184 research outputs found
The growth of galactic bulges through mergers in LCDM haloes revisited. II. Morphological mix evolution
The mass aggregation and merger histories of present-day distinct haloes
selected from the cosmological Millennium Simulations I and II are mapped into
stellar mass aggregation and galaxy merger histories of central galaxies by
using empirical stellar-to-halo and stellar-to-gas mass relations. The growth
of bulges driven by the galaxy mergers/interactions is calculated using
dynamical prescriptions. The predicted bulge demographics at redshift z~0 is
consistent with observations (Zavala+2012). Here we present the evolution of
the morphological mix (traced by the bulge-to-total mass ratio, B/T) as a
function of mass up to z=3. This mix remains qualitatively the same up to z~1:
B/T<0.1 galaxies dominate at low masses, 0.1<B/T<0.45 at intermediate masses,
and B/T>0.45 at large masses. At z>1, the fractions of disc-dominated and
bulgeless galaxies increase strongly, and by z~2 the era of pure disc galaxies
is reached. Bulge-dominated galaxies acquire such a morphology, and most of
their mass, following a downsizing trend. Since our results are consistent with
most of the recent observational studies of the morphological mix at different
redshifts, a LCDM-based scenario of merger-driven bulge assembly does not seem
to face critical issues. However, if the stellar-to-halo mass relation changes
too little with redshift, then some tensions with observations appear.Comment: 16 pages, 11 figures. Accepted for publication in MNRAS. The method
and the initial conditions are described in more detail. References adde
Application of Sustainability Framework for Quality Improvement in an Integrated Health System
Problem: Sustaining improvement in quality and patient safety is a critical challenge confronting healthcare today (Lennox, Maher & Reed, 2018). Failure to sustain the gains achieved with the improvement and results in harmful patient outcomes, wasted resources, and impact future improvement work (Lennox et al., 2018). Context: To address this challenge, the organization for this DNP project was a large, integrated healthcare system with 21 medical centers in Northern California with a redesigned a regional quality program to focus primarily on supporting the sustainability of patient safety initiatives that have been successfully implemented and spread. Interventions: Two frameworks from the Institute for Health Improvement (IHI) that promote leadership and frontline engagement have been selected to provide the basis for the sustainability approach for this project. In this context, two drivers of sustainability were implemented between January and May of 2020: a template for local sustainability oversight structure and the integration of clinical workgroups to engage and motivate frontline leaders and staff to sustain improvement. Measures: To measure the impact of this project on patient care outcomes, performance of the hospitals in patient harm prevention initiatives was monitored. To measure frontline engagement and their perception of leadership support, the scores in selected indices for engagement, team effectiveness, and organization of the annual safety culture and learning climate survey were obtained. APPLICATION OF SUSTAINABILITY FRAMEWORK. 5 Results: All 21 medical centers have reported having established the recommended local oversight structure by May 2020. Preliminary regional Safety Priority Index (SPI) in June was better than the target and showed a slight decrease from 2019 performance. People Pulse survey results in the selected indices in 2019 showed that in general, medical centers with preexisting formal oversight structure, scored higher than those without. Conclusions: Early indicators from this project are reflective of the current literature on this topic strongly suggesting that focused leadership support and motivated frontline are facilitators of sustainability of healthcare improvement (Scoville, Little, Rakner, Luther, & Mate, 2016; Hilton & Anderson, 2018)
The mass and environmental dependence on the secular processes of AGN in terms of morphology, colour, and specific star-formation rate
Galaxy mass and environment play a major role in the evolution of galaxies.
