Abstract
Abstract We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator. We detect 5023 stars within the galaxy, down to a 50% completeness limit of F814W ≈ 28.0, using counts in the surrounding field to correct for contamination due to background sources and Virgo intracluster stars. We derive an extinction-corrected F814W tip magnitude of m tip , 0 = 27.19 − 0.05 + 0.07 , yielding a distance of d = 17.7 − 0.4 + 0.6 Mpc. This places VCC 615 on the far side of the Virgo Cluster (d Virgo = 16.5 Mpc), at a Virgocentric distance of 1.3 Mpc and near the virial radius of the main body of Virgo. Coupling this distance with the galaxy’s observed radial velocity, we find that VCC 615 is on an outbound trajectory, having survived a recent passage through the inner parts of the cluster. Indeed, our orbit modeling gives a 50% chance the galaxy passed inside the Virgo core (r < 620 kpc) within the past gigayear, although very close passages directly through the cluster center (r < 200 kpc) are unlikely. Given VCC 615's undisturbed morphology, we argue that the galaxy has experienced no recent and sudden transformation into a UDG due to the cluster potential, but rather is a long-lived UDG whose relatively wide orbit and large dynamical mass protect it from stripping and destruction by the Virgo cluster tides. Finally, we also describe the serendipitous discovery of a nearby Virgo dwarf galaxy projected 90″ (7.2 kpc) away from VCC 615.
Highlights
The wide diversity in properties of different galaxy populations — their structure, kinematics, stellar populations, and environments — reflects the diversity in their formation and evolutionary histories
We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator
Given VCC 615’s undisturbed morphology, we argue that the galaxy has experienced no recent and sudden transformation into a UDG due to the cluster potential, but rather is a long-lived UDG whose relatively wide orbit and large dynamical mass protect it from stripping and destruction by Virgo cluster tides
Summary
The wide diversity in properties of different galaxy populations — their structure, kinematics, stellar populations, and environments — reflects the diversity in their formation and evolutionary histories. An alternative explanation is that UDGs are “failed” L∗ galaxies (van Dokkum et al 2015), where gas was lost from the system before a luminous galaxy could form inside an otherwise normal dark halo These scenarios need not be exclusive; observational data suggests a large diversity in kinematics and globular cluster populations for these diffuse galaxies, better explained by a combination of failed massive objects and lower mass dwarfs-sized galaxies (e.g., Lim et al 2018, 2020; Doppel et al 2021). Within Virgo, we couple this information with its measured line-of-sight velocity (2094 ± 3 km s−1; Toloba et al 2018) to constrain the galaxy’s orbit within the cluster, probe its dynamical history, and compare to scenarios for cluster UDG formation and evolution
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