arXiv subjects

Mathias Schultheis

Publications and source records attributed to Mathias Schultheis.

At least 19 recordsLinked to original sources

arXiv2026

When bars and spirals conspire: recurrent build-up of the nuclear regions of disc galaxies

The assembly history of the central regions of disc galaxies is regulated by dynamical processes that trigger gas infall events, leading to active star formation in nuclear stellar discs (NSD) and in nuclear stellar clusters (NSC). In the Milky Way, recent studies of its nuclear regions have revealed a complex star formation history (SFH), with an initial burst associated to the formation of the Galactic bar, followed by a non-constant star formation rate. In this work, we aim to study the formation and evolution of nuclear structures and their link with the formation of large-scale structures. Our goal is to investigate the effects of the bar and spiral arms on the gas dynamics and, as a result on the SFH of NSDs and NSCs. We run a simulation of an isolated Milky Way-like galaxy with the SWIFT N-Body+hydro simulation code, including star formation and stellar feedback from SNIa & SNII. We start from a live DM halo and a pre-existing stellar & gaseous disc with 20% gas fraction, which form a bar, a boxy/peanut bulge, spiral arms and nuclear structures. We study the SFH of these regions and how they relate to variations in the bar length, strength and pattern speed. We investigate the role of spiral arms and their interaction with the bar. We find that the SFH of the nuclear regions display a main burst at bar formation time, due to bar-driven gas inflows. After bar formation, we find secondary periodic formation bursts, that do not appear in the disc SFH. These bursts occur when the spiral arms and the bar, rotating at different pattern speeds, reconnect, triggering secondary gas inflow events. The interaction of spiral arms and the galactic bar can enhance non-axisymmetric features in the disc, triggering bar-driven gas infall even after the bar has formed. These bar-spiral reconnection events are imprinted into the SFH of the NSCs and NSDs as episodic star formation bursts.

astro-ph.GA↗

arXiv2026

SMUGGLE-Ring: Evolutionary link between nuclear star cluster and nuclear disk

We present a high-resolution hydrodynamical simulation of the formation and evolution of nuclear structures in a Milky Way-mass galaxy using the SMUGGLE model. The system naturally develops a bar in isolation of $\approx5$ kpc in length, driving sustained gas inflows toward the center that lead to the formation of a nuclear stellar disk (NSD) and a nuclear star cluster (NSC). By only considering stars born after bar formation, we can cleanly isolate the nuclear structures and recover a clear inside-out growth of the NSD. In line with observations, we find that stellar feedback induces repeated shocks that regulate the size of the nuclear gas disk and drive gas from its outer edge toward the NSC region. Over time, the NSD and NSC share similar mass growth and star formation histories, except during the accretion of a massive star cluster. Our results suggest that both the evolutionary timescale of the bar (and thus of the NSD) and the accretion history of star clusters are essential for obtaining tighter scaling relations for nuclear structures and their host galaxies. Finally, our results favor a lower bulge mass for the Milky Way than that of our model ($B/D\approx 0.045$) to explain the compact size of its nuclear disk.

astro-ph.GA↗

arXiv2026

Kinematic hints of a nuclear bar in the Milky Way

The Milky Way hosts a flattened nuclear stellar disc (NSD) that dominates the gravitational potential in the inner few hundred parsecs. Whether the NSD is purely axisymmetric or contains a nuclear bar remains an open question. We test for the presence of a nuclear bar using kinematic diagnostics by combining line-of-sight velocities from the KMOS NSD survey with proper motions from VIRAC2 to construct the $ (v_\ell, v_\mathrm{los}) $ velocity ellipse. After applying strict quality cuts to minimise contamination from large-scale bar stars, we measure the vertex deviation $ l_v $ and anisotropy $ β$ for several subsamples. For our primary sample ($ |\ell| < 0.9^\circ $, $ -0.4^\circ < b < 0.25^\circ $, $ \mathrm{[Fe/H]} > -0.3 $), we find a significant negative vertex deviation $ l_v = -54.8^{+13.1}_{-14.8}\,^\circ $ with moderate anisotropy $ β= 0.16^{+0.08}_{-0.05} $. A subsample restricted to the innermost four fields yields an even stronger signal with $ l_v = -64.3^{+12.1}_{-12.2}\,^\circ $ and $ β= 0.38^{+0.12}_{-0.07} $. The direction of maximum velocity dispersion is oriented along Galactic longitude, opposite to that observed in large-scale bar-dominated samples. These signatures are robust against extinction-driven incompleteness, primary-bar contamination, and the choice of metallicity threshold. They are inconsistent with an axisymmetric NSD or one oriented orthogonally to the primary bar, but match expectations for a nuclear bar oriented at $ α\approx 60^\circ $-$75^\circ$ to the Sun-Galactic-Centre line with its near side pointing toward positive Galactic longitude. While definitive confirmation awaits larger and more precise samples from upcoming surveys, our results provide the first kinematic indication of a possible nuclear bar in the Milky Way.

