Spiral Galaxies Research Articles

Photometric properties of classical bulge and pseudo-bulge galaxies at 0.5 ≤ z < 1.0

Context. We compare the photometric properties and specific star formation rate (sSFR) of classical- and pseudo-bulge galaxies with M∗ ≥ 109.5 M⊙ at 0.5 ≤ z < 1.0, selected from all five CANDELS fields. We also compare these properties of bulge galaxies at lower redshift selected from MaNGA survey in previous work. Aims. This paper aims to study the properties of galaxies with classical and pseudo-bulges at intermediate redshift, to compare the differences between different bulge types, and to understand the evolution of bulges with redshift. Methods. Galaxies are classified into classical bulge and pseudo-bulge samples according to the Sérsic index n of the bulge component based on results of two-component decomposition of galaxies, as well as the position of bulges on the Kormendy diagram. For the 105 classical bulge and 86 pseudo-bulge galaxies selected, we compare their size, luminosity, and sSFR of various components. Results. At a given stellar mass, most classical bulge galaxies have smaller effective radii, larger B/T, brighter and relatively larger bulges, and less active star formation than pseudo-bulge galaxies. In addition, the two types of galaxies have larger differences in sSFR at large radii than at the central region at both low- and mid-redshifts. Conclusions. The differences between the properties of the two types of bulge galaxies are generally smaller at mid-redshift than at low-redshift, indicating that they are evolving to more distinct populations towards the local universe. Bulge type is correlated with the properties of their outer disks, and the correlation is already present at redshifts as high as 0.5 < z < 1.

First Detection of Molecular Gas in the Giant Low Surface Brightness Galaxy Malin 1

After over three decades of unsuccessful attempts, we report the first detection of molecular gas emission in Malin 1, the largest spiral galaxy observed to date, and one of the most iconic giant low surface brightness galaxies. Using Atacama Large Millimeter/submillimeter Array, we detect significant 12CO (J = 1–0) emission in the galaxy’s central region and tentatively identify CO emission across three regions on the disk. These observations allow for a better estimate of the H2 mass and molecular gas mass surface density, both of which are remarkably low given the galaxy’s scale. By integrating data on its H i mass, we derive a very low molecular-to-atomic gas mass ratio. Overall, our results highlight the minimal presence of molecular gas in Malin 1, contrasting sharply with its extensive, homogeneous atomic gas reservoir. For the first time, we position Malin 1 on the Kennicutt–Schmidt diagram, where it falls below the main sequence for normal spirals, consistent with previous upper limits but now with more accurate figures. These findings are crucial for constraining our understanding of star formation processes in environments characterized by extremely low molecular gas densities and for refining models of galaxy formation, thereby improving predictions concerning the formation, evolution, and distribution of these giant, elusive galaxies.

The M•–σe relation for local type 1 AGNs and quasars

We analyzed Multi Unit Spectroscopic Explorer observations of 42 local z ≲ 0.1 type 1 active galactic nucleus (AGN) host galaxies taken from the Palomar-Green quasar sample and the close AGN reference survey. Our goal was to study the relation between the black hole mass (M•) and bulge stellar velocity dispersion (σe) for type 1 active galaxies. The sample spans black hole masses of 106.0 − 109.2 M⊙, bolometric luminosities of 1042.9 − 1046.0 erg s−1, and Eddington ratios of 0.006 − 1.2. We avoided AGN emission by extracting the spectra over annular apertures. We modeled the calcium triplet stellar features and measured stellar velocity dispersions of σ* = 60 − 230 km s−1 for the host galaxies. We find stellar velocity dispersion values in agreement with previous measurements for local (z ≲ 0.1) AGN host galaxies, but slightly lower compared with those reported for nearby X-ray-selected type 2 quasars. Using a novel annular aperture correction recipe to estimate σe from σ* that considers the bulge morphology and observation beam-smearing, we estimate flux-weighted σe = 60 − 250 km s−1. If we consider the bulge type when estimating M•, we find no statistical difference between the distributions of AGN hosts and the inactive galaxies on the M•–σe plane for M• ≲ 108 M⊙. Conversely, if we do not consider the bulge type when computing M•, we find that both distributions disagree. We find no correlation between the degree of offset from the M•–σe relation and Eddington ratio for M• ≲ 108 M⊙. The current statistics preclude firm conclusions from being drawn for the high-mass range. We argue these observations support notions that a significant fraction of the local type 1 AGNs and quasars have undermassive black holes compared with their host galaxy bulge properties.

