# HST resolves stars in a tiny body falling on the dwarf galaxy DDO 68

## TL;DR

HST imaging reveals a star stream associated with dwarf galaxy DDO 68, showing old stars and evidence of hierarchical merging at the dwarf galaxy scale.

## Contribution

First resolved imaging of a star stream in a dwarf galaxy, confirming its association with DDO 68 and providing insights into its star formation history.

## Key findings

- Stream contains predominantly old RGB stars.
- Star formation in the stream ceased over 1 Gyr ago.
- Stream's stellar mass is about 1% of DDO 68's mass.

## Abstract

We present new Hubble Space Telescope (HST) imaging of a stream-like system associated with the dwarf galaxy DDO 68, located in the Lynx-Cancer Void at a distance of D$\sim$12.65 Mpc from us. The stream, previously identified in deep Large Binocular Telescope images as a diffuse low surface brightness structure, is resolved into individual stars in the F606W (broad V) and F814W ($\sim$I) images acquired with the Wide Field Camera 3. The resulting V, I color-magnitude diagram (CMD) of the resolved stars is dominated by old (age$\gtrsim$1-2 Gyr) red giant branch (RGB) stars. From the observed RGB tip, we conclude that the stream is at the same distance as DDO 68, confirming the physical association with it. A synthetic CMD analysis indicates that the large majority of the star formation activity in the stream occurred at epochs earlier than $\sim$1 Gyr ago, and that the star formation at epochs more recent than $\sim$500 Myr ago is compatible with zero. The total stellar mass of the stream is $\sim10^{6} M_{\odot}$, about 1/100 of that of DDO~68. This is a striking example of hierarchical merging in action at the dwarf galaxy scales.