1,535 research outputs found
Interactions of the Galactic bar and spiral arm in NGC 3627
Aims: To gain insight into the expected gas dynamics at the interface of the Galactic bar and spiral arms in our own Milky Way galaxy, we examine as an extragalactic counterpart the evidence of multiple distinct velocity components in the cold dense molecular gas that populates a similar region at the end of the bar in the nearby galaxy NGC 3627.
Methods: We assembled a high-resolution view of molecular gas kinematics traced by CO(2-1) emission and extracted line-of-sight velocity profiles from regions of high and low gas velocity dispersion.
Results: The high velocity dispersions arise with often double-peaked or multiple line-profiles. We compare the centroids of the different velocity components to expectations based on orbital dynamics in the presence of bar and spiral potential perturbations. A model of the region as the interface of two gas-populated orbits families supporting the bar and the independently rotating spiral arms provides an overall good match to the data. An extent of the bar to the corotation radius of the galaxy is favored.
Conclusions: Using NGC 3627 as an extragalactic example, we expect situations like this to favor strong star formation events such as are observed in our own Milky Way since gas can pile up where the orbit families cross. The relative motions of the material following these orbits is most likely even more important for the build-up of high density in the region. The surface densities in NGC 3627 are also so high that shear at the bar end is unlikely to significantly weaken the star formation activity. We speculate that scenarios in which the bar and spiral rotate at two different pattern speeds may be the most favorable for intense star formation at such interfaces
Interactions of the Galactic bar and spiral arm in NGC 3627
Aims: To gain insight into the expected gas dynamics at the interface of the Galactic bar and spiral arms in our own Milky Way galaxy, we examine as an extragalactic counterpart the evidence of multiple distinct velocity components in the cold dense molecular gas that populates a similar region at the end of the bar in the nearby galaxy NGC 3627.
Methods: We assembled a high-resolution view of molecular gas kinematics traced by CO(2-1) emission and extracted line-of-sight velocity profiles from regions of high and low gas velocity dispersion.
Results: The high velocity dispersions arise with often double-peaked or multiple line-profiles. We compare the centroids of the different velocity components to expectations based on orbital dynamics in the presence of bar and spiral potential perturbations. A model of the region as the interface of two gas-populated orbits families supporting the bar and the independently rotating spiral arms provides an overall good match to the data. An extent of the bar to the corotation radius of the galaxy is favored.
Conclusions: Using NGC 3627 as an extragalactic example, we expect situations like this to favor strong star formation events such as are observed in our own Milky Way since gas can pile up where the orbit families cross. The relative motions of the material following these orbits is most likely even more important for the build-up of high density in the region. The surface densities in NGC 3627 are also so high that shear at the bar end is unlikely to significantly weaken the star formation activity. We speculate that scenarios in which the bar and spiral rotate at two different pattern speeds may be the most favorable for intense star formation at such interfaces
Setting the scale: Photometric and dynamical properties of high-redshift early-type galaxies
Sterrewacht Leiden - OU
M/L and Color Evolution for A Deep Sample of M* Cluster Galaxies at z~1: The Formation Epoch and the Tilt of the Fundamental Plane
We have measured velocity dispersions for a sample of 36 galaxies with J <
21.2 or Mr < -20.6 mag in MS1054-03, a massive cluster of galaxies at z = 0.83.
