The truncation of the disk of NGC 4565: Detected up to z=4 kpc, with star formation, and affected by the warp

Context: The hierarchical model of galaxy formation suggests that galaxies are continuously growing. However, our position inside the Milky Way prevents us from studying the disk edge. Truncations are low surface brightness features located in the disk outskirts of external galaxies. They indicate where the disk brightness abruptly drops and their location is thought to change dynamically. In previous analyses of Milky Way-like galaxies, truncations were detected up to 3 kpc above the mid-plane but whether they remain present beyond that height remains unclear. Aims: Our goal is to determine whether truncations can be detected above 3 kpc height in the Milky Way-like galaxy NGC 4565, thus establishing the actual disk thickness. We also aim to study how the truncation relates to disk properties such as star formation activity or the warp. Methods: We perform a vertical study of the disk of NGC 4565 edge in unprecedented detail. We explore the truncation radius at different heights above/below the disk mid-plane (0<z<8 kpc) and at different wavelengths. We use new ultra-deep optical data ($\mu_{g,\rm{lim}}=30.5$ mag arcsec$^{-2}$; $3 \sigma$ within $10 \times 10$ arcsec$^{2}$ boxes) in the $g$, $r$ and $i$ broad bands, along with near- and far-ultraviolet, H$\alpha$, and \ion{H}{i} observations. Results: We detect the truncation up to 4 kpc in the $g$, $r$ and $i$ ultra-deep bands which is 1 kpc higher than in any previous study for any galaxy. The radial position of the truncation remains constant up to 3 kpc while higher up it is located at a smaller radius. This result is independent of the wavelength but is affected by the presence of the warp. Conclusions: We propose an inside-out growth scenario for the formation of the disk of NGC 4565. Our results point towards the truncation feature being linked to a star-forming threshold and to the onset of the disk warp.Comment: 27 pages, 18 figures (incl. 2 appendix); accepted for publication in A&A; Fixed labels in Fig.

An almost dark galaxy with the mass of the Small Magellanic Cloud

Almost Dark Galaxies are objects that have eluded detection by traditional surveys such as the Sloan Digital Sky Survey (SDSS). The low surface brightness of these galaxies ($\mu_r$(0)$>26$ mag/arcsec^2), and hence their low surface stellar mass density (a few solar masses per pc^2 or less), suggests that the energy density released by baryonic feedback mechanisms is inefficient in modifying the distribution of the dark matter halos they inhabit. For this reason, almost dark galaxies are particularly promising for probing the microphysical nature of dark matter. In this paper, we present the serendipitous discovery of Nube, an almost dark galaxy with $$e~ 26.7 mag/arcsec^2. The galaxy was identified using deep optical imaging from the IAC Stripe82 Legacy Project. Follow-up observations with the 100m Green Bank Telescope strongly suggest that the galaxy is at a distance of 107 Mpc. Ultra-deep multi-band observations with the 10.4m Gran Telescopio Canarias favour an age of ~10 Gyr and a metallicity of [Fe/H]$\sim-1.1$. With a stellar mass of ~4x10^8 Msun and a half-mass radius of Re=6.9 kpc (corresponding to an effective surface density of ~0.9 Msun/pc^2), Nube is the most massive and extended object of its kind discovered so far. The galaxy is ten times fainter and has an effective radius three times larger than typical ultra-diffuse galaxies with similar stellar masses. Galaxies with comparable effective surface brightness within the Local Group have very low mass (~10^5 Msun) and compact structures (effective radius Re<1 kpc). Current cosmological simulations within the cold dark matter scenario, including baryonic feedback, do not reproduce the structural properties of Nube. However, its highly extended and flattened structure is consistent with a scenario where the dark matter particles are ultra-light axions with a mass of m$_B$=($0.8^{+0.4}_{-0.2}$)$\times10^{-23}$ eV.}Comment: Accepted for publication in A&A. Main figures are 8, 9 and 1

Toward Long-Term and Archivable Reproducibility

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