Eric Persson

New NIR light-curve templates for classical Cepheids

Aims. We present new near-infrared (NIR) light-curve templates for fundamental (FU, J, H, KS) and first overtone (FO, J) classical Cepheids. The new templates together with period-luminosity and period-Wesenheit (PW) relations provide Cepheid distances from single-epoch observations with a precision only limited by the intrinsic accuracy of the method adopted. Methods. The templates rely on a very large set of Galactic and Magellanic Cloud Cepheids (FU, ~600; FO, ~200) with well-sampled NIR (IRSF data set) and optical (V, I; OGLE data set) light-curves. To properly trace the change in the shape of the light-curve as a function of pulsation period, we split the sample of calibrating Cepheids into ten different period bins. The templates for the first time cover FO Cepheids and the short-period range of FU Cepheids (P ≤ 5 days). Moreover, the phase zero-point is anchored to the phase of the mean magnitude along the rising branch. The new approach has several advantages in sampling the light-curve of bump Cepheids when compared with the canonical phase of maximum light. We also provide new empirical estimates of the NIR-to-optical amplitude ratios for FU and FO Cepheids. We perform detailed analytical fits using seventh-order Fourier series and multi-Gaussian periodic functions. The latter are characterized by fewer free parameters (nine vs. fifteen). Results. The mean NIR magnitudes based on the new templates are up to 80% more accurate than single-epoch NIR measurements and up to 50% more accurate than the mean magnitudes based on previous NIR templates, with typical associated uncertainties ranging from 0.015 mag (J band) to 0.019 mag (KS band). Moreover, we find that errors on individual distance estimates for Small Magellanic Cloud Cepheids derived from NIR PW relations are essentially reduced to the intrinsic scatter of the adopted relations. Conclusions. Thus, the new templates are the ultimate tool for estimating precise Cepheid distances from NIR single-epoch observations, which can be safely adopted for future interesting applications, including deriving the 3D structure of the Magellanic Clouds.

THE IMACS CLUSTER BUILDING SURVEY. V. FURTHER EVIDENCE FOR STARBURST RECYCLING FROM QUANTITATIVE GALAXY MORPHOLOGIES

Using J- and K s-band imaging obtained as part of the IMACS Cluster Building Survey (ICBS), we measure S?rsic indices for 2160 field and cluster galaxies at 0.31 < z < 0.54. Using both mass- and magnitude-limited samples, we compare the distributions for spectroscopically determined passive, continuously star-forming, starburst, and post-starburst systems and show that previously established spatial and statistical connections between these types extend to their gross morphologies. Outside of cluster cores, we find close structural ties between starburst and continuously star-forming, as well as post-starburst and passive types, but not between starbursts and post-starbursts. These results independently support two conclusions presented in Paper II of this series: (1) most starbursts are the product of a non-disruptive triggering mechanism that is insensitive to global environment, such as minor mergers; (2) starbursts and post-starbursts generally represent transient phases in the lives of normal star-forming and quiescent galaxies, respectively, originating from and returning to these systems in closed recycling loops. In this picture, spectroscopically identified post-starbursts constitute a minority of all recently terminated starbursts, largely ruling out the typical starburst as a quenching event in all but the densest environments.

First-generation instruments for the Magellan telescopes: characteristics, operation, and performance

The Magellan Telescopes are a collaboration between the Observatories of the Carnegie Institution of Washington (OCIW), University of Arizona, Harvard University, University of Michigan, and Massachusetts Institute of Technology (MIT) consisting of two 6.5 meter telescopes located at Las Campanas Observatory, in the Chilean Andes. The Walter Baade telescope achieved first light in September 2000 and the Landon Clay telescope started science operations in September 2002. In addition to two modified spectroscopic instruments, the Boller and Chivens Spectrograph and the Low Dispersion Survey Spectrograph (LDSS-2), four first generation instruments are now deployed at the Magellan Telescopes. Here we briefly describe the operations and performance of MagIC - a direct imaging CCD camera, MIKE - a double echelle spectrograph, PANIC - a near-IR imager, and IMACS - a multi-purpose, multi-object imaging spectrograph.