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<jats:title>ABSTRACT</jats:title> <jats:p>Leo P is a low-luminosity dwarf galaxy discovered through the blind H <jats:sc>i</jats:sc> Arecibo Legacy Fast ALFA survey. The H <jats:sc>i</jats:sc> and follow-up optical observations have shown that Leo P is a gas-rich dwarf galaxy with active star formation, an underlying older population, and an extremely low oxygen abundance. We have obtained optical imaging with the <jats:italic>Hubble Space Telescope</jats:italic> to two magnitudes below the red clump in order to study the evolution of Leo P. We refine the distance measurement to Leo P to be 1.62 ± 0.15 Mpc, based on the luminosity of the horizontal branch stars and 10 newly identified RR Lyrae candidates. This places the galaxy at the edge of the Local Group, ∼0.4 Mpc from Sextans B, the nearest galaxy in the NGC 3109 association of dwarf galaxies of which Leo P is clearly a member. The star responsible for ionizing the H <jats:sc>ii</jats:sc> region is most likely an O7V or O8V spectral type, with a stellar mass ≳25 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. The presence of this star provides observational evidence that massive stars at the upper end of the initial mass function are capable of being formed at star formation rates as low as ∼10<jats:sup>−5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>. The best-fitting star formation history (SFH) derived from the resolved stellar populations of Leo P using the latest PARSEC models shows a relatively constant star formation rate over the lifetime of the galaxy. The modeled luminosity characteristics of Leo P at early times are consistent with low-luminosity dSph Milky Way satellites, suggesting that Leo P is what a low-mass dSph would look like if it evolved in isolation and retained its gas. Despite the very low mass of Leo P, the imprint of reionization on its SFH is subtle at best, and consistent with being totally negligible. The isolation of Leo P, and the total quenching of star formation of Milky Way satellites of similar mass, implies that the local environment dominates the quenching of the Milky Way satellites.</jats:p>

<jats:title>ABSTRACT</jats:title> <jats:p>Galaxy clusters are one of the prime sites to search for dark matter (DM) annihilation signals. Depending on the substructure of the DM halo of a galaxy cluster and the cross sections for DM annihilation channels, these signals might be detectable by the latest generation of <jats:italic>γ</jats:italic>-ray telescopes. Here we use three years of <jats:italic>Fermi</jats:italic>-Large Area Telescope data, which are the most suitable for searching for very extended emission in the vicinity of the nearby Virgo galaxy cluster. Our analysis reveals statistically significant extended emission which can be well characterized by a uniformly emitting disk profile with a radius of 3° that moreover is offset from the cluster center. We demonstrate that the significance of this extended emission strongly depends on the adopted interstellar emission model (IEM) and is most likely an artifact of our incomplete description of the IEM in this region. We also search for and find new point source candidates in the region. We then derive conservative upper limits on the velocity-averaged DM pair annihilation cross section from Virgo. We take into account the potential <jats:italic>γ</jats:italic>-ray flux enhancement due to DM sub-halos and its complex morphology as a merging cluster. For DM annihilating into <jats:inline-formula> <jats:tex-math> <?CDATA $b\bar{b},$?> </jats:tex-math> <?MML <mml:math> <mml:mi>b</mml:mi> <mml:mover accent="true"> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="true">&macr;</mml:mo> </mml:mrow> </mml:mover> <mml:mo>,</mml:mo> </mml:math>?> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj520463ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> assuming a conservative sub-halo model setup, we find limits that are between 1 and 1.5 orders of magnitude above the expectation from the thermal cross section for <jats:italic>m</jats:italic> <jats:sub>DM</jats:sub> ≲ 100 GeV. In a more optimistic scenario, we exclude <jats:inline-formula> <jats:tex-math> <?CDATA $\langle \sigma v\rangle \sim 3\times {10}^{-26}\;{\mathrm{cm}}^{3}\;{{\rm{s}}}^{-1}$?> </jats:tex-math> <?MML <mml:math> <mml:mrow> <mml:mo stretchy="true">&lang;</mml:mo> <mml:mi>&sgr;</mml:mi> <mml:mi>v</mml:mi> <mml:mo stretchy="true">&rang;</mml:mo> </mml:mrow> <mml:mo>&sim;</mml:mo> <!--MPSinvTimes--> <mml:mn>3</mml:mn> <mml:mo>&times;</mml:mo> <!--MPSinvTimes--> <!--RemoveAttribute_Superscriptshift--> <mml:msup superscriptshift="75%"> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <!--mstyle_mmlmsup_level_1_open--> <mml:mrow> <mml:mo form="prefix"> <!--RemoveAttribForm-->&minus;</mml:mo> <mml:mn>26</mml:mn> </mml:mrow> <!--mstyle_mmlmsup_level_1_close--> </mml:msup> <mml:mspace width="0.25em"/> <!--RemoveAttribute_Superscriptshift--> <mml:msup superscriptshift="75%"> <mml:mrow> <mml:mi>cm</mml:mi> </mml:mrow> <!--mstyle_mmlmsup_level_1_open--> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <!--mstyle_mmlmsup_level_1_close--> </mml:msup> <mml:mspace width="0.25em"/> <!--RemoveAttribute_Superscriptshift--> <mml:msup superscriptshift="75%"> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <!--mstyle_mmlmsup_level_1_open--> <mml:mrow> <mml:mo form="prefix"> <!--RemoveAttribForm-->&minus;</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <!--mstyle_mmlmsup_level_1_close--> </mml:msup> </mml:math>?> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj520463ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> for <jats:italic>m</jats:italic> <jats:sub>DM</jats:sub> ≲ 40 GeV for the same channel. Finally, we derive upper limits on the <jats:italic>γ</jats:italic>-ray-flux produced by hadronic cosmic-ray interactions in the inter cluster medium. We find that the volume-averaged cosmic-ray-to-thermal pressure ratio is less than ∼6%.</jats:p>