48 research outputs found
Fully compressive tides in galaxy mergers
The disruptive effect of galactic tides is a textbook example of
gravitational dynamics. However, depending on the shape of the potential, tides
can also become fully compressive. When that is the case, they might trigger or
strengthen the formation of galactic substructures (star clusters, tidal dwarf
galaxies), instead of destroying them. We perform N-body simulations of
interacting galaxies to quantify this effect. We demonstrate that tidal
compression occurs repeatedly during a galaxy merger, independently of the
specific choice of parameterization. With a model tailored to the Antennae
galaxies, we show that the distribution of compressive tides matches the
locations and timescales of observed substructures. After extending our study
to a broad range of parameters, we conclude that neither the importance of the
compressive tides (~15% of the stellar mass) nor their duration (~ 10 Myr) are
strongly affected by changes in the progenitors' configurations and orbits.
Moreover, we show that individual clumps of matter can enter compressive
regions several times in the course of a simulation. We speculate that this may
spawn multiple star formation episodes in some star clusters, through e.g.,
enhanced gas retention.Comment: 17 pages, 16 figures, accepted for publication in Ap
The Star Formation History in The Far Outer Disc of M33
The outer regions of disc galaxies are becoming increasingly recognized as
key testing sites for models of disc assembly and evolution. Important issues
are the epoch at which the bulk of the stars in these regions formed and how
discs grow radially over time. To address these issues, we use Hubble Space
Telescope Advanced Camera for Surveys imaging to study the star formation
history (SFH) of two fields at 9.1 and 11.6 kpc along M33's northern major
axis. These fields lie at ~ 4 and 5 V-band disc scale-lengths and straddle the
break in M33's surface brightness profile. The colour-magnitude diagrams (CMDs)
reach the ancient main sequence turnoff with a signal-to-noise ratio of ~ 5.
From detailed modelling of the CMDs, we find that the majority of stars in both
fields combined formed at z < 1. The mean age in the inner field, S1, is ~ 3
+/- 1 Gyr and the mean metallicity is [M/H] ~ -0.5 +/- 0.2 dex. The star
formation history of S1 unambiguously reveals how the inside-out growth
previously measured for M33's inner disc out to ~ 6 kpc extends out to the disc
edge at ~ 9 kpc. In comparison, the outer field, S2, is older (mean age ~ 7 +/-
2 Gyr), more metal-poor (mean [M/H] ~ -0.8 +/- 0.3 dex), and contains ~ 30
times less stellar mass. These results provide the most compelling evidence yet
that M33's age gradient reverses at large radii near the disc break and that
this reversal is accompanied by a break in stellar mass surface density. We
discuss several possible interpretations of this behaviour including radial
stellar mixing, warping of the gaseous disc, a change in star formation
efficiency, and a transition to another structural component. These results
offer one of the most detailed views yet of the peripheral regions of any disc
galaxy and provide a much-needed observational constraint on the last major
epoch of star formation in the outer disc.Comment: 15 pages, 9 figures, accepted to MNRAS, hi-res version available at
http://www.roe.ac.uk/~mkb/astroph/m33hires.pd
Interpreting the Evolution of the Size - Luminosity Relation for Disk Galaxies from Redshift 1 to the Present
A sample of very high resolution cosmological disk galaxy simulations is used
to investigate the evolution of galaxy disk sizes back to redshift 1 within the
Lambda CDM cosmology. Artificial images in the rest frame B band are generated,
allowing for a measurement of disk scale lengths using surface brightness
profiles as observations would, and avoiding any assumption that light must
follow mass as previous models have assumed. We demonstrate that these
simulated disks are an excellent match to the observed magnitude - size
relation for both local disks, and for disks at z=1 in the magnitude/mass range
of overlap. We disentangle the evolution seen in the population as a whole from
the evolution of individual disk galaxies. In agreement with observations, our
simulated disks undergo roughly 1.5 magnitudes/arcsec^2 of surface brightness
dimming since z=1. We find evidence that evolution in the magnitude - size
plane varies by mass, such that galaxies with M* > 10^9 M_sun undergo more
