1) Data from NASA's NuSTAR telescope helped resolve a contentious issue regarding the spin rates of supermassive black holes at the centers of galaxies. Measurements of the galaxy NGC 1365 showed its black hole is spinning at least 84% of maximum theoretical spin. 2) Fast black hole spin provides evidence they grew rapidly through dramatic feeding events rather than slower accretion, helping unravel mysteries of black hole formation and galaxy evolution. 3) Precise spectra from NuSTAR ruled out alternative gas absorption models for the first time, confirming some supermassive black holes do spin rapidly.

1. NEWS & VIEWS
AST ROPHYSICS
Black holes in a spin
The implications of the X-ray emission patterns of galaxies hosting supermassive black holes have been contentious. Data
from NASA’s NuSTAR telescope seem to resolve the issue — at least for one such galaxy. See Letter p.449
CHRISTOPHER S. REYNOLDS
ESO
S
upermassive black holes, with masses of
millions to billions times that of our Sun,
are believed to exist at the centre of essen-
tially every galaxy. When these monsters feast
upon the gas (and possibly the stars!) within
galactic centres, they release enormous quan-
tities of energy, producing a phenomenon
called an active galactic nucleus (AGN). Far
from being dark and difficult to detect, the
black holes in AGNs are the most luminous
spectacles in the Universe. On page 449 of
this issue, Risaliti et al.1 report observations
of the AGN at the centre of the nearby galaxy
NGC 1365 (Fig. 1) using the Nuclear Spectro-
scopic Telescope Array (NuSTAR), a newly
deployed X-ray observatory and NASA’s
latest addition to its fleet of space telescopes.
By exploiting NuSTAR’s ability to measure the
high-energy X-ray spectrum of an AGN with
unprecedented accuracy, the authors obtain an
unambiguous measurement of the spin rate of
this supermassive black hole, finding a spin
that is at least 84% of the maximum theoreti-
cally allowed value.
Why should we care so much about these
supermassive black holes or their spin? To
start with, their very presence is a mystery
that draws in the curious astrophysicist. It now
seems clear that the first black-hole ‘seeds’ were Figure 1 | Spiral galaxy NGC 1365. Risaliti et al.1 have measured the spin rate of the supermassive
created just a few hundred million years after black hole that lurks at the centre of NGC 1365, shown here in an optical image obtained with the
the Big Bang, although the process that created Very Large Telescope.
them is still not understood. Weighing in at
a mere 10,000 solar masses or so, these seeds be spinning very fast. On the other hand, by the gravity of the black hole, producing an
then gorged upon the gas within the young gal- growth through the infall of small, randomly enhancement in the gravitationally induced
axies and grew rapidly into the behemoths that oriented packets of gas (or even small black redshifting of the disk’s emission spectrum4,5.
we see today. Furthermore, our understanding holes) would produce black holes that rotate Through a detailed spectral analysis of
of galaxy formation and evolution is intimately much more slowly3. In this way, the black- the X-ray emission from the accretion disk,
linked to our understanding of supermassive hole spin is a ‘fossil remnant’ of its formation we can model these effects and determine the
black holes. The energy released by a growing processes. black-hole spin.
supermassive black hole can be so powerful Black-hole spin — which reveals itself in Black-hole spin measurements using this
that it disrupts the normal growth of the host the twisting of space-time close to the hole’s technique have been conducted6,7 for several
galaxy; in extreme cases, the AGN can termi- event horizon, beyond which no matter or light years using the 0.5–10-kiloelectronvolt part of
nate all subsequent growth of the galaxy. can escape — is a difficult quantity to meas- the X-ray spectrum accessible with the sensi-
Although essentially every detail of this ure. Our handle on spin comes from the fact tive spectrographs on previous X-ray missions
feasting process is uncertain, the spin of a that a spinning black hole ‘draws in’ the inner such as the Chandra X-ray Observatory,
supermassive black hole can help us unravel edge of the accretion disk, the flat rotating disk XMM-Newton and Suzaku. However, these
the mystery of its growth2. If a black hole grew through which gas flows into the black hole. measurements remained somewhat contro-
in one (or a small number of) dramatic feed- Because the accretion disk can get closer to versial because of the existence of an alterna-
ing event(s), it would acquire the angular the black hole when the black hole is spinning, tive interpretation of this part of the X-ray
momentum of the inflowing matter and would the disk’s emissions are more strongly affected spectrum. In this view, the X-ray-emitting
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2. NEWS & VIEWS RESEARCH
accretion disk is partially obscured by multi- VI R O LO GY
ple layers of gas. These absorbing layers would
introduce complexity into the X-ray spectrum
that could mimic the effects of rapid black-
hole spin8. This has cast a cloud (literally) over
Phages hijack
a host’s defence
measurements of the spins of supermassive
black holes.
The new data from NuSTAR have finally
resolved this issue, at least in this particular
AGN, NGC 1365. Using the superior high- The discovery that some viruses use a defence mechanism known as a CRISPR/Cas
energy X-ray capabilities of NuSTAR, Risaliti system beautifully illustrates the evolutionary tit-for-tat between viruses and
et al. have produced a high-quality spectrum the bacteria they infect. See Letter p .489
of the photons with energies in the range
3–80 keV. Above 10 keV, the signal-to-noise
ratio of the spectrum is unprecedented and M A N U E L A V I L L I O N & S Y LVA I N M O I N E A U CRISPR-targeted regions7 or through acqui-
allows a direct face-off between the alternative, sition of anti-CRISPR genes8. Metagenomic
I
gas-absorption models and the more stand- f you belong to the most abundant bio- studies have also identified CRISPR/Cas sys-
ard (spin-sensitive) models. The authors find logical entity on the planet, you need a few tems in viral genomes (see, for example, ref. 9),
that, for the gas-absorption models to work, tricks to stay on top. The global population but their biological relevance has not been
the accretion disk would have to be blanketed of bacterial viruses (bacteriophages, or phages) proposed. Enter Seed and colleagues, who
by a thick layer of gas such that only 2–3% of has been estimated to be 1031, outnumbering elegantly demonstrate that phage genomes
the generated X-rays actually make it out of their bacterial hosts by tenfold1. Bacteria have are not just ammunition and targets for
the system. Following this picture to its logi- developed a formidable arsenal of sophis- CRISPR/Cas sequences — certain V. cholerae
cal conclusion, the intrinsic emission in this ticated strategies to neutralize viruses2, but phages have hijacked the entire system for their
AGN would have to be so luminous that the phages always seem to find a way to evolve, own defence and persistence.
