Galactic bar resonances inferred from kinematically hot stars in Gaia EDR3

ABSTRACT Using a numerical simulation of an isolated barred disc galaxy, we first demonstrate that the resonances of the inner bar structure induce more prominent features in the action space distribution for the kinematically hotter stars, which are less sensitive to the local perturbation, such as the transient spiral arms. Then, we analyse the action distribution for the kinematically hotter stars selected from the Gaia EDR3 data as the stars with higher values of radial and vertical actions. We find several resonance features, including two new features, in the angular momentum distribution similar to what are seen in our numerical simulations. We show that the bar pattern speeds of about Ωbar ∼ 34 and 42 km s−1 kpc−1 explain all these features equally well. The resonance features we find correspond to the inner 4:1, co-rotation (CR), outer 4:1, outer Lindblad, and outer 4:3 (CR, outer 4:1, outer Lindblad, outer 4:3, and outer 1:1) resonances, when Ωbar ∼ 34 (42) km s−1 kpc−1 is assumed.

Resolving local and global kinematic signatures of satellite mergers with billion particle simulations

ABSTRACT In this work, we present two new ∼109 particle self-consistent simulations of the merger of a Sagittarius-like dwarf galaxy with a Milky Way (MW)-like disc galaxy. One model is a violent merger creating a thick disc, and a Gaia–Enceladus/Sausage-like remnant. The other is a highly stable disc which we use to illustrate how the improved phase space resolution allows us to better examine the formation and evolution of structures that have been observed in small, local volumes in the MW, such as the z−vz phase spiral and clustering in the vR−vϕ plane when compared to previous works. The local z−vz phase spirals are clearly linked to the global asymmetry across the disc: we find both 2-armed and 1-armed phase spirals, which are related to breathing and bending behaviours, respectively. Hercules-like moving groups are common, clustered in vR−vϕ in local data samples in the simulation. These groups migrate outwards from the inner galaxy, matching observed metallicity trends even in the absence of a galactic bar. We currently release the best-fitting ‘present-day’ merger snapshots along with the unperturbed galaxies for comparison.

Search for gas accretion imprints in voids: II. The galaxy Ark 18 as a result of a dwarf-dwarf merger

The low-mass low-surface brightness (LSB) disc galaxy Arakelian 18 (Ark 18) resides in the Eridanus void and because of its isolation represents an ideal case to study the formation and evolution mechanisms of such a galaxy type. Its complex structure consists of an extended blue LSB disc and a bright central elliptically-shaped part hosting a massive off-centered star-forming clump. We present the in-depth study of Ark 18 based on observations with the SCORPIO-2 long-slit spectrograph and a scanning Fabry-Perot interferometer at the Russian 6-m telescope complemented by archival multi-wavelength images and SDSS spectra. Ark 18 appears to be a dark matter dominated gas-rich galaxy without a radial metallicity gradient. The observed velocity field of the ionised gas is well described by two circularly rotating components moderately inclined with respect to each other and a possible warp in the outer disc. We estimated the age of young stellar population in the galaxy centre to be ∼140 Myr, while the brightest star-forming clump appears to be much younger. We conclude that the LSB disc is likely the result of a dwarf–dwarf merger with a stellar mass ratio of the components at least ∼5:1 that occurred earlier than 300 Myr ago. The brightest star forming clump was likely formed later by accretion of a gas cloud.

