2,613 research outputs found
Measuring Learning Gains in Chemical Education: A Comparison of Two Methods
Evaluating the effect of a pedagogical innovation is often done by looking for a significant difference in a content measure using a pre−post design. While this approach provides valuable information regarding the presence or absence of an effect, it is limited in providing details about the nature of the effect. A measure of the magnitude of the pre−post change, commonly called learning gain, could provide this additional information to chemical education researchers. In this paper, we compare two methods of measuring learning gains using data from large-scale administrations of the Chemical Concepts Inventory at four universities. The intent of this study is to compare various measures of learning gain, not to contrast the teaching effectiveness at the four universities. In this gain analysis, we introduce a method based on Rasch modeling and discuss the advantages offered by this type of analysis over more commonly used measures of learning gain
On the occurrence of buoyancy-induced oscillatory growth instability in directional solidification of alloys
Recent solidification experiments identified an oscillatory growth
instability during directional solidification of Ni-based superalloy CMSX4
under a given range of cooling rates. From a modeling perspective, the
quantitative simulation of dendritic growth under convective conditions remains
challenging, due to the multiple length scales involved. Using the dendritic
needle network (DNN) model, coupled with an efficient Navier-Stokes solver, we
reproduced the buoyancy-induced growth oscillations observed in CMSX4
directional solidification. These previous results have shown that, for a given
alloy and temperature gradient, oscillations occur in a narrow range of cooling
rates (or pulling velocity, ) and that the selected primary dendrite arm
spacing () plays a crucial role in the activation of the flow leading
to oscillations. Here, we show that the oscillatory behavior may be generalized
to other binary alloys within an appropriate range of by
reproducing it for an Al-4at.%Cu alloy. We perform a mapping of oscillatory
states as a function of and , and identify the regions of
occurrence of different behaviors (e.g., sustained or damped oscillations) and
their effect on the oscillation characteristics. Our results suggest a minimum
of for the occurrence of oscillations and confirm the correlation between
the oscillation type (namely: damped, sustained, or noisy) with the ratio of
average fluid velocity over . We describe the different
observed growth regimes and highlight similarities and contrasts with our
previous results for a CMSX4 alloy
Global Properties of the Rich Cluster ABCG 209 at z~0.2. Spectroscopic and Photometric Catalogue
This paper is aimed at giving an overview of the global properties of the
rich cluster of galaxies ABCG 209. This is achieved by complementing the
already available data with new medium resolution spectroscopy and NIR
photometry which allow us to i) analyse in detail the cluster dynamics,
distinguishing among galaxies belonging to different substructures and deriving
their individual velocity distributions, using a total sample of 148 galaxies
in the cluster region, of which 134 belonging to the cluster; ii) derive the
cluster NIR luminosity function; iii) study the Kormendy relation and the
photometric plane of cluster early-type galaxies (ETGs). Finally we provide an
extensive photometric (optical and NIR) and spectroscopic dataset for such a
complex system to be used in further analyses investigating the nature,
formation and evolution of rich clusters of galaxies. The observational
scenario confirms that ABCG 209 is presently undergoing strong dynamical
evolution with the merging of two or more subclumps. This interpretation is
also supported by the detection of a radio halo (Giovannini et al. 2006)
suggesting that there is a recent or ongoing merging. Cluster ETGs follow a
Kormendy relation whose slope is consistent with previous studies both at
optical and NIR wavelengths. We investigate the origin of the intrinsic scatter
of the photometric plane due to trends of stellar populations, using line
indices as indicators of age, metallicity and alpha/Fe enhancement. We find
that the chemical evolution of galaxies could be responsible for the intrinsic
dispersion of the Photometric Plane.Comment: 39 pages, 17 figures, MNRAS in pres
