The Densest Galaxy (1307.7707v2)

Abstract: We report the discovery of a remarkable ultra-compact dwarf galaxy around the massive Virgo elliptical galaxy NGC 4649 (M60), which we term M60-UCD1. With a dynamical mass of 2.0 x 10^8 M_sun but a half-light radius of only ~ 24 pc, M60-UCD1 is more massive than any ultra-compact dwarfs of comparable size, and is arguably the densest galaxy known in the local universe. It has a two-component structure well-fit by a sum of Sersic functions, with an elliptical, compact (r_h=14 pc; n ~ 3.3) inner component and a round, exponential, extended (r_h=49 pc) outer component. Chandra data reveal a variable central X-ray source with L_X ~ 10^38 erg/s that could be an active galactic nucleus associated with a massive black hole or a low-mass X-ray binary. Analysis of optical spectroscopy shows the object to be old (~> 10 Gyr) and of solar metallicity, with elevated [Mg/Fe] and strongly enhanced [N/Fe] that indicates light element self-enrichment; such self-enrichment may be generically present in dense stellar systems. The velocity dispersion (~ 70 km/s) and resulting dynamical mass-to-light ratio (M/L_V=4.9 +/- 0.7) are consistent with---but slightly higher than---expectations for an old, metal-rich stellar population with a Kroupa initial mass function. The presence of a massive black hole or a mild increase in low-mass stars or stellar remnants is therefore also consistent with this M/L_V. The stellar density of the galaxy is so high that no dynamical signature of dark matter is expected. However, the properties of M60-UCD1 suggest an origin in the tidal stripping of a nucleated galaxy with M_B ~ -18 to -19.

Analysis of Ultra-Compact Dwarf Galaxy M60-UCD1

This essay provides a detailed summary of the research conducted on the discovery of M60-UCD1, an ultra-compact dwarf galaxy situated near the massive Virgo elliptical galaxy NGC 4649 (M60). The paper meticulously examines the unique properties of M60-UCD1, highlighting its substantial mass, compact size, and implications for the understanding of galaxy formation and evolution.

M60-UCD1 stands out due to its exceptional density, characterized by a dynamical mass of $2.0\times10^{8} M_{\odot}$ and a half-light radius of approximately 24 pc, making it arguably the densest galaxy documented in the local universe. The galaxy's structure is best described by a combination of Sersic functions, comprising a compact elliptical inner component ($r_h=14$ pc; $n\sim3.3$) and a round, exponential outer component ($r_h=49$ pc). The presence of a potentially variable central X-ray source suggests the existence of an active galactic nucleus or a low-mass X-ray binary, indicating that M60-UCD1 might harbor a massive black hole.

The galaxy contains an old stellar population with an age greater than 10 Gyr, solar metallicity, increased [Mg/Fe], and significantly enhanced [N/Fe]. This chemical profile suggests that self-enrichment by light elements may be an inherent characteristic of dense stellar systems like M60-UCD1. The observed velocity dispersion ($\sigma\sim70$ -1) and the calculated dynamical mass-to-light ratio ($M/L_V=4.9\pm0.7$) are consistent with predictions for an old, metal-rich population according to a Kroupa IMF. These values may also indicate the presence of a massive black hole or a slight excess of low-mass stellar remnants.

The absence of a detectable dynamical signature of dark matter aligns with the hypothesis that M60-UCD1 developed from the tidal stripping of a nucleated galaxy, originally possessing a brightness $M_B\sim-18$ to $-19$. This scenario suggests that M60-UCD1 could be the remnant core of a more massive progenitor, stripped down to its current compact form.

Implications and Speculation for Future Research

The comprehensive analyses of M60-UCD1 offer profound insights into the formation mechanisms of ultra-compact dwarf galaxies and the evolutionary processes affecting them. The intriguing possibility of such galaxies harboring massive black holes could redefine our understanding of black hole distribution across different types of stellar systems. The observational evidence of self-enrichment emphasizes the complex chemical evolution occurring in dense stellar environments.

Future research could benefit from more extensive observations, such as integral-field spectroscopy, to explore the two-dimensional kinematics of M60-UCD1 and potentially confirm the presence of a supermassive black hole. Additionally, continued efforts to identify and paper similar ultra-compact dwarf galaxies might clarify their prevalence and role in the cosmic landscape.

Exploring the remnants of tidally stripped galaxies can enhance our grasp of galaxy interactions and the consequent transformation processes within galaxy clusters. The insights gained from M60-UCD1 could thus contribute significantly to the refinement of theoretical models pertaining to galaxy composition and structural dynamics.