© NASA/JPL-Caltech/STScI

A Satellite Galaxy, a diminutive galaxy encircling a larger galactic host, exemplifies a cosmic dance akin to the moon’s orbit around a planet or a planet’s orbit around a star. Within our Milky Way Galaxy and the Andromeda Galaxy, these satellite galaxies twirl, held in gravitational thrall by their larger counterparts, often lacking distinct shapes due to their modest size. Delving into the study of these satellite galaxies offers a pathway to estimating the total mass of our galaxy.

Yet, satellite galaxies aren’t the sole celestial entities tethered gravitationally to larger hosts; consider globular clusters. Consequently, astronomers delineate galaxies as gravitational clusters of stars manifesting properties beyond the purview of baryonic matter and Newton’s gravitational laws. Notably, observations of stellar and gas orbital velocities within spiral galaxies unveil velocity curves diverging significantly from theoretical predictions, prompting theoretical constructs like dark matter theory and Newtonian dynamics modifications. Thus, while sharing the status of host galaxy satellites, globular clusters remain distinct from satellite galaxies. The latter not only exhibit greater extension and diffusion but also cloak themselves in substantial dark matter halos believed to have been acquired during their formative epochs.

Around fifty satellite galaxies gracefully orbit the Milky Way, with the Large Magellanic Cloud reigning as the largest among them. Situated approximately 163,000 light-years from Earth, this celestial entity measures a mere 1/100th of the Milky Way’s size and lacks the defined spiral structure characteristic of our galactic home. Some theorists speculate that the gravitational influence exerted by the Milky Way and neighbouring galaxies may be distorting its shape.

In the realm of proximity, two contenders vie for the title of closest satellite galaxy. One cluster of stars is diminutive enough to earn the designation of a “dwarf galaxy” by astronomers. At the same time, the other lies so near that scholarly discourse continues regarding its classification as part of our galaxy or an independent dwarf galaxy.

The consensus among astronomers christens the Sagittarius Dwarf Spheroidal Galaxy as the agreed-upon contender positioned approximately 50,000 light-years distant from the Milky Way’s center. Its orbit traces a path both above and below the galactic disk, akin to a ring encircling a spinning top.

Yet, even closer to the Milky Way lies another congregation of stars, often referred to as the Canis Major Dwarf Galaxy. Hosting an estimated billion stars, it hugs the galactic periphery so tightly that it dwells nearer to our solar system than to the galactic nucleus, merely 25,000 light-years distant from Earth.

FORMATION OF SATELLITE GALAXY

The formation of satellite galaxies is a complex process influenced by various factors such as gravitational interactions, mergers, and the hierarchical structure of the universe.

• Hierarchical Structure Formation: In the early universe, small density fluctuations led to the formation of structures on various scales. Over time, smaller structures merged to form larger ones through gravitational attraction. This hierarchical growth forms the framework for satellite galaxy formation.
• Tidal Interactions and Galaxy Mergers: Satellite galaxies often form from smaller protogalactic clouds that are gravitationally attracted to larger galaxies. As these protogalactic clouds orbit the larger galaxy, tidal interactions can strip material from them, leading to the formation of satellite galaxies. Additionally, galaxy mergers within the hierarchical structure can create satellite galaxies as smaller galaxies are captured by larger ones.
• Accretion: Satellite galaxies can also form through the accretion of smaller galaxies or gas-rich clouds onto larger galaxies. As these smaller objects are drawn towards the gravitational potential of the larger galaxy, they may become satellite galaxies orbiting it.
• Environmental Influence: The environment surrounding galaxies can play a significant role in satellite galaxy formation. Galaxies located in dense regions such as galaxy clusters are more likely to have numerous satellite galaxies due to the increased gravitational interactions and mergers within these environments.
• Dynamical Processes: Once formed, satellite galaxies undergo dynamical processes within the gravitational field of the larger galaxy. These processes can include tidal stripping, where gravitational forces from the larger galaxy pull material away from the satellite, and dynamical friction, which causes the satellite’s orbit to decay over time, leading to eventual merger with the larger galaxy.

