Characterization of the sodium layer at Cerro Pachon, and impact on laser guide star performance

Detailed knowledge of the mesospheric sodium layer characteristics is crucial to estimate and optimize the performance of laser guide star (LGS) assisted adaptive optics (AO) systems. In this paper, we present an analysis of two sets of data on the mesospheric sodium layer. The first set comes from a laser experiment that was carried out at Cerro Tololo to monitor the abundance and altitude of the mesospheric sodium in 2001, during six runs covering a period of one year. These data are used to derive the mesospheric sodium column density, the sodium layer thickness and the temporal behaviour of the sodium layer mean altitude. The second set of data was gathered during the first year of the Gemini Multi-Conjugate Adaptive Optics (MCAO) System (GeMS) commissioning and operations. GeMS uses five LGSs to measure and compensate for atmospheric distortions. Analysis of the LGS wavefront sensor (WFS) data provides information about the sodium photon return and the spot elongation seen by the WFS. All these parameters show large variations on a yearly, nightly and hourly basis, affecting the LGS brightness, shape and mean altitude. The sodium photon return varies by a factor of 3-4 over a year, and can change by a factor of 2 over a night. In addition, the comparison of the photon returns obtained in 2001 with those measured a decade later using GeMS shows a significant difference in laser format efficiencies. We find that the temporal power spectrum of the sodium mean altitude follows a linear trend, in good agreement with the results reported by Pfrommer & Hickson

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. II. NGC 3256 Clusters

We present Gemini optical spectroscopy of 23 young star clusters in NGC3256. We find that the cluster ages range are from few Myr to ~150 Myr. All these clusters are relatively massive (2--40)x 10^{5} \msun$ and appear to be of roughly 1.5 \zo metallicity. The majority of the clusters in our sample follow the same rotation curve as the gas and hence were presumably formed in the molecular-gas disk. However, a western subsample of five clusters has velocities that deviate significantly from the gas rotation curve. These clusters may either belong to the second spiral galaxy of the merger or may have formed in tidal-tail gas falling back into the system. We discuss our observations in light of other known cluster populations in merging galaxies, and suggest that NGC 3256 is similar to Arp 220, and hence may become an Ultra-luminous Infrared Galaxy as the merger progresses and the star-formation rate increases. Some of the clusters which appeared as isolated in our ground-based images are clearly resolved into multiple sub-components in the HST-ACS images. The same effect has been observed in the Antennae galaxies, showing that clusters are often not formed in isolation, but instead tend to form in larger groups or cluster complexes.Comment: 20 pages, 10 figures, 3 tables; Accepted Ap

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. IV. Stephan's Quintet

We present a spectroscopic survey of 21 young massive clusters and complexes and one tidal dwarf galaxy candidate (TDG) in Stephan's Quintet, an interacting compact group of galaxies. All of the selected targets lie outside the main galaxies of the system and are associated with tidal debris. We find clusters with ages between a few and 125 Myr and confirm the ages estimated through HST photometry by Fedotov et al. (2011), as well as their modelled interaction history of the Quintet. Many of the clusters are found to be relatively long-lived, given their spectrosopically derived ages, while their high masses suggest that they will likely evolve to eventually become intergalactic clusters. One cluster, T118, is particularly interesting, given its age (\sim 125 Myr), high mass (\sim 2\times10^6 M\odot) and position in the extreme outer end of the young tidal tail. This cluster appears to be quite extended (Reff \sim 12 - 15 pc) compared to clusters observed in galaxy disks (Reff \sim 3 - 4 pc), which confirms an effect we previously found in the tidal tails of NGC 3256, where clusters are similarly extended. We find that star and cluster formation can proceed at a continuous pace for at least \sim 150 Myr within the tidal debris of interacting galaxies. The spectrum of the TDG candidate is dominated by a young population (\sim 7 Myr), and assuming a single age for the entire region, has a mass of at least 10^6 M\odot.Comment: 37 pages, 10 Figures, 7 Tabl

