Background: Meiosis is a specialized germ cell cycle that generates haploid gametes. In the initial stage of meiosis, meiotic prophase I (MPI), homologous chromosomes pair and recombine. Extensive changes in chromatin in MPI raise an important question concerning the contribution of epigenetic mechanisms such as DNA methylation to meiosis. Interestingly, previous studies concluded that in male mice, genome-wide DNA methylation patters are set in place prior to meiosis and remain constant subsequently. However, no prior studies examined DNA methylation during MPI in a systematic manner necessitating its further investigation.

DNA methylation regulates the organization and function of the genome. Yamanaka et al. now report that de novo methylation of male germ cells of mice involves the transient opening of heterochromatin at megabase-size differentially accessible domains (DADs). This chromatin remodeling likely facilitates de novo methylation of the germ cell genome.

Female reproductive success critically depends on the size and quality of a finite ovarian reserve. Paradoxically, mammals eliminate up to 80% of the initial oocyte pool through the enigmatic process of fetal oocyte attrition (FOA). Here, we interrogate the striking correlation of FOA with retrotransposon LINE-1 (L1) expression in mice to understand how L1 activity influences FOA and its biological relevance. We report that L1 activity triggers FOA through DNA damage-driven apoptosis and the complement system of immunity. We demonstrate this by combined inhibition of L1 reverse transcriptase activity and the Chk2-dependent DNA damage checkpoint to prevent FOA. Remarkably, reverse transcriptase inhibitor AZT-treated Chk2 mutant oocytes that evade FOA initially accumulate, but subsequently resolve, L1-instigated genotoxic threats independent of piRNAs and differentiate, resulting in an increased functional ovarian reserve. We conclude that FOA serves as quality control for oocyte genome integrity, and is not obligatory for oogenesis nor fertility.

We study the mass distribution in the late-type dwarf galaxy NGC 2976 through stellar kinematics obtained with the Visible Integral Field ReplicableUnit Spectrograph Prototype and anisotropic Jeans models as a test of cosmological simulations and baryonic processes that putatively alter small-scale structure. Previous measurements of the Ha emission-line kinematics have determined that the dark matter halo of NGC 2976 is most consistent with a cored density profile. We find that the stellar kinematics are best fit with a cuspy halo. Cored dark matter halo fits are only consistent with the stellar kinematics if the stellar mass-to-light ratio is significantly larger than that derived from stellar population synthesis, while the best-fitting cuspy model has no such conflict. The inferred mass distribution from a harmonic decomposition of the gaseous kinematics is inconsistent with that of the stellar kinematics. This difference is likely due to the gas disk not meeting the assumptions that underlie the analysis such as no pressure support, a constant kinematic axis, and planar orbits. By relaxing some of these assumptions, in particular the form of the kinematic axis with radius, the gas-derived solution can be made consistent with the stellar kinematic models. A strong kinematic twist in the gas of NGC 2976' s center suggests caution, and we advance the mass model based on the stellar kinematics as more reliable. The analysis of this first galaxy shows promising evidence that dark matter halos in late-type dwarfs may in fact be more consistent with cuspy dark matter distributions than earlier work has claimed.

We present properties of individual and composite rest-UV spectra of continuum-and narrowband-selected star-forming galaxies (SFGs) at a redshift of 2 < z < 3.5 discovered by the MUSYC collaboration in the Extended Chandra Deep Field-South. Among our sample of 81 UV-bright SFGs, 59 have R < 25.5, of which 32 have rest-frame equivalent widths of W-Ly alpha > 20 angstrom, the canonical limit to be classified as an Ly alpha-emitting galaxy. We divide our data set into subsamples based on properties that we are able to measure for each individual galaxy: Ly alpha equivalent width, rest-frame UV colors, and redshift. Among our subsample of galaxies with R < 25.5, those with rest frame W-Ly alpha > 20 angstrom have bluer UV continua, weaker low-ionization interstellar absorption lines, weaker C IV absorption, and stronger Si II* nebular emission than those with W-Ly alpha < 20 angstrom. We measure a velocity offset of Delta nu similar to 600 km s(-1) between Ly alpha emission and low-ionization absorption, which does not vary substantially among any of our subsamples. We find that the interstellar component, as opposed to the stellar component, dominates the high-ionization absorption line profiles. We find that the low-and high-ionization Si ionization states have similar kinematic properties, yet the low-ionization absorption is correlated with Ly alpha emission and the high-ionization absorption is not. These trends are consistent with outflowing neutral gas being in the form of neutral clouds embedded in ionized gas as previously suggested by Steidel et al. Moreover, our galaxies with bluer UV colors have stronger Ly alpha emission, weaker low-ionization absorption, and more prominent nebular emission line profiles. From a redshift of 2.7 < z < 3.5 to 2.0 < z < 2.7, our subsample of galaxies with W-Ly alpha < 20 angstrom shows no significant evolution in their physical properties or the nature of their outflows. Among our data set, UV-bright galaxies with W-Ly alpha > 20 angstrom exhibit weaker Ly alpha emission at lower redshifts, although we caution that this could be caused by spectroscopic confirmation of low Ly alpha equivalent width galaxies being harder at z similar to 3 than z similar to 2.

