Distance from a fishing community explains fish abundance in a no-take zone with weak compliance

There are numerous examples of no-take marine reserves effectively conserving fish stocks within their boundaries. However, no-take reserves can be rendered ineffective and turned into ‘paper parks’ through poor compliance and weak enforcement of reserve regulations. Long-term monitoring is thus essential to assess the effectiveness of marine reserves in meeting conservation and management objectives. This study documents the present state of the 15-year old no-take zone (NTZ) of South El Ghargana within the Nabq Managed Resource Protected Area, South Sinai, Egyptian Red Sea. Previous studies credited willing compliance by the local fishing community for the increased abundances of targeted fish within the designated NTZ boundaries compared to adjacent fished or take-zones. We compared benthic habitat and fish abundance within the NTZ and the adjacent take sites open to fishing, but found no significant effect of the reserve. Instead, the strongest evidence was for a simple negative relationship between fishing pressure and distance from the closest fishing village. The abundance of targeted piscivorous fish increased significantly with increasing distance from the village, while herbivorous fish showed the opposite trend. This gradient was supported by a corresponding negative correlation between the amount of discarded fishing gear observed on the reef and increasing distance from the village. Discarded fishing gear within the NTZ suggested decreased compliance with the no-take regulations. Our findings indicate that due to non-compliance the no-take reserve is no longer functioning effectively, despite its apparent initial successes and instead a gradient of fishing pressure exists with distance from the nearest fishing community

Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey

We report preliminary results of a deep near-infrared search for methane-absorbing brown dwarfs; almost five years after the discovery of Gl 229b, there are only a few confirmed examples of this type of object. New J band, wide-field images, combined with pre-existing R band observations, allow efficient identification of candidates by their extreme (R-J) colours. Follow-up measurements with custom filters can then confirm objects with methane absorption. To date, we have surveyed a total of 11.4 square degrees to J~20.5 and R~25. Follow-up CH_4 filter observations of promising candidates in 1/4 of these fields have turned up no methane absorbing brown dwarfs. With 90% confidence, this implies that the space density of objects similar to Gl 229b is less than 0.012 per cubic parsec. These calculations account for the vertical structure of the Galaxy, which can be important for sensitive measurements. Combining published theoretical atmospheric models with our observations sets an upper limit of alpha <= 0.8 for the exponent of the initial mass function power law in this domain.Comment: 11 pages + 2 figures To be published in Astrophysical Journal Letter

A Young Stellar Cluster in the Nucleus of NGC 4449

We have obtained 1-2 A resolution optical Echellette spectra of the nuclear star cluster in the nearby starburst galaxy NGC 4449. The light is clearly dominated by a very young (6-10 Myr) population of stars. For our age dating, we have used recent population synthesis models to interpret the observed equivalent width of stellar absorption features such as the HI Balmer series and the CaII triplet around 8500 A. We also compare the observed spectrum of the nuclear cluster to synthesized spectra of simple stellar populations of varying ages. All these approaches yield a consistent cluster age. Metallicity estimates based on the relative intensities of various ionization lines yield no evidence for significant enrichment in the center of this low mass galaxy: the metallicity of the nuclear cluster is about one fourth of the solar value, in agreement with independent estimates for the disk material of NGC 4449.Comment: 24 pages (incl. 7 figures), accepted by AJ, March 2001 issue revised version with minor changes and additions, one additional figur

The Inside-out Growth of the Galactic Disk

© 2019. The American Astronomical Society. All rights reserved.. We quantify the inside-out growth of the Milky Way's low-α stellar disk, modeling the ages, metallicities, and Galactocentric radii of APOGEE red clump stars with 6 kpc < R < 13 kpc. The current stellar distribution differs significantly from that expected from the star formation history due to the redistribution of stars through radial orbit mixing. We propose and fit a global model for the Milky Way disk, specified by an inside-out star formation history, radial orbit mixing, and an empirical, parametric model for its chemical evolution. We account for the spatially complex survey selection function, and find that the model fits all data well. We find distinct inside-out growth of the Milky Way disk; the best-fit model implies that the half-mass radius of the Milky Way disk has grown by 43% over the last 7 Gyr. Yet, such inside-out growth still results in a present-day age gradient weaker than 0.1 Gyr kpc-1. Our model predicts the half-mass and half-light sizes of the Galactic disk at earlier epochs, which can be compared to the observed redshift-size relations of disk galaxies. We show that radial orbit migration can reconcile the distinct disk-size evolution with redshift, also expected from cosmological simulations, with the modest present-day age gradients seen in the Milky Way and other galaxies