In the transition from star-forming to quenched galaxies, Active galactic
nuclei (AGN) have also a principal action. However, the connections between
these three actors are still uncertain. In this work we investigate the effects
of stellar mass and the large-scale environment (LSS), on the fraction of
optical nuclear activity in a population of isolated galaxies, where AGN would
not be triggered by recent galaxy interactions or mergers. As a continuation of
a previous work, we focus on isolated galaxies to study the effect of stellar
mass and the LSS in terms of morphology (early- and late-type), colour (red and
blue), and specific star formation rate (quenched and star-forming). To explore
where AGN activity is affected by the LSS we fix the stellar mass into low- and
high-mass galaxies. We use the tidal strength parameter to quantify their
effects. We found that AGN is strongly affected by stellar mass in 'active'
galaxies (namely late-type, blue, and star-forming), however it has no
influence for 'quiescent' galaxies (namely early-type, red, and quenched), at
least for masses down to . In relation to the LSS, we
found an increment on the fraction of SFN with denser LSS in low-mass star
forming and red isolated galaxies. Regarding AGN, we find a clear increment of
the fraction of AGN with denser environment in quenched and red isolated
galaxies, independently of the stellar mass. AGN activity would be 'mass
triggered' in 'active' isolated galaxies. This means that AGN is independent of
the intrinsic property of the galaxies, but on its stellar mass. On the other
hand, AGN would be 'environment triggered' in 'quiescent' isolated galaxies,
where the fraction of AGN in terms of sSFR and colour increases from void
regions to denser LSS, independently of its stellar mass.Comment: 14 pages, 9 figures (11 pages and 6 figures without appendix),
accepted for publication in Astronomy & Astrophysic
Galactic conformity measured in semi-analytic models
We study the correlation between the specific star formation rate of central
galaxies and neighbour galaxies, also known as 'galactic conformity', out to 20
Mpc/h using three semi-analytic models (SAMs, one from L-GALAXIES and other two
from GALFORM). The aim is to establish whether SAMs are able to show galactic
conformity using different models and selection criteria. In all the models,
when the selection of primary galaxies is based on an isolation criterion in
real space, the mean fraction of quenched galaxies around quenched primary
galaxies is higher than that around star-forming primary galaxies of the same
stellar mass. The overall signal of conformity decreases when we remove
satellites selected as primary galaxies, but the effect is much stronger in
GALFORM models compared with the L-GALAXIES model. We find this difference is
partially explained by the fact that in GALFORM once a galaxy becomes a
satellite remains as such, whereas satellites can become centrals at a later
time in L-GALAXIES. The signal of conformity decreases down to 60% in the
L-GALAXIES model after removing central galaxies that were ejected from their
host halo in the past. Galactic conformity is also influenced by primary
galaxies at fixed stellar mass that reside in dark matter haloes of different
masses. Finally, we explore a proxy of conformity between distinct haloes. In
this case the conformity is weak beyond ~ 3 Mpc/h (<3% in L-GALAXIES, <1-2% in
GALFORM models). Therefore, it seems difficult that conformity is directly
related with a long-range effect.Comment: 15 pages, 7 figures. Accepted for publication in MNRA
The nature of assembly bias - III. Observational properties
We analyse galaxies in groups in the Sloan Digital Sky Survey (SDSS) and find
a weak but significant assembly-type bias, where old central galaxies have a
higher clustering amplitude (61 9 per cent) at scales > 1 Mpc than young
central galaxies of equal host halo mass (
). The observational sample is volume-limited out to z=0.1 with 5 log. We construct a mock catalogue of galaxies that shows a
similar signal of assembly bias (46 9 per cent) at the same halo mass. We
then adapt the model presented by Lacerna & Padilla (Paper I) to redefine the
overdensity peak height, which traces the assembly bias such that galaxies in
equal density peaks show the same clustering regardless of their stellar age,
but this time using observational features such as a flux limit. The proxy for
peak height, which is proposed as a better alternative than the virial mass,
consists in the total mass given by the mass of neighbour host haloes in
cylinders centred at each central galaxy. The radius of the cylinder is
parametrized as a function of stellar age and virial mass. The best-fitting set
of parameters that make the assembly bias signal lower than 515 per cent for
both SDSS and mock central galaxies are similar. The idea behind the
parametrization is not to minimize the bias, but it is to use this method to
understand the physical features that produce the assembly bias effect. Even
though the tracers of the density field used here differ significantly from
those used in paper I, our analysis of the simulated catalogue indicates that
the different tracers produce correlated proxies, and therefore the reason
behind this assembly bias is the crowding of peaks in both simulations and the
SDSS.Comment: 12 pages, 11 figures. Accepted for publication in MNRA
Isolated elliptical galaxies in the local Universe
We have studied a sample of 89 very isolated, elliptical galaxies at z < 0.08
and compared their properties with elliptical galaxies located in a
high-density environment such as the Coma supercluster. Our aim is to probe the
role of environment on the morphological transformation and quenching of
elliptical galaxies as a function of mass. In addition, we elucidate the nature
of a particular set of blue and star-forming isolated ellipticals identified
here. We study physical properties of ellipticals such as color, specific star
formation rate, galaxy size, and stellar age, as a function of stellar mass and
environment based on SDSS data. We analyze the blue star-forming isolated
ellipticals in more detail, through photometric characterization using GALFIT,
and infer their star formation history using STARLIGHT. Among the isolated
ellipticals ~ 20% are blue, 8% are star forming, and ~ 10% are recently
quenched, while among the Coma ellipticals ~ 8% are blue and just <= 1% are
star forming or recently quenched. There are four isolated galaxies (~ 4.5%)
that are blue and star forming at the same time. These galaxies, with masses
between 7 x 10^9 and 2 x 10^10 h-2 M_sun, are also the youngest galaxies with
light-weighted stellar ages <= 1 Gyr and exhibit bluer colors toward the galaxy
center. Around 30-60% of their present-day luminosity, but only < 5% of their
present-day mass, is due to star formation in the last 1 Gyr. The processes of
morphological transformation and quenching seem to be in general independent of
environment since most of elliptical galaxies are 'red and dead', although the
transition to the red sequence should be faster for isolated ellipticals. In
some cases, the isolated environment seems to propitiate the rejuvenation of
ellipticals by recent (< 1 Gyr) cold gas accretion.Comment: 23 pages, 15 figures (16 pages and 9 figures without appendices).