CLASSIFICATION

Satellite galaxies are typically classified based on their size, distance from the host galaxy, and structural characteristics.

DWARF GALAXY

These are the most common types of satellite galaxies. They are smaller in size and contain fewer stars compared to their host galaxies. Dwarf galaxies are small galaxies composed of a few billion stars, contrasting with larger galaxies that can contain hundreds of billions of stars. These galaxies are typically found orbiting larger galaxies like the Milky Way or the Andromeda Galaxy. They are believed to have formed due to gravitational forces during the early stages of larger galaxy creation or as a result of galaxy collisions, emerging from streams of material and dark matter ejected from parent galaxies.

Dwarf galaxies can further be classified into:

DWARF ELLIPTICAL GALAXY

Dwarf elliptical galaxies (dEs) constitute a subgroup within the elliptical galaxy classification, distinguished by their smaller size compared to typical elliptical galaxies. Often observed in galaxy groups and clusters, they frequently accompany larger galaxies as satellite companions. These galaxies exhibit absolute blue magnitudes ranging from -18 to -14, rendering them dimmer than their counterparts. Structurally distinct from true elliptical galaxies, dwarf ellipticals boast masses around one billion solar masses and contain minimal gas, setting them apart from dwarf irregular galaxies. Noteworthy instances of dwarf elliptical galaxies include NGC 147, NGC 185, and NGC 205, all serving as companions to the Andromeda Galaxy.

DWARF IRREGULAR GALAXY

Dwarf irregular galaxies represent a category of galactic structures distinguished by their irregular shapes, modest rates of current star formation, and blue hues. These galaxies are typically classified according to their visual appearance and inherent characteristics, encompassing a broad range of luminosities. A prototypical dwarf irregular galaxy maintains a relatively steady pace of star formation, with stellar activity dispersed thinly across its optical disc. Unlike their starbursting counterparts, where star formation is concentrated, tranquil dwarf irregular galaxies exhibit minimal or no discernible colour gradients. The majority of isolated dwarf irregular galaxies display comparable star formation histories, suggesting a gradual yet continuous process of stellar birth over extended periods.

ULTRA- COMPACT DWARF GALAXY

An ultra-compact dwarf galaxy (UCD) stands out as a distinctive class of galactic formations, characterised by their exceptionally dense stellar populations and compact dimensions. Diverging from the typical traits of both dwarf galaxies and globular clusters, UCDs possess intermediate characteristics. They surpass the largest Milky Way globular clusters in size, brightness, and mass, yet they remain significantly more condensed than conventional dwarf galaxies of similar luminosity. The origins of UCDs are thought to stem from various processes, including the remnants of tidally stripped dwarf galaxies, merged stellar super-clusters, or the formation of genuine compact dwarf galaxies within the smallest peaks of primordial dark matter fluctuations.

One notable example of an ultra-compact dwarf galaxy is M60-UCD1, situated in proximity to the massive elliptical galaxy NGC 4649. M60-UCD1 stands as the most luminous UCD on record and one of the most massive, harbouring roughly 200 million times the mass of our Sun within a radius spanning only about 80 light-years.

SATELLITE GALAXY CLUSTERS

Satellite galaxy clusters represent conglomerates of numerous galaxies tethered together by gravitational forces. These expansive clusters typically house hundreds to thousands of galaxies, boasting masses spanning from 10^14 to 10^15 times that of the Sun. Ranking as the universe’s second-largest gravitationally bound structures, satellite galaxy clusters follow superclusters in scale. Distinguished from galactic clusters and globular clusters, which consist of galaxies and star clusters within galaxies, respectively, satellite galaxy clusters serve as assemblages of galaxies on a grander scale.

Prominent examples of satellite galaxy clusters include the Virgo Cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster, situated within the vicinity of our Universe. These clusters serve as pivotal arenas for probing cosmic phenomena such as dark matter, dark energy, and the local expansion of the Universe.