Probing the Dust Grain Alignment Mechanisms in Spiral Galaxies with M51 as the Case Study

Magnetic fields (B fields) in galaxies have recently been traced using far-infrared and submillimeter polarimetric observations with the Stratospheric Observatory for Infrared Astronomy, James Clerk Maxwell Telescope, and Atacama Large Millimeter/submillimeter Array. The main assumption is that dust grains are magnetically aligned with the local B-field in the interstellar medium (ISM). However, the range of physical conditions of the ISM, dust grain sizes, and B-field strengths in galaxies where this assumption is valid has not been characterized yet. Here, we use the well-studied spiral galaxy M51 as a case study. We find that the timescale for the alignment mechanism arising from magnetically aligned dust grains (B-RAT) dominates over other alignment mechanisms, including radiative precession (k-RAT) and mechanical alignment (v-MAT), as well as the randomization effect (gas damping). We estimate the sizes of the aligned dust grain to be in the range of 0.009–0.182 μm and 0.019–0.452 μm for arms and interarms, respectively. We show that the difference in the polarization fraction between arms and interarms may arise from the enhancement of small dust grain sizes in the arms resulting from large grains being broken into small grains as an effect of the grain alignment disruption (RAT-D) mechanism. We argue that the RAT-D mechanism needs to have additional effects, e.g., intrinsic variations of the B-field structure and turbulence, in the galaxy’s components to fully explain the polarization fraction variations within the arms and interarms.

Abundant Molecular Gas in the Central Region of Lenticular Galaxy PGC 39535

Lenticular galaxies (S0s) in the local Universe are generally absent of recent star formation and lack molecular gas. In this paper, we investigate one massive (M * ∼ 5 × 1010 M ⊙) star-forming S0, PGC 39535, with the Northern Extended Millimeter Array (NOEMA). Using optical data from the Sloan Digital Sky Survey IV Mapping Nearby Galaxies at the Apache Point Observatory survey, we find star formation mainly concentrates in the central region of PGC 39535. The total star formation rate estimated using extinction-corrected Hα flux is 1.57 M ⊙ yr−1. The results of the NOEMA observation suggest that the molecular gas mainly concentrates in the central regions as a gaseous bar and a ring-like structure, and shows similar kinematics as the stellar and ionized gas components. The total molecular gas mass estimated from CO(1–0) is (5.42 ± 1.52) × 109 M ⊙. We find PGC 39535 lies on the star-forming main sequence but falls below the Kennicutt–Schmidt relation of spiral galaxies, suggesting that the star formation efficiency may be suppressed by the massive bulge. The existence of a second Gaussian component in the CO spectrum of the central region indicates possible gas flows. Furthermore, our analyses suggest that PGC 39535 resides in the center of a massive group and the derived star formation history indicates it may experience a series of gas-rich mergers over the past 2–7 Gyr.

Dynamical friction in rotating ultralight dark matter galactic cores

Abstract Dynamical friction and stellar orbital motion in spiral galaxies with dark matter composed of ultralight bosons in the state of rotating Bose-Einstein condensate (BEC) are studied. It is found that the dynamical friction force is significantly affected by the topological charge of the vortex structure of the BEC core with the strongest effect at distances near the galactic center. It is also shown that the ultralight dark matter self-interaction plays an important role in studying the dynamical friction.

