A type of very bright compact stellar system (-14 ≤ MV ≥ -12) that is intermediate between globular clusters (GCs) and compact elliptical galaxies (cEs). With masses of M > 2 x 10^6 M_sun and radii >
10 parsecs (pc), UCDs are among the densest stellar systems in the Universe. Nevertheless, the nature and origin of these objects is still widely debated. Early interpretations suggested that UCDs could be the most massive GCs or possibly the tidally stripped remnants of dwarf galaxies. However, there is evidence that both formation mechanisms could contribute to the UCD population. Supermassive black holes (SMBHs) have been confirmed in most UCDs with masses M > 10^7 M_sun. The most massive UCD discovered to date, M59-UCD3 (M_* ~2 x 10^8 M_sun, radius ~ 25 pc), hosts a SMBH.
A particle belonging to the most energetic population of cosmic rays with an energy above ~10^20 electron-volts. The UHECRs constitute a real challenge for theoretical models, because their accelerati
on requires extreme conditions hardly fulfilled by known astrophysical objects.
A particle belonging to the most energetic population of cosmic rays with an energy above ~10^20 electron-volts. The UHECRs constitute a real challenge for theoretical models, because their accelerati
on requires extreme conditions hardly fulfilled by known astrophysical objects.
A galaxy that emits more than 90% of its energy in the infrared (8-1000 µm) and whose infrared luminosity exceeds 10^12 solar luminosities. Quasars can also have such high or even higher bolometric lu
minosities. However LIRGs and ULIRGs emit the bulk of their energy in the infrared. Most of ULIRGs are found in merging and interacting galaxy systems. It is thought that their luminosity results from galactic collisions, which increase the rate of star formation.