astro-ph.GA↗

arXiv2026

Self-consistent modelling of the Milky Way's Nuclear Stellar Disc

The Nuclear Stellar Disc (NSD) is a flattened high-density stellar structure that dominates the gravitational field of the Milky Way at Galactocentric radius $30\lesssim R\lesssim 300$ pc. We construct axisymmetric self-consistent equilibrium dynamical models of the NSD in which the distribution function is an analytic function of the action variables. We fit the models to the normalised kinematic distributions (line-of-sight velocities + VIRAC2 proper motions) of stars in the NSD survey of Fritz et al., taking the foreground contamination due to the Galactic Bar explicitly into account using an $N$-body model. The posterior marginalised probability distributions give a total mass of $M_{\rm NSD} = 10.5^{+1.1}_{-1.0} \times10^8 \,{\rm M_\odot}$, roughly exponential radial and vertical scale-lengths of $R_{\rm disc} = 88.6^{+9.2}_{-6.9}$ pc and $H_{\rm disc}=28.4^{+5.5}_{-5.5}$ pc respectively, and a velocity dispersion $σ\simeq 70$ km/s that decreases with radius. We find that the assumption that the NSD is axisymmetric provides a good representation of the data. We quantify contamination from the Galactic Bar in the sample, which is substantial in most observed fields. Our models provide the full 6D (position+velocity) distribution function of the NSD, which can be used to generate predictions for future surveys. We make the models publicly available as part of the software package AGAMA.

astro-ph.GA↗

arXiv2026

Kinematic diagnostics for non-axisymmetry in the Milky Way's nuclear stellar disc

There is now strong evidence that the Milky Way (MW) hosts a nuclear stellar disc (NSD). However, whether the NSD is purely axisymmetric or contains a nuclear bar remains unresolved. Since approximately $50\%$ of barred galaxies with MW-like mass in the local Universe host a nuclear bar, investigating whether the MW hosts one is of interest. We conduct a systematic analysis to identify robust kinematic diagnostics capable of determining whether the MW hosts a nuclear bar. Using N-body simulations, we explore the kinematic signatures indicative of a nuclear bar. Using the phase-space coordinates longitude $(\ell)$, latitude $(b)$, proper motions ($μ_\ell$ and $μ_{\rm b})$ and line-of-sight velocity $(v_{\rm los})$, we test various diagnostics assuming different nuclear bar orientations. We also evaluate how sample size, dust extinction and bar amplitude influence the efficacy of the diagnostics. We identify two independent kinematic diagnostics capable of revealing a nuclear bar in the MW: (1) the vertex deviation, $l_{\rm v}$, of the ($v_{\ell}-v_{\rm los}$) velocity ellipse; and (2) The asymmetry in the $μ_{\ell}$ vs $\ell$ distribution. While both are impacted by the sample size and extinction, the vertex deviation proves more robust, especially when combining stars from multiple observational fields. We also assess the correlation between the line-of-sight velocity and the $h_3$ Gauss-Hermite moment ("skewness") of the line-of-sight velocity but find no clear distinction between an NSD and a nuclear bar based on this metric. Our results suggest that data from the current KMOS survey may allow a marginal detection of a nuclear bar using the vertex deviation method. A companion paper provides further validation and detailed analysis of this approach. Nonetheless, future surveys will provide the high quality data necessary to fully exploit the diagnostics outlined in this study.

astro-ph.GA↗

arXiv2026

The SMUGGLE-Ring project: Bar and bulge effects on nuclear disk and ring formation