SPIDER - IV. Optical and NIR color gradients in Early-type galaxies: New Insights into Correlations with Galaxy Properties
We present an analysis of stellar population gradients in 4,546 Early-Type
Galaxies with photometry in along with optical spectroscopy. A new
approach is described which utilizes color information to constrain age and
metallicity gradients. Defining an effective color gradient, ,
which incorporates all of the available color indices, we investigate how
varies with galaxy mass proxies, i.e. velocity dispersion,
stellar (M_star) and dynamical (M_dyn) masses, as well as age, metallicity, and
alpha/Fe. ETGs with M_dyn larger than 8.5 x 10^10, M_odot have increasing age
gradients and decreasing metallicity gradients wrt mass, metallicity, and
enhancement. We find that velocity dispersion and alpha/Fe are the main drivers
of these correlations. ETGs with 2.5 x 10^10 M_odot =< M_dyn =< 8.5 x 10^10
M_odot, show no correlation of age, metallicity, and color gradients wrt mass,
although color gradients still correlate with stellar population parameters,
and these correlations are independent of each other. In both mass regimes, the
striking anti-correlation between color gradient and alpha-enhancement is
significant at \sim 4sigma, and results from the fact that metallicity gradient
decreases with alpha/Fe. This anti-correlation may reflect the fact that star
formation and metallicity enrichment are regulated by the interplay between the
energy input from supernovae, and the temperature and pressure of the hot X-ray
gas in ETGs. For all mass ranges, positive age gradients are associated with
old galaxies (>5-7 Gyr). For galaxies younger than \sim 5 Gyr, mostly at
low-mass, the age gradient tends to be anti-correlated with the Age parameter,
with more positive gradients at younger ages.Comment: Accepted for Publication in the Astronomical Journa
SPIDER VII - Revealing the Stellar Population Content of Massive Early-type Galaxies out to 8Re
Radial trends of stellar populations in galaxies provide a valuable tool to
understand the mechanisms of galaxy growth. In this paper, we present the first
comprehensive analysis of optical-optical and optical-NIR colours, as a
function of galaxy mass, out to the halo region (8Re) of early-type galaxies
(ETGs). We select a sample of 674 massive ETGs (M*>3x10^10MSun) from the
SDSS-based SPIDER survey. By comparing with a large range of population
synthesis models, we derive robust constraints on the radial trends in age and
metallicity. Metallicity is unambiguously found to decrease outwards, with a
measurable steepening of the slope in the outer regions (Re<R<8Re). The
gradients in stellar age are found to be more sensitive to the models used, but
in general, the outer regions of ETGs feature older populations compared to the
cores. This trend is strongest for the most massive galaxies in our sample
(M*>10^11MSun). Furthermore, when segregating with respect to large scale
environment, the age gradient is more significant in ETGs residing in higher
density regions. These results shed light on the processes leading from the
formation of the central core to the growth of the stellar envelope of massive
galaxies. The fact that the populations in the outer regions are older and more
metal-poor than in the core suggests a process whereby the envelope of massive
galaxies is made up of accreted small satellites (i.e. minor mergers) whose
stars were born during the first stages of galaxy formation.Comment: 20 pages, 13 figures, 10 tables. Accepted for publication in MNRA
The nature and origins of the low surface brightness outskirts of massive, central galaxies in Subaru HSC
We explore the stellar mass density and colour profiles of 118 low redshift,
massive, central galaxies, selected to have assembled 90 percent of their
stellar mass 6 Gyr ago, finding evidence of the minor merger activity expected
to be the driver behind the size growth of quiescent galaxies. We use imaging
data in the bands from the Subaru Hyper Suprime-Cam survey and
perform SED fitting to construct spatially well-resolved radial profiles in
colour and stellar mass surface density. Our visual morphological
classification reveals that percent of our sample displays tidal
features, similar to previous studies, percent of the remaining
sample display a diffuse stellar halo and only percent display no
features, down to a limiting 28 mag
arcsec. We find good agreement between the stacked colour profiles of
