The interactions and mergers of gas rich galaxies are known to produce star formation which often leads to nuclear activity as well. The star formation is ideally mapped using FUV and NUV emission, since UV traces star formation for longer timescales compared to Hα emission. It is also emitted over a broader range of stellar masses in galaxies. In this study we present FUV and NUV observations of merging and interacting galaxies in our nearby universe conducted using the UVIT. We present the example of a merging system MRK212 that has dual AGN and the triple AGN system NGC7733-7734. The UV emission is associated with the tidal arms, individual nuclei, resonance rings, nuclear spirals as well as AGN/stellar feedback. We also find that radio emission is often closely associated with the UV emission, arising from both star formation as well as AGN activity, and perhaps kpc-scale AGN feedback. We find that a comparison of optical IFU imaging with FUV in NGC7733-7734 reveals unique properties associated with the interaction including the third AGN buried in a tidal arm.

In the standard cosmological model of galaxy evolution, mergers and interactions play a fundamental role in shaping galaxies. Galaxies that are currently isolated are thus interesting, allowing us to identify how internal or external processes impact galactic structure. However, current observational limits may be obscuring crucial information in the low-mass or low-brightness regime. We use the AMIGA catalog of isolated galaxies to explore the impact of different factors on the structure of these galaxies. In particular, we study the type of disk break based on the degree of isolation and the presence of interactions which are only detectable in the ultra-low surface brightness regime. We present the first results of an extensive observational campaign of ultra-deep optical imaging targeting a sample of 25 low-redshift (z < 0.035) isolated galaxies. The nominal surface brightness limits achieved are comparable to those to be obtained in the 10-year LSST coadds ( mag arcsec−2; 3σ ; 10” × 10”). We find that isolated galaxies have a considerably higher fraction of purely exponential disk profiles and a lower presence of up-bending breaks than field or cluster galaxies. Our extreme imaging depth allows us to detect the presence of previously unreported interactions with minor companions in some of the galaxies in our sample (∼40% of the galaxies show signs of interaction). The results of our work fit with the general framework of galactic structure in which up-bending breaks (Type III) would be produced by mergers and down-bending breaks (Type II) due to a threshold in star formation that would tend to become single exponential disk (Type I) in case of cessation or decrease of star formation.

To study the role of H i content in galaxy interactions, we select galaxy pairs and control galaxies from the SDSS-IV MaNGA IFU survey, adopting kinematic asymmetry as a new effective indicator to describe the merger stage. With archival data from the HI-MaNGA survey and new observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we investigate the differences in H i gas fraction (fH i), star formation rate (SFR), and H i star formation efficiency (SFEH i) between pairs and controls. Our results suggest that on average the H i gas fraction of major-merger pairs is marginally decreased by ∼ 15% relative to isolated galaxies, and paired galaxies during pericentric passage show weakly decreased fH i (−0.10 ± 0.05 dex), significantly enhanced SFR (0.42 ± 0.11 dex), and SFEH i (0.48 ± 0.12 dex). We propose the marginally detected H i depletion may originate from the gas consumption in fueling the enhanced H2 reservoir of galaxy pairs.

Stars interact with their planets through gravitation, radiation, and magnetic fields. Although magnetic activity decreases with time, reducing associated high-energy (e.g., coronal XUV emission, flares), stellar winds persist throughout the entire evolution of the system. Their cumulative effect will be dominant for both the star and for possible orbiting exoplanets, affecting in this way the expected habitability conditions. However, observations of stellar winds in low-mass main sequence stars are limited, which motivates the usage of models as a pathway to explore how these winds look like and how they behave. Here we present the results from a grid of 3D state-of-the-art stellar wind models for cool stars (spectral types F to M). We explore the role played by the different stellar properties (mass, radius, rotation, magnetic field) on the characteristics of the resulting magnetized winds (mass and angular momentum losses, terminal speeds, wind topology) and isolate the most important dependencies between the parameters involved. These results will be used to establish scaling laws that will complement the lack of stellar wind observational constraints.