2MASX J00423991 + 3017515: an offset active galactic nucleus in an interacting system

ABSTRACT We present a spectroscopic and imaging study of an abnormal active galactic nucleus (AGN), 2MASX J00423991 + 3017515. This AGN is newly identified in the hard X-rays by the Swift BAT All-Sky survey and found in an edge-on disc galaxy interacting with a nearby companion. Here, we analyse the first optical spectra obtained for this system (taken in 2011 and 2016), high-resolution imaging taken with the Hubble Space Telescope and Chandra X-ray Observatory, and 1 imaging with the Very Large Array. Two unique properties are revealed: the peaks of the broad Balmer emission lines (associated with gas orbiting very near the supermassive black hole) are blueshifted from the corresponding narrow line emission and host galaxy absorption by 1540 km s−1, and the AGN is spatially displaced from the apparent centre of its host galaxy by 3.8 kpc. We explore several scenarios to explain these features, along with other anomalies, and propose that 2MASX J00423991 + 3017515 may be an AGN with an unusually strong wind residing in a uniquely configured major merger, or that it is an AGN recoiling from either a gravitational ‘slingshot’ in a three-body interaction or from a kick due to the asymmetric emission of gravitational waves following the coalescence of two progenitor supermassive black holes.

Did Sgr cause the vertical waves in the solar neighbourhood?

ABSTRACT The vertical distribution of stars in the solar neighbourhood is not in equilibrium but contains a wave signature in both density and velocity space originating from a perturbation. With the discovery of the phase-space spiral in Gaia data release (DR) 2, determining the origin of this perturbation has become even more urgent. We develop and test a fast method for calculating the perturbation from a passing satellite on the vertical component of a part of a disc galaxy. This fast method allows us to test a large variety of possible perturbations to the vertical disc very quickly. We apply our method to the range of possible perturbations to the solar neighbourhood stemming from the recent passage of the Sagittarius dwarf galaxy (Sgr), varying its mass, mass profile, and present-day position within their observational uncertainties, and its orbit within different realistic models for the Milky Way’s gravitational potential. We find that we are unable to reproduce the observed asymmetry in the vertical number counts and its concomitant breathing mode in velocity space for any plausible combination of Sgr and Milky Way properties. In all cases, either the amplitude or the perturbation wavelength of the number-count asymmetry and of the oscillations in the mean vertical velocity produced by the passage of Sgr are in large disagreement with the observations from Gaia DR2. We conclude that Sgr cannot have caused the observed oscillations in the vertical disc or the Gaia phase-space spiral.

The Phantom of RAMSES user guide for galaxy simulations using Milgromian and Newtonian gravity

This document describes the general process of setting up, running, and analysing disc galaxy simulations using the freely available program PHANTOM OF RAMSES (POR). This implements Milgromian Dynamics (MOND) with a patch to the RAMSES grid-based N-body and hydrodynamical code that uses adaptive mesh refinement. We discuss the procedure of setting up isolated and interacting disc galaxy initial conditions for POR, running the simulations, and analysing the results. This manual also concisely documents all previously developed MOND simulation codes and the results obtained with them.

Breaking the degeneracy between gas inflow and outflows with stellar metallicity: insights on M 101

ABSTRACT An analytical chemical evolution model is constructed to investigate the radial distribution of gas-phase and stellar metallicity for star-forming galaxies. By means of the model, the gas-phase and stellar metallicity can be obtained from the stellar-to-gas mass ratio. Both the gas inflow and outflow processes play an important role in building the final gas-phase metallicity, and there exists degeneracy effect between the gas inflow and outflow rates for star-forming galaxies. On the other hand, stellar metallicity is more sensitive to the gas outflow rate than to the gas inflow rate, and this helps to break the parameter degeneracy for star-forming galaxies. We apply this analysis method to the nearby disc galaxy M 101 and adopting the classical χ2 methodology to explore the influence of model parameters on the resulted metallicity. It can be found that the combination of gas-phase and stellar metallicity is indeed more effective for constraining the gas inflow and outflow rates. Our results also show that the model with relatively strong gas outflows but weak gas inflow describes the evolution of M 101 reasonably well.

