Starbursts in the Local Universe: new HST/ACS observations of the irregular galaxy NGC 4449
Starbursts in the Local Universe: new HST/ACS observations of the irregular galaxy NGC 4449
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本地宇宙中的星暴：不规则星系 NGC 4449 的新 HST/ACS 观测
DOI：
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发表时间：
2007
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通讯作者：
M. Sirianni
中科院分区：
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作者：
F. Annibali;A. Aloisi;J. Mack;M. Tosi;R. V. D. Marel;L. Angeretti;C. Leitherer;M. Sirianni
本文介绍了利用哈勃太空望远镜（HST）上的高级巡天照相机（ACS）对麦哲伦星暴星系NGC4449中的恒星进行测光。该星系已在F435 W（B）、F555 W（V）和F814 W（I）宽带滤光片和F658 N（Halalpha）窄带滤光片中成像。我们的测光包括约300，000个在（B，V）色星等图（CMD）中的物体，直到V < 28，以及约400，000个在（V，I）CMD中的物体，直到I < 27。约200，000颗恒星的子样本已在所有三个波段中同时进行了光度测量。在CMDs中观察到的特征意味着各种恒星年龄，至少可达1 Gyr，甚至可能与哈勃时间一样古老。CMD形态和红巨星分支颜色的空间变化指向年龄梯度的存在：年轻和中等年龄的恒星往往集中在银河系中心，而老年恒星则到处都是。碳星平均光度的空间变化表明，没有很强的金属丰度梯度（< 0.2 dex）。此外，我们发现了一个有趣的解决星星集群的西侧的星系，由一个对称的潮汐或螺旋功能组成的年轻恒星包围。NGC 4449中年龄小于1000万年的恒星的位置与H α辐射密切相关。我们推导出NGC 4449到红巨星分支顶端的距离为D=3.82 pm 0.27 Mpc。这一结果与我们从碳星的亮度中推导出的距离一致。
We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I) broad-band filters, and in the F658N (Halpha) narrow-band filter. Our photometry includes ~300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V < 28, and ~400,000 objects in the (V, I) CMD, down to I < 27 . A subsample of ~200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (< 0.2 dex). Also, we detect an interesting resolved star cluster on the West side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Halpha emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D=3.82 pm 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.