LSDCat: Detection and cataloguing of emission-line sources in integral-field spectroscopy datacubes

We present a robust, efficient, and user-friendly algorithm for detecting faint emission-line sources in large integral-field spectroscopic datacubes together with the public release of the software package LSDCat (Line Source Detection and Cataloguing). LSDCat uses a 3-dimensional matched filter approach, combined with thresholding in signal-to-noise, to build a catalogue of individual line detections. In a second pass, the detected lines are grouped into distinct objects, and positions, spatial extents, and fluxes of the detected lines are determined. LSDCat requires only a small number of input parameters, and we provide guidelines for choosing appropriate values. The software is coded in Python and capable to process very large datacubes in a short time. We verify the implementation with a source insertion and recovery experiment utilising a real datacube taken with the MUSE instrument at the ESO Very Large Telescope.Comment: 14 pages. Accepted for publication in Astronomy & Astrophysics. The LSDCat software is available at https://bitbucket.org/Knusper2000/lsdcat, v2 corrected typos and language editin

An IFU investigation of possible Lyman continuum escape from Mrk 71/NGC 2366

Mrk 71/NGC 2366 is the closest Green Pea (GP) analog and candidate Lyman Continuum (LyC) emitter. Recently, 11 LyC-leaking GPs have been detected through direct observations of the ionizing continuum, making this the most abundant class of confirmed LyC-emitters at any redshift. High resolution, multi-wavelength studies of GPs can lead to an understanding of the method(s), through which LyC escapes from these galaxies. The proximity of Mrk 71/NCG 2366 offers unprecedented detail on the inner workings of a GP analog, and enables us to identify the mechanisms of LyC escape. We use 5825-7650{\AA} integral field unit PMAS observations to study the kinematics and physical conditions in Mrk 71. An electron density map is obtained from the [S II] ratio. A fortuitous second order contamination by the [O II]3727 doublet enables the construction of an electron temperature map. Resolved maps of sound speed, thermal broadening, "true" velocity dispersion, and Mach number are obtained and compared to the high resolution magneto-hydrodynamic SILCC simulations. Two regions of increased velocity dispersion indicative of outflows are detected to the north and south of the super star cluster, knot B, with redshifted and blueshifted velocities, respectively. We confirm the presence of a faint broad kinematical component, which is seemingly decoupled from the outflow regions, and is fainter and narrower than previously reported in the literature. Within uncertainties, the low- and high-ionization gas move together. Outside of the core of Mrk 71, an increase in Mach numbers is detected, implying a decrease in gas density. Simulations suggest this drop in density can be as high as ~4 dex, down to almost optically thin levels, which would imply a non-zero LyC escape fraction along the outflows... [abridged]Comment: Accepted for publication in A&A. 17 pages, 16 figures, 4 table

Deciphering Lyman α\alpha blob 1 with deep MUSE observations

Context: Lyman $\alpha$ blobs (LABs) are large-scale radio-quiet Lyman $\alpha$ (Ly$\alpha$) nebula at high-$z$ that occur predominantly in overdense proto-cluster regions. Especially the prototypical SSA22a-LAB1 at $z=3.1$ has become an observational reference for LABs across the electromagnetic spectrum. Aims: We want to understand the powering mechanisms that drive the LAB to gain empirical insights into galaxy formation processes within a rare dense environment at high-$z$. Methods: LAB 1 was observed for 17.5h with the VLT/MUSE integral-field spectrograph. We produced optimally extracted narrow band images in Ly$\alpha$ $\lambda1216$ and HeII $\lambda1640$. By using a moment based analysis we mapped the kinematics of the blob. Results: We detect Ly$\alpha$ emission to surface-brightness limits of $10^{-19}$erg s$^{-1}$cm$^{-2}$arcsec$^{-2}$. At this depth we reveal a bridge between LAB 1 and its northern neighbour LAB 8, as well as a shell-like filament towards the south of LAB 1. We find a coherent large scale east-west $\sim$1000 km s$^{-1}$ velocity gradient that is aligned perpendicular to the major axis of the blob. We detect HeII emission in three distinct regions, but we can only provide upper limits for CIV. Conclusions: Various gas excitation mechanisms are at play in LAB 1: Ionising radiation and feedback effects dominate near the embedded galaxies, while Ly$\alpha$ scattering is contributing at larger distances. However, HeII/Ly$\alpha$ ratios combined with upper limits on CIV/Ly$\alpha$ can not discriminate between AGN ionisation and feedback driven shocks. The alignment of the angular momentum vector parallel to the morphological principal axis appears odds with the predicted norm for high-mass halos, but likely reflects that LAB\,1 resides at a node of multiple intersecting filaments of the cosmic web. (Abridged)Comment: Revised version. Accepted for publication in A&

