The Fifth Candidate for an Intermediate-mass Black Hole in the Galactic Center

Abstract: We report the results of high-resolution molecular line observations of the high-velocity compact cloud HCN-0.085-0.094 with the Atacama Large Millimeter/submillimeter Array. The HCN J=4-3, HCO$^+$ J=4-3, and CS J=7-6 line images reveal that HCN-0.085-0.094 consists mainly of three small clumps with extremely broad velocity widths. Each of the three clumps has a 5.5 GHz radio continuum counterpart in its periphery toward Sgr A$^*$. The positional relationship indicates that their surfaces have been ionized by ultraviolet photons from young stars in the central cluster, suggesting the clumps are in close proximity to the Galactic nucleus. One of the three clumps has a ring-like structure with a very steep velocity gradient. This kinematical structure suggests an orbit around a point-like object with a mass of $\sim 10^4$ $M_\odot$. The absence of stellar counterparts indicates that the point-like object may be a quiescent black hole. This discovery adds another intermediate-mass black hole candidate in the central region of our Galaxy.

• The paper reveals that the high-velocity cloud HCN--0.085--0.094 shows significant velocity gradients, indicating the presence of a ~10^4 M☉ point-like object.
• The study employs high-resolution ALMA observations in HCN, HCO⁺, and CS lines to uncover clumps with dynamic properties yet lacking multi-wavelength counterparts.
• The findings highlight the need for follow-up studies to refine IMBH identification methods and deepen understanding of black hole formation and star cluster dynamics.

The Fifth Candidate for an Intermediate-mass Black Hole in the Galactic Center

The paper by Takekawa et al. investigates the presence of an intermediate-mass black hole (IMBH) candidate through high-resolution observations at the Galactic Center. The focal point of the study is a high-velocity compact cloud (HVCC), specifically identified as HCN--0.085--0.094, observed using the Atacama Large Millimeter/submillimeter Array (ALMA). This research builds on previous studies suggesting gravitational interactions involving massive, point-like objects, which may be IMBHs, as a potential explanation for such HVCC phenomena.

Observational Findings

HCN--0.085--0.094 was examined through molecular line observations in HCN, HCO$^+$, and CS line emissions. The three identified clumps within this HVCC exhibit significantly broad velocity widths, with T1 having the most substantial velocity gradient. Unlike conventional clouds, these clumps lack counterparts at other wavelengths, complicating direct correlations with known stellar phenomena. Notably, the 5.5 GHz radio continuum investigation unravels ionized surfaces near these clumps, indicative of interactions with ultraviolet emissions from the central star cluster around Sgr A$^*$.

Potential IMBH Indication

The critical highlight pertains to the T1 clump, which, based on its structural characteristics and velocity gradient, suggests the presence of a massive point-like entity likely responsible for observed dynamical effects. The estimated mass of this entity is approximately $10^4\, M_\odot$, observed through the Keplerian motion model of the identified ring-like structure in T1. Given the lack of associated stellar bodies, this dynamics hint at a quiet black hole, formulating the fifth candidate IMBH near the Galactic Center.

Implications and Future Directions

This discovery potentially expands our understanding of IMBH prevalence, providing clues to the possible mechanics of star cluster mass dynamics and black hole formation. While the study provides credible evidence towards IMBH existence in the Galactic Center, further investigations would benefit from targeted multi-wavelength monitoring to refine motion mappings and verify the absence of non-dark stellar entities. Ensuing studies could establish whether this point mass is an isolated IMBH or a composite mass cluster involving non-luminous remnants, such as neutron stars or smaller black holes, which collectively simulate the gravitational footprint of an IMBH. The contemplation of dense stellar environs migrating through galactic dynamics adds an additional layer to these cosmological inquiries.

Overall, the presented work contributes noteworthy observations and invites extended exploration methodologies in the quest to untangle the complex dynamics within our Galactic Center, emphasizing the intricate interplay between high-velocity cloud formation and potential IMBH presence.