Sub-picosecond compression by velocity bunching in a photo-injector

We present an experimental evidence of a bunch compression scheme that uses a traveling wave accelerating structure as a compressor. The bunch length issued from a laser-driven radio-frequency electron source was compressed by a factor >3 using an S-band traveling wave structure located immediately downstream from the electron source. Experimental data are found to be in good agreement with particle tracking simulations.Comment: 19 pages, 9 figures, submitted to Phys. Rev. Spec. Topics A&

Dissecting the Red Sequence--II. Star Formation Histories of Early-Type Galaxies Throughout the Fundamental Plane

This analysis uses spectra of ~16,000 nearby SDSS quiescent galaxies to track variations in galaxy star formation histories along and perpendicular to the Fundamental Plane (FP). We sort galaxies by their FP properties (sigma, R_e, and I_e) and construct high S/N mean galaxy spectra that span the breadth and thickness of the FP. From these spectra, we determine mean luminosity-weighted ages, [Fe/H], [Mg/H], and [Mg/Fe] based on single stellar population models using the method described in Graves & Schiavon (2008). In agreement with previous work, the star formation histories of early-type galaxies are found to form a two-parameter family. The major trend is that mean age, [Fe/H], [Mg/H], and [Mg/Fe] all increase with sigma. However, no stellar population property shows any dependence on R_e at fixed sigma, suggesting that sigma and not dynamical mass (M_dyn ~ sigma^2 R_e) is the better predictor of past star formation history. In addition to the main trend with sigma, galaxies also show a range of population properties at fixed sigma that are strongly correlated with surface brightness residuals from the FP, such that higher surface brightness galaxies have younger mean ages, higher [Fe/H], higher [Mg/H], and lower [Mg/Fe] than lower-surface brightness galaxies. These latter trends are a major new constraint on star-formation histories.Comment: 23 pages, 14 figures. Accepted to Ap

Dissecting the Red Sequence. IV. The Role of Truncation in the Two-Dimensional Family of Early-Type Galaxy Star Formation Histories

In the three-dimensional parameter space defined by velocity dispersion, effective radius (R_e), and effective surface brightness (I_e), early-type galaxies are observed to populate a two-dimensional fundamental plane (FP) with finite thickness. In Paper III of this series, we showed that the thickness of the FP is predominantly due to variations in the stellar mass surface density (Sigma_*) inside the effective radius R_e. These variations represent differences in the dark matter fraction inside R_e (or possibly differences in the initial mass function) from galaxy to galaxy. This means that galaxies do not wind up below the FP at lower surface brightness due to the passive fading of their stellar populations; they are structurally different. Here, we show that these variations in Sigma_* at fixed dynamical mass (M_dyn) are linked to differences in the galaxy stellar populations, and therefore to differences in their star formation histories. We demonstrate that the ensemble of stellar population and Sigma_* variations through the FP thickness can be explained by a model in which early-type galaxies at fixed M_dyn have their star formation truncated at different times. The thickness of the FP can therefore be interpreted as a sequence of truncation times. Galaxies below the FP have earlier truncation times for a given M_dyn, resulting in lower Sigma_*, older ages, lower metallicities in both [Fe/H] and [Mg/H], and higher [Mg/Fe]. We show that this model is quantitatively consistent with simple expectations for chemical enrichment in galaxies. We also present fitting functions for luminosity-weighted age, [Fe/H], [Mg/H], and [Mg/Fe] as functions of the FP parameters velocity dispersion, R_e, and I_e. These provide a new tool for estimating the stellar population properties of quiescent early-type galaxies for which high-quality spectra are not available.Comment: 21 pages, 9 figures. Accepted to Ap

Millisecond accuracy video display using OpenGL under Linux

To measure people’s reaction times to the nearest millisecond, it is necessary to know exactly when a stimulus is displayed. This article describes how to display stimuli with millisecond accuracy on a normal CRT monitor, using a PC running Linux. A simple C program is presented to illustrate how this may be done within X Windows using the OpenGL rendering system. A test of this system is reported that demonstrates that stimuli may be consistently displayed with millisecond accuracy. An algorithm is presented that allows the exact time of stimulus presentation to be deduced, even if there are relatively large errors in measuring the display time

The Peak Brightness and Spatial Distribution of AGB Stars Near the Nucleus of M32

The bright stellar content near the center of the Local Group elliptical galaxy M32 is investigated with 0.12 arcsec FWHM H and K images obtained with the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving near the tip of the asymptotic giant branch (AGB), are resolved to within 2 arcsec of the nucleus, and it is concluded that the peak stellar brightness near the center of M32 is similar to that in the outer regions of the galaxy. Moreover, the projected density of bright AGB stars follows the visible light profile to within 2 arcsec of the nucleus, indicating that the brightest stars are well mixed throughout the galaxy. Thus, there is no evidence for an age gradient, and the radial variations in spectroscopic indices and ultraviolet colors that have been detected previously must be due to metallicity and/or some other parameter. We suggest that either the bright AGB stars formed as part of a highly uniform and coherent galaxy-wide episode of star formation, or they originated in a separate system that merged with M32.Comment: 9 pages of text, 3 figures. ApJ (Letters) in pres