The Lyman Alpha Reference Sample. VIII. Characterizing Lyman-Alpha Scattering in Nearby Galaxies

We examine the dust geometry and Ly{\alpha} scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < $z$ < 0.2) Ly{\alpha} emitting and starbursting systems with Hubble Space Telescope Ly{\alpha}, H{\alpha}, and H{\beta} imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media while others of them show significantly lower Ly{\alpha} emission compared to their Balmer decrement. With the LARS imaging, we present Ly{\alpha}/H{\alpha} and H{\alpha}/H{\beta} maps with spatial resolutions as low as $\sim$ 40 pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Ly{\alpha} photons only scatter. We show that the dust that affects the escape of Ly{\alpha} is more restricted to the galaxies' central regions, while the larger Ly{\alpha} halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Ly{\alpha} scatters in the halo, and find that this "characteristic" scattering distance correlates with the measured size of the Ly{\alpha} halo. We note that there exists a slight anti-correlation between the scattering distance of Ly{\alpha} and global dust properties.Comment: 32 pages, 51 figures, accepted to Ap

The MUSE-Wide survey: A measurement of the Lyα\alpha emitting fraction among z>3z>3 galaxies

We present a measurement of the fraction of Lyman $\alpha$ (Ly$\alpha$) emitters ($X_{\rm{Ly} \alpha}$) amongst HST continuum-selected galaxies at $3<z<6$ with the Multi-Unit Spectroscopic Explorer (MUSE) on the VLT. Making use of the first 24 MUSE-Wide pointings in GOODS-South, each having an integration time of 1 hour, we detect 100 Ly$\alpha$ emitters and find $X_{\rm{Ly} \alpha}\gtrsim0.5$ for most of the redshift range covered, with 29 per cent of the Ly$\alpha$ sample exhibiting rest equivalent widths (rest-EWs) $\leq$ 15\AA. Adopting a range of rest-EW cuts (0 - 75\AA), we find no evidence of a dependence of $X_{\rm{Ly} \alpha}$ on either redshift or UV luminosity.Comment: 10 pages, 5 figures (MNRAS, updated as per version in press

The MUSE Hubble Ultra Deep Field Survey X. Lyα\alpha Equivalent Widths at 2.9<z<6.62.9 < z < 6.6

We present rest-frame Ly$\alpha$ equivalent widths (EW) of 417 Ly$\alpha$ emitters (LAEs) detected with Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) at $2.9 < z < 6.6$ in the Hubble Ultra Deep Field. Based on the deep MUSE spectroscopy and ancillary Hubble Space Telescope (HST) photometry data, we carefully measured EW values taking into account extended Ly$\alpha$ emission and UV continuum slopes ($\beta$). Our LAEs reach unprecedented depths, both in Ly$\alpha$ luminosities and UV absolute magnitudes, from log($L_{\rm Ly\alpha}$/erg s$^{-1}$) $\sim$41.0 to 43.0 and from Muv $\sim$ -16 to -21 (0.01-1.0 $L^{*}_{\rm z=3}$). The EW values span the range of $\sim$ 5 to 240 \AA\ or larger, and their distribution can be well fitted by an exponential law $N = N_{\rm 0}$ exp($-$EW/$w_{\rm 0}$). Owing to the high dynamic range in Muv, we find that the scale factor, $w_{\rm 0}$, depends on Muv in the sense that including fainter Muv objects increases $w_{\rm 0}$, i.e., the Ando effect. The results indicate that selection functions affect the EW scale factor. Taking these effects into account, we find that our $w_{\rm 0}$ values are consistent with those in the literature within $1\sigma$ uncertainties at $2.9 < z < 6.6$ at a given threshold of Muv and $L_{\rm Ly\alpha}$. Interestingly, we find 12 objects with EW $>200$ \AA\ above $1\sigma$ uncertainties. Two of these 12 LAEs show signatures of merger or AGN activity: the weak CIV $\lambda 1549$ emission line. For the remaining 10 very large EW LAEs, we find that the EW values can be reproduced by young stellar ages ($< 100$ Myr) and low metallicities ($\lesssim 0.02$ $Z_{\rm \odot}$). Otherwise, at least part of the Ly$\alpha$ emission in these LAEs needs to arise from anisotropic radiative transfer effects, fluorescence by hidden AGN or quasi-stellar object activity, or gravitational cooling.Comment: 22 pages, 12 figures, 9 tables, accepted for publication in A&A (MUSE UDF Series Paper X

Ubiquitous giant Ly α\alpha nebulae around the brightest quasars at z∼3.5z\sim3.5 revealed with MUSE

