螺旋星系 NGC 4424 的潜在黑洞播种通过下落星团,The Astrophysical Journal

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螺旋星系 NGC 4424 的潜在黑洞播种通过下落星团
The Astrophysical Journal ( IF 4.8 ) Pub Date : 2021-12-16 , DOI: 10.3847/1538-4357/ac235b
Alister W. Graham ₁ , Roberto Soria _{2,

3} , Bogdan C. Ciambur _{4,

5} , Benjamin L. Davis _{1,

6} , Douglas A. Swartz ₇

Affiliation

星系可以通过相互的引力吸引和随后的结合而成长。在围绕一个常规的高表面亮度星系运行时，可以剥离一个低质量星系的主体。然而，下落星系的恒星心脏，如果以紧密结合的核星团为代表，则更具弹性。从档案哈勃太空望远镜图像中，我们发现了一个红色的、潮汐拉伸的星团，位于距合并后螺旋星系 NGC 4424 中心约 5 英寸（投影约 400 pc）处，并指向中心。星团，我们称之为“Nikhuli”的近红外光度为 (6.88 ± 1.85) × 10 ⁶ 大号 _⊙,F160W并且可能代表捕获/结合星系的核心。此外，从我们的钱德拉 X 射线天文台图像中，可以看到 Nikhuli 包含一个高能 X 射线点源，其中

大号0.5-8千伏=6.31-3.77+7.50×1038

erg s ^-1（90% 置信度）。我们认为这更有可能是一个活跃的大质量黑洞，而不是 X 射线双星。Nikhuli 的恒星结构缺乏向外指向的彗星状外观，有利于坠落而不是引力波反冲事件的喷射。与低质量早期型星系的小规模合并可能播下了一个巨大的黑洞，助长了一个 X 形的假核球，并正在缝合一个小核球。NGC 4424 的恒星质量和速度色散预测中心黑洞为 (0.6–1.0) × 10 ⁵ 米 _⊙，类似于预期的尼胡里中等质量黑洞，暗示了无核球晚型星系的黑洞供应机制。我们可能会通过捕获和下沉来目睹黑洞的播种，而核星团则是运载工具。

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Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an Infalling Star Cluster

Galaxies can grow through their mutual gravitational attraction and subsequent union. While orbiting a regular high-surface-brightness galaxy, the body of a low-mass galaxy can be stripped away. However, the stellar heart of the infalling galaxy, if represented by a tightly bound nuclear star cluster, is more resilient. From archival Hubble Space Telescope images, we have discovered a red, tidally stretched star cluster positioned ∼5″ (∼400 pc in projection) from, and pointing toward the center of, the post-merger spiral galaxy NGC 4424. The star cluster, which we refer to as “Nikhuli,” has a near-infrared luminosity of (6.88 ± 1.85) × 10⁶ L _⊙,F160W and likely represents the nucleus of a captured/wedded galaxy. Moreover, from our Chandra X-ray Observatory image, Nikhuli is seen to contain a high-energy X-ray point source, with

L0.5−8keV=6.31−3.77+7.50×1038

erg s⁻¹ (90% confidence). We argue that this is more likely to be an active massive black hole than an X-ray binary. Lacking an outward-pointing comet-like appearance, the stellar structure of Nikhuli favors infall rather than the ejection from a gravitational-wave recoil event. A minor merger with a low-mass early-type galaxy may have sown a massive black hole, aided an X-shaped pseudobulge, and be sewing a small bulge. The stellar mass and the velocity dispersion of NGC 4424 predict a central black hole of (0.6–1.0) × 10⁵ M _⊙, similar to the expected intermediate-mass black hole in Nikhuli, and suggestive of a black hole supply mechanism for bulgeless late-type galaxies. We may potentially be witnessing black hole seeding by capture and sinking, with a nuclear star cluster the delivery vehicle.

更新日期：2021-12-16

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