We present the first results from the SMUGGLE-Ring project, a suite of simulations employing the SMUGGLE ISM and stellar feedback model to explore nuclear structures in Milky Way-mass galaxies. We discuss results from three simulations evolved for 5 Gyr in isolation, in which we vary the classical bulge mass, while keeping the disk and halo structures identical. Nuclear stellar disks and rings emerge exclusively in our bulge models, with more massive bulges associated with earlier formation and more extended initial gas reservoirs shortly after bar formation. After gas depletion via active star formation, the nuclear stellar disks bifurcate into pressure-supported nuclear star clusters (NSCs, $v_{\phi}/\sigma_R < 0.7$) and rotationally supported nuclear stellar rings (NSRs, $v_{\phi}/\sigma_R = 1.2$--1.7, radii 0.64--0.76 kpc). The bulgeless model fails to build up and sustain stable nuclear gas disks against feedback disruptions. The enclosed stellar mass of NSCs ($\sim10^{9}\Msun$) dominates over that of NSRs ($\sim10^{8}\Msun$). The star formation rates decline over time due to gas depletion (NSCs 0.1--1 $\Msun$/yr, NSRs 0.01--$0.1 \Msun$/yr). Kinematics reveal outward-shifting rotation peaks with $\sigma$-drops in NSRs, while a fraction of stars in NSCs exhibits radial shift after 3 Gyr. These findings support inside-out NSD formation via secular bar evolution, with NSRs tracing the star-forming outer edge of the nuclear gas disk and NSCs forming the kinematically hotter inner component. The range of nuclear stellar disk sizes (0.25--0.76 kpc) falls within the observationally inferred ranges, but the existence of larger rings would require external gas flow and/or a longer period of evolution. Future SMUGGLE-Ring extensions will incorporate varying gas fractions, tidal/merger effects, and the circumgalactic medium to further elucidate nuclear diversity and outliers.

astro-ph.GA↗

arXiv2025

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.

astro-ph.GA↗

arXiv2025

Towards systematic searches for LISA white dwarf binaries with multiband photometry

Ultra-compact double white dwarfs (DWDs) represent key targets for multi-messenger astrophysics, as they may be observed both through gravitational waves and the electromagnetic (EM) spectrum. The future Laser Interferometer Space Antenna (LISA) will detect thousands of these systems, and they are predicted to be the most numerous science targets of the mission. We develop a strategy to identify LISA source candidates in multiband photometric surveys. We constructed a synthetic EM catalogue of white dwarf (WD) detections based on a population synthesis code combined with a semi-analytical model of the Milky Way and a consistent cooling model for the evolution. We compared sources in the LISA band with other WD observations in magnitude-colour and colour-colour plots. From a full sky survey with $u \le$24.5, we find that 57$\%$ of the sources in the LISA band occupy a specific region in colour-colour diagrams. Inside this area, we find that $\sim 63\%$ (6.5 $\times 10^4$) of EM observations are LISA candidates, $\sim 31\%$ ($ 3.2 \times 10^4$) are DWDs slightly outside the LISA frequency range, and only a small contamination comes from single WDs and wider binaries. We find that the colour distributions of close DWDs represent a powerful tool to distinguish potential LISA sources from the broader WD population. This is an avenue to select candidates for further follow-up and identification.

astro-ph.HE↗

arXiv2025

Nuclear Stellar Discs

We review our current understanding of nuclear stellar discs (NSDs), rotating, and flattened stellar structures found in the central regions of both early- and late-type galaxies. We examine their demographics, kinematics, stellar populations, metallicity gradients and star formation histories. We derive scaling relations linking NSDs to properties of their host galaxies, and compare them with analogous relations for nuclear star clusters. The relationship between NSDs and other central galactic components, including nuclear rings, nuclear bars, and nuclear star clusters, is explored. The role of NSDs as tracers of the secular evolution of barred galaxies is highlighted, emphasising how they can be used to constrain properties of galactic bars such as their ages. Special attention is given to the Milky Way's NSD, which serves as a unique case study thanks to its proximity and the ability to resolve individual stars. The review covers the Milky Way's NSD structure, kinematics, dynamics and stellar content, addressing ongoing debates about the presence of a nuclear bar and implications of the latter for central gas dynamics. We argue that NSDs form by in-situ star formation, most of them because of bar-driven gas inflow, but possibly in some cases because of external acquisition of gas during a gas-rich merger. The review concludes by outlining open questions, future research directions and the exciting prospects provided by upcoming observational facilities.