A&A, in pres
The nature of assembly bias - II. Halo spin
We study an assembly-type bias parametrized by the dimensionless spin
parameter that affects massive structures. In numerical simulations higher spin
haloes are more strongly clustered than lower spin haloes of equal mass. We
detect a difference of over a 30 per cent in the clustering strength for dark
matter haloes of 10^13-10^14 Msun, which is similar to the result of Bett et
al. We explore whether the dependence of clustering strength on halo spin is
removed if we apply the redefinition of overdensity peak height proposed by
Lacerna & Padilla (Paper I) obtained using assembly ages. We find that this is
not the case due to two reasons. Firstly, only a few objects of low-virial mass
are moved into the mass range where the spin introduces an assembly bias after
using this redefinition. Secondly, this formalism does not alter the mass of
massive objects. We then repeat the process of finding the redefined peak
height of Paper I but using the spin. In this case, the new masses show no
spin-related assembly bias but they introduce a previously absent assembly bias
with respect to relative age. From this result, we conclude that the
assembly-type bias with respect to the halo spin has a different origin than
with respect to assembly age. The former may be due to the material from
filaments, which is accreted by massive haloes, that is enhanced in
high-density environments, thus causing more extreme spin values without
significantly changing the formation age of the halo. In addition, high-mass
objects may correspond, in some cases, to a different peak height than that
suggested by their mass in numerical simulations, providing a possible
explanation for the assembly bias with respect to spin. (abridged)Comment: 5 pages, 5 figures. Accepted for publication in MNRAS letter
The less significant role of large-scale environment than optical AGN in nearby, isolated elliptical galaxies
The formation and evolution of elliptical galaxies in low-density
environments are less understood than classical elliptical galaxies in
high-density environments. Isolated galaxies are defined as galaxies without
massive neighbors within scales of galaxy groups. The effect of the environment
at several Mpc scales on their properties has been barely explored. Here we
study the role of large-scale environment in some physical properties of 573
isolated elliptical galaxies out to z=0.08. We use three environmental
estimators of the large-scale structure within a projected radius of 5 Mpc
around isolated galaxies: the tidal strength parameter, the projected density
eta_k, and the distance to the fifth nearest neighbor galaxy. We find 80% of
galaxies at lower densities correspond to 'red and dead' elliptical galaxies.
Blue and red galaxies do not tend to be located in different environments
according to eta_k. Almost all the isolated ellipticals in the densest
large-scale environments are red or quenched, where a third of them are
low-mass galaxies. The percentage of isolated elliptical galaxies located in
the AGN region of the BPT diagram is 64%. We have identified 33 blue,
star-forming isolated ellipticals using both color and sSFR. Half of them are
star-forming nuclei in the BPT diagram, which is 5% of the galaxies in this
diagram. The large-scale environment is not playing the primary role to
determine the color or sSFR of isolated elliptical galaxies. The large-scale
environment seems to be negligible from a stellar mass scale around 10^10.6
Msun, probably because of the dominant presence of AGN at higher masses. For
lower masses, the processes of cooling and infall of gas from large scales are
very inefficient in ellipticals. AGN might also be an essential ingredient to
keep most of the low-mass isolated elliptical galaxies quenched.Comment: 15 pages, 6 figures (10 pages and 4 figures without appendices).
Accepted for publication in A&
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