Close Binary Fractions in accreted and in-situ Halo Stars

Abstract The study of binary stars in the Galactic halo provides crucial insights into the dynamical history and formation processes of the Milky Way. In this work, we aim to investigate the binary fraction in a sample of accreted and in-situ halo stars, focusing on short-period binaries. Utilising data from Gaia DR3, we analysed the radial velocity (RV) uncertainty σRV distribution of a sample of main-sequence stars. We used a novel Bayesian framework to model the dependence in σRV of single and binary systems allowing us to estimate binary fractions F in a sample of bright (GRVS< 12) Gaia sources. We selected the samples of in-situ and accreted halo stars based on estimating the 6D phase space information and affiliating the stars to the different samples on an action-angle vs energy (Lz − E) diagram. Our results indicate a higher, though not significant, binary fraction in accreted stars compared to the in-situ halo sample. We further explore binary fractions using cuts in E and Lz, and find a higher binary fraction in both high-energy and prograde orbits that might be explained by differences in metallicity. By cross-matching our Gaia sample with APOGEE DR17 catalogue, we confirm the results of previous studies on higher binary fractions in metal-poor stars and find the fractions of accreted and in-situ halo stars consistent with this trend. Our finding provides new insights into binary stars’ formation processes and dynamical evolution in the primordial Milky Way Galaxy and its accreted dwarf Galaxies.

Asymmetry in Galaxy Spin Directions: A Fully Reproducible Experiment Using HSC Data

The asymmetry in the large-scale distribution of the directions in which spiral galaxies rotate has been observed by multiple telescopes, all showing a consistent asymmetry in the distribution of galaxy spin directions as observed from Earth. Here, galaxies with a redshift from HSC DR3 are annotated by their direction of rotation, and their distribution is analyzed. The results show that galaxies that rotate in the opposite direction relative to the Milky Way as observed from Earth are significantly more prevalent compared to galaxies that rotate in the same direction relative to the Milky Way. The asymmetry also forms a dipole axis that becomes stronger when the redshift gets higher. These results are aligned with observations from virtually all premier digital sky surveys, as well as space telescopes such as the HST and the JWST. This shows that the distribution of galaxy spin directions as observed from Earth is not symmetrical, and has a possible link to the rotational velocity of the Milky Way. This experiment provides data, code, and a full protocol that allows the results to be easily reproduced in a transparent manner. This practice is used to overcome the “reproducibility crisis” in science.

Bar Presence in Local Galaxies: Dependence on Morphology in Field Galaxies

Abstract We analyzed the fractions of barred galaxies in the local universe using a volume-limited sample of galaxies from the Sloan Digital Sky Survey Data Release 3. We examined 116 field galaxies with redshifts between 0.0207 and 0.030, using r and z-band images. Overall, the bar fraction was 26% in the r-band and 19% in the z-band. For distinct morphological groups, barred spiral galaxies had fractions of 33% in the r-band and 22% in the z-band, while barred lenticular galaxies had 25% in the r-band and 12% in the z-band. We observed that the bar fraction in spiral galaxies increases for stellar masses log(M∗/M⊙) > 10.5 and for galaxies with red colors (u − r) > 2.0. Additionally, most barred galaxies have a bulge-to-total ratio B/T ≤ 0.2. Our results indicate that the bar fraction is more dependent on internal morphology than on the galaxy environment.

Galaxy Classification Using EWGC

The Enhanced Wide-field Galaxy Classification Network (EWGC) is a novel architecture designed to classify spiral and elliptical galaxies using Wide-field Infrared Survey Explorer (WISE) images. The EWGC achieves an impressive classification accuracy of 90.02%, significantly outperforming the previously developed WGC network and underscoring its superior performance in galaxy morphology classification. Remarkably, the network demonstrates a consistent accuracy of 90.02% when processing both multi-target and single-target images. Such robustness indicates the EWGC’s versatility and potential for various applications in galaxy classification tasks.