Mass loss plays a key role in the evolution of massive stars and their environment. High mass-loss events are traced by complex circumstellar ejecta and intricate line profiles across the upper Hertzsprung-Russell diagram for massive stars in different evolutionary stages. The basic physics of radiation-driven stellar wind for hot stars is well understood. However, the driving mechanisms and related instabilities for their enhanced mass-loss episodes and the driving mechanisms for the mass loss of cool stars are still debated. In this review, the mass-loss characteristics and the possible mechanisms will be surveyed for an observational set of prominent massive stellar populations that experience outflows, strong stellar winds, and periods of enhanced and eruptive mass loss; massive young stellar objects, OB-type stars, red supergiants, warm hypergiants, luminous blue variables, and Wolf-Rayet stars.

The stellar wind from low-mass stars affects the evolution of the whole stellar system in various ways. To better describe its quantitative contributions, we need to understand the theoretical aspects of stellar wind formation. Here, we present an overview of the theoretical models of stellar wind. The classical thermally-driven wind model fails in reproducing the anti-correlation between the coronal temperature and wind speed observed in the solar wind, thus needs modification with magnetic-energy injection. Specifically, energy input by Alfvén wave is likely to be important. Indeed, a number of solar-wind observations are well reproduced by the Alfvén-wave models, although it could be risky to directly apply the Alfvén-wave models to general low-mass stars. For a better description of stellar wind from low-mass stars with a variety of activity levels, the hybrid model would be better, in which we consider the effect of flux emergence as well as Alfvén wave.

The CO-to-H2 conversion factor (αCO) is crucial for accurate estimation of the amount and properties of molecular gas. However, αCO is known to vary with environmental conditions, and previous kpc-scale studies have revealed lower αCO in the centers of some barred galaxies, including NGC 3351, 3627, and 4321. We present ALMA Band 3, 6, and 7 observations toward the inner ∼2 kpc of these galaxies tracing 12CO, 13CO, and C18O lines at ∼100 pc resolution. We show that dynamical effects resulting from turbulence/shear can lead to substantially lower αCO in the bar-driven inflows of NGC 3351 due to lower optical depth. A clear, positive correlation between αCO and 12CO optical depth is seen in all three galaxy centers. We also find that the CO/13CO(2–1) ratio mainly traces the 12CO optical depth, and thus it may be a useful observable in predicting αCO variation in galaxy centers.

We investigated the evolution of HS Hya system’s inclination based on analysis of its light curves in the period 1964–2021. HS Hya is EA type eclipsing binary star, belonging to separate group with changing orbital inclination. We used our recent observations as well as the data from sky surveys.

Feedback from supernovae (SNe) is an essential mechanism that self-regulates the growth of galaxies. We build an SN feedback model based on high-resolution simulations of superbubble and SN-driven outflows for the physical understanding of the galaxy–CGM connection. Using an Eulerian hydrodynamic code Athena++, we find universal scaling relations for the time evolution of superbubble momentum, when the momentum and time are scaled by those at the shell-formation time. We then develop an SN feedback model utilizing Voronoi tessellation, and implement it into the GADGET3-Osaka smoothed particle hydrodynamic code. We show that our stochastic thermal feedback model produces galactic outflow that carries the metals high above the galactic plane but with weak suppression of star formation. Additional mechanical feedback further suppresses star formation. Therefore, we argue that both thermal and mechanical feedback is necessary for the SN feedback model of galaxy evolution when an individual SN bubble is unresolved.

