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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.

We investigate the differences in the stellar population properties, the structure, and the environment between massive compact star-forming galaxies (cSFGs) with or without active galactic nucleus (AGN) at 2 < z < 3 in the five 3D-Hubble Space Telescope/CANDELS fields. In a sample of 221 massive cSFGs, we constitute the most complete AGN census so far, identifying 66 AGNs by the X-ray detection, the mid-infrared color criterion, and/or the spectral energy distribution fitting, while the rest (155) are non-AGNs. Further dividing these cSFGs into two redshift bins, i.e., 2 < z < 2.5 and 2.5 ≤ z < 3, we find that in each redshift bin the cSFGs with AGNs have similar distributions of the stellar mass, the specific star formation rate, and the ratio of LIR to LUV to those without AGNs. After having performed a two-dimensional surface brightness modeling for those cSFGs with X-ray-detected AGNs (37) to correct for the influence of the central point-like X-ray AGN on measuring the structural parameters of its host galaxy, we find that in each redshift bin the cSFGs with AGNs have comparable distributions of all concerned structural parameters, i.e., the Sérsic index, the 20%-light radius, the Gini coefficient, and the concentration index, to those without AGNs. With a gradual consumption of available gas and dust, the structure of cSFGs, indicated by the above structural parameters, seem to be slightly more concentrated with decreasing redshift. At 2 < z < 3, the similar environment between cSFGs with and without AGNs suggests that their AGN activities are potentially triggered by internal secular processes, such as gravitational instabilities or/and dynamical friction.

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.

The Virgo galaxy cluster, for example, hosts more than 1,000 galaxies of various masses and sizes, spread around a massive elliptical galaxy, known as Messier 87, at its core. Wang etai, found more illustra tiveevidence that nucleated dwarf galaxies transform into ultra-compact dwarf galaxies using observations of the Virgo galaxy cluster obtained with ground-based telescopes. The European Space Agency's Euclid space telescope that launched injulyand NASA's upcoming Nancy Grace Roman Space Telescope will push such analyses to lower-mass ultra-compact dwarfs, which is a class that probably comprises objects with a mixture of origins, including star clusters and remnant nuclei.

Galaxy mergers produce pairs of supermassive black holes (SMBHs), which may be witnessed as dual quasars if both SMBHs are rapidly accreting. The kiloparsec (kpc)-scale separation represents a physical regime sufficiently close for merger-induced effects to be important 1 yet wide enough to be directly resolvable with the facilities currently available. Whereas many kpc-scale, dual active galactic nuclei—the low-luminosity counterparts of quasars—have been observed in low-redshift mergers 2 , no unambiguous dual quasar is known at cosmic noon ( z  ≈ 2), the peak of global star formation and quasar activity 3 , 4 . Here we report multiwavelength observations of Sloan Digital Sky Survey (SDSS) J0749 + 2255 as a kpc-scale, dual-quasar system hosted by a galaxy merger at cosmic noon ( z  = 2.17). We discover extended host galaxies associated with the much brighter compact quasar nuclei (separated by 0.46″ or 3.8 kpc) and low-surface-brightness tidal features as evidence for galactic interactions. Unlike its low-redshift and low-luminosity counterparts, SDSS J0749 + 2255 is hosted by massive compact disk-dominated galaxies. The apparent lack of stellar bulges and the fact that SDSS J0749 + 2255 already follows the local SMBH mass–host stellar mass relation, suggest that at least some SMBHs may have formed before their host stellar bulges. While still at kpc-scale separations where the host-galaxy gravitational potential dominates, the two SMBHs may evolve into a gravitationally bound binary system in around 0.22 Gyr. The authors report multiwavelength observations of SDSS J0749 + 2255, hosted by massive compact disk-dominated galaxies, as a kpc-scale, dual-quasar system hosted by a galaxy merger at cosmic noon.

NRC publication: Yes

Understanding how super-massive black holes form and grow in the early Universe has become a major challenge 1 , 2 since it was discovered that luminous quasars existed only 700 million years after the Big Bang 3 , 4 . Simulations indicate an evolutionary sequence of dust-reddened quasars emerging from heavily dust-obscured starbursts that then transition to unobscured luminous quasars by expelling gas and dust 5 . Although the last phase has been identified out to a redshift of 7.6 (ref. 6 ), a transitioning quasar has not been found at similar redshifts owing to their faintness at optical and near-infrared wavelengths. Here we report observations of an ultraviolet compact object, GNz7q, associated with a dust-enshrouded starburst at a redshift of 7.1899 ± 0.0005. The host galaxy is more luminous in dust emission than any other known object at this epoch, forming 1,600 solar masses of stars per year within a central radius of 480 parsec. A red point source in the far-ultraviolet is identified in deep, high-resolution imaging and slitless spectroscopy. GNz7q is extremely faint in X-rays, which indicates the emergence of a uniquely ultraviolet compact star-forming region or a Compton-thick super-Eddington black-hole accretion disk at the dusty starburst core. In the latter case, the observed properties are consistent with predictions from cosmological simulations 7 and suggest that GNz7q is an antecedent to unobscured luminous quasars at later epochs. An unusual ultraviolet compact object associated with a dusty starburst has been observed at a redshift of about 7.2, with a luminosity that falls between that of quasars and galaxies, possibly in transition between the two. 