Chemical Evolution of R-process Elements in Stars (CERES). II. The impact of stellar evolution and rotation on light and heavy elements

Carbon, nitrogen, and oxygen are the most abundant elements throughout the universe, after hydrogen and helium. Studying these elements in low-metallicity stars can provide crucial information on the chemical composition in the early Galaxy and possible internal mixing processes that can alter the surface composition of the stars. This work aims to investigate the chemical abundance patterns for CNO elements and Li in a homogeneously analyzed sample of 52 metal-poor halo giant stars. From these results, we have been able to determine whether internal mixing processes have taken place in these stars. We used high-resolution spectra with a high signal-to-noise ratio (S/N) to carry out a spectral synthesis to derive detailed C, N, O, and Li abundances for a sample of stars with metallicities in the range of$-$3.58 leq Fe/H leq $-$1.79\,dex. Our study was based on the assumption of one-dimensional (1D) local thermodynamic equilibrium (LTE) atmospheres. Based on carbon and nitrogen abundances, we investigated the deep mixing taking place within stars along the red giant branch (RGB). The individual abundances of carbon decrease towards the upper RGB while nitrogen shows an increasing trend, indicating that carbon has been converted into nitrogen. No signatures of ON-cycle processed material were found for the stars in our sample. We computed a set of galactic chemical evolution (GCE) models, implementing different sets of massive star yields, both with and without including the effects of stellar rotation on nucleosynthesis. We confirm that stellar rotation is necessary to explain the highest N/Fe and N/O ratios observed in unmixed halo stars. The predicted level of N enhancement varies sensibly in dependence of the specific set of yields that are adopted. For stars with stellar parameters similar to those of our sample, heavy elements such as Sr, Y, and Zr appear to have unchanged abundances despite the stellar evolution mixing processes. The unmixed RGB stars provide very useful constraints on chemical evolution models of the Galaxy. As they are more luminous than unevolved (main sequence and turnoff) stars, they also allow for stars to be probed at greater distances. The stellar CN-cycle clearly changes the atmospheric abundances of the lighter elements, but no changes were detected with respect to the heavy elements.

Filling in the blanks

Context. In the solar neighbourhood, only ∼2% of stars in the Gaia survey have a line-of-sight velocity (vlos) contained within the RVS catalogue. These limitations restrict conventional dynamical analysis, such as finding and studying substructures in the stellar halo. Aims. We aim to present and test a method to infer a probability density function (PDF) for the missing vlos of a star with 5D information within 2.5 kpc. This technique also allows us to infer the probability that a 5D star is associated with the Milky Way’s stellar Disc or the stellar Halo, which can be further decomposed into known stellar substructures. Methods. We use stars from the Gaia DR3 RVS catalogue to describe the local orbital structure in action space. The method is tested on a 6D Gaia DR3 RVS sample and a 6D Gaia sample crossmatched to ground-based spectroscopic surveys, stripped of their true vlos. The stars predicted vlos, membership probabilities, and inferred structure properties are then compared to the true 6D equivalents, allowing the method’s accuracy and limitations to be studied in detail. Results. Our predicted vlos PDFs are statistically consistent with the true vlos, with accurate uncertainties. We find that the vlos of Disc stars can be well-constrained, with a median uncertainty of 26 km s−1. Halo stars are typically less well-constrained with a median uncertainty of 72 km s−1, but those found likely to belong to Halo substructures can be better constrained. The dynamical properties of the total sample and subgroups, such as distributions of integrals of motion and velocities, are also accurately recovered. The group membership probabilities are statistically consistent with our initial labelling, allowing high-quality sets to be selected from 5D samples by choosing a trade-off between higher expected purity and decreasing expected completeness. Conclusions. We have developed a method to estimate 5D stars’ vlos and substructure membership. We have demonstrated that it is possible to find likely substructure members and statistically infer the group’s dynamical properties.

The R-process Alliance: Fifth Data Release from the Search for R-process-enhanced Metal-poor Stars in the Galactic Halo with the GTC* * This paper includes data gathered with the 10.4 m Gran Telescopio Canarias located at La Palma, Canary Islands, Spain.