evolution in size than luminosity, while dwarf galaxies tend to evolve
potentially more in luminosity. The disks grow in such a way as to stay on
roughly the same stellar mass - size relation with time. Finally, due to an
evolving stellar mass - SFR relation, a galaxy at a given stellar mass (or
size) at z=1 will reside in a more massive halo and have a higher SFR, and thus
a higher luminosity, than a counterpart of the same stellar mass at z=0.Comment: Version resubmitted to ApJ, after referee's comment
Building a circular supply chain:Achieving resilient operations with the circular economy
This paper highlights the fundamental contribution that supply chain professionals can make to the transition to a circular economy. It aims to provide a general understanding of how the circular economy and supply chain management fields are related to one another. By exploring the concept of a circular supply chain, the paper illustrates the role of supply chain professionals in operationalising circular economy initiatives within their organisations, as well as the opportunities and challenges they may encounter along the way. The paper also provides initialrecommendations for and examples of companies overcoming some of these challenges, based on the experiences of supply chain professionals involved in the research
Cosmological Galaxy Formation Simulations Using SPH
We present the McMaster Unbiased Galaxy Simulations (MUGS), the first 9
galaxies of an unbiased selection ranging in total mass from 5
M to 2 M simulated using n-body smoothed
particle hydrodynamics (SPH) at high resolution. The simulations include a
treatment of low temperature metal cooling, UV background radiation, star
formation, and physically motivated stellar feedback. Mock images of the
simulations show that the simulations lie within the observed range of
relations such as that between color and magnitude and that between brightness
and circular velocity (Tully-Fisher). The greatest discrepancy between the
simulated galaxies and observed galaxies is the high concentration of material
at the center of the galaxies as represented by the centrally peaked rotation
curves and the high bulge-to-total ratios of the simulations determined both
kinematically and photometrically. This central concentration represents the
excess of low angular momentum material that long has plagued morphological
studies of simulated galaxies and suggests that higher resolutions and a more
accurate description of feedback will be required to simulate more realistic
galaxies. Even with the excess central mass concentrations, the simulations
suggest the important role merger history and halo spin play in the formation
of disks.Comment: 16 pages, 16 figures, submitted to MNRAS, movies available at
http://mugs.mcmaster.ca . Comments welcome
Forming a Large Disc Galaxy from a z<1 Major Merger
Using high resolution SPH simulations in a fully cosmological Lambda CDM
context we study the formation of a bright disk dominated galaxy that
originates from a "wet" major merger at z=0.8. The progenitors of the disk
galaxy are themselves disk galaxies that formed from early major mergers
between galaxies with blue colors. A substantial thin stellar disk grows
rapidly following the last major merger and the present day properties of the
final remnant are typical of early type spiral galaxies, with an i band B/D
~0.65, a disk scale length of 7.2 kpc, g-r = 0.5 mag, an HI line width
(W_{20}/2) of 238 km/sec and total magnitude i = -22.4. The key ingredients for
the formation of a dominant stellar disk component after a major merger are: i)
substantial and rapid accretion of gas through cold flows followed at late
times by cooling of gas from the hot phase, ii) supernova feedback that is able
to partially suppress star formation during mergers and iii) relative fading of
the spheroidal component. The gas fraction of the progenitors' disks does not
exceed 25% at z<3, emphasizing that the continuous supply of gas from the local
environment plays a major role in the regrowth of disks and in keeping the
galaxies blue. The results of this simulation alleviate the problem posed for
the existence of disk galaxies by the high likelihood of interactions and
mergers for galaxy sized halos at relatively low z.Comment: MNRAS in press. Minor additions to text. Movie at:
http://www.astro.washington.edu/fabio/movies/Merger.mp
Do Mismatches between Pre- and Post-Natal Environments Influence Adult Physiological Functioning?
