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The European Space Agency has invested heavily in two cornerstones missions: Herschel and Planck. The legacy data from these missions provides an unprecedented opportunity to study cosmic dust in galaxies so that we can, for example, answer fundamental questions about the origin of the chemical elements, physical processes in the interstellar medium (ISM), its effect on stellar radiation, its relation to star formation and how this relates to the cosmic far-infrared background. In this paper we describe the DustPedia project, which enables us to develop tools and computer models that will help us relate observed cosmic dust emission to its physical properties (chemical composition, size distribution, and temperature), its origins (evolved stars, supernovae, and growth in the ISM), and the processes that destroy it (high-energy collisions and shock heated gas). To carry out this research, we combine the Herschel/Planck data with that from other sources of data, and provide observations at numerous wavelengths ( ≤ 41 ) across the spectral energy distribution, thus creating the DustPedia database. To maximize our spatial resolution and sensitivity to cosmic dust, we limit our analysis to 4231 local galaxies ( v < 3000 km s−1) selected via their near-infrared luminosity (stellar mass). To help us interpret this data, we developed a new physical model for dust (THEMIS), a new Bayesian method of fitting and interpreting spectral energy distributions (HerBIE) and a state-of-the-art Monte Carlo photon-tracing radiative transfer model (SKIRT). In this, the first of the DustPedia papers, we describe the project objectives, data sets used, and provide an insight into the new scientific methods we plan to implement.

This book provides a comprehensive, self-contained introduction to one of the most exciting frontiers in astrophysics today: the quest to understand how the oldest and most distant galaxies in our universe first formed. Until now, most research on this question has been theoretical, but the next few years will bring about a new generation of large telescopes that promise to supply a flood of data about the infant universe during its first billion years after the big bang. This book bridges the gap between theory and observation. It is an invaluable reference for students and researchers on early galaxies. The First Galaxies in the Universestarts from basic physical principles before moving on to more advanced material. Topics include the gravitational growth of structure, the intergalactic medium, the formation and evolution of the first stars and black holes, feedback and galaxy evolution, reionization, 21-cm cosmology, and more. Provides a comprehensive introduction to this exciting frontier in astrophysicsBegins from first principlesCovers advanced topics such as the first stars and 21-cm cosmologyPrepares students for research using the next generation of large telescopesDiscusses many open questions to be explored in the coming decade

Dwarf galaxies pose substantial challenges for cosmological models. In particular, current models predict a dark-matter density that is divergent at the center, which is in sharp contrast with observations that indicate a core of roughly constant density. Energy feedback, from supernova explosions and stellar winds, has been proposed as a major factor shaping the evolution of dwarf galaxies. We present detailed cosmological simulations with sufficient resolution both to model the relevant physical processes and to directly assess the impact of stellar feedback on observable properties of dwarf galaxies. We show that feedback drives large-scale, bulk motions of the interstellar gas, resulting in substantial gravitational potential fluctuations and a consequent reduction in the central matter density, bringing the theoretical predictions in agreement with observations.

This paper presents provides a comprehensive survey of dwarf galaxies, which represent the most numerous and diverse systems in the Universe. We discuss their definitions and morphological classifications, emphasizing the unique properties that distinguish them from globular clusters and giant galaxies. Special attention is given to their formation and evolutionary processes in the framework of hierarchical structure formation and ΛCDM cosmology, including the role of environmental mechanisms and stellar feedback. Star formation histories are explored based on observations and simulations, highlighting both bursty and extended activity across different dwarf types. We further examine the crucial role of dark matter in shaping the dynamics and structure of dwarf galaxies, as well as the core–cusp and missing satellites problems. Finally, we summarize insights from numerical simulations and theoretical models, which provide a bridge between observations and cosmological predictions. This synthesis demonstrates that dwarf galaxies remain essential laboratories for testing galaxy formation theories and probing the nature of dark matter.

