Cool dust heating and temperature mixing in nearby star-forming galaxies
Cool dust heating and temperature mixing in nearby star-forming galaxies
复制标题
DOI：
10.1051/0004-6361/201424734
复制
发表时间：
2014-09
影响因子：
6.5
通讯作者：
L. Hunt;B. Draine;S. Bianchi;K. Gordon;K. Gordon;G. Aniano;G. Aniano;D. Calzetti;D. Dale;G. Helou;J. Hinz;R. Kennicutt;H. Roussel;C. Wilson;A. Bolatto;M. Boquien;K. Croxall;M. Galametz;A. Paz;J. Koda;J. Muńoz-Mateos;K. Sandstrom;K. Sandstrom;M. Sauvage;L. Vigroux;S. Zibetti
中科院分区：
文献类型：
--
作者：
L. Hunt;B. Draine;S. Bianchi;K. Gordon;K. Gordon;G. Aniano;G. Aniano;D. Calzetti;D. Dale;G. Helou;J. Hinz;R. Kennicutt;H. Roussel;C. Wilson;A. Bolatto;M. Boquien;K. Croxall;M. Galametz;A. Paz;J. Koda;J. Muńoz-Mateos;K. Sandstrom;K. Sandstrom;M. Sauvage;L. Vigroux;S. Zibetti
星系中星际介质的物理条件与环境辐射场和尘埃颗粒的加热密切相关。为了在广泛的物理条件下表征星系中的尘埃特性，我们在这里介绍了61个星系的整个样本的径向表面亮度分布，这些星系来自附近星系的关键洞察：Herschel远红外巡天（KINGFISH）。我们工作的主要目标是表征的颗粒发射率，尘埃温度，和星际辐射场（ISRF）负责加热尘埃。我们首先用指数函数拟合径向轮廓，以比较恒星和冷尘埃盘的尺度，如3.6 μm和250 μm的表面亮度所测量的。我们的研究结果表明，恒星和尘埃的scalelength是可比的，与1.04的平均比率，虽然有几个星系的尘埃恒星scalelength比为1.5或更高。然后，我们拟合远红外光谱能量分布（SED）在每个环形区域与单温度修改黑体使用可变（MBBV）和固定（MBBF）发射率指数β，以及与物理驱动的灰尘模型。KINGFISH廓线非常适合于研究尘埃温度T_（dust）和β的趋势，因为它们跨越了加热大部分尘埃质量的ISRF强度U_（min）的~200倍。拟合的结果表明，平均而言，尘埃光学厚度T_（dust）、尘埃光学厚度τ_（dust）和U_（min）随半径的增大而减小。发射率指数β在某些星系中也随半径减小，但在另一些星系中则随半径增大，或在内部区域增大而在外部区域减小。尽管物理模型的颗粒发射率固定（平均β ~ 2.1），但它们能够很好地适应β ≥ 1的平坦光谱斜率。对数据和模型中尘埃发射率波长变化的分析表明，与通常的T_（dust）- β简并度相反，较平坦的斜率（β = 1.5）与较低的温度有关。这种趋势与U_（min）的变化有关，因为在KINGFISH星系的U_（min）的整个采样范围内，β和U_（min）是非常密切相关的：低U_（min）与平坦的β α 1有关。这两个结果都有力地表明，MBBV拟合中的低表观β值（平坦斜率）是由沿着视线的温度混合引起的，而不是由晶粒特性的内在变化引起的。最后，对尘埃模型和数据的比较表明，在500微米处，金属丰度低（12 + log（O/H）108）和远红外表面亮度低（10500）的情况下，略微超出10%。
Physical conditions of the interstellar medium in galaxies are closely linked to the ambient radiation field and the heating of dust grains. In order to characterize dust properties in galaxies over a wide range of physical conditions, we present here the radial surface brightness profiles of the entire sample of 61 galaxies from Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the characterization of the grain emissivities, dust temperatures, and interstellar radiation fields (ISRFs) responsible for heating the dust. We first fit the radial profiles with exponential functions in order to compare stellar and cool-dust disk scalelengths, as measured by 3.6 μm and 250 μm surface brightnesses. Our results show that the stellar and dust scalelengths are comparable, with a mean ratio of 1.04, although several galaxies show dust-to-stellar scalelength ratios of 1.5 or more. We then fit the far-infrared spectral energy distribution (SED) in each annular region with single-temperature modified blackbodies using both variable (MBBV) and fixed (MBBF) emissivity indices β, as well as with physically motivated dust models. The KINGFISH profiles are well suited to examining trends of dust temperature T_(dust) and β because they span a factor of ~200 in the ISRF intensity heating the bulk of the dust mass, U_(min). Results from fitting the profile SEDs suggest that, on average, T_(dust), dust optical depth τ_(dust), and U_(min) decrease with radius. The emissivity index β also decreases with radius in some galaxies, but in others is increasing, or rising in the inner regions and falling in the outer ones. Despite the fixed grain emissivity (average β ~ 2.1) of the physically-motivated models, they are well able to accommodate flat spectral slopes with β ≲ 1. An analysis of the wavelength variations of dust emissivities in both the data and the models shows that flatter slopes (β ≲ 1.5) are associated with cooler temperatures, contrary to what would be expected from the usual T_(dust) – β degeneracy. This trend is related to variations in U_(min) since β and U_(min) are very closely linked over the entire range in U_(min) sampled by the KINGFISH galaxies: low U_(min) is associated with flat β ≲ 1. Both these results strongly suggest that the low apparent β values (flat slopes) in MBBV fits are caused by temperature mixing along the line of sight, rather than by intrinsic variations in grain properties. Finally, a comparison of dust models and the data show a slight ~10% excess at 500 μm for low metallicity (12 + log (O/H) ≲ 8) and low far-infrared surface brightness (Σ500).