astro-ph.GA↗

arXiv2025

The First Chemical Census the Milky Way's Nuclear Star Cluster

An important step in understanding the formation and evolution of the Nuclear Star Cluster (NSC) is to investigate its chemistry and chemical evolution. Additionally, exploring the relationship of the NSC to the other structures in the Galactic Center and the Milky Way disks is of great interest. Extreme optical extinction has previously prevented optical studies, but near-IR high-resolution spectroscopy is now possible. Here, we present a detailed chemical abundance analysis of 19 elements - more than four times as many as previously published - for 9 stars in the NSC of the Milky Way, observed with the IGRINS spectrometer on the Gemini South telescope. This study provides new, crucial observational evidence to shed light on the origin of the NSC. We demonstrate that it is possible to probe a variety of nucleosynthetic channels, reflecting different chemical evolution timescales. Our findings reveal that the NSC trends for the elements F, Mg, Al, Si, S, K, Ca, Ti, Cr, Mn, Co, Ni, Cu, and Zn, as well as the s-process elements Ba, Ce, Nd, and Yb, generally follow the inner bulge trends within uncertainties. This suggests a likely shared evolutionary history and our results indicate that the NSC population is consistent with the chemical sequence observed in the inner Galaxy (the inner-disk sequence). However, we identify a significant and unexplained difference in the form of higher Na abundances in the NSC compared to the inner-bulge. This is also observed in few Galactic globular clusters, and may suggest a common enrichment process at work in all these systems.

astro-ph.GA↗

arXiv2025

Bar-spiral interaction produces radial migration and star formation bursts

Central bars and spirals are known to strongly impact the evolution of their host galaxies, both in terms of dynamics and star formation. Their typically different pattern speeds cause them to regularly overlap, which induces fluctuations in bar parameters. In this paper, we analyze both numerical simulations of disk galaxies and observational data to study the effect of bar-spiral physical overlap on stellar radial migration and star formation in the bar vicinity, as a function of time and galactic azimuth. We study three different numerical models, two of which are in a cosmological context, as well as APOGEE DR17 data and the WISE catalog of Galactic HII regions. We find that periodic boosts in stellar radial migration occur when the bar and spiral structure overlap. This mechanism causes net inward migration along the bar leading side, while stars along the bar trailling side and minor axis are shifted outward. The signature of bar-spiral induced migration is seen between the bar's inner Lindbald resonance and well outside its corotation, beyond which other drivers take over. We also find that, in agreement with simulations, APOGEE DR17 stars born at the bar vicinity (mostly metal-rich) can migrate out to the solar radius while remaining on cold orbits. For the Milky Way, 13% of stars in the solar vicinity were born inside the bar, compared to 5-20% in the simulations. Bar-spiral reconnections also result in periodic starbursts at the bar ends with an enhancement of up to a factor of 4, depending on the strength of the spiral structure. Similarly to the migration bursts, these do not always happen simultaneously at the two sides of the bar, hinting at the importance of odd spiral modes. Data from the WISE catalog suggest this phhenomenon is also relevant in our own Galaxy.

astro-ph.GA↗

arXiv2024

A Wide Metallicity Range for Gyr-old Stars in the Nuclear Star Cluster

We report metallicities for three $\sim$Gyr-old stars in the Milky Way nuclear star cluster (NSC) using high-resolution near-infrared spectroscopy. We derive effective temperatures from a calibration with Sc line strength, which yields results in good agreement with other methods, and metallicities from spectral fits to Fe I lines. Our derived metallicities range from -1.2 < [Fe/H] < +0.5, a span of 1.7 dex. In addition we use isochrone projection to obtain masses of 1.6 to 4.3 M$_\odot$, and ages assuming single-star evolution. The oldest of these stars is 1.5 Gyr while the youngest and most metal-rich is only 100 Myr. The wide range in metallicity poses interesting questions concerning the chemical evolution and enrichment of the NSC and adds to the evidence for the presence of a young, metal-rich population in the NSC. We suggest that the candidate intermediate-age, metal-poor ([Fe/H] = -1.2) star may be best explained as a blue straggler from an underlying old population.

astro-ph.GA↗

arXiv2024

The Pristine Inner Galaxy Survey (PIGS) VIII: Characterising the orbital properties of the ancient, very metal-poor inner Milky Way