Furthermore, satellite galaxy clusters may exhibit distinctive features like cold fronts, shock waves, and non-thermal diffuse radio emissions like radio halos and relics. These phenomena have been scrutinized using sophisticated instruments such as the Chandra X-ray Observatory, yielding invaluable insights into particle acceleration mechanisms within these colossal cosmic entities. The exploration of satellite galaxy clusters facilitates the unravelling of enigmatic cosmic processes, including galaxy evolution, gravitational lensing effects, and the distribution of mass throughout the Universe.

PECULIAR OR INTERACTING SATELLITE GALAXIES

Peculiar or interacting satellite galaxies denote galactic entities showcasing atypical forms, dimensions, or compositions as a result of interactions with neighbouring galaxies or internal dynamics. Such peculiar galaxies can be categorized into two primary groups: interacting galaxies and active galactic nuclei (AGN). Interacting galaxies undergo structural modifications due to the gravitational influence of nearby galaxies, resulting in phenomena such as tidal gravitational bridges, tails, and instances of galactic mergers. Conversely, AGNs represent peculiar galaxies featuring energetic events within their nuclei, characterized by phenomena such as the emission of hot jets of material at high velocities.

TRANSITIONAL-TYPE SATELLITE GALAXIES

Transitional-type satellite galaxies represent a distinct classification of galactic entities that display attributes hinting at a shift between various evolutionary stages, such as transitioning from active star-forming phases to quiescent states. These galaxies are discerned based on criteria including their colours, rates of star formation, and morphologies, all of which suggest a developmental transition.

Within this category, three types of transitional galaxies have been recognized: green valley (GV) galaxies, blue quiescent (BQ) galaxies, and spectroscopic post-starburst (PSB) galaxies. Green valley galaxies are characterized by their rest-frame colours, whereas BQ galaxies inhabit the blue end of the quiescent spectrum in terms of colour space. PSB galaxies are identified through their post-starburst spectra, indicative of recent vigorous star formation followed by a swift cessation phase.

GALACTIC CANNIBALISM

Galactic cannibalism, also known as galactic cannibalization or galaxy merging, is a phenomenon in astrophysics where one galaxy, typically larger and more massive, engulfs or merges with another galaxy, often smaller or less massive. This process profoundly influences the structure, evolution, and dynamics of galaxies and is a fundamental aspect of galaxy formation and evolution theories.

• Gravitational Interactions: The primary driver behind galactic cannibalism is gravitational interactions between galaxies. As galaxies are bound together by gravity, they can exert gravitational forces on each other, leading to interactions ranging from close encounters to outright mergers.
• Galaxy Collisions: When two galaxies approach each other closely, their gravitational attraction can distort their shapes and pull stars, gas, and dust from each other. In some cases, the gravitational forces can be so strong that the galaxies undergo a direct collision, resulting in a merger. This collision can trigger intense bursts of star formation and the formation of new structures within the merged galaxy.
• Tidal Effects: As galaxies interact gravitationally, they can experience tidal effects, where the gravitational forces exerted by one galaxy distort the other’s shape and pull material away from it. This process, known as tidal stripping, can strip stars, gas, and dust from the smaller galaxy, leaving behind tidal tails or streams of material that can eventually be assimilated by the larger galaxy.
• Galactic Evolution: Galactic cannibalism plays a crucial role in the evolution of galaxies. Through mergers, galaxies can grow in size and mass, leading to the formation of larger and more massive galaxies over cosmic time. Mergers can also trigger bursts of star formation and the activation of active galactic nuclei (AGN), where supermassive black holes at the centers of galaxies become actively accreting material.
• Observational Evidence: Observational studies have provided extensive evidence for galactic cannibalism. Astronomers have observed numerous examples of interacting and merging galaxies at various stages of the process. These observations include galaxies with distorted shapes, tidal tails, and ongoing mergers, as well as remnants of past mergers such as elliptical galaxies with complex structures.
• Simulations and Models: Computational simulations and theoretical models have also been instrumental in understanding galactic cannibalism. These simulations can simulate the dynamics of interacting galaxies, including the formation of tidal features, the triggering of star formation, and the long-term evolution of merged galaxies. Comparisons between simulations and observations help refine our understanding of the underlying physical processes involved.