We have extracted point-spread-function-fitted stellar photometry from near-ultraviolet, optical, and near-infrared images, obtained with the Hubble Space Telescope, of the nearby (D ≈ 5.5 Mpc) SBm galaxy NGC?1311. The ultraviolet and optical data reveal a population of hot main-sequence (MS) stars with ages of 2-10 Myr. We also find populations of blue supergiants with ages between 10 and 40 Myr and red supergiants with ages between 10 and 100 Myr. Our near-infrared data show evidence of star formation going back ~1 Gyr, in agreement with previous work. Fits to isochrones indicate a metallicity of Z ≈ 0.004. The ratio of blue to red supergiants is consistent with this metallicity. This indicates that NGC?1311 follows the well-known luminosity-metallicity relation for late-type dwarf galaxies. About half of the hot MS stars and blue supergiants are found in two regions in the inner part of NGC?1311. These two regions are each about 200 pc across, and thus have crossing times roughly equal to the 10 Myr age we find for the dominant young population. The luminosity functions of the supergiants indicate a slowly rising star formation rate (~10–3 M ☉ yr–1) from ~100 Myr ago until ~15 Myr ago, followed by a strong enhancement (~10–2 M ☉ yr–1) at ~10 Myr ago. We see no compelling evidence for gaps in the star-forming history of NGC?1311 over the last 100 Myr, and, with lower significance, none over the last Gyr. This argues against a bursting mode, and in favor of a gasping or breathing mode for the recent star formation history.
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机译：我们从哈勃太空望远镜获得的附近（D≈5.5 Mpc）SBm星系NGC？1311的近紫外，光学和近红外图像中提取了点扩展函数拟合的恒星光度法。紫外线和光学数据揭示了一群年龄在2到10 Myr的热主序列（MS）星。我们还发现了年龄在10到40 Myr之间的蓝色超巨人和年龄在10到100 Myr之间的红色超巨人。我们的近红外数据表明，恒星形成的证据可以追溯到大约1 Gyr，这与以前的工作一致。拟合等时线表示Z≈0.004的金属度。蓝色和红色超级巨人的比例与此金属性一致。这表明NGC？1311遵循晚期矮星系的众所周知的光度-金属性关系。在NGC？1311内部的两个区域中发现了大约一半的MS热星和蓝色超巨星。这两个区域的跨度均为200 pc，因此穿越时间大致等于我们发现的占主导地位的年轻人口的10 Myr年龄。超巨星的光度函数表明恒星形成速率从〜100 Myr之前一直缓慢上升（〜10–3 M☉yr–1），直到〜15 Myr之前，然后是一个强烈的增强（〜10–2 M yr yr–1）。 ），时间大约是10迈尔。我们没有令人信服的证据表明最近100 Myr的NGC？1311形成恒星的历史有差距，而重要性较低的是最后一个Gyr没有差距。这与爆发模式相抵触，并支持最近的恒星形成历史采用喘气或呼吸模式。
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