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A Mechanism-Based Approach to the Identification of Age–Period–Cohort Models
This article offers a new approach to the identification of age-period-cohort (APC) models that builds on Pearl's work on nonparametric causal models, in particular his front-door criterion for the identification of causal effects. The goal is to specify the mechanisms through which the age, period, and cohort variables affect the outcome and in doing so identify the model. This approach allows for a broader set of identification strategies than has typically been considered in the literature and, in many circumstances, goodness of fit tests are possible. The authors illustrate the utility of the approach by developing an APC model for political alienation.Sociolog
A Dynamic Galaxy: Stellar Age Patterns Across the Disk of M101
Using deep, narrowband imaging of the nearby spiral galaxy M101, we present
stellar age information across the full extent of the disk of M101. Our
narrowband filters measure age-sensitive absorption features such as the Balmer
lines and the slope of the continuum between the Balmer break and 4000 \r{A}
break. We interpret these features in the context of inside-out galaxy
formation theories and dynamical models of spiral structure. We confirm the
galaxy's radial age gradient, with the mean stellar age decreasing with radius.
In the relatively undisturbed main disk, we find that stellar ages get
progressively older with distance across a spiral arm, consistent with the
large-scale shock scenario in a quasi-steady spiral wave pattern. Unexpectedly,
we find the same pattern across spiral arms in the outer disk as well, beyond
the corotation radius of the main spiral pattern. We suggest that M101 has a
dynamic, or transient, spiral pattern with multiple pattern speeds joined
together via mode coupling to form coherent spiral structure. This scenario
connects together the radial age gradient inherent to inside-out galaxy
formation with the across-arm age gradients predicted by dynamic spiral arm
theories across the full radial extent of the galaxy.Comment: 21 pages, 12 figures, 1 table. Accepted to Ap
Diffuse Light in the Virgo Cluster
We present deep optical imaging of the inner 1.5 x 1.5 degrees of the Virgo
cluster to search for diffuse intracluster light (ICL). Our image reaches a 1
sigma depth of mu_v=28.5 mag/arcsec^2 -- 1.5 mag/arcsec^2 deeper than previous
surveys -- and reveals an intricate web of diffuse intracluster light. We see
several long (>100 kpc) tidal streamers, as well as a myriad of smaller-scale
tidal tails and bridges between galaxies. The diffuse halo of M87 is traced out
to nearly 200 kpc, appearing very irregular on these scales, while significant
diffuse light is also detected around the M84/M86 pair. Several galaxies in the
core are embedded in common envelopes, suggesting they are true physical
subgroups. The complex substructure of Virgo's diffuse ICL reflects the
hierarchical nature of cluster assembly, rather than being the product of
smooth accretion around a central galaxy.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Bypassing the requirement for aminoacyl-tRNA by a cyclodipeptide synthase enzyme
C. J. H. and C. M. C. are funded by the Wellcome trust (210486/Z/18/Z), ES is funded by the Cunningham trust (PhD-CT-18-41).Cyclodipeptide synthases (CDPSs) produce a variety of cyclic dipeptide products by utilising two aminoacylated tRNA substrates. We sought to investigate the minimal requirements for substrate usage in this class of enzymes as the relationship between CDPSs and their substrates remains elusive. Here, we investigated the Bacillus thermoamylovorans enzyme, BtCDPS, which synthesises cyclo(L-Leu–L-Leu). We systematically tested where specificity arises and, in the process, uncovered small molecules (activated amino esters) that will suffice as substrates, although catalytically poor. We solved the structure of BtCDPS to 1.7 Å and combining crystallography, enzymatic assays and substrate docking experiments propose a model for how the minimal substrates interact with the enzyme. This work is the first report of a CDPS enzyme utilizing a molecule other than aa-tRNA as a substrate; providing insights into substrate requirements and setting the stage for the design of improved simpler substrates.Publisher PDFPeer reviewe
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