A dwarf galaxy companion to NGC 4449 is reported, and is found to be in a transient stage of tidal disruption. Aftermath of an intergalactic collision The nearby starburst galaxy NGC 4449 is surrounded by a neutral HI cloud complex with a variety of features including rings, shells and a counter-rotating core. This complexity is suggestive of interactions with another galaxy, possibly the dwarf irregular galaxy DDO 125, at a distance of 40 kiloparsecs. But new observations of the region around NGC 4449 reveal a previously unknown companion, a tidally distorted smaller galaxy, named NGC 4449B. Its properties, such as its S-shaped morphology (resembling the Sagittarius dwarf near the Milky Way), suggest that it has had a close encounter with NGC 4449. NGC 4449 is a nearby Magellanic irregular starburst galaxy 1 with a B-band absolute magnitude of −18 and a prominent, massive, intermediate-age nucleus 2 at a distance from Earth of 3.8 megaparsecs (ref. 3 ). It is wreathed in an extraordinary neutral hydrogen (H  i ) complex, which includes rings, shells and a counter-rotating core, spanning ∼90 kiloparsecs (kpc; refs 1 , 4 ). NGC 4449 is relatively isolated 5 , although an interaction with its nearest known companion—the galaxy DDO 125, some 40 kpc to the south—has been proposed as being responsible for the complexity of its H  i structure 6 . Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of −13.4 and a half-light radius of 2.7 kpc, with a full extent of around 8 kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf 7 near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies 7 , 8 but has hitherto been seen only in a dissolving globular cluster 9 . We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20 kpc from the galaxy’s centre.

We report on the discovery of three large diffuse dwarf (LDD) galaxies located in isolated triple systems. They have effective diameters of 3.6 ÷ 10.0 kpc and effective surface brightness of 26.2 ÷ 27.3 m /sq. arcsec. We note that the LDD galaxies tend to occur in small groups with a very low dispersion of radial velocities. The total (orbital) mass of the triplets approximately equals to their integral stellar mass within velocity measurement errors. The presence of LDD galaxies in cold multiple systems seems mysterious.

The stellar and integral photometry of 30 irregular dwarf galaxies was performed in the ( ) and ( ) filters based on archival images from the Hubble Space Telescope. Distances were determined for 12 galaxies using the TRGB method. Branches of blue supergiants were identified on the plotted Hertzsprung–Russell diagrams, and the average luminosities of the three brightest stars were calculated. The color indices and luminosities of galaxies in the and filters were determined in circular apertures with the maximum radius . A diagram of the dependence between the luminosities of galaxies and their brightest stars was developed, which shows that the luminosity of the brightest stars increases with increasing luminosity of the parent galaxies. A comparison of the dependence for 30 irregular galaxies with a similar one for 150 spiral and irregular galaxies, published by Tikhonov et al. in 2021, shows their similarity. This result confirms the hypothesis that the absence of bright massive stars in dwarf irregular galaxies cannot be explained by the small number of stars in these galaxies. Using the results of Hunter et al. (2019 and 2021), we developed a dependence between the luminosity of galaxies and the mass of the third-largest H I cloud of these galaxies. The presented correlation, as well as the well-known Larson correlation proposed in 1982, between the mass of molecular hydrogen clouds and the mass of stars born in them, give us reason to conclude that the dependence between the luminosity of galaxies and the brightest stars is a consequence of the correlation between the luminosity (mass) of galaxies and the average mass of the gaseous clouds of these galaxies.