Understanding the abundance pattern of metal-poor stars and the production of heavy elements through various nucleosynthesis processes offers crucial insights into the chemical evolution of the Milky Way, revealing primary sites and major sources of rapid neutron-capture process (r-process) material in the Universe. In this fifth data release from the R-Process Alliance (RPA), we present the detailed chemical abundances of 41 faint (down to V = 15.8) and extremely metal-poor (down to [Fe/H] = −3.3) halo stars selected from the RPA. We obtained high-resolution spectra for these objects with the HORuS spectrograph on the Gran Telescopio Canarias. We measure the abundances of light, α, Fe-peak, and neutron-capture elements. We report the discovery of five carbon-enhanced metal-poor, one limited-r, three r-I, and four r-II stars, and six Mg-poor stars. We also identify one star of a possible globular cluster origin at an extremely low metallicity at [Fe/H] = −3.0. This adds to the growing evidence of a lower-limit metallicity floor for globular cluster abundances. We use the abundances of Fe-peak elements and the α-elements to investigate the contributions from different nucleosynthesis channels in the progenitor supernovae. We find the distribution of [Mg/Eu] as a function of [Fe/H] to have different enrichment levels, indicating different possible pathways and sites of their production. We also reveal differences in the trends of the neutron-capture element abundances of Sr, Ba, and Eu of various r-I and r-II stars from the RPA data releases, which provide constraints on their nucleosynthesis sites and subsequent evolution.

The host of GRB 171205A in 3D

Long GRB hosts at z < 1 are usually low-mass, low-metallicity star-forming galaxies. Here we present the most detailed, spatially resolved study of the host of GRB 171205A so far, a grand-design barred spiral galaxy at z = 0.036. Our analysis includes MUSE integral field spectroscopy complemented with high-spatial-resolution UV/VIS HST imaging and CO(1−0) and H I 21 cm data. The GRB is located in a small star-forming region in a spiral arm of the galaxy at a deprojected distance of ∼8 kpc from the center. The galaxy shows a smooth negative metallicity gradient and the metallicity at the GRB site is half solar, slightly below the mean metallicity at the corresponding distance from the center. Star formation in this galaxy is concentrated in a few H II regions between 5 and 7 kpc from the center and at the end of the bar, inwards from the GRB region; however the H II region hosting the GRB is in the top 10% of the regions with the highest specific star-formation rate. The stellar population at the GRB site has a very young component (< 5 Myr) that contributes a significant part of the light. Ionized and molecular gas show only minor deviations at the end of the bar. A parallel study found an asymmetric H I distribution and some additional gas near the position of the GRB, which might explain the star-forming region of the GRB site. Our study shows that long GRBs can occur in many types of star-forming galaxies; however the actual GRB sites have consistently low metallicity, high star formation rates, and a young population. Furthermore, gas inflow or interactions triggering the star formation producing the GRB progenitor might not be evident in ionized or even molecular gas but only in H I.

Neutral atomic and molecular gas dynamics in the nearby spiral galaxies NGC 1512, NGC 4535, and NGC 7496

Neutral atomic gas (H I) effectively traces galactic dynamics across mid to large galactocentric radii. However, its limitations in observing small-scale changes within the central few kiloparsecs, coupled with the often observed H I deficit in galactic centers, necessitates the use of molecular gas emission as a preferred tracer in these regions. Understanding the dynamics of both neutral atomic and molecular gas is crucial for a more complete understanding of how galaxies evolve, funnel gas from the outer disk into their central parts, and eventually form stars. In this work we aim to quantify the dynamics of both, the neutral atomic and molecular gas, in the nearby spiral galaxies NGC 1512, NGC 4535, and NGC 7496 using new MeerKAT H I observations together with ALMA CO (2-1) observations from the PHANGS collaboration. We use the analysis tool 3DBarolo to fit tilted ring models to the H I and CO observations. A combined approach of using the H I to constrain the true disk orientation parameters before applying these to the CO datasets is tested. This paper sets expectations for the results of the upcoming high-resolution H I coverage of many galaxies in the PHANGS-ALMA sample using MeerKAT or VLA, to establish a robust methodology for characterizing galaxy orientations and deriving dynamics from combing new H I with existing CO data.