Ninety per cent of baryons are located outside galaxies, either in the circumgalactic or intergalactic medium 1 , 2 . Theory points to galactic winds as the primary source of the enriched and massive circumgalactic medium 3 – 6 . Winds from compact starbursts have been observed to flow to distances somewhat greater than ten kiloparsecs 7 – 10 , but the circumgalactic medium typically extends beyond a hundred kiloparsecs 3 , 4 . Here we report optical integral field observations of the massive but compact galaxy SDSS J211824.06+001729.4. The oxygen [O  ii ] lines at wavelengths of 3726 and 3729 angstroms reveal an ionized outflow spanning 80 by 100 square kiloparsecs, depositing metal-enriched gas at 10,000 kelvin through an hourglass-shaped nebula that resembles an evacuated and limb-brightened bipolar bubble. We also observe neutral gas phases at temperatures of less than 10,000 kelvin reaching distances of 20 kiloparsecs and velocities of around 1,500 kilometres per second. This multi-phase outflow is probably driven by bursts of star formation, consistent with theory 11 , 12 . Theory predicts that winds expel baryons from galaxies into intergalactic space; now optical observations of the massive, but compact, galaxy SDSS J211824.06+001729.4 show that it is ejecting an enormous ionized outflow of gas.

Radio-loud compact radio sources (CRSs) are characterised by morphological compactness of the jet structure centred on the active nucleus of the galaxy. Most of the local elliptical galaxies are found to host a CRS with nuclear luminosities lower than those of typical quasars, ≲1042ergs-1. Recently, low-luminosity CRSs with a LINER-like optical spectrum have been named Fanaroff–Riley (FR) type 0 to highlight their lack of substantially extended radio emission at kpc scales, in contrast with the other Fanaroff–Riley classes, full-fledged FR Is and FR II radio galaxies. FR 0s are the most abundant class of radio galaxies in the local Universe, and characterised by a higher core dominance, poorer Mpc-scale environment and smaller (sub-kpc scale, if resolved) jets than FR Is. However, FR 0s share similar host and nuclear properties with FR Is. A different accretion–ejection paradigm from that in place in FR Is is invoked to account for the parsec-scale FR 0 jets. This review revises the state-of-the-art knowledge about FR 0s, their nature, and which open issues the next generation of radio telescopes can solve in this context.

The massive relic galaxy NGC 1277 has few blue globular clusters, indicating that it has undergone little mass accretion and is a candidate ‘red nugget’ in the nearby Universe. Red globular clusters around a 'red nugget' galaxy Globular clusters are relatively dense masses of stars bound together in a spherical shape by gravity, and are found around most galaxies. They are thought to form in two phases. The initial burst of star formation in the early collapse phase of galaxy evolution leads to a population of metal-rich red clusters. A second phase of cluster formation is triggered when gas is accreted from mergers with smaller galaxies. These clusters are metal-poor and blue, and tend to dominate the populations of large galaxies. Michael Beasley and colleagues measured the optical colours of the globular clusters around the galaxy NGC 1277, which is thought to be a local counterpart to the 'red nugget' galaxies seen at high redshift. The clusters around NGC 1277 are exclusively red, suggesting that the galaxy has accreted little gas since it first formed. Massive galaxies are thought to form in two phases: an initial collapse of gas and giant burst of central star formation, followed by the later accretion of material that builds up their stellar and dark-matter haloes 1 , 2 , 3 , 4 . The systems of globular clusters within such galaxies are believed to form in a similar manner. The initial central burst forms metal-rich (spectrally red) clusters, whereas more metal-poor (spectrally blue) clusters are brought in by the later accretion of less-massive satellites 5 , 6 , 7 , 8 , 9 , 10 . This formation process is thought to result in the multimodal optical colour distributions that are seen in the globular cluster systems of massive galaxies 8 , 11 , 12 . Here we report optical observations of the massive relic-galaxy candidate NGC 1277—a nearby, un-evolved example of a high-redshift ‘red nugget’ galaxy 13 , 14 , 15 , 16 , 17 . We find that the optical colour distribution of the cluster system of NGC 1277 is unimodal and entirely red. This finding is in strong contrast to other galaxies of similar and larger stellar mass, the cluster systems of which always exhibit (and are generally dominated by) blue clusters 11 . We argue that the colour distribution of the cluster system of NGC 1277 indicates that the galaxy has undergone little (if any) mass accretion after its initial collapse, and use simulations of possible merger histories to show that the stellar mass due to accretion is probably at most ten per cent of the total stellar mass of the galaxy. These results confirm that NGC 1277 is a genuine relic galaxy and demonstrate that blue clusters constitute an accreted population in present-day massive galaxies.