Rare, midsize black hole caught devouring a star

Jan 27, 2020
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Technique could reveal missing population thought to be key in assembling largest black holes

by
DANIEL CLERY
10 NOV 2022

tidal disruption.jpeg
In this artist’s conception, a star that strays too close to a supermassive black hole ends its life in a spectacular light show known as a tidal disruption event.NASA/JPL-CALTECH

When it comes to the size of black holes, there’s a conspicuous gap in the middle. Astronomers have discovered dozens of small ones and tens of gargantuan ones, but only a handful of midsize ones.

Now, researchers have added another potential midsize black hole to the pile, and it’s ravenous: In a distant dwarf galaxy, astronomers have caught this cosmic beast in the act of devouring a star and spewing out brightly glowing crumbs. If more such untidy eaters can be found this way, it could bolster a long-held theory: that midsize black holes are the seeds from which their supermassive cousins grow.

Knowing how many dwarf galaxies harbor midsize black holes “will become a breakthrough,” says Igor Chilingarian, an astrophysicist at Harvard University who was not involved in the study. “Not only will it answer the black hole seeding question,” he says, but could also help understand how galaxies form.

The midsize black hole was bagged by the Young Supernova Experiment (YSE), a collaboration of astronomers that is primarily looking for stars that explode at the end of their lives. The team uses Pan-STARRS, a pair of 1.8-meter telescopes in Hawaii, to look at the same patch of sky every few days; the hope is to catch a supernova explosion in the first hours or days after it starts.

But in June 2020 the astronomers caught something else in their net: a rapidly brightening object in a dwarf galaxy nearly 1 billion light-years away. “We were very, very lucky,” says lead author Charlotte Angus of the University of Copenhagen. “We just jumped on it.” They continued to observe the object, dubbed AT 2020neh, over the following days and weeks using several ground-based telescopes as well as the Hubble Space Telescope. Its light curve—how its brightness changes over time—peaked after just over 13 days and then began a long slow decline.

AT 2020neh.jpeg
Astronomers discovered a star being ripped apart by a black hole in the galaxy SDSS J152120.07+140410.5, 850 m

The shape of the light curve and features in the light’s spectrum didn’t match those from a supernova; it seemed more like a tidal disruption event (TDE), the light show put on when a giant black hole containing millions or even billions of solar masses rips apart a star, consuming some of it and spraying out the rest in a bright superheated arc.

But the object reached its peak brightness more than twice as fast as in a typical TDE. Theorists who model these events predict smaller black holes produce fast-peaking TDEs. Using such models, the team calculated that AT 2020neh’s light curve could have been produced by a black hole with a mass of between 100,000 and 1 million Suns, they report today in Nature Astronomy. “I would say this is the most likely scenario,” Chilingarian says. “We still know too little about these events to be 100% certain.”

Astronomers believe all normal size galaxies have a supermassive black hole at their hearts. But it’s an open question whether dwarf galaxies, such as the one in which AT 2020neh was found, all contain midsize black holes. Because dwarf galaxies are small and faint, “they’re very difficult to detect,” Angus says.

With their apparent midsize TDE, the YSE researchers have stumbled on a new way to detect midsize black holes in dwarf galaxies. If they can detect a large enough sample, they may find whether the size of central black holes grows in step with galaxy size, something already seen in larger galaxies. If that relationship extends from dwarf galaxies right up through large ones, it supports the idea that galaxies get big through the merger of smaller ones, as opposed to coalescing from one gigantic cloud of gas. How galaxies form and grow is one of the great unknowns in astrophysics, one that astronomers hope new sharp-eyed space telescopes such as the recently launched James Webb Space Telescope will shine light on.

The YSE project will likely make only a small contribution to that project; Angus estimates it may detect just a handful more dwarf galaxy TDEs. But when the Vera C. Rubin Observatory, a survey telescope with an 8.4-meter mirror, starts up next year, it will be able to see deeper into space over a wider area. That scope is expected to find as many as 80,000 TDEs in its 10-year survey, so prospects are good.

See: https://www.science.org/content/article/rare-midsize-black-hole-caught-devouring-star?utm_source=sfmc&utm_medium=email&utm_campaign=DailyLatestNews&utm_content=alert&et_rid=255259432&et_cid=4485140

The original paper by the YSE team:

A fast-rising tidal disruption event from a candidate intermediate-mass black hole

by:

Massive black holes (BHs) at the centres of massive galaxies are ubiquitous. The population of BHs within dwarf galaxies, on the other hand, is not yet known. Dwarf galaxies are thought to harbour BHs with proportionally small masses, including intermediate-mass BHs, with masses 102 < MBH < 106 solar masses (M⊙). Identification of these systems has historically relied on the detection of light emitted from accreting gaseous disks close to the BHs. Without this light, they are difficult to detect. Tidal disruption events, the luminous flares produced when a star strays close to a BH and is shredded, are a direct way to probe massive BHs. The rise times of these flares theoretically correlate with the BH mass. Here we present AT 2020neh, a fast-rising tidal disruption event candidate, hosted by a dwarf galaxy. AT 2020neh can be described by the tidal disruption of a main sequence star by a 104.7–105.9 M⊙ BH. We find the observable rate of fast-rising nuclear transients like AT 2020neh to be low, at ≲2 × 10−8 events Mpc−3 yr−1. Finding non-accreting BHs in dwarf galaxies is important to determine how prevalent BHs are within these galaxies, and to constrain models of BH formation. AT 2020neh-like events may provide a galaxy-independent method of measuring the masses of intermediate-mass BHs.