Purpose: Mismatches between pre- and post-natal environments have implications for disease in adulthood. However, less is known about how this mismatch can affect physiological systems more generally, especially at younger ages. We hypothesised that mismatches between pre- and post-natal environments, as measured by the measures of birthweight and adult leg length, would be associated with poorer biomarker levels across five key physiological systems in young adults. Methods: Data were collected from 923, 36 year-old respondents from the West of Scotland Twenty-07 Study. The biomarkers were: systolic blood pressure (sBP); forced expiratory volume (FEV1); glycated haemoglobin (HbA1c); glomerular filtration rate (eGFR); and gamma- glutamyltransferase (GGT). These biomarkers were regressed against pre-natal conditions (birthweight), post-natal conditions (leg length) and the interaction between pre- and post-natal measures. Sex, childhood socioeconomic position and adult lifestyle characteristics were adjusted for as potential effect modifiers and confounders, respectively. Results: There were no associations between birthweight and leg length and sBP, FEV1, HbA1c, or GGT. Higher birthweight and longer leg length were associated with better kidney function (eGFR). However, there was no evidence for mismatches between birthweight and leg length to be associated with worse sBP, FEV1, HbA1c, eGFR or GGT levels (P>0.05). Conclusions: Our hypothesis that early signs of physiological damage would be present in young adults given mismatches in childhood environments, as measured by growth markers, was not proven. This lack of association could be because age 36 is too young to identify significant trends for future health, or the associations simply not being present. © 2014 Robertson, Benzeval
The SINS survey: modeling the dynamics of z~2 galaxies and the high-z Tully-Fisher relation
We present the modeling of SINFONI integral field dynamics of 18 star forming
galaxies at z ~ 2 from Halpha line emission. The galaxies are selected from the
larger sample of the SINS survey, based on the prominence of ordered rotational
motions with respect to more complex merger induced dynamics. The quality of
the data allows us to carefully select systems with kinematics dominated by
rotation, and to model the gas dynamics across the whole galaxy using suitable
exponential disk models. We obtain a good correlation between the dynamical
mass and the stellar mass, finding that large gas fractions Mgas~M*) are
required to explain the difference between the two quantities. We use the
derived stellar mass and maximum rotational velocity Vmax from the modeling to
construct for the first time the stellar mass Tully-Fisher relation at z ~ 2.2.
The relation obtained shows a slope similar to what is observed at lower
redshift, but we detect an evolution of the zero point. We find that at z ~ 2.2
there is an offset in log(M*) for a given rotational velocity of 0.41+-0.11
with respect to the local Universe. This result is consistent with the
predictions of the latest N-body/hydrodynamical simulations of disk formation
and evolution, which invoke gas accretion onto the forming disk in filaments
and cooling flows. This scenario is in agreement with other dynamical evidence
from SINS, where gas accretion from the halo is required to reproduce the
observed properties of a large fraction of the z ~ 2 galaxies.Comment: 10 pages, 6 figures, submitted to Ap
Host galaxies, clustering, Eddington ratios, and evolution of radio, X-ray, and infrared-selected AGNs
We explore the connection between different classes of active galactic nuclei
(AGNs) and the evolution of their host galaxies, by deriving host galaxy
properties, clustering, and Eddington ratios of AGNs selected in the radio,
X-ray, and infrared. We study a sample of 585 AGNs at 0.25 < z < 0.8 using
redshifts from the AGN and Galaxy Evolution Survey (AGES) and data in the radio
(WSRT 1.4 GHz), X-rays (Chandra XBootes), and mid-IR (IRAC Shallow Survey). The
radio, X-ray, and IR AGN samples show modest overlap, indicating that to the
flux limits of the survey, they represent largely distinct classes of AGNs. We
derive host galaxy colors and luminosities, as well as Eddington ratios
(lambda), for obscured or optically faint AGNs. We also measure the two-point
cross-correlation between AGNs and galaxies on scales of 0.3-10 h^-1 Mpc, and
derive typical dark matter halo masses. We find that: (1) radio AGNs are mainly
found in luminous red galaxies, are strongly clustered (with M_halo ~ 3x10^13
h^-1 M_sun), and have very low lambda <~ 10^-3; (2) X-ray-selected AGNs are
preferentially found in galaxies in the "green valley" of color-magnitude space
and are clustered similarly to typical AGES galaxies (M_halo ~ 10^13 h^-1
M_sun), with 10^-3 <~ lambda <~ 1; (3) IR AGNs reside in slightly bluer, less
luminous galaxies than X-ray AGNs, are weakly clustered (M_halo <~ 10^12 h^-1
M_sun), and have lambda > 10^-2. We interpret these results in terms of a
simple model of AGN and galaxy evolution, whereby a "quasar" phase and the
growth of the stellar bulge occurs when a galaxy's dark matter halo reaches a
critical mass between ~10^12 and 10^13 M_sun. Subsequently, star formation
ceases and AGN accretion shifts from radiatively efficient (optical- and IR-
bright) to radiatively inefficient (optically-faint, radio-bright) modes.Comment: 30 emulateapj pages, 21 figures, 3 tables, v2: minor changes match
version to appear in Ap