PEARLS: A Potentially Isolated Quiescent Dwarf Galaxy with a TRGB Distance of 30 Mpc

Abstract: A wealth of observations have long suggested that the vast majority of isolated classical dwarf galaxies ($M_*=10^7$-$10^9$ M$\odot$) are currently star-forming. However, recent observations of the large abundance of "Ultra-Diffuse Galaxies" beyond the reach of previous large spectroscopic surveys suggest that our understanding of the dwarf galaxy population may be incomplete. Here we report the serendipitous discovery of an isolated quiescent dwarf galaxy in the nearby Universe, which was imaged as part of the PEARLS GTO program. Remarkably, individual red-giant branch stars are visible in this near-IR imaging, suggesting a distance of $30\pm4$ Mpc, and a wealth of archival photometry point to an sSFR of $2\times10^{-11}$ yr$^{-1}$ and SFR of $4\times10^{-4}$ M$\odot$ yr$^{-1}$. Spectra obtained with the Lowell Discovery Telescope find a recessional velocity consistent with the Hubble Flow and ${>}1500$ km/s separated from the nearest massive galaxy in SDSS, suggesting that this galaxy was either quenched from internal mechanisms or had a very high-velocity ($>1000$ km/s) interaction with a nearby massive galaxy in the past. This analysis highlights the possibility that many nearby quiescent dwarf galaxies are waiting to be discovered and that JWST has the potential to resolve them.

• The paper introduces PEARLSDG, a rare isolated quiescent dwarf galaxy that challenges the assumption that similar dwarfs are predominantly star-forming.
• It employs JWST imaging and TRGB distance measurements to establish a 30 Mpc distance and spectroscopic follow-up to confirm a negligible star formation rate.
• The findings imply that internal quenching processes might dominate in isolated environments, prompting a reevaluation of traditional galaxy evolution models.

Discovery and Characterization of an Isolated Quiescent Dwarf Galaxy

The detection and analysis presented in the described paper introduce an intriguing example of an isolated quiescent dwarf galaxy, identified serendipitously in imaging acquired by the Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS) program using the James Webb Space Telescope (JWST). This discovery challenges prevailing assumptions that isolated classical dwarf galaxies, particularly those with stellar masses in the range $M_*=10^7$--$10^9$ M$_\odot$, are predominantly star-forming entities.

Observational Findings

The focal dwarf galaxy, colloquially named PEARLSDG, is situated at a distance of $30 \pm 4$ Mpc, determined through prominent near-infrared observations that resolve individual red-giant branch stars, a capability attributed to the enhanced sensitivity of the JWST. The recessional velocity of this galaxy aligns with the Hubble Flow, suggesting its location in a relatively unperturbed cosmic environment, devoid of significant interactions with nearby massive galaxies. This inference is underscored by its separation of over 1500 km/s from the nearest massive galaxy cataloged in the SDSS.

Quiescent Nature and Star Formation Rate

Spectroscopic follow-up using the Lowell Discovery Telescope provided further substantiation of its isolation and quiescent character. Spectral energy distribution fitting confirms an exceedingly low specific star formation rate (sSFR) in the range of $2 \times 10^{-11}$ yr$^{-1}$, with a stellar formation rate (SFR) lower than $4 \times 10^{-4}$ M$_\odot$ yr$^{-1}$. These measurements underscore the absence of recent prolific star-forming activity—an attribute that deviates from the norm observed in isolated dwarfs.

Implications for Galaxy Formation and Evolution

The existence of PEARLSDG raises intriguing questions regarding its formation and evolution pathways, particularly within the context of internal versus external quenching mechanisms in dwarf galaxies. Traditionally, environmental factors like ram-pressure stripping and tidal interactions have been posited as primary quenching agents. However, the isolation of PEARLSDG suggests the potential dominance of internal processes or high-speed interactions leading to quenching—a hypothesis that warrants further investigation through more comprehensive spectroscopic studies and dynamical modeling.

Future Prospects and Theoretical Considerations

The serendipitous recognition of PEARLSDG hints at the broader existence of numerous such isolated, quiescent dwarf galaxies previously undetected due to selection biases in traditional surveys. This discovery foreshadows the critical role the JWST and similar observatories could play in unveiling a more holistic representation of the dwarf galaxy population, enhancing theoretical frameworks on galaxy evolution and the influence of cosmic environments on morphological and star-forming properties.

In conclusion, PEARLSDG exemplifies a significant anomaly in the current understanding of dwarf galaxies, marking an essential step towards a nuanced comprehension of these systems' intricacies and their evolutionary trajectories. Future advancements and surveys could reveal further exceptions, compelling a reevaluation of galaxy formation paradigms to include mechanisms that might sustain quiescence in once-active dwarf galaxies.