The oldest stars in the Milky Way (born in the first few billion years) are expected to have a high density in the inner few kpc, spatially overlapping with the Galactic bulge. We use spectroscopic data from the Pristine Inner Galaxy Survey (PIGS) to study the dynamical properties of ancient, metal-poor inner Galaxy stars. We compute distances using StarHorse, and orbital properties in a barred Galactic potential. With this paper, we release the spectroscopic AAT/PIGS catalogue (13 235 stars). We find that most PIGS stars have orbits typical for a pressure-supported population. The fraction of stars confined to the inner Galaxy decreases with decreasing metallicity, but many very metal-poor stars (VMP, [Fe/H] < -2.0) stay confined (~ 60% stay within 5 kpc). The azimuthal velocity v$_ϕ$ also decreases between [Fe/H] = -1.0 and -2.0, but is constant for VMP stars (at ~ 40 km/s). The carbon-enhanced metal-poor (CEMP) stars in PIGS appear to have similar orbital properties compared to normal VMP stars. Our results suggest a possible transition between two spheroidal components - a more metal-rich, more concentrated, faster rotating component, and a more metal-poor, more extended and slower/non-rotating component. We propose that the former may be connected to pre-disc in-situ stars (or those born in large building blocks), whereas the latter may be dominated by contributions from smaller galaxies. This is an exciting era where large metal-poor samples, such as in this work (as well as upcoming surveys, e.g., 4MOST), shed light on the earliest evolution of our Galaxy.

astro-ph.GA↗

arXiv2024

Composition of Giants 1$^{\circ}$ North of the Galactic Center: Detailed Abundance Trends for 21 Elements Observed with IGRINS

We report the first high resolution, detailed abundances of 21 elements for giants in the Galactic bulge/bar within $1^\circ$ of the Galactic plane, where high extinction has rendered such studies challenging. Our high S/N and high-resolution, near-infrared spectra of 7 M giants in the inner-bulge, located at ($l,b$)=(0,+1$^{\circ}$), are observed using the IGRINS spectrograph. We report the first multi-chemical study of the inner Galactic bulge, by investigating relative to a robust new Solar Neighborhood sample, the abundance trends of 21 elements, including the relatively difficult to study heavy elements. The elements studied are: F, Mg, Si, S, Ca, Na, Al, K, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Ce, Nd, and Yb. We investigate bulge membership of all seven stars using distances and orbital simulations, and find that the most metal-poor star may be a halo interloper. Our investigation shows that the inner-bulge also as close as $1^\circ$ North of the Galactic Center displays a similarity to the inner disk sequence, following the high [$α$/Fe] envelope of the Solar vicinity metal-rich population, though no firm conclusions for a different enrichment history are evident from this sample. We find a small fraction of metal-poor stars (\feh$<-0.5$) but most of our stars are mainly of super-solar metallicity. Fluorine is found to be enhanced at high metallicity compared to the solar neighbourhood, but confirmation with a larger sample is required. We will apply this approach to explore populations of the Nuclear Stellar Disk and the Nuclear Star Cluster.

astro-ph.GA↗

arXiv2023

Diffuse Interstellar Bands in Gaia DR3 RVS spectra Machine-learning based new measurements

Diffuse interstellar bands (DIBs) are weak and broad interstellar absorption features in astronomical spectra originating from unknown molecules. To measure DIBs in spectra of late-type stars more accurately and more efficiently, we developed a Random Forest model to isolate the DIB features from the stellar components and applied this method to 780 thousand spectra collected by the Gaia Radial Velocity Spectrometer (RVS) that were published in the third data release (DR3). After subtracting the stellar components, we modeled the DIBs $λ$8621 and $λ$8648. After quality control, we selected 7619 reliable measurements. The rest-frame wavelength of DIB $λ$8621 was updated as $λ_0\,{=}\,8623.141\,{\pm}\,0.030$ AA in vacuum, corresponding to 8620.766 AA in air, which was determined by 77 DIB measurements toward the Galactic anti-center. With the peak finding method and a coarse analysis, DIB $λ$8621 was found to correlate better with the neutral hydrogen than the molecular hydrogen (represented by $^{12}$CO $J\,{=}\,(1{-}0)$ emission). We also obtained 179 reliable measurements of DIB $λ$8648 in the RVS spectra of individual stars for the first time, further confirming this very broad DIB feature. A rough estimation of $λ_0$ for DIB $λ$8648 was 8646.31 AA in vacuum, corresponding to 8643.93 AA in air, assuming that the carriers of $λ$8621 and $λ$8648 are co-moving. We confirmed the impact of stellar residuals on the DIB measurements in Gaia DR3, which led to a distortion of the DIB profile and a shift of the center ($\lesssim0.5$ AA), but the EW was consistent with our new measurements. With our measurements and analyses, we propose that the machine-learning-based approach can be widely applied to measure DIBs in numerous spectra from spectroscopic surveys.

