
the dense environment have undergone many merging events and the mixing of
stars through the merging have created the gentle color gradients.Comment: 3 pages, 2 figures. Proceedings of the 6th East Asian Meeting of
Astronomy, held at Seoul National University, Korea, from October 18-22, 200
First Weak-lensing Results from "See Change": Quantifying Dark Matter in the Two Z>1.5 High-redshift Galaxy Clusters SPT-CL J2040-4451 and IDCS J1426+3508
We present a weak-lensing study of SPT-CLJ2040-4451 and IDCSJ1426+3508 at
z=1.48 and 1.75, respectively. The two clusters were observed in our "See
Change" program, a HST survey of 12 massive high-redshift clusters aimed at
high-z supernova measurements and weak-lensing estimation of accurate cluster
masses. We detect weak but significant galaxy shape distortions using IR images
from the WFC3, which has not yet been used for weak-lensing studies. Both
clusters appear to possess relaxed morphology in projected mass distribution,
and their mass centroids agree nicely with those defined by both the galaxy
luminosity and X-ray emission. Using an NFW profile, for which we assume that
the mass is tightly correlated with the concentration parameter, we determine
the masses of SPT-CL J2040-4451 and IDCS J1426+3508 to be
M_{200}=8.6_{-1.4}^{+1.7}x10^14 M_sun and 2.2_{-0.7}^{+1.1}x10^14 M_sun,
respectively. The weak-lensing mass of SPT-CLJ2040-4451 shows that the cluster
is clearly a rare object. Adopting the central value, the expected abundance of
such a massive cluster at z>1.48 is only ~0.07 in the parent 2500 sq. deg.
survey. However, it is yet premature to claim that the presence of this cluster
creates a serious tension with the current LCDM paradigm unless that tension
will remain in future studies after marginalizing over many sources of
uncertainties such as the accuracy of the mass function and the
mass-concentration relation at the high mass end. The mass of IDCSJ1426+3508 is
in excellent agreement with our previous ACS-based weak-lensing result while
the much higher source density from our WFC3 imaging data makes the current
statistical uncertainty ~40% smaller.Comment: Accepted to Ap
Tracing Recent Star Formation of Red Early-type Galaxies out to 1
We study the mid-infrared (IR) excess emission of early-type galaxies (ETGs)
on the red-sequence at 1 using a spectroscopic sample of galaxies in the
fields of Great Observatories Origins Deep Survey (GOODS). In the mass-limited
sample of 1025 galaxies with 10 and
, we identify 696 24 m detected (above the
5) galaxies and find them to have a wide range of NUV- and -[12
m] colors despite their red optical colors. Even in the sample of
very massive ETGs on the red sequence with 10
, more than 18% show excess emission over the photospheric emission
in the mid-IR. The combination with the results of red ETGs in the local
universe suggests that the recent star formation is not rare among quiescent,
red ETGs at least out to if the mid-IR excess emission results from
intermediate-age stars or/and from low-level ongoing star formation. Our
colorcolor diagram including near-UV and mid-IR emissions are efficient not
only for identifying ETGs with recent star formation, but also for
distinguishing quiescent galaxies from dusty star-forming galaxies.Comment: 25 pages, 9 figures, accepted for publication in Ap
The formation of the brightest cluster galaxy and intracluster light in cosmological N-body simulations with the Galaxy Replacement Technique
We investigate the formation channels of the intracluster light (ICL) and the
brightest cluster galaxy (BCG) in clusters at . For this, we perform
multi-resolution cosmological N-body simulations using the "Galaxy Replacement
Technique" (GRT). We study the formation channels of the ICL and BCG as a
function of distance from the cluster center and the dynamical state of the
clusters at . To do this, we trace back the stars of the ICL and BCG, and
identify the stellar components in which they existed when they first fell into
the clusters. We find that the progenitors of the ICL and BCG in the central
region of the cluster fell earlier and with a higher total mass ratio of the
progenitors to the cluster compared to the outer region. This causes a negative
radial gradient in the infall time and total mass ratio of the progenitors.
Although stellar mass of the progenitors does not show the same radial gradient
in all clusters, massive galaxies ()
are the dominant formation channel of the ICL and BCG for all clusters, except
for our most relaxed cluster. For clusters that are dynamically more unrelaxed,
we find that the progenitors of the ICL and BCG fall into their clusters more
recently, and with a higher mass and mass ratio. Furthermore, we find that the
diffuse material of massive galaxies and group-mass halos that is formed by
pre-processing contributes significantly to the ICL in the outer region of the
unrelaxed clusters.Comment: 18 pages, 8 figures, 4 tables. Accepted for publication in The
Astrophysical Journal, December 3rd, 202
Precise Mass Determination of SPT-CL J2106-5844, the Most Massive Cluster at z \u3e 1
We present a detailed high-resolution weak-lensing study of SPT-CL J2106-5844 at z = 1.132, claimed to be the most massive system discovered at z \u3e 1 in the South Pole Telescope Sunyaev–Zel\u27dovich survey. Based on the deep imaging data from the Advanced Camera for Surveys and Wide Field Camera 3 on board the Hubble Space Telescope, we find that the cluster mass distribution is asymmetric, composed of a main clump and a subclump ~640 kpc west thereof. The central clump is further resolved into two smaller northwestern and southeastern substructures separated by ~150 kpc. We show that this rather complex mass distribution is more consistent with the cluster galaxy distribution than a unimodal distribution as previously presented. The northwestern substructure coincides with the brightest cluster galaxy and the X-ray peak while the southeastern one agrees with the location of the peak in number density. These morphological features and the comparison with the X-ray emission suggest that the cluster might be a merging system. We estimate the virial mass of the cluster to be , where the second error bar is the systematic uncertainty. Our result confirms that the cluster SPT-CL J2106-5844 is indeed the most massive cluster at z \u3e 1 known to date. We demonstrate the robustness of this mass estimate by performing a number of tests with different assumptions on the centroids, mass–concentration relations, and sample variance