The Clearing Timescale for Infrared-selected Star Clusters in M83 with HST

We present an analysis of Hubble Space Telescope data from Wide Field Camera 3 (WFC3)/Ultraviolet Imaging Spectrometer, WFC3/IR, and the Advanced Camera for Surveys, investigating the young stellar cluster (YSC) population in the face-on spiral galaxy M83. Within the field of view of the IR pointings, we identify 454 sources with compact F814W continuum and Paβ line emission with a S/N ≥ 3 as possible YSC candidates embedded in dust. We refine this selection to 97 candidates based on their spectral energy distributions, multiwavelength morphology, and photometric uncertainties. For sources that are detected in all bands and have mass >102.8 M ⊙ (53 sources), we find that by 2 Myr 75% of IR-selected star clusters have an A V ≤ 1 and that by 3 Myr the fraction rises to ∼82%. This evidence of early clearing implies that presupernova (pre-SN) feedback from massive stars is responsible for clearing the majority of the natal gas and dust that surround IR-selected star clusters in M83. Further, this result is consistent with previous estimates based on WFC3 observations and adds to the growing body of literature suggesting pre-SN feedback to be crucial for YSC emergence in normal star-forming galaxies. Finally, we find a weak correlation between the YSC concentration index and age over the first 10 Myr, which matches previous studies and indicates little or no change in the size of YSCs in M83 during their early evolution.

Comparison of physical characteristics of mass and luminosity function of disk systems in barred and unbarred spiral galaxies

إحدى أهم الطرق لتقصي توزيع المجرات عبر الزمن الكوني هي دالة اللمعان LF بدلالة كتلة القرص الباريوني ψS(Mb)، القدر . لقد درسنا تقديرًا لكثافة كتلة الباريون في عينة من المجرات الحلزونية القضيبية وغير القضيبية من الادبيات السابقة، والتي تتضمن فعليًا، لكل صنف من الاجرام السماوية ذات المحتوى الباريون المرئي، جزءًا لا يتجزأ من ناتج دالة الضيائية (LF) ونسبة الكتلة إلى الضوء. استخدمت تقنية الانحدار المتعدد لحزمة البرامج الإحصائية في دراستنا ونتائجنا، مثل برنامج تحليل قواعد البيانات والرسوم البيانية)برامج Statistics Win و(Origin Pro . وفقًا للتحليل الإحصائي، هناك علاقة إيجابية قوية وارتباط وثيق للغاية (α MB~1), ، وغالبًا ما تظهر المجرات الحلزونية القرصية القضيبية وغير القضيبية قدراً مطلقاً بحدود MB < -18 mag . "الركبة" لدالة الضيائية للمجرات الحلزونية تبين قطعًا كبيرًا عند كتلة باريونية تبلغ Mb > 1010 Mʘ للمجرات الحلزونية القضيبية وغير القضيبية. يوفر هذا دليلاً يدعم الفرضية القائلة بأن اللوالب الحلزونية لنظام القرص بدأت تتشكل داخل عتبة كتلة متزايدة. نظرًا لأن زيادة دالة الكتلة الأولية للنجم مع الانزياح نحو الأحمر تكون أسرع بكثير، فقد أشارت النتائج التي توصلنا إليها إلى أن دالة الكتلة الأولية المنتقلة ψS(Mb) للمجرات القضيبية وغير القضيبيةعند انزياح أحمر مرتفع z > 0.027 للمجرات االقضيبية وz > 0.02للمجرات غير القضيبية والذي يبدو أنه يتناقص مقارنة بالكون الحرج.

Galactic-Seismology Substructures and Streams Hunter with LAMOST and Gaia. I. Methodology and Local Halo Results