An off-centred bulge or a satellite? Hydrodynamical N-body simulations of the disc galaxy NGC 5474

ABSTRACT We present dynamical models of the star-forming galaxy NGC 5474 based on N-body hydrodynamical numerical simulations. We investigate the possible origin of the compact round stellar structure, generally interpreted as the bulge of the galaxy, but unusually off-set by ${\simeq} 1\, {\rm kpc}$ in projection from the visual and the kinematic centres of both the star and the gas discs. We argue that it is very unlikely that the putative bulge is in a coplanar orbit in the disc plane, showing that such a configuration would be hardly compatible with its smooth and regular spatial distribution, and, in case its mass is above $10^8\, \mathrm{ M}_{\odot }$, also with the regular ${\rm H\,{\small I}}$ velocity field of NGC 5474. Instead, if the putative bulge is in fact an early-type satellite galaxy orbiting around NGC 5474, not only the off-set can be easily produced by projection effects, but our simulations suggest that the gravitational interaction between the two systems can explain also the warped ${\rm H\,{\small I}}$ distribution of NGC 5474 and the formation of its loose spiral arms. As a by-product of the simulations, we find that the peculiar overdensity of old stars detected in the south-west region of NGC 5474 may be explained with the interaction between NGC 5474 and a smaller stellar system, unrelated to the putative bulge, accreted in the disc plane.

Spiral instabilities: linear and non-linear effects

ABSTRACT We present a study of the spiral responses in a stable disc galaxy model to co-orbiting perturbing masses that are evenly spaced around rings. The amplitudes of the responses, or wakes, are proportional to the masses of the perturbations, and we find that the response to a low-mass ring disperses when it is removed – behaviour that is predicted by linear theory. Higher mass rings cause non-linear changes through scattering at the major resonances, provoking instabilities that were absent before the scattering took place. The separate wake patterns from two rings orbiting at differing frequencies produce a net response that is an apparently shearing spiral. When the rings have low mass, the evolution of the simulation is both qualitatively and quantitatively reproduced by linear superposition of the two separate responses. We argue that apparently shearing transient spirals in simulations result from the superposition of two or more steadily rotating patterns, each of which is best accounted for as a normal mode of the non-smooth disc.

Evidence for galaxy quenching in the green valley caused by a lack of a circumgalactic medium

ABSTRACT The relationship between a galaxy’s properties and its circumgalactic medium (CGM) provides a unique view of how galaxies evolve. We present an interesting edge-on (i = 86°) disc galaxy (G1547) where the CGM is probed by a background quasar at a distance of 84 kpc and within 10° of the galaxy major axis. G1547 does not have any detectable CGM absorption down to stringent limits, covering H i (EWr <0.02 Å, log(N(H i)/cm−2) < 12.6) and a range of low and high ionization absorption lines (O i, C ii, N ii, Si ii, C iii, N iii, Si iii, C iv, Si iv, N v, and O vi). This system is rare, given the covering fraction of $1.00_{-0.04}^{+0.00}$ for sub-L* galaxies within 50–100 kpc of quasar sightlines. G1547 has a low star formation rate (SFR, 1.1 M⊙ yr−1), specific SFR (sSFR, 1.5 × 10−10 yr−1), and ΣSFR (0.06 M⊙ yr−1 kpc−2) and does not exhibit active galactic nucleus or star-formation-driven outflows. Compared to the general population of galaxies, G1547 is in the green valley and has an above average metallicity with a negative gradient. When compared to other H i absorption-selected galaxies, we find that quiescent galaxies with log(sSFR/yr−1) < −11 have a low probability (4/12) of possessing detectable H i in their CGM, while all galaxies (40/40) with log(sSFR/yr−1) > −11 have H i absorption. We conclude that sSFR is a good indicator of the presence of H i CGM. Interestingly however, G1547 is the only galaxy with log(sSFR/yr−1) > −11 that has no detectable CGM. Given the properties of G1547, and its absent CGM, it is plausible that G1547 is undergoing quenching due to a lack of accreting fuel for star formation, with an estimated quenching time-scale of 4 ± 1 Gyr. G1547 provides a unique perspective into the external mechanisms that could explain the migration of galaxies into the green valley.