VERY LARGE ARRAY AND ATCA SEARCH FOR NATAL STAR CLUSTERS IN NEARBY STAR-FORMING GALAXIES
VERY LARGE ARRAY AND ATCA SEARCH FOR NATAL STAR CLUSTERS IN NEARBY STAR-FORMING GALAXIES
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非常大的阵列和 ATCA 搜索附近恒星形成星系中的原生星团
DOI：
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发表时间：
2011
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通讯作者：
D. Pisano
中科院分区：
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作者：
A. Aversa;A. Aversa;A. Aversa;Kelsey E. Johnson;Kelsey E. Johnson;C. Brogan;W. Goss;D. Pisano;D. Pisano
为了研究当地环境与出生星团性质之间的关系，我们利用甚大阵列和澳大利亚望远镜紧凑阵列对20 Mpc内25个恒星形成星系进行了射电观测。本塔尔恒星形成区可以通过其特征的热射电发射来识别，这体现在它们在厘米波长的光谱指数中。我们样本中的宿主星系是根据它们孕育年轻恒星的可能性来选择的。在恒星形成区域，大质量嵌入恒星的电离通量为这些源的主要热自由-自由发射提供了能量，导致我们计算出的光谱指数为α <−0.2（其中Sν∝να）。根据目前的灵敏度，我们发现在该样本的25个星系中，只有5个星系的射电源的光谱指数仅与热源一致，4个星系的射电源仅与非热源一致，6个星系的射电源由于光谱指数的不确定性而性质模糊，16个星系没有探测到射电源。对于那些似乎以热辐射为主的源，我们推断出星团的电离通量和产生观测到的射电通量密度所需的等效O7.5 V恒星的数量。我们探测到的最明亮的射电星团拥有相当于~ 7 × 103颗O7.5 V的恒星，而最小的星团只有相当于~ 102颗O7.5 V的恒星；因此，这些恒星形成区域跨越了从大型OB星团到中等“超级星团”的范围。根据目前的检测极限，我们还为最近可能形成的星团的质量设定了上限；对于许多星系，我们可以明确地排除比银河系恒星形成区W49A （~ 5 × 104 M☉）质量大得多的出生星团的存在。在这些星系中缺乏当前的大质量星团形成表明，它们当前的恒星形成强度已经下降到接近或低于银河系，并且/或者产生热射电发射的进化状态是短暂的。
In order to investigate the relationship between the local environment and the properties of natal star clusters, we obtained radio observations of 25 star-forming galaxies within 20 Mpc using the Very Large Array and the Australia Telescope Compact Array. Natal star-forming regions can be identified by their characteristic thermal radio emission, which is manifest in their spectral index at centimeter wavelengths. The host galaxies in our sample were selected based upon their likelihood of harboring young star formation. In star-forming regions, the ionizing flux of massive embedded stars powers the dominant thermal free–free emission of those sources, resulting in a spectral index of α ≳ −0.2 (where Sν ∝ να), which we compute. With the current sensitivity, we find that of the 25 galaxies in this sample only 5 have radio sources with spectral indices that are only consistent with a thermal origin, 4 have radio sources that are only consistent with a non-thermal origin, 6 have radio sources whose nature is ambiguous due to uncertainties in the spectral index, and 16 have no detected radio sources. For those sources that appear to be dominated by thermal emission, we infer the ionizing flux of the star clusters and the number of equivalent O7.5 V stars that are required to produce the observed radio flux densities. The most radio-luminous clusters that we detect have an equivalent of ∼7 × 103 O7.5 V stars, and the smallest only have an equivalent of ∼102 O7.5 V stars; thus these star-forming regions span the range of large OB associations to moderate “super star clusters.” With the current detection limits, we also place upper limits on the masses of clusters that could have recently formed; for a number of galaxies we can conclusively rule out the presence of natal clusters significantly more massive than the Galactic star-forming region W49A (∼5 × 104 M☉). The dearth of current massive cluster formation in these galaxies suggests that either their current star formation intensities have fallen to near or below that of the Milky Way and/or the evolutionary state that gives rise to thermal radio emission is short-lived.