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In this work we study the striking case of a narrow blue stream around the NGC 7241 galaxy and its foreground dwarf companion. We want to figure out if the stream was generated by tidal interaction with NGC 7241 or it first interacted with the foreground dwarf companion and later both fell together towards NGC 7241. We use four sets of observations, including a follow-up spectroscopic study with the MEGARA instrument at the 10.4-m Gran Telescopio Canarias. Our data suggest that the compact object we detected in the stream is a foreground Milky Way halo star. Near this compact object we detect emission lines overlapping a bluer and fainter blob of the stream that is clearly visible in both ultra-violet and optical deep images. From its heliocentric systemic radial velocity (Vsyst= 1548.58+/-1.80 km s^-1) and new UV and optical broad-band photometry, we conclude that this over-density could be the actual core of the stream, with an absolute magnitude of M_g ~ -10 and a (g-r) = 0.08 +/- 0.11, consistent with a remnant of a low-mass dwarf satellite undergoing a current episode of star formation. From the width of the stream and assuming a circular orbit, we calculate that the progenitor mass can be the typical of a dwarf galaxy, but it could also be substantially lower if the stream is on a very radial orbit or it was created by tidal interaction with the companion dwarf instead of with NGC 7241. Finally, we find that blue stellar streams containing star formation regions are commonly predicted by high-resolution cosmological simulations of galaxies lighter than the Milky Way. This scenario is consistent with the processes explaining the bursty star formation history of some dwarf satellites, which are followed by a gas depletion and a fast quenching once they enter within the virial radius of their host galaxies for the first time., Comment: 12 pages, 5 figures, accepted for publication in Astronomy & Astrophysics

Cosmological simulations suggest that most of the matter in the Universe is distributed along filaments connecting galaxies. Illuminated by the cosmic UV background (UVB), these structures are expected to glow in fluorescent Lyman alpha emission with a Surface Brightness (SB) that is well below current observational limits for individual detections. Here, we perform a stacking analysis of the deepest MUSE/VLT data using three-dimensional regions (subcubes) with orientations determined by the position of neighbouring Lyman alpha galaxies (LAEs) at 3

A noise-based non-parametric technique for detecting nebulous objects, for example, irregular or clumpy galaxies, and their structure in noise is introduced. "Noise-based" and "non-parametric" imply that this technique imposes negligible constraints on the properties of the targets and that it employs no regression analysis or fittings. The sub-sky detection threshold is defined and initial detections are found, independently of the sky value. False detections are then estimated and removed using the ambient noise as a reference. This results in a purity level of 0.89 for the final detections as compared to 0.29 for SExtractor when a completeness of 1 is desired for a sample of extremely faint and diffuse mock galaxy profiles. The difference in the mean of the undetected pixels with the known background of mock images is decreased by 4.6 times depending on the diffuseness of the test profiles, quantifying the success in their detection. A non-parametric approach to defining substructure over a detected region is also introduced. NoiseChisel is our software implementation of this new technique. Contrary to the existing signal-based approach to detection, in its various implementations, signal-related parameters such as the image point spread function or known object shapes and models are irrelevant here. Such features make this technique very useful in astrophysical applications such as detection, photometry, or morphological analysis of nebulous objects buried in noise, for example, galaxies that do not generically have a known shape when imaged., Comment: NoiseChisel is part of GNU Astronomy Utilities (see https://www.gnu.org/software/gnuastro/ ). All the data-generated numbers and figures in this paper are now exactly reproducible with a `make' command. See `reproduce/README' in the arXiv source files for details and http://astr.tohoku.ac.jp/~akhlaghi/reproduce/1505.01664/ for up-to-date information. All results generated by free software

Despite expanding research activity in gravitational lens modeling, there is no particular software which is considered a standard. Much of the gravitational lens modeling software is written by individual investigators for their own use. Some gravitational lens modeling software is freely available for download but is widely variable with regard to ease of use and quality of documentation. This review of 13 software packages was undertaken to provide a single source of information. Gravitational lens models are classified as parametric models or non-parametric models, and can be further divided into research and educational software. Software used in research includes the GRAVLENS package (with both gravlens and lensmodel), Lenstool, LensPerfect, glafic, PixeLens, SimpLens, Lensview, and GRALE. In this review, GravLensHD, G-Lens, Gravitational Lensing, lens and MOWGLI are categorized as educational programs that are useful for demonstrating various aspects of lensing. Each of the 13 software packages is reviewed with regard to software features (installation, documentation, files provided, etc.) and lensing features (type of model, input data, output data, etc.) as well as a brief review of studies where they have been used. Recent studies have demonstrated the utility of strong gravitational lensing data for mass mapping, and suggest increased use of these techniques in the future. Coupled with the advent of greatly improved imaging, new approaches to modeling of strong gravitational lens systems are needed. This is the first systematic review of strong gravitational lens modeling software, providing investigators with a starting point for future software development to further advance gravitational lens modeling research.

We present scaling relations between stellar-mass (Mstar) and the size of galaxies at 0.3 < z < 3 for half- (R_50) and 90 percent-light (R_90) radii, using a deep K-band selected catalogue taken with the Subaru Telescope and MOIRCS in the GOODS-North region. The logarithmic slope R \propto Mstar^{0.1-0.2} is independent of redshift in a wide mass range of Mstar ~ 10^8-10^11 Msun, irrespective of galaxy populations (star-forming, quiescent). The offset change is < 50 percent. Provided that optical light in the rest frame traces the stellar mass of galaxies, the universal relation demonstrates that the stellar mass was built up in galaxies over their cosmic histories in a similar manner on average irrelevant to galaxy mass. The small offset in each stellar mass bin from the universal relation shows weak size evolution at a given mass. There is a moderate increase of 30-50 percent for R_50 and R_90 for less massive galaxies (Mstar < 10^10 Msun) from z~3 to z~1, while the sizes remains unchanged or slightly decrease towards z~0.3. For massive galaxies (Mstar > 10^11 Msun), the evolution is ~70-80 % increase in R_90 from z~3 to z~0.3, though that in R_50 is weaker. The evolution of compactness factor, R_50/R_90, which becomes smaller at lower redshift, is suggestive of minor merging effect in the outer envelope of massive galaxies., Comment: 16 pages, 12 figures, 3 tables, accepted for publication in MNRAS