Direct Ly $\alpha$ imaging of intergalactic gas at $z\sim2$ has recently revealed giant cosmological structures around quasars, e.g. the Slug Nebula (Cantalupo et al. 2014). Despite their high luminosity, the detection rate of such systems in narrow-band and spectroscopic surveys is less than 10%, possibly encoding crucial information on the distribution of gas around quasars and the quasar emission properties. In this study, we use the MUSE integral-field instrument to perform a blind survey for giant Ly $\alpha$ nebulae around 17 bright radio-quiet quasars at $3<z<4$ that does not suffer from most of the limitations of previous surveys. After data reduction and analysis performed with specifically developed tools, we found that each quasar is surrounded by giant Ly $\alpha$ nebulae with projected sizes larger than 100 physical kpc and, in some cases, extending up to 320 kpc. The circularly averaged surface brightness profiles of the nebulae appear very similar to each other despite their different morphologies and are consistent with power laws with slopes $\approx-1.8$. The similarity between the properties of all these nebulae and the Slug Nebula suggests a similar origin for all systems and that a large fraction of gas around bright quasars could be in a relatively "cold" (T$\sim$10$^4$K) and dense phase. In addition, our results imply that such gas is ubiquitous within at least 50 kpc from bright quasars at $3<z<4$ independently of the quasar emission opening angle, or extending up to 200 kpc for quasar isotropic emission.Comment: 19 pages, 9 figures, 3 Tables, accepted to Ap

The Lyman alpha reference sample. VII. Spatially resolved Hα\alpha kinematics

We present integral field spectroscopic observations with the Potsdam Multi Aperture Spectrophotometer of all 14 galaxies in the $z\sim 0.1$ Lyman Alpha Reference Sample (LARS). We produce 2D line of sight velocity maps and velocity dispersion maps from the Balmer $\alpha$ (H$\alpha$) emission in our data cubes. These maps trace the spectral and spatial properties of the LARS galaxies' intrinsic Ly$\alpha$ radiation field. We show our kinematic maps spatially registered onto the Hubble Space Telescope H$\alpha$ and Lyman $\alpha$ (Ly$\alpha$) images. Only for individual galaxies a causal connection between spatially resolved H$\alpha$ kinematics and Ly$\alpha$ photometry can be conjectured. However, no general trend can be established for the whole sample. Furthermore, we compute non-parametric global kinematical statistics -- intrinsic velocity dispersion $\sigma_0$, shearing velocity $v_\mathrm{shear}$, and the $v_\mathrm{shear}/\sigma_0$ ratio -- from our kinematic maps. In general LARS galaxies are characterised by high intrinsic velocity dispersions (54\,km\,s$^{-1}$ median) and low shearing velocities (65\,km\,s$^{-1}$ median). $v_\mathrm{shear}/\sigma_0$ values range from 0.5 to 3.2 with an average of 1.5. Noteworthy, five galaxies of the sample are dispersion dominated systems with $v_\mathrm{shear}/\sigma_0 <1$ and are thus kinematically similar to turbulent star forming galaxies seen at high redshift. When linking our kinematical statistics to the global LARS Ly$\alpha$ properties, we find that dispersion dominated systems show higher Ly$\alpha$ equivalent widths and higher Ly$\alpha$ escape fractions than systems with $v_\mathrm{shear}/\sigma_0 > 1$. Our result indicates that turbulence in actively star-forming systems is causally connected to interstellar medium conditions that favour an escape of Ly$\alpha$ radiation.Comment: 26 pages, 15 figures, accepted for publication in A&

On the evolution of the size of Lyman alpha halos across cosmic time: no change in the circumgalactic gas distribution when probed by line emission

Lyman $\alpha$ (Ly$\alpha$) is now routinely used as a tool for studying high-redshift galaxies and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Ly$\alpha$ emitters indicate that significant extended Ly$\alpha$ halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-$z$ samples while optimizing the observations for detection of low surface brightness Ly$\alpha$ emission. The Ly$\alpha$ escape fractions range between 0.7\% and 37\%, and we detect extended Ly$\alpha$ emission around six out of seven targets. We find Ly$\alpha$ halo to UV scale length ratios around 6:1 which is marginally lower than high-redshift observations, and halo flux fractions between 60\% and 85\% -- consistent with high-redshift observations -- when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Ly$\alpha$ halo fractions. We find that deeper H$\alpha$ data would be required to firmly determine the origin of Ly$\alpha$ halo emission, however, there are indications that H$\alpha$ is more extended than the central FUV profile, potentially indicating conditions favorable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation.Comment: 20 page, 14 figures, 6 tables. Accepted for publication in MNRA

The Astropy Problem

The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software