[Paper Review] Evidence for Ultra-Diffuse Galaxy Formation Through Tidal Heating of Normal Dwarfs

Michael G. Jones, Paul Bennet+6 more|arXiv (Cornell University)|Apr 26, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy172 references33 citations

TL;DR

This study investigates two ultra-diffuse galaxies (UDGs), NGC 2708-Dw1 and NGC 5631-Dw1, found near stellar streams, using Hubble Space Telescope imaging and VLA H i mapping. It finds their globular cluster counts are consistent with normal dwarf galaxies and lacks clear H i connections to their hosts, strongly disfavoring a tidal dwarf galaxy origin and supporting the hypothesis that these UDGs formed through tidal heating of pre-existing normal dwarfs.

ABSTRACT

We have followed up two ultra-diffuse galaxies (UDGs), detected adjacent to stellar streams, with Hubble Space Telescope (HST) imaging and HI mapping with the Jansky Very Large Array (VLA) in order to investigate the possibility that they might have a tidal origin. With the HST F814W and F555W images we measure the globular cluster (GC) counts for NGC 2708-Dw1 and NGC 5631-Dw1 as $2^{+1}_{-1}$ and $5^{+1}_{-2}$, respectively. NGC 2708-Dw1 is undetected in HI down to a 3$\sigma$ limit of $\log (M_\mathrm{HI}/\mathrm{M_\odot}) = 7.3$, and there is no apparent HI associated with the nearby stellar stream. There is a 2$\sigma$ HI feature coincident with NGC 5631-Dw1. However, this emission is blended with a large gaseous tail emanating from NGC 5631 and is not necessarily associated with the UDG. The presence of any GCs and the lack of clear HI connections between the UDGs and their parent galaxies strongly disfavor a tidal dwarf galaxy origin, but cannot entirely rule it out. The GC counts are consistent with those of normal dwarf galaxies, and the most probable formation mechanism is one where these UDGs were born as normal dwarfs and were later tidally stripped and heated. We also identify an over-luminous ($M_\mathrm{V} = -11.1$) GC candidate in NGC 2708-Dw1, which may be a nuclear star cluster transitioning to an ultra-compact dwarf as the surrounding dwarf galaxy gets stripped of stars.

Motivation & Objective

• To test whether ultra-diffuse galaxies (UDGs) associated with stellar streams originated as tidal dwarf galaxies (TDGs).
• To determine the globular cluster (GC) populations of two UDGs to assess their formation history.
• To search for H i gas connections between the UDGs and their host galaxies to evaluate tidal origin hypotheses.
• To assess whether the UDGs were pre-existing diffuse systems or were transformed from normal dwarfs via tidal interactions.
• To evaluate competing formation mechanisms—such as feedback, spin parameter, or failed L* galaxies—against observational constraints.

Proposed method

• Acquired high-resolution HST/WFC3 F814W and F555W imaging to detect and count globular clusters in NGC 2708-Dw1 and NGC 5631-Dw1.
• Performed deep H i mapping with the Karl G. Jansky Very Large Array (VLA) to search for neutral gas associations between the UDGs and their host galaxies or stellar streams.
• Used DOLPHOT software to perform point-source photometry and derive GC counts with statistical uncertainties.
• Measured the 3σ H i detection limit for NGC 2708-Dw1 at log(MHI/M⊙) = 7.3, indicating it is undetected in H i.
• Analyzed spatial coincidence of H i emission with NGC 5631-Dw1 and its host NGC 5631, assessing whether the gas is physically linked.
• Compared observed GC counts and H i properties to theoretical expectations for TDGs, normal dwarfs, and tidally heated systems.

Experimental results

Research questions

• RQ1 Do the globular cluster systems of NGC 2708-Dw1 and NGC 5631-Dw1 support a tidal dwarf galaxy origin, as expected for such systems?
• RQ2 Is there detectable H i gas connecting NGC 2708-Dw1 or NGC 5631-Dw1 to their host galaxies or to the surrounding stellar streams?
• RQ3 Are the GC counts in these UDGs consistent with those of normal dwarf galaxies, or do they indicate a more massive halo typical of failed L* galaxies or tidal dwarfs?
• RQ4 Does the spatial location of H i emission relative to the UDG and its host galaxy support a tidal origin, particularly given that TDGs are typically found at the tips of tidal tails?
• RQ5 What is the most plausible formation mechanism for these UDGs, given their GC systems and H i properties?

Key findings

• NGC 2708-Dw1 has a globular cluster count of 2+1−1, consistent with normal dwarf galaxies of similar luminosity.
• NGC 5631-Dw1 has a globular cluster count of 5+1−2, also consistent with typical dwarf galaxy GC populations.
• NGC 2708-Dw1 is undetected in H i down to a 3σ limit of log(MHI/M⊙) = 7.3, with no apparent H i connection to the nearby stellar stream or host galaxy NGC 2708.
• A 2σ H i feature is coincident with NGC 5631-Dw1 but is blended with a large gaseous tail from NGC 5631 and not definitively associated with the UDG.
• The low GC count and lack of H i connection strongly disfavor a tidal dwarf galaxy formation scenario for both UDGs.
• The most plausible formation mechanism is tidal heating of pre-existing normal dwarf galaxies, consistent with simulations of tidal stripping in group environments.