AT 2020neh was first reported by the Zwicky Transient Facility (ZTF; 1) on 19th June 2020 at right ascension 15h21m20.07s and declination +14◦04’10.74” (J2000), and was confirmed with Young Supernova Experiment data (YSE; 2), which showed an initial ealier detection on 17th June 2020. The location of the transient, confirmed in late time imaging from the Hubble Space Telescope (Figure 1), is coincident with the nucleus of the galaxy, lying within 0.1” of the centre. Host-galaxy spectral lines constrain the redshift of the event to z = 0.062 (∼280 Mpc).

AT 2020neh reached peak brightness on 1st July, 2020, and was monitored with multi-wavelength follow-up observations for over 400 days from peak in the rest frame (for details of the follow-up campaign, see Methods). The full ultraviolet and optical light curves for AT 2020neh are shown in Extended Data Figure 1.

We present our spectroscopic follow-up observations of AT 2020neh in Figure 2. The classi- fication spectrum, obtained using the Nordic Optical Telescope on 25th June 2020, 6 days before maximum light, shows a strong blue continuum with a clearly blended helium ii λ4685 and nitro- gen iii λ4640 emission feature, and no traces of hydrogen. This blended emission feature has been observed for several optical TDEs (3, 4, 5, 6, 7), and is attributed to a fluorescence mechanism re- quiring both a high-energy radiation source and a high gas density (8). Given the nuclear location of the transient (Figure 1), (continued below Figure 1)

Screenshot 2022-11-11 at 18.59.23.png
Figure 1 - The wider image shows the environment of AT 2020neh in optical PS1 r-band imaging. The dwarf host galaxy of the transient is highlighted. The first inset shows the apparent location of AT 2020neh within its host galaxy in the optical. The host centroid is marked with a black cross whilst the location of AT 2020neh is marked with a red cross (shown with 1σ astrometric uncertainties). The location of the transient is coincident with the host nucleus. A second inset (orange boarder) shows deep ultraviolet imaging from the Hubble Space Telescope of AT 2020neh at +416d. The transient is still clearly detected at the centre of the host, surrounded by a ring of star formation approximately 600pc from the nucleus.

Screenshot 2022-11-11 at 19.06.25.png
Rest frame wavelength
Figure 2 | The spectroscopic evolution of AT 2020neh. Common TDE emission features (H, He, N) are marked. The strong He II and N III emission seen pre-maximum light disappears after the peak, with Balmer emission appearing at much later epochs, becoming increasingly asymmetric and blue-shifted as the TDE evolves. Spectra have been offset for clarity with rest-frame phases indicated. Crossed circles mark telluric features still present within the spectra.


...we interpret these features under a TDE classification for the transient AT 2020neh. The spectra become featureless after maximum light, evolving to gradually reveal a broad hydrogen, Hα, emission line at +36 d (and later in Hβ too). This Hα emission dominates the late-time spectra of AT 2020neh, exhibiting an asymmetric profile which is blueshifted with re- spect to the rest frame by ∼4000 km s−1 (see Extended Data Table 1). The profile of this emission is consistent with emission lines arising from optically thick outflowing material, which has been seen in several other TDEs (e.g. 7, 10, 11, 12). The lack of elements heavier than hydrogen within the late time (>200 d) spectra is consistent with a TDE classification, as these elements would only be expected if the event arose from a stellar explosion.
Screenshot 2022-11-11 at 19.10.05.png
Extended Data Figure 2 | Spectroscopic comparison with other transients. Top: The early time spectrum of AT 2020neh alongside other TDEs which show Bowen fluorescence and an example of an alternative to this interpretation (CCSN flash-ionisation, as exemplified in SN 1998S). Middle: Comparison of the post-maximum features in AT 2020neh with other TDEs, a CCSN and the FBOT AT 2018cow. AT 2020neh presents a Hα profile in emission, lacking the typical P-Cygni profile of a SN II but much broader than the FBOT. The rest of the spectrum is featureless, lacking absorption features from iron-group elements which would be expected of a CCSN during the photospheric phase. Bottom: The ‘nebular’ phase spectrum of AT 2020neh alongside a nebular phase CCSN. Expected emission features from SN during this phase are marked. Right: The evolution of the Hα profile of AT 2020neh. This feature appears late in the post-peak evolution, and retains a strongly blue-shifted profile, unseen in normal SNe. The narrow profile of AT 2018cow is shown for comparison.

See: https://www.nature.com/articles/s41550-022-01811-y

See: https://ui.adsabs.harvard.edu/abs/2022arXiv220900018A/abstract

See: https://arxiv.org/pdf/2209.00018.pdf

Data from the Young Supernova Experiment (YSE), a collaboration of astronomers who search for stars that explode at the end of their lives, enabled the team to detect the first signs of light as the black hole began to eat the star. Capturing this initial moment was pivotal to unlocking how big the black hole was, because the duration of these events can be used to measure the mass of the central black hole. Optical data was supplied by W. M. Keck Observatory in Hawaii, the Nordic Optical Telescope, UC's Lick Observatory, NASA's Hubble Space Telescope, the International Gemini Observatory, the 200 inch Palomar Observatory, and the Pan-STARRS Survey at Haleakala Observatory.
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