our sample to those derived from previous studies and an expected smooth,
declining stellar mass surface density profile in the central regions (< 3
R). However, we also see a flattening of the profile ( M kpc) in the outskirts (up to 10
R), which is revealed by our method of specifically targeting
tidal/accretion features. We find similar levels of tidal features and
behaviour in the stellar mass surface density profiles in a younger comparison
sample, however a lack of diffuse haloes. We also apply stacking techniques,
similar to those in previous studies, finding such procedures wash out tidal
features and thereby produces smooth declining profiles. The stellar material
in the outskirts contributes on average M or a few
percent of the total stellar mass and has similar colours to SDSS satellites of
similar stellar mass.Comment: 14 pages, 9 figures, 1 tabl
The luminosity and stellar mass Fundamental Plane of early-type galaxies
From a sample of ~50000 early-type galaxies from the SDSS, we measured the
traditional Fundamental Plane in four bands. We then replaced luminosity with
stellar mass, and measured the "stellar mass" FP. The FP steepens slightly as
one moves from shorter to longer wavelengths: the orthogonal fit has slope 1.40
in g and 1.47 in z. The FP is thinner at longer wavelengths: scatter is 0.062
dex in g, 0.054 dex in z. The scatter is larger at small galaxy sizes/masses;
at large masses measurement errors account for essentially all of the observed
scatter. The FP steepens further when luminosity is replaced with stellar mass,
to slope ~ 1.6. The intrinsic scatter also reduces further, to 0.048 dex. Since
color and stellar mass-to-light ratio are closely related, this explains why
color can be thought of as the fourth FP parameter. However, the slope of the
stellar mass FP remains shallower than the value of 2 associated with the
virial theorem. This is because the ratio of dynamical to stellar mass
increases at large masses as M_d^0.17. The face-on view of the stellar mass
kappa-space suggests that there is an upper limit to the stellar density for a
given dynamical mass, and this decreases at large masses: M_*/R_e^3 ~ M_d^-4/3.
We also study how the estimated coefficients a and b of the FP are affected by
other selection effects (e.g. excluding small sigma biases a high; excluding
fainter L biases a low). These biases are seen in FPs which have no intrinsic
curvature, so the observation that a and b scale with L and sigma is not, by
itself, evidence that the Plane is warped. We show that the FP appears to curve
sharply downwards at the small mass end, and more gradually downwards towards
larger masses. Whereas the drop at small sizes is real, most of the latter
effect is due to correlated errors.Comment: 17 pages, 15 figures, MNRAS in press. Added appendix on possible
sample contamination by disk
A z=1.82 Analog of Local Ultra-massive Elliptical Galaxies
We present observations of a very massive galaxy at z=1.82 which show that
its morphology, size, velocity dispersion and stellar population properties
that are fully consistent with those expected for passively evolving
progenitors of today's giant ellipticals. These findings are based on a deep
optical rest-frame spectrum obtained with the Multi-Object InfraRed Camera and
Spectrograph (MOIRCS) on the Subaru telescope of a high-z passive galaxy
candidate (pBzK) from the COSMOS field, for which we accurately measure its
redshift of z=1.8230 and obtain an upper limit on its velocity dispersion
sigma_star<326 km/s. By detailed stellar population modeling of both the galaxy
broad-band SED and the rest-frame optical spectrum we derive a
star-formation-weighted age and formation redshift of t_sf~1-2 Gyr and
z_form~2.5-4, and a stellar mass of M_star~(3-4)x10^{11} M_sun. This is in
agreement with a virial mass limit of M_vir<7x10^{11}M_sun, derived from the
measured sigma_star value and stellar half-light radius, as well as with the
dynamical mass limit based on the Jeans equations. In contrast with previously
reported super-dense passive galaxies at z~2, the present galaxy at z=1.82
appears to have both size and velocity dispersion similar to early-type
galaxies in the local Universe with similar stellar mass. This suggests that
z~2 massive and passive galaxies may exhibit a wide range of properties, then
possibly following quite different evolutionary histories from z~2 to z=0.Comment: ApJ Letters in press; 9 pages; 4 figures; 1 table; emulateapj.cl
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