1. NEWS & VIEWS
AST ROPHYSICS
The final plunge
A gas cloud has been spotted approaching the Milky Way’s central black hole. Observations of its closest approach,
expected to occur in mid-2013, may offer insight into the black hole’s immediate surroundings. See Letter p .51
MARK MORRIS Gillessen and colleagues’ pro-
F
400 jected 2013 approach of a gas cloud
rom observations made at with known dynamical character-
radio, infrared and X-ray istics into this arena could offer a
wavelengths, we know that fresh perspective on the geometry
matter is continuously falling into 200 and energetics of the accretion
the supermassive black hole that flow. It will be a challenging inves-
Declination o set (mas)
lies at the centre of the Milky Way. tigation; the emission that can be
As that gaseous matter plunges observed in several parts of the
towards the black hole, it heats up 0 electromagnetic spectrum will
and emits copious electromagnetic provide a rather narrow window
radiation, allowing us to ‘see’ the into the ferment of complex physics
black hole’s immediate surround- taking place there.
ings as a source known as Sagit- The scale of the incoming cloud
–200
tarius A* (or Sgr A*). During the of gas and dust is very modest:
decade or so over which research- Gillessen et al.2 estimate it to be
ers have followed this emission, about the size of the Solar System
Sgr A* has flickered on timescales (with a radius about 120 times the
–400
of minutes to hours — presum- 600 400 200 0 –200 mean Earth–Sun distance), and to
ably because of inhomogeneities Right ascension o set (mas) have a mass only about three times
or instabilities in its accretion that of Earth. It therefore has the
flow. But on longer timescales its status of a tiny fragment of a dense
average emission has been fairly Figure 1 | On its way to the Galactic black hole. The sequence of green blobs, interstellar cloud. This raises sig-
which are not drawn to scale, represents the successive locations of the cloud
steady1. This may not be the case of gas and dust reported by Gillessen et al.2 as it approaches the Milky Way’s nificant questions about whether
for much longer: on page 51 of black hole (black spot). The dashed black line is the trajectory that a point it will remain coherent as it under-
this issue, Gillessen et al.2 report the mass would follow in the absence of tidal effects. The cloud becomes elongated goes its presumably terminal plunge
discovery of a small, coherent blob because of the tidal force acting on it, especially as it nears the point of closest through the violent, hot, low-
of ionized gas and dust accelerating approach, where it is violently sheared and the trajectories of different parts density medium around Sgr A*.
along a trajectory that they project of the cloud diverge (dashed green lines). The authors argue that some of the Such a small cloud cannot pos-
will carry it close to the black hole†. gas will circle the black hole and provoke an accretion event that might cause sibly hold itself together with its
Gillessen and colleagues estimate a brightening of Sgr A*, the currently rather dim source associated with the own gravity. External gas pressure
that the gas blob’s closest approach black hole, in mid-to-late 2013. The angular offsets from the black hole in right and the compression resulting
ascension and declination are shown in units of milliarcseconds (mas).
to the black hole — about 260 times from the impact of the ambient
the mean Earth–Sun distance — gas through which the cloud is
will occur in mid-2013. By that time, and per- thus explaining why our Galactic black hole is moving, perhaps assisted by an enveloping
haps well before that, the blob will be disrupted extremely dim. That we can detect it at all is due magnetic field, might promote its confine-
by a combination of dynamical instabilities largely to its proximity — it is about 100 times ment. But the inevitable doom of such a blob
and the black hole’s tidal forces. Consequently, closer to Earth than the nearest supermassive of gas is its inexorable tendency towards frag-
according to the authors, the gas will be black hole in another galactic nucleus, that of mentation, as dynamical instabilities, such as
dispersed over a broad range of orbital traject- the Andromeda galaxy. We also know that the the Rayleigh–Taylor and Kelvin–Helmholtz
ories (Fig. 1), and some of it could accrete onto accreting gas is heated so much that the individ- instabilities, cleave and pare the cloud into a
the black hole in mid-to-late 2013, possibly ual gas particles almost all approach the speed of cluster of smaller and more ill-defined frag-
leading to a marked, long-duration brightening light, and that the observed emission emanates ments, like a disintegrating satellite entering
of Sgr A*. from a magnetohydrodynamic maelstrom3. Earth’s atmosphere. The fragmentation of a
2
The cautiously predicted flare-up would be But the basic parameters of this small-accretion- disintegrating gas cloud would continue on
a valuable probe of the environment immedi- rate regime are still being sought4: does the ever shorter scales of size and time until little
ately surrounding the black hole, about which accreting gas form a well-defined disk or an was left but a faint, broad breeze representing
little is known. What we do know is that the outflowing jet, as it often does in the more the initial momentum of the incoming cloud.
average rate of mass accretion driving the prominent cases observed in many other galax- One might wonder why this hasn’t already
emission from Sgr A* is exceptionally small, ies? Or is it better described as a more isotropic happened to this tiny cloud.
†This article and the paper 2 under discussion were inflow comprising stochastically distributed Gillessen et al.2 have undertaken an initial
published online on 14 December 2011. density fluctuations? numerical investigation of how the black
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2. NEWS & VIEWS RESEARCH
hole’s tidal force would disrupt a small cloud, consider the possibility that a much larger Mark Morris is in the Department
and their results seem to be consistent with mass than the cloud — a relatively faint star or of Physics and Astronomy,
their observation of a progressive elongation a stellar-mass black hole — binds the dusty gas University of California, Los Angeles,
of the blob, and perhaps with a gaseous tail that Gillessen et al. have observed in the form Los Angeles, California 90095–1547, USA.
following the blob in its orbit. But it will be of a circumstellar disk as the ensemble orbits e-mail: morris@astro.ucla.edu
necessary to add numerical hydrodynamic towards Sgr A*. Even then, the black hole’s
1. Meyer, L. et al. Astrophys. J. 694, L87–L91
calculations of this phenomenon in order to tidal force near closest approach may be able (2009).
assess whether the tiny cloud that the authors to wrench some of the gas from the disk, and 2. Gillessen, S. et al. Nature 481, 51–54 (2012).
have found could survive the violent plunge fuel an accretion-induced brightening of the 3. Melia, F. & Falcke, H. Annu. Rev. Astron. Astrophys.