We present a novel, deep-learning-based method—dubbed Galactic-Seismology Substructures and Streams Hunter, or GS3 Hunter for short—to search for substructures and streams in stellar kinematics data. GS3 Hunter relies on a combined application of Siamese neural networks to transform the phase space information and the K-means algorithm for the clustering. As a validation test, we apply GS3 Hunter to a subset of the Feedback in Realistic Environments (FIRE) cosmological simulations. The stellar streams and substructures thus identified are in good agreement with corresponding results reported earlier by the FIRE team. In the same vein, we apply our method to a subset of local halo stars from the Gaia Early Data Release 3 and GALAH DR3 data sets and recover several previously known dynamical groups, such as Thamnos 1+2, the hot thick disk, ED-1, L-RL3, Helmi 1+2, Gaia-Sausage-Enceladus, Sequoia, Virgo Radial Merger, Cronus, and Nereus. Finally, we apply our method without fine-tuning to a subset of K giant stars located in the inner halo region, obtained from the LAMOST Data Release 5 data set. We recover three previously known structures (Sagittarius, Hercules-Aquila Cloud, and the Virgo Overdensity), but we also discover a number of new substructures. We anticipate that GS3 Hunter will become a useful tool for the community dedicated to the search for stellar streams and structures in the Milky Way (MW) and the Local Group, thus helping advance our understanding of the stellar inner and outer halos and the assembly and tidal stripping history in and around the MW.

Uncovering the first-infall history of the LMC through its dynamical impact in the Milky Way halo

ABSTRACT The gravitational interactions between the LMC and the Milky Way cause dynamical perturbations in the MW halo, leading to biased distributions of stellar density and kinematics. We run 50 high-resolution N-body simulations exploring varying masses and halo shapes of the MW and LMC to study the evolution of LMC-induced perturbations. By measuring mean velocities of simulated halo stars, we identify a discontinuity between the first-infall and second-passage scenarios of the LMC’s orbital history. In the first-infall, the Galactocentric latitudinal velocity hovers around 16 km s$^{-1}$ for stars at 50–100 kpc, while it subsides to about 8 km s$^{-1}$ in the second-passage scenario. We demonstrate that, this reduced perturbation magnitude in the second-passage scenario is mainly due to the short dynamical times of the Galactic inner halo and the lower velocity of the LMC during its second infall into the MW. Using a subset of $\sim 1100$ RR Lyrae stars located in the outer halo (50 kpc $\le R_{\mathrm{GC}}\lt $ 100 kpc) with precise distance estimates from Gaia, we find the mean latitudinal velocity ($v_{b}$) in the Galactocentric frame to be $\langle v_{b} \rangle =18.1 \pm 4.1$ km s$^{-1}$. The observation supports the first-infall scenario with a massive LMC ($\sim$$2.1 \times 10^{11} \, \mathrm{M}_{\odot }$) at infall, an oblate MW halo with a virial mass $M_{200}\lt 1.4\times 10^{12}\,\mathrm{M}_{\odot }$ and a flattening parameter $q\gt 0.7$. Our study indicates that LMC-induced kinematic disturbances can reveal its orbital history and key characteristics, as well as those of the MW. This approach shows promise in helping determine fundamental parameters of both galaxies.

A spectroscopic and kinematic survey of fast hot subdwarfs

Hot subdwarfs (sdO/B) are the stripped helium cores of red giants formed via binary interactions. Close hot subdwarf binaries with massive white dwarf companions have been proposed as possible progenitors of thermonuclear supernovae type Ia (SN Ia). If the supernova is triggered by stable mass transfer from the helium star, the companion should survive the explosion and should be accelerated to high velocities. The hypervelocity star US 708 is regarded as the prototype for such an ejected companion. To find more of those objects we conducted an extensive spectroscopic survey. Candidates for such fast stars have been selected from the spectroscopic database of the Sloan Digital Sky Survey (SDSS) and several ground-based proper-motion surveys. Follow-up spectroscopy has been obtained with several 4m- to 10m-class telescopes. Combining the results from quantitative spectroscopic analyses with space-based astrometry from Gaia Early Data Release 3 (EDR3) we determined the atmospheric and kinematic parameters of 53 fast hot subdwarf stars. None of these stars is unbound to the Galaxy, although some have Galactic restframe velocities close to the Galactic escape velocity. 21 stars are apparently single objects, that crossed the Galactic disc within their lifetimes in the sdO/B stage and could be regarded as potential candidates for the SN Ia ejection scenario. However, the properties of the full sample are more consistent with a pure old Galactic halo population. We therefore conclude that the fast sdO/B stars we found are likely to be extreme halo stars.