The scaling relation of central massive black holes (MBHs) and their host galaxies is well-studied for supermassive BHs (SMBHs, M BH ≥ 10 6 M ⊙ ). However, this relation has large uncertainties in the mass range of the intermediate-mass BHs (IMBHs, M BH ∼ 10 3 –10 6 M ⊙ ). Since Green pea (GP) galaxies are luminous compact dwarf galaxies, which may be likely to host less massive SMBHs or even IMBHs, we systematically search for MBHs in a large sample of 2190 GP galaxies at z < 0.4, selected from LAMOST and SDSS spectroscopic surveys. Here, we report a newly discovered sample of 59 MBH candidates with broad H α lines. This sample has a median stellar mass of 10 8.83±0.11 M ⊙ and hosts MBHs with single-epoch virial masses ranging from M BH ∼ 10 4.7 to 10 8.5 M ⊙ (median 10 5.85±0.64 M ⊙ ). Among the 59 MBH candidates, 36 have black hole masses M BH ≤ 10 6 M ⊙ (IMBH candidates), one of which even has M BH ≲ 10 5 M ⊙ . We find that the M BH - M * relation of our MBH sample is consistent with the M BH - M bulge relation for SMBHs, while is above the M BH - M * relation for MBHs in dwarf galaxies in the same mass range. Furthermore, we show that 25 MBH candidates, including 4 IMBH candidates, have additional evidence of black hole activities, assessed through various methods such as the broad-line width, BPT diagram, mid-infrared color, X-ray luminosity, and radio emission. Our studies show that it is very promising to find IMBHs in GP galaxies, and the BH sample so obtained enables us to probe the connection between the MBHs and compact dwarf galaxies in the low-redshift Universe.

It was found that satellites of nearby galaxies can form flattened co-rotating structures called disks of satellites or planes of satellites. Their existence is not expected by the current galaxy formation simulations in the standard dark matter-based cosmology. On the contrary, modified gravity offers a promising alternative: the objects in the disks of satellites are tidal dwarf galaxies, that is, small galaxies that form from tidal tails of interacting galaxies. After introducing the topic, we review here our work on simulating the formation of the disks of satellites of the Milky Way and Andromeda galaxies. The initial conditions of the simulation were tuned to reproduce the observed positions, velocities and disk orientations of the galaxies. The simulation showed that the galaxies had a close flyby 6.8 Gyr ago. One of the tidal tails produced by the Milky Way was captured by Andromeda. It formed a cloud of particles resembling the disk of satellites at Andromeda by its size, orientation, rotation and mass. A hint of a disk of satellites was formed at the Milky Way too. In addition, the encounter induced a warp in the disk of the simulated Milky Way that resembles the real warp by its magnitude and orientation. We present here, for the first time, the proper motions of the members of the disk of satellites of Andromeda predicted by our simulation. Finally, we point out some of the remaining open questions which this hypothesis, for the formation of disks of satellites, brings up.

The dynamics of the merger of a dwarf disc galaxy with a massive spiral galaxy of the Milky Way type were studied in detail. The remnant of such interaction after numerous crossings of the satellite through the disc of the main galaxy was a compact stellar core, the characteristics of which were close to small compact elliptical galaxies (cEs) or large ultra-compact dwarfs (UCDs). Such transitional cE/UCD objects with an effective radius of 100–200 pc arise as a result of stripping the outer layers of the stellar core during the destruction of a dwarf disc galaxy. Numerical models of the satellite before interaction included baryonic matter (stars and gas) and dark mass. We used N-body to describe the dynamics of stars and dark matter, and we used smoothed-particle hydrodynamics to model the gas components of both galaxies. The direct method of calculating the gravitational force between all particles provided a qualitative resolution of spatial structures up to 10 pc. The dwarf galaxy fell onto the gas and stellar discs of the main galaxy almost along a radial trajectory with a large eccentricity. This ensured that the dwarf crossed the disc of the main galaxy at each pericentric approach over a time interval of more than 9 billion years. We varied the gas mass and the initial orbital characteristics of the satellite over a wide range, studying the features of mass loss in the core. The presence of the initial gas component in a dwarf galaxy significantly affects the nature of the formation and evolution of the compact stellar core. The gas-rich satellite gives birth to a more compact elliptical galaxy compared to the merging gas-free dwarf galaxy. The initial gas content in the satellite also affects the internal rotation in the stripped nucleus. The simulated cE/UCD galaxies contained very little gas and dark matter at the end of their evolution.