Our data are of uniformly high quality down to our selection limit, our 16-hour
exposures typically yielding errors of only \delta(dispersion)~10% for L* and
fainter galaxies. By combining our measurements with data from the literature,
we have 53 cluster galaxies with measured dispersions, and HST/ACS-derived
sizes, colors and surface brightnesses. This sample is complete for the typical
L* galaxy at z~1, unlike most previous z~1 cluster samples which are complete
only for the massive cluster members (>1e11 M_sun). We find no evidence for a
change in the tilt of the fundamental plane (FP). Nor do we find evidence for
evolution in the slope of the color-dispersion relation and M/L_B-dispersion
relations; measuring evolution at a fixed dispersion should minimize the impact
of size evolution found in other work. The M/L_B at fixed dispersion evolves by
\Delta log10 M/L_B=-0.50 +/- 0.03 between z=0.83 and z=0.02 or d(log10
M/L_B)=-0.60 +/- 0.04 dz, and we find \Delta (U-V)_z=-0.24 +/- 0.02 mag at
fixed dispersion in the rest-frame, matching the expected evolution in M/L_B
within 2.25 standard deviations. The implied formation redshift from both the
color and M/L_B evolution is z*=2.0 +/- 0.2 +/- 0.3 (sys), during the epoch in
which the cosmic star-formation activity peaked, with the systematic
uncertainty showing the dependence of z* on the assumptions we make about the
stellar populations. The lack of evolution in either the tilt of the FP or in
the M/L- and color-dispersion relations imply that the formation epoch depends
weakly on mass, ranging from z*=2.3 +1.3 -0.3 at 300 km/s to z*=1.7 +0.3 -0.2
at 160 km/s and implies that the IMF similarly varies slowly with galaxy mass.Comment: revised; typos corrected, references updated, and other cosmetic
changes to meet ApJ format ApJ accepted, 22 pages in emulate ApJ format, 8
color figures, 1 b/w figur
Conformational studies of pathogenic expanded polyglutamine protein deposits from Huntington’s disease
Huntington’s disease, like other neurodegenerative diseases, continues to lack an effective cure. Current treatments that address early symptoms ultimately fail Huntington’s disease patients and their families, with the disease typically being fatal within 10–15 years from onset. Huntington’s disease is an inherited disorder with motor and mental impairment, and is associated with the genetic expansion of a CAG codon repeat encoding a polyglutamine-segment-containing protein called huntingtin. These Huntington’s disease mutations cause misfolding and aggregation of fragments of the mutant huntingtin protein, thereby likely contributing to disease toxicity through a combination of gain-of-toxic-function for the misfolded aggregates and a loss of function from sequestration of huntingtin and other proteins. As with other amyloid diseases, the mutant protein forms non-native fibrillar structures, which in Huntington’s disease are found within patients’ neurons. The intracellular deposits are associated with dysregulation of vital processes, and inter-neuronal transport of aggregates may contribute to disease progression. However, a molecular understanding of these aggregates and their detrimental effects has been frustrated by insufficient structural data on the misfolded protein state. In this review, we examine recent developments in the structural biology of polyglutamine-expanded huntingtin fragments, and especially the contributions enabled by advances in solid-state nuclear magnetic resonance spectroscopy. We summarize and discuss our current structural understanding of the huntingtin deposits and how this information furthers our understanding of the misfolding mechanism and disease toxicity mechanisms
The Dependence of Galaxy Morphology and Structure on Environment and Stellar Mass
From the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5), we extract a
sample of 4594 galaxies at redshifts 0.02<z<0.03, complete down to a stellar
mass of M=10^10 Msol. We quantify their structure (Sersic index), morphology
(Sersic index + ``Bumpiness''), and local environment. We show that morphology
and structure are intrinsically different galaxy properties, and we demonstrate
that this is a physically relevant distinction by showing that these properties
depend differently on galaxy mass and environment. Structure mainly depends on
galaxy mass whereas morphology mainly depends on environment. This is driven by
variations in star formation activity, as traced by color, which only weakly
affects the structure of a galaxy but strongly affects its morphological
appearance. The implication of our results is that the existence of the
morphology-density relation is intrinsic and not just due to a combination of
more fundamental, underlying relations. Our findings have consequences for
high-redshift studies, which often use some measure of structure as a proxy for
morphology. A direct comparison with local samples selected through visually
classified morphologies may lead to biases in the inferred evolution of the
morphological mix of the galaxy population, and misinterpretations in terms of
how galaxy evolution depends on mass and environment.Comment: Accepted for publication in ApJ Letters, 5 pages, 5 figures. Minor
changes made to match published versio
The Physical Origins of The Morphology-Density Relation: Evidence for Gas Stripping from the SDSS
We provide a physical interpretation and explanation of the
morphology-density relation for galaxies, drawing on stellar masses, star
formation rates, axis ratios and group halo masses from the Sloan Digital Sky
Survey. We first re-cast the classical morphology-density relation in more
quantitative terms, using low star formation rate (quiescence) as a proxy for
early-type morphology and dark matter halo mass from a group catalog as a proxy
for environmental density: for galaxies of a given stellar mass the quiescent
fraction is found to increase with increasing dark matter halo mass. Our novel
result is that - at a given stellar mass - quiescent galaxies are significantly
flatter in dense environments, implying a higher fraction of disk galaxies.