NICI: combining coronagraphy, ADI, and SDI

The Near-Infrared Coronagraphic Imager (NICI) is a high-contrast AO imager at the Gemini South telescope. The camera includes a coronagraphic mask and dual channel imaging for Spectral Differential Imaging (SDI). The instrument can also be used in a fixed Cassegrain Rotator mode for Angular Differential Imaging (ADI). While coronagraphy, SDI, and ADI have been applied before in direct imaging searches for exoplanets. NICI represents the first time that these 3 techniques can be combined. We present preliminary NICI commissioning data using these techniques and show that combining SDI and ADI results in significant gains.Comment: Proc. SPIE, Vol. 7014, 70141Z (2008

A Gemini/GMOS Study of Intermediate Luminosity Early-Type Virgo Cluster Galaxies. I. Globular Cluster and Stellar Kinematics

We present a kinematic analysis of the globular cluster systems and diffuse stellar light of four intermediate luminosity (sub-$L^{\ast}$) early-type galaxies in the Virgo cluster based on Gemini/GMOS data. Our galaxy sample is fainter ($-23.8<M_K<-22.7$) than most previous studies, nearly doubling the number of galaxies in this magnitude range that now have GC kinematics. The data for the diffuse light extends to $4R_e$, and the data for the globular clusters reaches 8--$12R_e$. We find that the kinematics in these outer regions are all different despite the fact that these four galaxies have similar photometric properties, and are uniformly classified as "fast rotators" from their stellar kinematics within $1R_e$. The globular cluster systems exhibit a wide range of kinematic morphology. The rotation axis and amplitude can change between the inner and outer regions, including a case of counter-rotation. This difference shows the importance of wide-field kinematic studies, and shows that stellar and GC kinematics can change significantly as one moves beyond the inner regions of galaxies. Moreover, the kinematics of the globular cluster systems can differ from that of the stars, suggesting that the formation of the two populations are also distinct.Comment: 24 pages, 21 figures, 9 table, ApJ in pres

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies. III. The Antennae

We present optical spectroscopy of 16 star clusters in the merging galaxies NGC 4038/39 ("The Antennae") and supplement this dataset with HST imaging. The age and metallicity of each cluster is derived through a comparison between the observed Balmer and metal line strengths with simple stellar population models. We then estimate extinctions and masses using the photometry. We find that all but three clusters have ages between ~3-200 Myr, consistent with the expected increase in the star-formation rate due to the merger. Most of the clusters have velocities in agreement with nearby molecular and HI gas that has been previously shown to be rotating within the progenitor galaxies, hence star/cluster formation is still taking place within the galactic disks. However, three clusters have radial velocities that are inconsistent with being part of the rotating gas disks, which is surprising given their young (200-500Myr) ages. Interestingly, we find a stellar association with the same colors (V-I) near one of these three clusters, suggesting that the cluster and association were formed concurrently and have remained spatially correlated. We find evidence for spatially distributed cluster formation throughout the duration of the merger. The impact of various assumptions about the star/cluster formation rate on the interpretation of the cluster age distribution are explored, and we do not find evidence for long term "infant mortality" as has been previously suggested. Models of galaxy mergers that include a prescription for star formation can provide an overall good fit to the observed cluster age distribution.Comment: 15 pages, 11 figures, ApJ in pres

NFIRAOS adaptive optics for the Thirty Meter Telescope

NFIRAOS (Narrow-Field InfraRed Adaptive Optics System) will be the first-light multi-conjugate adaptive optics system for the Thirty Meter Telescope (TMT). NFIRAOS houses all of its opto-mechanical sub-systems within an optics enclosure cooled to precisely -30°C in order to improve sensitivity in the near-infrared. It supports up to three client science instruments, including the first-light InfraRed Imaging Spectrograph (IRIS). Powering NFIRAOS is a Real Time Controller that will process the signals from six laser wavefront sensors, one natural guide star pyramid WFS, up to three low-order on-instrument WFS and up to four guide windows on the client instrument’s science detector in order to correct for atmospheric turbulence, windshake, optical errors and plate-scale distortion. NFIRAOS is currently preparing for its final design review in late June 2018 at NRC Herzberg in Victoria, British Columbia in partnership with Canadian industry and TMT