High-Resolution H I Observations of the Galaxy NGC 404: A Dwarf S0 with Abundant Interstellar Gas
High-Resolution H I Observations of the Galaxy NGC 404: A Dwarf S0 with Abundant Interstellar Gas
复制标题
银河 NGC 404 的高分辨率 H I 观测：具有丰富星际气体的矮星 S0
DOI：
10.1086/421358
复制
发表时间：
2004
期刊：
影响因子：
--
通讯作者：
Spain
中科院分区：
文献类型：
--
作者：
M. S. D. Rio;E. Brinks;J. C. Inaoe;Puebla;Mexico;Upaep;Iac;Tenerife;Spain
作为对一个早期类型星系中气体含量的详细研究的一部分，我们给出了S0星系NGC404的21厘米HI线图，空间分辨率为15.“2‘×14.”4(α×δ)，速度分辨率为2.6公里S-1。HI已被追溯到半径R∼8R25或48个磁盘刻度长度，使其成为报告的最大HI区段之一(假设距离3.3MPC时为800“或12.6KPC)。大约75%的HI驻留在甜甜圈中，可以看到甜甜圈与∼R25和∼4R25的内外半径分别接近正面。光学星系恰好与甜甜圈的洞穴相吻合。剩下的25%的中性气体位于与甜甜圈同心的环空中，椭圆度略大，从∼4R25的半径延伸到∼8R25。总的HI质量为1.5 2±0.0 4×10 8M⊙，这意味着太阳单位的MHI/Lb=0.2 2。我们认为，这些气体中的大部分(如果不是全部)来自外部，最有可能来自一个矮星不规则星系与MB∼-15.5MAG的合并。速度场呈现出一条急剧下降的观测旋转曲线，与开普勒式下降相一致。然而，由于星系接近面对面，因此在确定固有自转曲线和倾角时存在简并现象。因此，我们分析了两种极端情况，产生了倾斜的环模型拟合，迫使开普勒式或平坦的旋转曲线通过观测；这两种方法都产生了同样可信的拟合。在两种模型拟合中，环的运动长轴的位置角与甜甜圈的运动长轴的位置角不同，范围为160°到120°(对于甜甜圈，这些值是100°到60°)。假设距离为3.3Mpc，根据开普勒式转动曲线，得到总质量为3×1010M的⊙。这意味着以太阳为单位的全球MT/LB比率为∼44，这是高的，可能反映了星系的低蓝色光度(∼比平均S0光度低15倍)。平坦旋转曲线的值要高出4倍。
As part of a detailed study of the gas content in a sample of early-type galaxies, we present 21 cm H I line maps of the S0 galaxy NGC 404 at a spatial resolution of 15.″2′ × 14.″4 (α × δ) and a velocity resolution of 2.6 km s-1. The H I has been traced out to a radius R ∼ 8R25 or 48 disk scale lengths, making it one of the largest H I extents reported (800″ or 12.6 kpc at the assumed distance of 3.3 Mpc). Approximately 75% of the H I resides in a doughnut, which is seen close to face-on with inner and outer radii of ∼R25 and ∼4R25, respectively. The optical galaxy fits snugly within the hole of the doughnut. The remaining 25% of the neutral gas is found in an annulus concentric with the doughnut and with a somewhat larger ellipticity, extending from a radius of ∼4R25 to ∼8R25. A total H I mass of 1.52 ± 0.04 × 108 M⊙ is found, which implies an MHI/LB = 0.22 in solar units. We argue that most, if not all, of this gas is of external origin, most likely from the merger of a dwarf irregular galaxy with MB ∼ -15.5 mag. The velocity field shows a steeply declining observed rotation curve, compatible with Keplerian decline. However, because the galaxy is close to face-on, there is a degeneracy in the determination of the intrinsic rotation curve and inclination. We therefore analyzed two extreme cases, producing tilted-ring model fits forcing either a Keplerian or a flat rotation curve through the observations; both approaches result in equally plausible fits. In both model fits, the position angle of the kinematical major axis of the annulus is distinct from that of the doughnut and ranges from 160° to 120° (for the doughnut these values are 100° to 60°). Assuming a distance of 3.3 Mpc, a total mass of 3 × 1010 M⊙ is found on the basis of the Keplerian rotation curve. This implies a global MT/LB ratio of ∼44 in solar units, which is high and likely a reflection of the low blue luminosity of the galaxy (∼15 times lower than the average S0 luminosity). Values for a flat rotation curve are a factor of 4 higher.