Epitaxial Growth of Pentacene on Alkali Halide Surfaces Studied by Kelvin Probe Force Microscopy

In the field of molecular electronics thin films of molecules adsorbed on insulating surfaces are used as the functional building blocks of electronic devices. A control of the structural and electronic properties of the thin films is required for a reliable operating mode of such devices. Here, noncontact atomic force and Kelvin probe force microscopies have been used to investigate the growth and electronic properties of pentacene on KBr(001) and KCl(001) surfaces. Mainly molecular islands of upright standing pentacene are formed, whereas a new phase of tilted molecules appear near step edges on some KBr samples. Local contact potential differences (LCPD) have been studied with both Kelvin experiments and density-functional theory calculations. Large LCPD are found between the substrate and the differently oriented molecules, which may be explained by a partial charge transfer from the pentacene to the surface. The monitoring of the changes of the pentacene islands during dewetting shows that multilayers build up at the expense of monolayers. Moreover, in the Kelvin images, previously unknown line defects appear, which unveil the epitaxial growth of pentacene crystals.Comment: This document is the unedited author's version of a Submitted Work that was subsequently accepted for publication in ACSNano, copyright American Chemical Society after peer review. To access the final edited and published work see doi belo

Core handling and processing for the WAIS Divide ice-core project

On 1 December 2011 the West Antarctic Ice Sheet (WAIS) Divide ice-core project reached its final depth of 3405 m. The WAIS Divide ice core is not only the longest US ice core to date, but is also the highest-quality deep ice core, including ice from the brittle ice zone, that the US has ever recovered. The methods used at WAIS Divide to handle and log the drilled ice, the procedures used to safely retrograde the ice back to the US National Ice Core Laboratory (NICL) and the methods used to process and sample the ice at the NICL are described and discussed

Core handling and processing for the WAIS Divide ice-core project

On 1 December 2011 the West Antarctic Ice Sheet (WAIS) Divide ice-core project reached its final depth of 3405 m. The WAIS Divide ice core is not only the longest US ice core to date, but is also the highest-quality deep ice core, including ice from the brittle ice zone, that the US has ever recovered. The methods used at WAIS Divide to handle and log the drilled ice, the procedures used to safely retrograde the ice back to the US National Ice Core Laboratory (NICL) and the methods used to process and sample the ice at the NICL are described and discussed

Lyman alpha emitting galaxies at 0.2 < z < 0.35 from GALEX spectroscopy

The GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of about 8 A in the FUV (1350 - 1750 A) and about 20 A in the NUV (1950 - 2750 A) is used for a systematic search of Ly-a emitting galaxies at low redshift. This aims at filling a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in 5 deep exposures (5.65 sq.deg) has resulted in 96 Ly-a emitting galaxy candidates in the FUV domain, after accounting for broad-line AGNs. The Ly-a EWs (equivalent width) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, except a possible decrease in the most luminous that may be due to small-number statistics. Their distribution in EW is similar to that at z about 3 but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a sub-sample of 66 objects in the range 0.2 < z < 0.35 has been used to build a Ly-a LF (luminosity function). The incompleteness due to objects with significant Ly-a emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with H-a LF in the same redshift domain is consistent with an average Ly-a/H-a of about 1 in about 15 % of the star-forming galaxies. A comparison with high-redshift Ly-a LFs implies an increase of the Ly-a luminosity density by a factor of about 16 from z about 0.3 to z about 3. By comparison with the factor 5 increase of the UV luminosity density in the same redshift range, this suggests an increase of the average Ly-a escape fraction with redshift.Comment: 18 pages, 9 figures, accepted for publication in Ap

The UV-Optical Galaxy Color-Magnitude Diagram. I. Basic Properties

We have analyzed the bivariate distribution of galaxies as a function of ultraviolet-optical colors and absolute magnitudes in the local universe. The sample consists of galaxies with redshifts and optical photometry from the Sloan Digital Sky Survey (SDSS) main galaxy sample matched with detections in the near-ultraviolet (NUV) and far-ultraviolet (FUV) bands in the Medium Imaging Survey being carried out by the Galaxy Evolution Explorer (GALEX) satellite. In the (NUV − r)_(0.1) versus M_(r,0.1) galaxy color-magnitude diagram, the galaxies separate into two well-defined blue and red sequences. The (NUV − r)_(0.1) color distribution at each M_(r,0.1) is not well fit by the sum of two Gaussians due to an excess of galaxies in between the two sequences. The peaks of both sequences become redder with increasing luminosity, with a distinct blue peak visible up to M_(r,0.1) ~ − 23. The r_(0.1)-band luminosity functions vary systematically with color, with the faint-end slope and characteristic luminosity gradually increasing with color. After correcting for attenuation due to dust, we find that approximately one-quarter of the color variation along the blue sequence is due to dust, with the remainder due to star formation history and metallicity. Finally, we present the distribution of galaxies as a function of specific star formation rate and stellar mass. The specific star formation rates imply that galaxies along the blue sequence progress from low-mass galaxies with star formation rates that increase somewhat with time to more massive galaxies with a more or less constant star formation rate. Above a stellar mass of ~10^(10.5) M_☉, galaxies with low ratios of current to past averaged star formation rate begin to dominate