39, 309–352 (2001).
long enough to deposit a significant amount Galactic black hole. Many telescopes are likely 4. Genzel, R., Eisenhauer, F. & Gillessen, S. Rev. Mod.
of matter near the black hole. If not, one might to be watching. ■ Phys. 82, 3121–3195 (2010).
SYNTHETIC B IO LO GY significantly. This produced a device with a
large dynamic range. What’s more, because
Bacteria collaborate the device averaged the outputs of a popula-
tion of cells, noise was reduced and the sensor’s
response was decoupled from the growth state
to sense arsenic
of individual cells. The authors scaled up their
device so that it included more than 12,000
communicating bacterial colonies, covering
an area of 2.4 × 1.2 centimetres.
A method developed to allow rapid communication between bacterial cells across Several advances, yet to be achieved, would
long distances enables the cells to detect arsenic collectively, and to report it as improve the ability to connect this living
an oscillatory output. See Article p .39 sensor to an electronic system. One problem is
that the minimum response time of the sensor
to an input signal is slow, because gene expres-
CHRISTOPHER A. VOIGT a network of genes and proteins that produced sion — which takes about 20 minutes to occur
L
regular pulses of molecules — and used this as — is required. Another issue is that the out-
iving cells can be exploited to sense and a time-keeping mechanism to control cell–cell put involves fluorescence, which is awkward
process environmental stimuli, including communication between bacteria. This yielded for electronic devices to use; the ideal output
poorly defined microenvironments, bio- populations of bacteria that expressed a fluo- would be a direct electrical signal. To this end,
logical markers of disease, defects in materials rescent protein in unison, and so produced cells have been metabolically engineered so that
and complex small molecules. But obtaining a synchronized pulses of light. they can be induced to release electrons, which
reliable signal from individual cells has proved Theoretically, such an oscillator would enable can then be read by an electronic sensor4. The
a challenge. On page 39 of this issue, Prindle a sensor to use the frequency of oscillations electron-transport system found in bacterial
et al.1 report a solution to this problem: a sen- as signals, making the sensor less sensitive to nanowires (extracellular appendages that con-
sor composed of millions of bacterial cells environmental noise and exposure time than duct electricity5) has also been harnessed6 to
that communicate with each other over long systems based on steady-state signals. A prob- link cells to an electronic system. Nevertheless,
distances (up to 2.4 centimetres). The cells lem with the previous oscillator3, however, was these strategies still require the expression of a
respond to the presence of arsenic by altering that cell–cell coupling relied on the diffusion of gene that triggers electron flux, so the resulting
the rate at which they produce synchronized a small molecule through cellular media, a pro- sensors are relatively slow to respond to signals.
pulses of fluorescence. cess that is too slow to allow rapid, long-range More broadly, there are several collaborative
Cells have been engineered to sense many coupling of millions of cells. Molecular diffu- research efforts aiming to develop better tool-
environmental signals, including light, chemi- sion in the gas phase is much faster, so Prindle boxes for building interfaces between cellular
cals, touch, metal ions and pH. For example, et al.1 used this mechanism to accelerate the and electronic components. One such project
sensors have been made in which human coupling between separate colonies of bacteria. is to build a millimetre-scale robot that swims
olfactory receptors are expressed in yeast In this way, the authors were able to couple like a lamprey, using a combination of human
cells2. Most cellular sensors are based on a 2.5 million cells of the bacterium Escherichia muscle cells, yeast-based sensors, an elec-
protein or messenger RNA that responds to coli, which were arranged as an array of colo- tronic brain and flexible materials (nicknamed
a signal by causing the expression of a gene. nies across a distance of 5 millimetres. As in ‘cyberplasm’)7. Another project is to develop
Such genetic sensors often suffer from low the previously reported oscillator3, the output genetic sensors, along with genetic circuits to
dynamic range (that is, there is little change in of the system1 was the coordinated, oscillat- apply signal processing within the cell8 and
output between the absence and presence of a ing expression of a fluorescent protein, which new approaches to link cellular outputs to an
signal) and nonspecificity (they are activated the authors detected using a microscope. The electronic system, with the ultimate objective
by multiple signals). Furthermore, because period of the oscillations was quite long (more of controlling robots9. As the integration of
cells are living systems, individual responses than an hour), but the degree of synchroniza- cellular and electronic systems matures, it will
may vary because of stochastic effects or tion was high — the colonies produced light be interesting to see how circuitry in future
differences in growth states. pulses within 2 minutes of each other. devices is divided between biological and
Prindle et al.1 have addressed the problem To demonstrate a potential application of electronic components. ■
of dynamic range by applying the principles their system, Prindle et al. ‘rewired’ their net-
of signal processing to a biosensor based on work to incorporate elements that respond Christopher A. Voigt is in the Synthetic Biology
genetic circuits. Such circuits use biochemical to arsenic. The resulting system acted as an Center, Department of Biological Engineering,
interactions to produce functions analogous to arsenic sensor: once the concentration of Massachusetts Institute of Technology,
their electronic counterparts. Previously, the arsenic reached a threshold value, the ampli- Cambridge, Massachusetts 02139, USA.
same group built a robust genetic oscillator3 — tude and period of the oscillations increased e-mail: cavoigt@gmail.com
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