An Ultraviolet Study of Star-Forming Regions in M31

We present a comprehensive study of star-forming (SF) regions in the nearest large spiral galaxy M31. We use GALEX far-UV (1344-1786 \AA, FUV) and near-UV (1771-2831 \AA, NUV) imaging to detect young massive stars and trace the recent star formation across the galaxy. The FUV and NUV flux measurements of the SF regions, combined with ground-based data for estimating the reddening by interstellar dust from the massive stars they contain, are used to derive their ages and masses. The GALEX imaging, combining deep sensitivity and coverage of the entire galaxy, provides a complete picture of the recent star formation in M31 and its variation with environment throughout the galaxy. The FUV and NUV measurements are sensitive to detect stellar populations younger than a few hundred Myrs. We detected 894 SF regions, with size > 1600 pc^{2} above an average FUV flux limit of ~ 26 ABmag arcsecond^{-2}, over the whole 26 kpc galaxy disk. We derive the star-formation history of M31 within this time span. The star formation rate (SFR) from the youngest UV sources (age < 10 Myr) is comparable to that derived from H_{alpha}, as expected. We show the dependence of the results on the assumed metallicity. When star formation detected from IR measurements of the heated dust is added to the UV-measured star formation (from the unobscured populations) in the recent few Myrs, we find the SFR has slightly decreased in recent epochs, with a possible peak between 10 and 100 Myrs, and an average value of SFR ~ 0.6 or 0.7 M_{sun} yr^{-1} (for metallicity Z=0.02 or 0.05 respectively) over the last 400 Myrs.Comment: 39 pages, 14 figures, accepted for publication in ApJ, high resolution version available at this http://dolomiti.pha.jhu.edu/papers/2009_KangBianchiRey.pd

Probing the Intermediate-Age Globular Clusters in NGC 5128 from Ultraviolet Observations

We explore the age distribution of the globular cluster (GC) system of the nearby elliptical galaxy NGC 5128 using ultraviolet (UV) photometry from Galaxy Evolution Explorer (GALEX) observations, with UV - optical colors used as the age indicator. Most GCs in NGC 5128 follow the general trends of GCs in M31 and Milky Way in UV - optical color-color diagram, which indicates that the majority of GCs in NGC 5128 are old similar to the age range of old GCs in M31 and Milky Way. A large fraction of spectroscopically identified intermediate-age GC (IAGC) candidates with ~ 3-8 Gyr are not detected in the FUV passband. Considering the nature of intermediate-age populations being faint in the far-UV (FUV) passband, we suggest that many of the spectroscopically identified IAGCs may be truly intermediate in age. This is in contrast to the case of M31 where a large fraction of spectroscopically suggested IAGCs are detected in FUV and therefore may not be genuine IAGCs but rather older GCs with developed blue horizontal branch stars. Our UV photometry strengthens the results previously suggesting the presence of GC and stellar subpopulation with intermediate age in NGC 5128. The existence of IAGCs strongly indicates the occurrence of at least one more major star formation episode after a starburst at high redshift.Comment: 8 pages, 3 figures, accepted for ApJ Lette

NGC3801 caught in the act: A post-merger starforming early-type galaxy with AGN-jet feedback

In the current models of galaxy formation and evolution, AGN feedback is crucial to reproduce galaxy luminosity function, colour-magnitude relation and M-sigma relation. However, if AGN-feedback can indeed expel and heat up significant amount of cool molecular gas and consequently quench star formation, is yet to be demonstrated observationally. Only in four cases so far (Cen A, NGC 3801, NGC 6764 and Mrk 6), X-ray observations have found evidences of jet-driven shocks heating the ISM. We chose the least-explored galaxy, NGC 3801, and present the first ultraviolet imaging and stellar population analysisis of this galaxy from GALEX data. We find this merger-remnant early-type galaxy to have an intriguing spiral-wisp of young star forming regions (age ranging from 100--500 Myr). Taking clues from dust/PAH, HI and CO emission images we interpret NGC 3801 to have a kinamatically decoupled core or an extremely warped gas disk. From the HST data we also show evidence of ionised gas outflow similar to that observed in HI and molecular gas (CO) data, which may have caused the decline of star formation leading to the red optical colour of the galaxy. However, from these panchromatic data we interpret that the expanding shock shells from the young ($\sim$2.4 million years) radio jets are yet to reach the outer gaseous regions of the galaxy. It seems, we observe this galaxy at a rare stage of its evolutionary sequence where post-merger star formation has already declined and new powerful jet feedback is about to affect the gaseous star forming outer disk within the next 10 Myr, to further transform it into a red-and-dead early-type galaxy.Comment: 5 pages letter paper, 4 figures, 1 table, (submitted