VLT Diffraction Limited Imaging and Spectroscopy in the NIR: Weighing the black hole in Centaurus A with NACO

We present high spatial resolution near-infrared spectra and images of the nucleus of Centaurus A (NGC 5128) obtained with NAOS-CONICA at the VLT. The adaptive optics corrected data have a spatial resolution of 0.06" (FWHM) in K- and 0.11" in H-band, four times higher than previous studies. The observed gas motions suggest a kinematically hot disk which is orbiting a central object and is oriented nearly perpendicular to the nuclear jet. We model the central rotation and velocity dispersion curves of the [FeII] gas orbiting in the combined potential of the stellar mass and the (dominant) black hole. Our physically most plausible model, a dynamically hot and geometrically thin gas disk, yields a black hole mass of M_bh = (6.1 +0.6/-0.8) 10^7 M_sun. As the physical state of the gas is not well understood, we also consider two limiting cases: first a cold disk model, which completely neglects the velocity dispersion; it yields an M_bh estimate that is almost two times lower. The other extreme case is to model a spherical gas distribution in hydrostatic equilibrium through Jeans equation. Compared to the hot disk model the best-fit black hole mass increases by a factor of 1.5. This wide mass range spanned by the limiting cases shows how important the gas physics is even for high resolution data. Our overall best-fitting black hole mass is a factor of 2-4 lower than previous measurements. With our revised M_bh estimate, Cen A's offset from the M_bh-sigma relation is significantly reduced; it falls above this relation by a factor of ~2, which is close to the intrinsic scatter of this relation. (Abridged)Comment: 12 pages, 14 figures, including minor changes following the referee report; accepted for publication in The Astrophysical Journa

Identifying stellar streams in the 1st RAVE public data release

We searched for and detected stellar streams or moving groups in the solar neighbourhood, using the data provided by the 1st RAVE public data release. This analysis is based on distances to RAVE stars estimated from a color-magnitude relation that was calibrated on Hipparcos stars. Our final sample consists of 7015 stars selected to be within 500 pc of the Sun and to have distance errors better than 25%. Together with radial velocities from RAVE and proper motions from various data bases, there are estimates for all 6 phase-space coordinates of the stars in the sample. We characterize the orbits of these stars through suitable proxies for their angular momentum and eccentricity, and compare the observed distribution to the expectations from a smooth distribution. On this basis we identify at least four "phase space overdensities" of stars on very similar orbits in the Solar neighbourhood. We estimate the statistical significance of these overdensities by Monte Carlo simulations. Three of them have been identified previously: the Sirius and Hercules moving group and a stream found independently in 2006 by Arifyanto and Fuchs and Helmi et al. In addition, we have found a new stream candidate on a quite radial orbit, suggesting an origin external to the Milky Way's disk. Also, there is evidence for the Arcturus stream and the Hyades-Pleiades moving group in the sample. This analysis, using only a minute fraction of the final RAVE data set, shows the power of this experiment to probe the phase-space substructure of stars around the Sun.Comment: accepted for publication in Ap

Axisymmetric Three-Integral Models for Galaxies

We describe an improved, practical method for constructing galaxy models that match an arbitrary set of observational constraints, without prior assumptions about the phase-space distribution function (DF). Our method is an extension of Schwarzschild's orbit superposition technique. As in Schwarzschild's original implementation, we compute a representative library of orbits in a given potential. We then project each orbit onto the space of observables, consisting of position on the sky and line-of-sight velocity, while properly taking into account seeing convolution and pixel binning. We find the combination of orbits that produces a dynamical model that best fits the observed photometry and kinematics of the galaxy. A key new element of this work is the ability to predict and match to the data the full line-of-sight velocity profile shapes. A dark component (such as a black hole and/or a dark halo) can easily be included in the models. We have tested our method, by using it to reconstruct the properties of a two-integral model built with independent software. The test model is reproduced satisfactorily, either with the regular orbits, or with the two-integral components. This paper mainly deals with the technical aspects of the method, while applications to the galaxies M32 and NGC 4342 are described elsewhere (van der Marel et al., Cretton & van den Bosch). (abridged)Comment: minor changes, accepted for publication in the Astrophysical Journal Supplement