Discovery and Characterization of Two Ultra Faint-Dwarfs Outside the Halo of the Milky Way: Leo M and Leo K. (arXiv:2307.08738v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+McQuinn_K/0/1/0/all/0/1">Kristen B. W. McQuinn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mao_Y/0/1/0/all/0/1">Yao-Yuan Mao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cohen_R/0/1/0/all/0/1">Roger E. Cohen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shih_D/0/1/0/all/0/1">David Shih</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buckley_M/0/1/0/all/0/1">Matthew R. Buckley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dolphin_A/0/1/0/all/0/1">Andrew E. Dolphin</a>
We report the discovery of two ultra-faint dwarf galaxies, Leo M and Leo K,
that lie outside the halo of the Milky Way. Using Hubble Space Telescope
imaging of the resolved stars, we create color-magnitude diagrams reaching the
old main sequence turn-off of each system and (i) fit for structural parameters
of the galaxies; (ii) measure their distances using the luminosity of the
Horizontal Branch stars; (iii) estimate integrated magnitudes and stellar
masses; and (iv) reconstruct the star formation histories. Based on their
location in the Local Group, neither galaxy is currently a satellite of the
Milky Way, although Leo K is located ~22 kpc from the low-mass galaxy Leo T and
these two systems may have had a past interaction. Leo M and Leo K have stellar
masses of 1.5+/-0.2 x 10^4 Msun and 1.0+/-0.2 x 10^4 Msun, and were quenched
10.9 (+1.8/-0.6) Gyr and 12.6 (+0.2/-5.8) Gyr ago, respectively. Given that the
galaxies are not satellites of the MW, it is unlikely that they were quenched
by environmental processing. Instead, such low masses and early quenching
timescales are consistent with the scenario that a combination of reionization
and stellar feedback shut down star formation at early cosmic times.
We report the discovery of two ultra-faint dwarf galaxies, Leo M and Leo K,
that lie outside the halo of the Milky Way. Using Hubble Space Telescope
imaging of the resolved stars, we create color-magnitude diagrams reaching the
old main sequence turn-off of each system and (i) fit for structural parameters
of the galaxies; (ii) measure their distances using the luminosity of the
Horizontal Branch stars; (iii) estimate integrated magnitudes and stellar
masses; and (iv) reconstruct the star formation histories. Based on their
location in the Local Group, neither galaxy is currently a satellite of the
Milky Way, although Leo K is located ~22 kpc from the low-mass galaxy Leo T and
these two systems may have had a past interaction. Leo M and Leo K have stellar
masses of 1.5+/-0.2 x 10^4 Msun and 1.0+/-0.2 x 10^4 Msun, and were quenched
10.9 (+1.8/-0.6) Gyr and 12.6 (+0.2/-5.8) Gyr ago, respectively. Given that the
galaxies are not satellites of the MW, it is unlikely that they were quenched
by environmental processing. Instead, such low masses and early quenching
timescales are consistent with the scenario that a combination of reionization
and stellar feedback shut down star formation at early cosmic times.
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