Supposing that the denser environments differ simply by a higher incidence of
quiescent disk galaxies that are structurally similar to star-forming disk
galaxies of similar mass, explains simultaneously and quantitatively these
quiescence -nvironment and shape-environment relations. Our findings add
considerable weight to the slow removal of gas as the main physical driver of
the morphology-density relation, at the expense of other explanations.Comment: published in ApJ: http://adsabs.harvard.edu/abs/2010ApJ...714.1779
Low Star Formation Rates for z=1 Early-Type Galaxies in the Very Deep GOODS-MIPS Imaging: Implications for their Optical/Near-Infrared Spectral Energy Distributions
We measure the obscured star formation in z~1 early-type galaxies. This
constrains the influence of star formation on their optical/near-IR colors,
which, we found, are redder than predicted by the model by Bruzual & Charlot
(2003). From deep ACS imaging we construct a sample of 95 morphologically
selected early-type galaxies in the HDF-N and CDF-S with spectroscopic
redshifts in the range 0.85<z<1.15. We measure their 24 micron fluxes from the
deep GOODS-MIPS imaging and derive the IR luminosities and star formation
rates. The fraction of galaxies with >2 sigma detections (~25 muJy} is
17(-4,+9)%. Of the 15 galaxies with significant detections at least six have an
AGN. Stacking the MIPS images of the galaxies without significant detections
and adding the detected galaxies without AGN we find an upper limit on the mean
star formation rate (SFR) of 5.2+/-3.0 Msol yr^-1, and on the mean specific SFR
of 4.6+/-2.2 * 10^-11 yr^-1. Under the assumption that the average SFR will
decline at the same rate as the cosmic average, the in situ growth in stellar
mass of the early-type galaxy population is less than 14+/-7% between z=1 and
the present. We show that the typically low IR luminosity and SFR imply that
the effect of obscured star formation (or AGN) on their rest-frame
optical/near-IR SEDs is negligible for ~90% of the galaxies in our sample.
Hence, their optical/near-IR colors are most likely dominated by evolved
stellar populations. This implies that the colors predicted by the Bruzual &
Charlot (2003) model for stellar populations with ages similar to those of z~1
early-type galaxies (~1-3 Gyr) are most likely too blue, and that stellar
masses of evolved, high-redshift galaxies can be overestimated by up to a
factor of ~2.Comment: Accepted for publication in ApJ, 8 pages, 4 figures, 1 tabl
Mental Health and Access to Active Labor Market Programs
This paper examines the often-overlooked precondition for successful implementation of active labor market policy, namely equal access to labor market programs. Focusing on a cohort of social assistance recipients, we compare program participation between individuals who were eligible for vocational training and had reported psychological distress, to possible participants with other health or social challenges. The study covers a period of six years. The results indicate that social services prioritize training for those without mental health problems. This is true independently of observed differences between the two groups in terms of demographic and human capital characteristics, work motivation and self-efficacy. Hence, the study concludes that there seems to be a mental health access bias in program participation among disadvantaged groups. Policy makers and future research should address possible organizational barriers to equal program acces
On the use of ultracentrifugal devices for routine sample preparation in biomolecular magic-angle-spinning NMR
A number of recent advances in the field of magic-angle-spinning (MAS) solid-state NMR have enabled its application to a range of biological systems of ever increasing complexity. To retain biological relevance, these samples are increasingly studied in a hydrated state. At the same time, experimental feasibility requires the sample preparation process to attain a high sample concentration within the final MAS rotor. We discuss these considerations, and how they have led to a number of different approaches to MAS NMR sample preparation. We describe our experience of how custom-made (or commercially available) ultracentrifugal devices can facilitate a simple, fast and reliable sample preparation process. A number of groups have since adopted such tools, in some cases to prepare samples for sedimentation-style MAS NMR experiments. Here we argue for a more widespread adoption of their use for routine MAS NMR sample preparation
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