Improving the electrochemical properties of graphite/LiCoO2 cells in ionic liquid-containing electrolytes

The electrochemical performance of graphite/lithium cobalt oxide (LiCoO2) cells in N-methoxymethyl-N,N-dimethylethylammonium bis(trifluoromethane-sulfonyl) imide (MMDMEA-TFSI)-containing electrolytes is significantly enhanced by the formation of a fluoroethylene carbonate (FEC)-derived protective film on an anode during the first cycle. The electrochemical intercalation of MMDMEA cations into the graphene layer is readily visualized by ex situ transmission electron microscopy (TEM). Moreover, differences in the X-ray diffraction (XRD) patterns of graphite electrodes in cells charged with and without FEC in dimethyl carbonate (DMC)/MMDMEA-TFSI are clearly discernible. Conclusively, the presence of FEC in MMDMEA-TFSI-containing electrolytes leads to a remarkable enhancement of discharge capacity retention for graphite/LiCoO2 cells as compared with ethylene carbonate (EC) and vinylene carbonate (VC).close222

Activating layered LiNi0.5Co0.2Mn0.3O2 as a host for Mg intercalation in rechargeable Mg batteries

Layered crystal structures are some of the most intensively studied intercalation hosts for guest ion insertion. For Mg insertion, layered transition metal sulfides or selenides have been used for reversible Mg intercalation; however, far less intercalation hosts have been found for layered oxides most likely because of the strong interaction between Mg2+ and the oxide host. Here, we demonstrate that layered LixNi0.5Co0.2Mn0.3O2 (NCM523), which is an important commercial electrode for Li-ion batteries but has been regarded as electrochemically inactive in rechargeable Mg batteries, can function as a reversible host for Mg2+ if water opens up the layers and screens the electrostatic interaction between Mg2+ and the host. Upon the formation of water-intercalated NCM523, the discharge capacity dramatically increases utilizing the multi-redox reaction of Ni2+/Ni3+/Ni4+, which exhibits an average voltage of similar to 3.1 V (vs. Mg/Mg2+) in rechargeable Mg batteries with an energy density of 589 Wh kg(-1) in the first discharge. (C) 2017 Elsevier Ltd. All rights reserved

Lithium-excess olivine electrode for lithium rechargeable batteries

Lithium iron phosphate (LFP) has attracted tremendous attention as an electrode material for next-generation lithium-rechargeable battery systems due to the use of low-cost iron and its electrochemical stability. While the lithium diffusion in LFP, the essential property in battery operation, is relatively fast due to the one-dimensional tunnel present in the olivine crystal, the tunnel is inherently vulnerable to the presence of Fe-Li anti-site defects (Fe ions in Li ion sites), if any, that block the lithium diffusion and lead to inferior performance. Herein, we demonstrate that the kinetic issue arising from the Fe-Li defects in LFP can be completely eliminated in lithium-excess olivine LFP. The presence of an excess amount of lithium in the Fe ion sites (Li-Fe) energetically destabilizes the Fe-Li-related defects, resulting in reducing the amount of Fe defects in the tunnel. Moreover, we observe that the spinodal decomposition barrier is notably reduced in lithium-excess olivine LFP. The presence of Li-Fe and the absence of Fe-Li in lithium-excess olivine LFP additionally induce faster kinetics, resulting in an enhanced rate capability and a significantly reduced memory effect. The lithium-excess concept in the electrode crystal brings up unexpected properties for the pristine crystal and offers a novel and interesting approach to enhance the diffusivity and open up additional diffusion paths in solid-state ionic conductors.