标题： 本地星系群矮不规则星系 IC5152、UGCA438 和凤凰座矮星系中的 CO 含量 显示英文标题

标题： The CO content of the Local Group dwarf irregular galaxies IC5152, UGCA438, and the Phoenix dwarf

摘要： 我们使用ATNF莫帕射电望远镜对三个本地星系群中的矮不规则星系IC5152、凤凰矮星系和UGCA438进行了CO(1->0)发射的搜索。我们的扫描主要覆盖了这些星系的光学范围以及凤凰矮星系西边的剥离HI云。除了在凤凰矮星系的一个位置上有一个可能但不显著的发射峰外，这些星系的CO谱中没有检测到显著的发射。对于速度宽度为6 km/s的情况，我们得出IC5152、凤凰矮星系和UGCA438的4sigma上限分别为0.03 K km/s、0.04 K km/s和0.06 K km/s。这比之前对IC5152的观测提高了超过一个数量级；另外两个星系此前尚未在毫米波长进行过观测。假设银河系的CO到H_2转换因子，我们得出IC5152、凤凰矮星系和UGCA438的分子气体质量上限分别为6.2 x 10^4 M_sun、3.7 x 10^3 M_sun和1.4 x 10^5 M_sun。我们研究了这些星系缺乏CO发射的两种可能原因。一方面，尽管存在大量的中性气体，这些系统中可能存在真正的分子气体缺乏。然而，在IC5152的情况下，它正在积极形成恒星，因此至少在恒星形成区域应该存在分子气体。另一方面，非常低金属度的矮星系（-2 <= [Fe/H] <= -1）中CO到H_2的转换因子可能会大幅增加，使得CO成为矮星系中分子气体含量的不良示踪物。 显示英文摘要

摘要： We present a search for CO(1->0) emission in three Local Group dwarf irregular galaxies: IC5152, the Phoenix dwarf, and UGCA438, using the ATNF Mopra radio telescope. Our scans largely cover the optical extent of the galaxies and the stripped HI cloud West of the Phoenix dwarf. Apart from a tentative but non-significant emission peak at one position in the Phoenix dwarf, no significant emission was detected in the CO spectra of these galaxies. For a velocity width of 6 km/s, we derive 4sigma upper limits of 0.03 K km/s, 0.04 K km/s and 0.06 K km/s for IC5152, the Phoenix dwarf and UGCA438, respectively. This is an improvement of over a factor of 10 compared with previous observations of IC5152; the other two galaxies had not yet been observed at millimeter wavelengths. Assuming a Galactic CO-to-H_2 conversion factor, we derive upper limits on the molecular gas mass of 6.2 x 10^4 M_sun, 3.7 x 10^3 M_sun and 1.4 x 10^5 M_sun for IC5152, the Phoenix dwarf and UGCA438, respectively. We investigate two possible causes for the lack of CO emission in these galaxies. On the one hand, there may be a genuine lack of molecular gas in these systems, in spite of the presence of large amounts of neutral gas. However, in the case of IC5152 which is actively forming stars, molecular gas is at least expected to be present in the star forming regions. On the other hand, there may be a large increase in the CO-to-H_2 conversion factor in very low-metallicity dwarfs (-2 <= [Fe/H] <= -1), making CO a poor tracer of the molecular gas content in dwarf galaxies.