Due to observational constraints, our detailed knowledge of stellar populations, formation, and evolution of galaxies is limited to a few dozen galaxies located in the Local Group. The Local Group of galaxies offers a unique opportunity to construct the formation histories and probe the structure and dynamics of many dwarf galaxies surrounding the Milky Way and Andromeda and of isolated dwarf galaxies. In this regard, we monitored the majority of galaxies in the Local Group, including the M33 galaxy and satellites galaxies surrounding the Milky Way and Andromeda galaxy, as well as isolated dwarf galaxies. We identified stellar populations and based on light curve analysis, the cool evolved stars pulsating in the fundamental mode were identified. In this paper, first, we will present the results we obtained for SFH and dust production rate in individual galaxies separately to answer how different types of galaxies have been formed and evolved over cosmic time. Then, we will discuss whether the mass return from dusty evolved stars can provide enough gas reservoirs to sustain the star formation or even rejuvenate the dwarf galaxy, as some seem to harbor relatively young stars.

In this invited review talk I summarize some of the recent observational advances in understanding mass loss from low-mass stars. This can take the form of a relatively steady wind, or stochastically occurring coronal mass ejections (CMEs). In recent years, there has been an expansion of observational signatures used to probe mass loss in low-mass stars. These observational tools span the electromagnetic spectrum. There has also been a resurgence of interest in this topic because of its potential impact on exoplanet space weather and habitability. The numerous recent observational and theoretical results also point to the complexities involved, rather than using simple scalings from solar understanding. This underscores the need to understand reconnection and eruption processes on magnetically active stars as a tool to putting our Sun in context.

The long-term behavior of a colliding wind binary WR 25 is presented using archival X-ray data obtained over a time span of : 16 years. The present analysis reveals phase-locked variations repeating consistently over many consecutive orbits of the source (with binary orbital period : 208 days). A significant deviation of the X-ray flux with respect to the theoretical 1/D trend (D is the binary separation) close to periastron passage has been observed. This may occur due to the shifting of the adiabatic wind collision to the radiative regime in that part of the orbit. Further, no signature of X-ray emission in 10.0-79.0 keV energy range attributable to inverse Compton scattering is detected by NuSTAR.

We explore the relationship between globular cluster total number, NGC, and central black hole mass, M•, in spiral galaxies. Including cosmic scatter, log M• ∝ (1.64 ± 0.24) log NGC. Whereas in ellipticals the correlation is linear [log M• ∝ (1.02 ± 0.10) log NGC], and hence could be due to statistical convergence through mergers, this mechanism cannot explain the much steeper correlation in spirals. Additionally, we derive total stellar galaxy mass, M*, from its two-slope correlation with NGC (Hudson et al. 2014). In the M• versus M* parameter space, with M* derived from NGC, M• ∝ (1.48 ± 0.18) log M* for ellipticals, and M• ∝ (1.21 ± 0.16) log M* for spirals. The observed agreement between ellipticals and spirals may imply that black holes and galaxies co-evolve through “calm” accretion, AGN feedback and other secular processes.

We studied the problem of two spherical celestial bodies in the general case when the masses of the bodies change non-isotropically at different rates in the presence of reactive forces. The problem was investigated by methods of perturbation theory based on aperiodic motion along a quasi-conic section, using the equation of perturbed motion in the form of Newton’s equations. The problem is described by the variables a, e, i, π, ω, λ, which are analogs of the corresponding Keplerian elements and the equations of motion in these variables are obtained. Averaging over the mean longitude, we obtained the evolution equations of the two-body problem with variable masses in the presence of reactive forces. The obtained evolution equations have the exact analytic integral ${a^3 e^4 = a^3_0 e^4_0} = {const}$.

We studied the probability distribution function of the column density (N-PDF) of molecular clouds based on a fit with a multi-log-normal function using the Nobeyama 45-m Cygnus X CO survey data. We identified 124 molecular clouds in 13CO data using the DENDROGRAM and SCIMES algorithms. The N-PDF was constructed for 11 extended (≥ 0.4 deg2) molecular clouds of these identified clouds. We found that every N-PDF is well-fitted with one or two log-normal (LN) distributions. We investigated the distributions of the column density, C18O dense cores, and radio continuum source in each cloud and found that the N-PDF was less correlated with the star-forming activity. The LN N-PDF parameters showed two impressive features. First, the LN distribution at the low-density part had the same mean column density (∼1021.5 cm−2 ) for almost all the molecular clouds. Second, the wider LN distribution tended to show the lower mean density of the structures.