astro-ph.GA↗

arXiv2023

Warp and flare of the old Galactic disc as traced by the red clump stars

Our study aims to investigate the outer disc structure of the Milky Way galaxy using the red clump (RC) stars. We analysed the distribution of the largest sample of RC stars to date, homogeneously covering the entire Galactic plane in the range of $40^\circ \le \ell \le 340^\circ$ and $-10^\circ \le b \le +10^\circ$. This sample allows us to model the RC star distribution in the Galactic disc to better constrain the properties of the flare and warp of the Galaxy. Our results show that the scale length of the old stellar disc weakly depends on azimuth, with an average value of $1.95 \pm0.26$ kpc. On the other hand, a significant disc flaring is detected, where the scale height of the disc increases from 0.38 kpc in the solar neighbourhood to $\sim 2.2$ kpc at R $\approx 15$ kpc. The flare exhibits a slight asymmetry, with $\sim 1$ kpc more scale height below the Galactic plane as compared to the Northern flare. We also confirm the warping of the outer disc, which can be modelled with $Z_w = (0.0057 \pm 0.0050)~ [R-(7358 \pm 368) (pc)]^{1.40 \pm 0.09} \sin(ϕ- (-2^\circ.03 \pm 0^\circ .18))$. Our analysis reveals a noticeable north-south asymmetry in the warp, with a greater amplitude observed in the southern direction compared to the northern. Comparing our findings with younger tracers from the literature, we observe an age dependency of both the flare and warp. An increase in flare strength with age suggests the secular evolution of the disc as the preferred mechanism for forming the flare. The increase of the maximum warp amplitude with age indicates that the warp dynamics could be the possible cause of the variation in the warp properties with age.

astro-ph.GA↗

arXiv2023

The Outer spiral arm of the Milky Way using Red Clump stars

Aims: Our aim is to provide an observational view of the old Disc structure of the Milky Way galaxy using the distribution of red clump stars. The spiral arms, warp structure, and other asymmetries present in the Disc are re-visited using a systematic study of red clump star counts over the disc of the Galaxy. Methods: We developed a method to systematically extract the red clump stars from 2MASS ($J-K_s, ~J$) colour-magnitude diagram of $1^\circ \times 1^\circ$ bins in $\ell \times b$ covering the range $40^\circ \le \ell \le 320^\circ$ and $-10^\circ \le b \le 10^\circ$. 2MASS data continues to be important since it is able to identify and trace the red clump stars to much farther distances than any optical survey of the Disc. The foreground star contamination in the selected sample is removed by utilising the accurate astrometric data from Gaia EDR3. Results: We have generated a face-on-view (XY-plane) of the Galaxy depicting the density distribution and count ratio above and below the Galactic plane. The resulting over-density of red clump stars traces the continuous morphology of the Outer arm from the second to the third Galactic quadrant. This is the first study to map the Outer arms across the disc using red clump stars. Through this study, we are able to trace the Outer arm well into the 3rd Galactic quadrant for the first time. Apart from the spiral structures, we also see a wave-like asymmetry above and below the Galactic plane with respect to longitudes indicating the warp structure. The warp structure is studied systematically by tracing the ratio of red clump stars above and below the Galactic plane. We provide the first direct observational evidence of the asymmetry in the Outer spiral arms confirming that the spiral arms traced by the older population are also warped, similar to the Disc.

astro-ph.GA↗

arXiv2023

Searching for clues of past binary supermassive black hole mergers in nuclear star clusters

Galaxy mergers are common processes in the Universe. As a large fraction of galaxies hosts at their centres a central supermassive black hole (SMBH), mergers can lead to the formation of a supermassive black hole binary (SMBHB). The formation of such a binary is more efficient when the SMBHs are embedded in a nuclear star cluster (NSC). NSCs are dense and massive stellar clusters present in the majority of the observed galaxies. Their central densities can reach up to $10^7\,M_\odot/{\rm pc}^3$ and their masses can be as large as a few $10^7\,M_\odot$. The direct detection of an SMBHB is observationally challenging. In this work, we illustrate how the large scale structural and dynamical properties of an NSC can help to identify nucleated galaxies that recently went through a merger that possibly led to the formation of a central SMBHB. Our models show that the merger can imprint signatures on the shape, density profile, rotation and velocity structure of the NSC. The strength of the signatures depends on the mass ratio between the SMBHs and on the orbital initial conditions of the merger. In addition, the number of hypervelocity stars produced in the mergers is linked to the SMBHB properties. The merger can also contribute to the formation of the nuclear stellar disc of the galaxy.

astro-ph.GA↗