The Crab Nebula\u27s Dynamical Age as Measured from its Northern Filamentary Jet

We present a deep [O III] 4959,5007 image of the northern filamentary jet in the Crab Nebula taken with the 8.2m Subaru telescope. Using this image and an image taken with the KPNO 4m in 1988 (Fesen & Staker 1993), we have computed proper motions for 35 locations in the jet. The results suggest that when compared to the main body of the remnant, the jet experienced less outward acceleration from the central pulsar\u27s rapidly expanding synchrotron nebula. The jet\u27s apparent expansion rate yields an undecelerated explosion date for the Crab Nebula of 1055 plus or minus 24 C.E., a date much closer to the appearance of the historic 1054 C.E. guest star than the 1120 - 1140 C.E. dates estimated in previous studies using filaments located within the remnant\u27s main nebula. Our proper motion measurements suggest the jet likely formed during the 1054 supernova explosion and represents the remnant\u27s highest velocity knots possibly associated with a suspected N-S bipolar outflow from the supernova explosion

Chandra View of DA 530: A Sub-Energetic Supernova Remnant with a Pulsar Wind Nebula?

Based on a Chandra ACIS observation, we report the detection of an extended X-ray feature close to the center of the remnant DA 530 with 5.3 sigma above the background within a circle of 20'' radius. This feature, characterized by a power-law with the photon index gamma=1.6+-0.8 and spatially coinciding with a nonthermal radiosource, most likely represents a pulsar wind nebula. We have further examined the spectrum of the diffuse X-ray emission from the remnant interior with a background-subtracted count rate of ~0.06 counts s^-1 in 0.3-3.5 keV. The spectrum of the emission can be described by a thermal plasma with a temperature of ~0.3-0.6 keV and a Si over-abundance of >~7 solar. These spectral characteristics, together with the extremely low X-ray luminosity, suggest that the remnant arises from a supernova with an anomalously low mechanical energy (<10^50 ergs). The centrally-filled thermal X-ray emission of the remnant may indicate an early thermalization of the SN ejecta by the circum-stellar medium. Our results suggest that the remnant is likely the product of a core-collapsed SN with a progenitor mass of 8-12 Msun. Similar remnants are probably common in the Galaxy, but have rarely been studied.Comment: 23 pages, 7 figures, accepted for publication in ApJ; complete the abstract on astro-ph and correct some typo

The Keck Lyman Continuum Spectroscopic Survey (KLCS): The Emergent Ionizing Spectrum of Galaxies at z∼3z\sim3

We present results of a deep spectroscopic survey designed to quantify the statistics of the escape of ionizing photons from star-forming galaxies at z~3. We measure the ratio of ionizing to non-ionizing UV flux density _obs, where f900 is the mean flux density evaluated over the range [880,910] A. We quantify the emergent ratio of ionizing to non-ionizing UV flux density by analyzing high-S/N composite spectra formed from sub-samples with common observed properties and numbers sufficient to reduce the statistical uncertainty in the modeled IGM+CGM correction to obtain precise values of _out, including a full-sample average _out=$0.057\pm0.006$. We further show that _out increases monotonically with Ly$\alpha$ rest equivalent width, inducing an inverse correlation with UV luminosity as a by-product. We fit the composite spectra using stellar spectral synthesis together with models of the ISM in which a fraction f_c of the stellar continuum is covered by gas with column density N(HI). We show that the composite spectra simultaneously constrain the intrinsic properties of the stars (L900/L1500)_int along with f_c, N(HI), E(B-V), and $f_{esc,abs}$, the absolute escape fraction of ionizing photons. We find a sample-averaged $f_{esc,abs} =0.09\pm0.01$, and that subsamples fall along a linear relation $\langle f_{esc,abs}\rangle \sim 0.75[W(Ly\alpha)/110 A]$. We use the FUV luminosity function, the distribution function $n[W(Ly\alpha)]$, and the relationship between $W(Ly\alpha)$ and _out to estimate the total ionizing emissivity of $z\sim3$ star-forming galaxies with Muv < -19.5: $\epsilon_{LyC}\sim 6\times10^{24}$ ergs/s/Hz/Mpc$^3$, exceeding the contribution of QSOs by a factor of $\sim 3$, and accounting for $\sim50$% of the total $\epsilon_{LyC}$ at $z\sim3$ estimated using indirect methods.Comment: 45 pages, 31 figures, ApJ, in pres

New records of fishes from the Maldive Islands, with notes on other species

Part 1: We report here information on the occurrence of the deep demersal fish species known to date from the Maldivian Exclusive Economic Zone below a depth of 180 m. Collections of Maldivian deep demersal fishes are held by The Natural History Museum, London (BMNH); the Bernice P. Bishop Museum, Honolulu; the Field Museum of Natural History, Chicago; the Marine Research Section, Ministry of Fisheries and Agriculture, Male, Republic of Maldives; the South African Museum, Cape Town; and the Zoological Survey of India, at the Indian Museum, Calcutta. Specimens from all of these institutions have been studied by the authors. In addition, the authors carried out sampling of the slope shark fishery during March - April 1996, which resulted in a significant new collection of shark material. A total of 99 deep demersal species are reported here which includes 36 new records for the Maldives. The six most speciose families are the Macrouridae (7 species), Congridae (5), Lutjanidae (5), Squalidae (4), Ogocephalidae (4) and Halosauridae (4).Part 2: Seventy-eight fish species are recorded from the Maldives for the first time. A further 30, which have been recorded in the literature but not included in previous reviews of Maldivian fishes, are listed. The total known shore and epipelagic fish fauna of the Maldives now stands at 1007 species. The total known demersal and epipelagic fish fauna is raised to 1090.Rhodes University Libraries (Digitisation

Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE

We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0< z < 2.6, star-formation rates 2 < SFR < 200 M_sun/yr, and stellar masses 8.6 < log(M*/M_sun) < 11.4, with high-quality spectra in both H- and K-band atmospheric windows. We show unambiguously that the locus of z~2.3 galaxies in the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight, relationship between the ratios [NII]6585/Halpha and [OIII]/Hbeta as compared to local galaxies. Using photoionization models, we argue that the offset of the z~2.3 locus relative to z~ 0 is explained by a combination of harder ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H than applies to most local galaxies, and that the position of a galaxy along the z~2.3 star-forming BPT locus is surprisingly insensitive to gas-phase oxygen abundance. The observed nebular emission line ratios are most easily reproduced by models in which the net ionizing radiation field resembles a blackbody with effective temperature T_eff = 50000-60000 K and N/O close to the solar value at all O/H. We critically assess the applicability of commonly-used strong line indices for estimating gas-phase metallicities, and consider the implications of the small intrinsic scatter in the empirical relationship between excitation-sensitive line indices and stellar mass (i.e., the "mass-metallicity" relation), at z~2.3.Comment: 41 pages, 25 figures, accepted for publication in the Astrophysical Journal. Version with full-resolution figures available at http://www.astro.caltech.edu/~ccs/mos_bpt_submit.pd

Pengalaman tahun pertama pelajar antarabangsa Universiti Malaysia Sabah

Tahun pertama di universiti adalah tempoh masa kritikal dalam kehidupan pelajar. Pelajar tidak hanya tertumpu dari segi intelek, malahan mereka juga turut mewujudkan dan mengekalkan hubungan peribadi, identiti, membuat keputusan mengenai kerjaya dan gaya hidup, kesihatan dan kesejahteraan diri, serta pembangunan terhadap falsafah kehidupan yang bersepadu. Institusi Pendidikan Tinggi di Malaysia berperanan sebagai medium dalam mewujudkan dan menyediakan persekitaran yang dapat membantu pelajar-pelajar antarabangsa merasai pengalaman tahun pertama yang bermakna di universiti tempatan.Kajian ini bertujuan untuk mendapatkan maklum balas daripada pelajar antarabangsa di Universiti Malaysia Sabah (UMS) pengalaman tahun pertama mereka.Kajian ini memberikan pandangan yang berharga terhadap penyesuian pelajar antarabangsa mengenai pelarasan bukan sahaja dalam sistem pendidikan yang baharu tetapi juga terhadap faktor- faktor seperti budaya dan psikososial. Kajian ini membincangkan beberapa faktor yang memberi kesan kepada pelajar antarabangsa pada tahun pertama mereka di universiti dalam usaha untuk menjadikan pengalaman yang bermakna serta meningkatkan pencapaian akademik mereka. Melalui kaedah kualitatif menerusi kajian kes ke atas 20 orang pelajar antarabangsa di Universiti Malaysia Sabah yang terlibat secara sukarela untuk ditemubual dengan menggunakan kaedah temubual semi struktur, hasil kajian diperolehi. Hasil dapatan penyelidikan mendapati bahawa aspek penyesuaian pelarasan persekitaran, faktor psikososial dan budaya, bahasa, akademik dan juga masalah sosial adalah aspek utama yang dihadapi oleh pelajar antarabangsa. Perbincangan dalam kertas kerja ini turut menawarkan dengan beberapa cadangan berhubung Institusi Pengajian Tinggi Awam di Malaysia dalam mewujudkan pengalaman yang bermakna bagi pelajar tahun pertama pelajar antarabangsa

Imaging Spectroscopy of Ionized Gaseous Nebulae around Optically Faint AGNs at Redshift z ∼ 2

We present Keck/OSIRIS laser guide-star assisted adaptive optics (LGSAO) integral-field spectroscopy of [O III] λ5007 nebular emission from 12 galaxies hosting optically faint (R = 20–25; ν L_ν ~ 10^(44) – 10^(46) erg s^(−1)) active galactic nuclei (AGNs) at redshift z ~ 2–3. In combination with deep Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) rest-frame optical imaging, Keck/MOSFIRE rest-optical spectroscopy, and Keck/KCWI rest-UV integral-field spectroscopy, we demonstrate that both the continuum and emission-line structures of these sources exhibit a wide range of morphologies, from compact, isolated point sources to double-AGN merging systems with extensive ~50 kpc tidal tails. One of the 12 galaxies previously known to exhibit a proximate damped Lyα system coincident in redshift with the galaxy shows evidence for both an extended [O III] narrow-line emission region and spatially offset Lyα emission (with morphologically distinct blueshifted and redshifted components) indicative of large-scale gas flows photoionized by the central AGN. We do not find widespread evidence of star formation in the host galaxies surrounding these AGNs; the [O III] velocity dispersions tend to be high (σ = 100–500 km s^(-1)), the continuum morphologies are much more compact than a mass-matched star-forming comparison sample, and the diagnostic nebular emission-line ratios are dominated by an AGN-like ionizing spectrum. The sample is most consistent with a population of AGNs that radiate at approximately their Eddington limit and photoionize extended [O III] nebulae whose characteristic sizes scale approximately as the square root of the AGN luminosity