Star formation laws are empirical relations between the cold gas (HI+H2) content of a galaxy and its star formation rate (SFR), being crucial for any model of galaxy formation and evolution. A well known example of such laws is the Schmidt-Kennicutt law, which is based on the projected surface densities. However, it has been long unclear whether a more fundamental relation exists between the intrinsic volume densities. By assuming the vertical hydrostatic equilibrium, we infer radial profiles for the thickness of gaseous discs in a sample of 23 local galaxies, and use these measurements to convert the observed surface densities of the gas and the SFR into the de-projected volume densities. We find a tight correlation linking these quantities, that we call the volumetric star formation law. This relation and its properties have crucial implications for our understanding of the physics of star formation.

We report a CO(3-2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with ALMA. The observed 1 kpc2 region is at about 1.24R25 from the disk center, where CO(2-1) was previously not detected. The detection and non-detection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A – with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud masses, estimated from CO(3-2), range from 8.2×102 to 2.3×104M⊙. The most massive clouds appear similar to Orion A in SF activity as well as in gas mass. The common cloud mass structure also justifies the use of high-J CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-z universe. This study is the first demonstration that CO(3-2) is an efficient tracer of molecular clouds even in low-metallicity environments. This study is published in the Astronomical Journal, entitled “First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83" by J. Koda, L. Watson, F. Combes, M. Rubio, S. Boissier, M. Yagi, D. Thilker, A. M Lee, Y. Komiyama, K. Morokuma-Matsui, and C. Verdugo.

Red supergiants (RSGs) are evolved massive stars in a stage preceding core-collapse supernova. Understanding evolved-phases of these cool stars is key to understanding the cosmic matter cycle of our Universe, since they enrich the cosmos with newly formed elements. However, the physical processes that trigger mass loss in their atmospheres are still not fully understood, and remain one of the key questions in stellar astrophysics. We use a new method to study the extended atmospheres of these cold stars, exploring the effect of a stellar wind for both a simple radiative equilibrium model and a semi-empirical model that accounts for a chromospheric temperature structure. We then can compute the intensities, fluxes and visibilities matching the observations for the different instruments at the Very Large Telescope Interferometer (VLTI). Specifically, when comparing with the atmospheric structure of HD 95687 based on published VLTI/AMBER data, we find that our model can accurately match these observations in the K-band, showing the enormous potential of this methodology to reproduce extended atmospheres of RSGs.

The evolution of star formation properties of galaxies depends on the environment where galaxies reside, and generally star formation of galaxies in dense environment decreases more quickly. Interestingly, the star formation property of high-redshift galaxies clusters vary largely even though they are at similar redshift. We have found that the large-scale environment surrounding each galaxy cluster can contribute to make this cluster-by-cluster variation. This correlation is found in the results from observational data as well as in the simulations of galaxy formation. We suggest the ‘Web-feeding model’ to explain this trend. Star-forming galaxies falling into the galaxy cluster from surrounding large-scale structure make the quiescent galaxy fraction of the cluster lower than relatively isolated clusters.

Ultra-luminous infrared galaxies (ULIRGs) are extreme in many ways. The major mergers trigger star formation at very high rates that cause the ISM to be dominated by infrared (IR) photons. We show the ammonia spectra toward the two cores of Arp 220, the nearest ULIRG, in three Very Large Array (VLA) bands (Ku, K, Ka). Typical decay times of the non-metastable transitions ∼ 100 s and are therefore usually difficult to observe. The FIR excitation of Arp 220, however, shows that non-metastable states are widely populated up to a limiting energy of ∼ 1500 K. We assume that this atypical ammonia spectrum is due to the strong FIR field that re-excites the ammonia molecule on timescales much shorter than the already short decay times. The resulting level population causes a break-down of the typical assumptions made for the use of ammonia as a molecular thermometer.