The Calibration and Data Products of the Galaxy Evolution Explorer

We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases have identical pipeline calibrations that are significantly improved over the GR1 data release. GALEX continues to survey the sky in the Far Ultraviolet (FUV, ~154 nm) and Near Ultraviolet (NUV, ~232 nm) bands, providing simultaneous imaging with a pair of photon counting, microchannel plate, delay line readout detectors. These 1.25 degree field-of-view detectors are well-suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 mAB in the FUV and NUV, respectively. We have detected a long term drift of order 1% FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" RMS in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.Comment: Accepted to the ApJS (a special GALEX issue

The Ultraviolet Imaging Telescope: Instrument and Data Characteristics

The Ultraviolet Imaging Telescope (UIT) was flown as part of the Astro observatory on the Space Shuttle Columbia in December 1990 and again on the Space Shuttle Endeavor in March 1995. Ultraviolet (1200-3300 Angstroms) images of a variety of astronomical objects, with a 40 arcmin field of view and a resolution of about 3 arcsec, were recorded on photographic film. The data recorded during the first flight are available to the astronomical community through the National Space Science Data Center (NSSDC); the data recorded during the second flight will soon be available as well. This paper discusses in detail the design, operation, data reduction, and calibration of UIT, providing the user of the data with information for understanding and using the data. It also provides guidelines for analyzing other astronomical imagery made with image intensifiers and photographic film.Comment: 44 pages, LaTeX, AAS preprint style and EPSF macros, accepted by PAS

Ly alpha-emitting galaxies at 0.2 < z < 0.35 from GALEX spectroscopy

We have used the GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of similar to 8 angstrom in the far-ultraviolet (FUV; 1350-1750 angstrom) and similar to 20 angstrom in the near-ultraviolet (NUV; 1950-2750 angstrom) for a systematic search of Ly alpha-emitting galaxies at low redshift. Our aim is to fill a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in five deep exposures (5.65 deg(2)) has resulted in 96 Ly alpha-emitting galaxy candidates in the FUV domain after accounting for broad-line AGNs. The Ly alpha equivalent widths (EWs) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, with the exception of a possible decrease in the most luminous objects that may be due to small-number statistics. The objects' distribution in EW is similar to that at z similar to 3, but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a subsample of 66 objects in the range 0.2 < z < 0.35 has been used to build a Ly alpha luminosity function (LF). The incompleteness due to objects with significant Ly alpha emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with H alpha LFs in the same redshift domain is consistent with an average Ly alpha/H alpha of similar to 1 in about 15% of the star-forming galaxies. A comparison with high-redshift Ly alpha LFs implies an increase of the Ly alpha luminosity density by a factor of about 16 from z similar to 0.3 to z similar to 3. By comparison with the factor of 5 increase in the UV luminosity density in the same redshift range, this suggests an increase of the average Ly alpha escape fraction with redshift

The Young and the Dustless: Interpreting Radio Observations of UltraViolet Luminous Galaxies

Ultraviolet Luminous Galaxies (UVLGs) have been identified as intensely star-forming, nearby galaxies. A subset of these, the supercompact UVLGs, are believed to be local analogs of high redshift Lyman Break Galaxies. Here we investigate the radio continuum properties of this important population for the first time. We have observed 42 supercompact UVLGs with the VLA, all of which have extensive coverage in the UV/optical by GALEX and SDSS. Our analysis includes comparison samples of multiwavelength data from the Spitzer First Look Survey and from the SDSS-Galex matched catalogs. In addition we have Spitzer MIPS data for 24 of our galaxies and find that they fall on the radio-FIR correlation of normal star-forming galaxies. We find that our galaxies have lower radio-to-UV ratios and lower Balmer decrements than other local galaxies with similar (high) star formation rates. Optical spectra show they have lower Dn(4000) and HdeltaA indices, higher Hbeta emission-line equivalents widths, and higher [OIII]5007/Hbeta emission-line ratios than normal star forming galaxies. Comparing these results to galaxy spectral evolution models we conclude that supercompact UVLGs are distinguished from normal star forming galaxies firstly by their high specific star formation rates. Moreover, compared to other types of galaxies with similar star formation rates, they have significantly less dust attenuation. In both regards they are similar to Lyman Break Galaxies. This suggests that the process that causes star formation in the supercompact UVLGs differs from other local star forming galaxies, but may be similar to Lyman Break Galaxies.Comment: 37 pages, 13 figures, accepted and scheduled to appear in ApJS December 2007 (GALEX Special Issue