We measure the radial profile of the (CO)-C-12(1-0) to H-2 conversion factor (X-CO) in NGC 628. The H alpha emission from the VENGA integral field spectroscopy is used to map the star formation rate (SFR) surface density (Sigma(SFR)). We estimate the molecular gas surface density (Sigma(H2)) from Sigma(SFR) by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure X-CO. We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (H alpha versus far-UVplus 24 mu m), and CO maps from different telescopes (single-dish and interferometers). The observed X-CO profile is robust against these systematics, drops by a factor of two from R similar to 7 kpc to the center of the galaxy, and is well fit by a gradient Delta log(X-CO) = 0.06 +/- 0.02 dex kpc(-1). We study how changes in X-CO follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in X-CO can be explained by a combination of decreasing metallicity, and decreasing Sigma(H2) with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of X-CO in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star-forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single X-CO value when large changes in gas surface density or metallicity are present.

We present the survey design, data reduction, and spectral fitting pipeline for the VIRUS-P Exploration of Nearby Galaxies (VENGA). VENGA is an integral field spectroscopic survey, which maps the disks of 30 nearby spiral galaxies. Targets span a wide range in Hubble type, star formation activity, morphology, and inclination. The VENGA data cubes have 5 ''.6 FWHM spatial resolution, similar to 5 angstrom FWHM spectral resolution, sample the 3600 angstrom-6800 angstrom range, and cover large areas typically sampling galaxies out to similar to 0.7R(25). These data cubes can be used to produce two-dimensional maps of the star formation rate, dust extinction, electron density, stellar population parameters, the kinematics and chemical abundances of both stars and ionized gas, and other physical quantities derived from the fitting of the stellar spectrum and the measurement of nebular emission lines. To exemplify our methods and the quality of the data, we present the VENGA data cube on the face-on Sc galaxy NGC 628 (a.k.a. M 74). The VENGA observations of NGC 628 are described, as well as the construction of the data cube, our spectral fitting method, and the fitting of the stellar and ionized gas velocity fields. We also propose a new method to measure the inclination of nearly face-on systems based on the matching of the stellar and gas rotation curves using asymmetric drift corrections. VENGA will measure relevant physical parameters across different environments within these galaxies, allowing a series of studies on star formation, structure assembly, stellar populations, chemical evolution, galactic feedback, nuclear activity, and the properties of the interstellar medium in massive disk galaxies.

We present 65 optical spectra of the Type Ia SN 2012fr, 33 of which were obtained before maximum light. At early times, SN 2012fr shows clear evidence of a high-velocity feature (HVF) in the Si lambda 6355 line that can be cleanly decoupled from the lower velocity "photospheric" component. This Si lambda 6355 HVF fades by phase - 5; subsequently, the photospheric component exhibits a very narrow velocity width and remains at a nearly constant velocity of similar to 12,000 km s(-1) until at least five weeks after maximum brightness. The Ca II infrared triplet exhibits similar evidence for both a photospheric component at v approximate to 12,000 km s(-1) with narrow line width and long velocity plateau, as well as an HVF beginning at v approximate to 31,000 km s(-1) two weeks before maximum. SN 2012fr resides on the border between the "shallow silicon" and "core-normal" subclasses in the Branch et al. classification scheme, and on the border between normal and high-velocity Type Ia supernovae (SNe Ia) in the Wang et al. system. Though it is a clear member of the "low velocity gradient" group of SNe Ia and exhibits a very slow light-curve decline, it shows key dissimilarities with the overluminous SN 1991T or SN 1999aa subclasses of SNe Ia. SN 2012fr represents a well-observed SN Ia at the luminous end of the normal SN Ia distribution and a key transitional event between nominal spectroscopic subclasses of SNe Ia.

The Visible Integral-field Replicable Unit Spectrograph (VIRUS) consists of a baseline build of 150 identical spectrographs (arrayed as 75 units, each with a pair of spectrographs) fed by 33,600 fibers, each 1.5 arcsec diameter, deployed over the 22 arcminute field of the upgraded 10 m Hobby-Eberly Telescope (HET). The goal is to deploy 82 units. VIRUS has a fixed bandpass of 350-550 nm and resolving power R similar to 700. VIRUS is the first example of industrial-scale replication applied to optical astronomy and is capable of spectral surveys of large areas of sky. This approach, in which a relatively simple, inexpensive, unit spectrograph is copied in large numbers, offers significant savings of engineering effort, cost, and schedule when compared to traditional instruments.

We search for evidence of diffuse Ly alpha emission from extended neutral hydrogen surrounding Ly alpha emitting galaxies (LAEs) using deep narrow-band images of the Extended Chandra Deep Field South. By stacking the profiles of 187 LAEs at z = 2.06, 241 LAEs at z = 3.10, and 179 LAEs at z = 3.12, and carefully performing low-surface brightness photometry, we obtain mean surface brightness maps that reach 9.9, 8.7, and 6.2 x 10(-19) erg cm(-2) s(-1) arcsec(-2) in the emission line. We undertake a thorough investigation of systematic uncertainties in our surface brightness measurements and find that our limits are 5-10 times larger than would be expected from Poisson background fluctuations; these uncertainties are often underestimated in the literature. At z similar to 3.1, we find evidence for extended halos with small-scale lengths of 5-8 kpc in some but not all of our sub-samples. We demonstrate that sub-samples of LAEs with low equivalent widths and brighter continuum magnitudes are more likely to possess such halos. At z similar to 2.1, we find no evidence of extended Ly alpha emission down to our detection limits. Through Monte-Carlo simulations, we also show that we would have detected large diffuse LAE halos if they were present in our data sets. We compare these findings to other measurements in the literature and discuss possible instrumental and astrophysical reasons for the discrepancies.