associated radiation pressure would blow the persist and abound. Studies of the complex V. cholerae is the cause of cholera, which
AGN apart. Although indirect arguments evolutionary dynamics between phages and affects hundreds of thousands of people
against gas-absorption models had been bacteria led to the discovery3 of a widespread each year10. Phages are among the factors
put forward previously5,9,10, this is by far the bacterial defence system called CRISPR/Cas. that may modulate the burden of cholera
cleanest observational demonstration that On page 489 of this issue, Seed et al.4 report in endemic regions, so understanding the
such models fail. the remarkable finding that some phages that interactions between the bacteria and their
With this cloud removed, it seems an infect the bacterial pathogen Vibrio cholerae infecting phages is of interest. Seed et al. ana-
unavoidable result that at least some super- have also acquired a functional CRISPR/Cas lysed the genomes of 11 phages isolated from
massive black holes are spinning rapidly and system in their own genome which allows stool samples of patients with cholera, and
must have grown in rapid accretion events. them to neutralize an unrelated antivirus found that five con-
This raises fundamental theoretical questions system in their bacterial host. “These results tained a CRISPR/
about how gas is fed onto a supermassive black CRISPR (clustered regularly interspaced demonstrate that Cas system. When
hole without fragmenting into smaller packets short palindromic repeats) sequences are phages can hijack the authors exam-
(or even stars) that would randomize its angu- often flanked by cas (CRISPR-associated) a functional, ined the sequence
lar momentum3. These results also encourage genes5. These directly repeating nucleotide of the spacers in
adaptive
us to push further and deeper with our X-ray sequences are separated by short stretches of
immune-evasion the phages, they
observations, necessitating the development non-repetitive DNA called spacers. CRISPR/ found that they
of more powerful X-ray observatories, so that Cas regions have been found in 40% of bac-
system to benefit matched regions in
we can use the diagnostic power of black-hole terial species and 90% of archaea. These their own the genome of the
spin to uncover the story of supermassive- CRISPR/Cas-harbouring microorganisms multiplication.” host bacteria. Speci­
black-hole growth. ■ can acquire small pieces of DNA directly from fically, the spacer
the genomes of invading phages3 or plasmids6 sequences matched an 18-kilobase ‘genomic
Christopher S. Reynolds is in the (small non-chromosomal DNA molecules island’ that is also present in several other
Department of Astronomy and the Joint Space that can be transmitted between bacteria) and strains of V. cholerae.
Science Institute, University of Maryland, insert them as spacers within a CRISPR site. This genomic element resembles phage-
College Park, Maryland 20742, USA. The spacers are then transcribed and pro- inducible chromosomal islands (PICIs), which
e-mail: chris@astro.umd.edu cessed, leading to the production of small RNA are found in some bacteria, including some
molecules called CRISPR RNAs (crRNAs). Staphylococcus aureus strains. In S. aureus,
1. Risaliti, G. et al. Nature 452, 449–451 (2013). The crRNAs and Cas proteins act together as these regions are known as SaPIs, and they
2. Volonteri, M., Madau, P., Quataert, E. & Rees, M. J.
Astrophys. J. 620, 69–77 (2005). a surveillance system that is primed to quickly represent pathogenicity islands that contain
3. King, A. R. & Pringle, J. E. Mon. Not. R. Astron. Soc. target — through base-pairing — similar virulence-factor-encoding genes11. When
373, L90–L92 (2006). nucleic acids from subsequent invaders, and a SaPI-containing cell is infected by certain
4. Tanaka, Y. et al. Nature 375, 659–661 (1995).
5. Fabian, A. C. et al. Mon. Not. R. Astron. Soc. 277,
then to cleave them6. Thus, the bacteria gain phages, the SaPI sequence excises from the
L11–L15 (1995). genetic information from invaders and use bacterial chromosome, circularizes and rep-
6. Miller, J. M. et al. Astrophys. J. 606, L131–L134 it to mount a defence. This process is remi- licates — presumably to exit the infected
(2004).
7. Brenneman, L. W. & Reynolds, C. S. Astrophys. J.
niscent of our own immune system and, as a bacterium. During this process, the bacte-
652, 1028–1043 (2006). result, CRISPR/Cas has been called an adaptive rium also activates a largely uncharacterized
8. Miller, L., Turner, T. J. & Reeves J. N. Astron. microbial immune system. defence system in an attempt to stop phage
Astrophys. 483, 437–452 (2008). A few stealthy phages have been discov- propagation, and thereby ensure its own per-
9. Reynolds, C. S. et al. Mon. Not. R. Astron. Soc. 397,
L21–L25 (2009). ered that can bypass this bacterial protection sistence and the persistence of the surrounding
10. Reynolds, C. S. Astrophys. J. 759, L15 (2012). mechanism through mutation or deletion of phage-susceptible bacterial population12. Seed
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