Discovery of disc truncations above the galaxies’ mid-plane in Milky Way-like galaxies

Abstract

Disc truncations are the closest feature to an edge that galaxies have, but the nature of this phenomenon is not yet understood. In this paper we explore the truncations in two nearby (D ∼15 Mpc) Milky Way-like galaxies: NGC 4565 and NGC 5907. We cover a wide wavelength range from the NUV and optical to 3.6 µm. We find that the radius of the truncation (26 ± 0.5 kpc) is independent of wavelength. Surprisingly, we identify (at all wavelengths) the truncation at altitudes as high as 3 kpc above the mid-plane, which implies that the thin disc in those outer regions has a width of at least this value. We find the characteristic U shaped radial colour profile associated with a star formation threshold at the location of the truncation. Further supporting such an origin, the stellar mass density at the position of the truncation is ∼1-2 M pc −2 , in good agreement with the critical gas density for transforming gas into stars. Beyond the truncation, the stellar mass in the mid-plane of the disc drops to just 0.1-0.2 % of the total stellar mass of the galaxies. The detection of the truncation at high altitude in combination with the U shape of the radial colour profile allows us to establish, for the first time, an upper limit to the present-day growth rate of galactic discs. We find that, if the discs of these galaxies are growing inside-out, their growth rate is less than 0.6-1 kpc Gyr −1 .

A physically motivated definition for the size of galaxies in an era of ultradeep imaging

“…In this paper we propose the radial position of the isomass contour at 1 M pc −2 as a size indicator motivated by its proximity to the location of the gas density threshold for star formation in galaxies found theoretically (see e.g. Schaye 2004) and because this value is representative of the location of the disc truncation in galaxies similar to the Milky Way (Martínez-Lombilla et al 2019). However, this value is not guaranteed to be representative of the gas density threshold for star formation in galaxies with very different stellar mass (like dwarfs) or those who were formed during an enormous burst of star formation at high-z (as is the case of the most massive ellipticals).…”

The haloes and environments of nearby galaxies (HERON) – II. The outer structure of edge-on galaxies

“…B2, similar to fig. 3 in Martínez-Lombilla et al (2019). Now let us briefly consider our results for each galaxy.…”

“…Edge-on galaxies are unique targets in the sense that they allow us to directly study their vertical structure without contamination from unknown inclinations (van der Kruit & Searle 1981a; Barteldrees & Dettmar 1994;de Grijs & van der Kruit 1996;Bizyaev & Mitronova 2002;Mosenkov et al 2010;Bizyaev et al 2014) and to uniquely decompose them into different structural components such as thin E-mail: aleksandr.mosenkov@tdtu.edu.vn truncations Kregel et al 2002;Comerón et al 2012;Martínez-Lombilla et al 2019), disc flaring (Comerón et al 2011c;López-Corredoira & Molgó 2014), X-shaped or boxy structure of a bar (Bureau et al 2006;Savchenko et al 2017). The disc flattening, which can be measured in the case of the edge-on view , is related to the ratio of dark-matter halo to the total galaxy mass (Zasov et al 2002;Mosenkov et al 2010) and the minor-merger history (Read et al 2008).…”

Modification of Gravitational Field Equation due to Invariance of Light Speed and New System of Universe Evolution

“…Melissa Ness said that this structure implies that large galaxies are not formed by the merger of small galaxies, because once the merger occurs, the structure will inevitably be destroyed, and we must abandon the existing theory of galaxy formation and establish a new logic system. e observations of Martinez-Lombila and others [3] show that the radius of disk galaxies similar to the Milky Way galaxy is expanding at a speed of 500 m/s; such a high speed cannot be the speed at which matter accumulates at the edge. If matter accumulates at this speed at the edge, it should be the same everywhere on the disk.…”