9,796 research outputs found
The Variation in Molecular Gas Depletion Time among Nearby Galaxies: What are the Main Parameter Dependencies?
We re-analyze correlations between global molecular gas depletion time (Tdep)
and galaxy parameters including stellar mass, specific star formation rate
(sSFR), stellar mass surface density and concentration index. The analysis is
based on the COLD GASS survey, which includes galaxies with stellar mass in the
range 10^10 - 10^11.5 Msun with molecular gas mass estimates derived from
CO(1-0) line measurements. We improve on previous work by Saintonge et al.
(2011b) by estimating SFRs using the combination of GALEX FUV and WISE 22
micron data and by deriving Tdep within a fixed aperture set by the IRAM beam
size. In our new study we find correlations with much smaller scatter.
Dependences of the Tdep on galaxy structural parameters such as stellar mass
surface density and concentration index are now weak or absent. Differences
with previous work arise because dust extinction correlates strongly with
galaxy structural parameters. We further demonstrate that the 'primary' global
parameter correlation is between Tdep and sSFR; all other remaining
correlations can be shown to be induced by this primary dependence. This
implies that galaxies with high current-to-past-averaged star formation
activity, will drain their molecular gas reservoir sooner. We then analyze
molecular gas depletion times on 1-kpc scales in galactic disks using data from
the HERACLES survey. There is remarkably good agreement between the global Tdep
versus sSFR relation for the COLD GASS galaxies and that derived for 1 kpc
scale grid regions in disks. The strong correlation between Tdep and sSFR
extends continuously over a factor of 10 in Tdep from log(SFR/M*) = -11.5 to
-9, i.e. from nearly quiescent patches of the disk to disk regions with very
strong star formation. This leads to the conclusion that the local molecular
gas depletion time in galactic disks is dependent on the local fraction of
young-to-old stars.Comment: Revised to version published in MNRA
The variation in molecular gas depletion time among nearby galaxies: II the impact of galaxy internal structures
We use a data set of nearby galaxies drawn from the HERACLES, ATLAS3D, and
COLD GASS surveys to study variations in molecular gas depletion time (Tdep) in
galaxy structures such as bulges, grand-design spiral arms, bars and rings.
Molecular gas is traced by CO line emission and star formation rate (SFR) is
derived using the combination of far-ultraviolet and mid-infrared (MIR) data.
The contribution of old stars to MIR emission for the ATLAS3D sample is
corrected using 2MASS K-band images. We apply a two-dimensional image
decomposition algorithm to decompose galaxies into bulges and discs. Spiral
arms, bars and rings are identified in the residual maps, and molecular gas
depletion times are derived on a square grid of 1 kpc^2 size. In previous work,
we showed that Tdep correlates strongly with specific star formation rate
(sSFR). We now find that at a given sSFR, the bulge has shorter Tdep than the
disc. The shift to shorter depletion times is most pronounced in the inner
bulge (R < 0.1Re). Grids from galaxies with bars and rings are similar to those
from galactic bulges in that they have reduced Tdep at a given sSFR. In
contrast, the Tdep versus sSFR relation in the discs of galaxies with spiral
arms is displaced to longer Tdep at fixed sSFR. We then show that the
differences in the Tdep-sSFR relation for bulges, discs, arms, bars and rings
can be linked to variations in "stellar", rather than gas surface density
between different structures. Our best current predictor for Tdep, both
globally and for 1 kpc grids, is given by Tdep=
-0.36log(Sigma_SFR)-0.5log(Sigma_*)+5.87.Comment: 14 pages, 13 figures, revised to version accepted in MNRA
Threshold Regression for Survival Analysis: Modeling Event Times by a Stochastic Process Reaching a Boundary
Many researchers have investigated first hitting times as models for survival
data. First hitting times arise naturally in many types of stochastic
processes, ranging from Wiener processes to Markov chains. In a survival
context, the state of the underlying process represents the strength of an item
or the health of an individual. The item fails or the individual experiences a
clinical endpoint when the process reaches an adverse threshold state for the
first time. The time scale can be calendar time or some other operational
measure of degradation or disease progression. In many applications, the
process is latent (i.e., unobservable). Threshold regression refers to
first-hitting-time models with regression structures that accommodate covariate
data. The parameters of the process, threshold state and time scale may depend
on the covariates. This paper reviews aspects of this topic and discusses
fruitful avenues for future research.Comment: Published at http://dx.doi.org/10.1214/088342306000000330 in the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Adaptive to Customers: The Roles of Learning Climate and Customer Knowledge
[[abstract]]Based on the theory of organizational socialization, the present study evaluates the effect of learning climate on salespeople's adaptive selling behaviors by reviewing and incorporating their knowledge of customers. The study also explores the mediating role of customer knowledge for learning climate and adaptive selling behaviors. A total of 350 salespeople in 35 consumer electronics and appliances stores located in Taiwan were analyzed using hierarchical linear modeling (HLM), producing results that support the proposed model. More specifically, learning climate was positively related to customer knowledge and adaptive selling behaviors, and customer knowledge was directly related to adaptive selling behaviors. In addition, learning climate was associated with adaptive selling behaviors through customer knowledge. These findings highlight the importance of enhancing the learning climate and salespeople's customer knowledge to enable retailing organizations to improve salespeople's adaptive selling behaviors.[[notice]]補正完畢[[incitationindex]]EI[[ispeerreviewed]]Y[[booktype]]電子版[[countrycodes]]NL
A systematic study of the inner rotation curves of galaxies observed as part of the GASS and COLD GASS surveys
We present a systematic analysis of the rotation curves of 187 galaxies with
masses greater than 10^10 M_sol, with atomic gas masses from the GALEX Arecibo
Sloan Survey (GASS), and with follow-up long-slit spectroscopy from the MMT.
Our analysis focuses on stellar rotation curves derived by fitting stellar
template spectra to the galaxy spectra binned along the slit. In this way, we
are able to obtain accurate rotation velocity measurements for a factor of 2
more galaxies than possible with the Halpha line. Galaxies with high atomic gas
mass fractions are the most dark-matter dominated galaxies in our sample and
have dark matter halo density profiles that are well fit by Navarro, Frenk &
White profiles with an average concentration parameter of 10. The inner slopes
and of the rotation curves correlate more strongly with stellar population age
than with galaxy mass or structural parameters. At fixed stellar mass, the
rotation curves of more actively star-forming galaxies have steeper inner
slopes than less actively star-forming galaxies. The ratio between the galaxy
specific angular momentum and the total specific angular momentum of its dark
matter halo, R_j, correlates strongly with galaxy mass, structure and gas
content. Low mass, disk-dominated galaxies with atomic gas mass fractions
greater than 20% have median values of R_j of around 1, but massive,
bulge-dominated galaxies have R_j=0.2-0.3. We argue that these trends can be
understood in a picture where gas inflows triggered by disk instabilities lead
to the formation of passive, bulge-dominated galaxies with low specific angular
momentum.Comment: 10 pages, 12 figures, submitted to MNRA
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