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Fig. 1:
This photo shows a spectacular pair of interacting galaxies
that are located 300 million light-years away in the constellation
Coma Berenices, and have been nicknamed "The Mice" because of the
long tails of stars and gas emanating from each galaxy. Numerous young
blue stars and star clusters, spawned by the tidal interaction, are
seen in the spiral arms, as well as in the long tails of stars. It is
thought that The Mice presage what may happen to our own Milky Way
Galaxy several billion years from now when it collides with our
nearest large neighbour, the Andromeda Galaxy (M31). This picture is
assembled from three sets of images taken on April 7, 2002, by the
Advanced Camera for Surveys (ACS), the newest camera on NASA's Hubble
Space Telescope (HST).
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Fig. 2:
The y-axis shows the average counts of neighbours around the
galaxies that are currently forming new stars. The x-axis shows the
distances from these galaxies at which the neighbours are found. The
distances are in unit of Mpc, a million parsecs (1 parsec approximates to
3.3 light-years). Different symbols are for galaxies with different
specific star formation rates, log(SFR/M*), which is the ratio
of the rate at which galaxies form stars with respect to the total
mass of their tars. Note that the specific star
formation rates, as indicated in the figure, are on a logarithmic
scale. For a typical bright galaxy like our Milky Way which contains a
few tens of billions of stars like our Sun, log(SFR/M*) = -9
means that a few tens of new stars are formed per year in the galaxy,
while log(SFR/M*) = -10 means a factor of 10 smaller in this
number. It is clear from this figure that galaxies form stars at
higher rates if they have close neighbours.
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Fig. 3:
As Fig. 2, but for active galactic nuclei (AGN).
In the figure, different symbols are for AGN with different
accretion rates, as indicated, which are estimated by the luminosity
of Oxygen emission line divided by the mass of the black hole.
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Active galactic nuclei (AGN) are among the most spectacular objects in
the Universe. They produce prodigious luminosities (in some cases as
much as 10,000 times the luminosity of a typical galaxy) in tiny
volumes (much less than one cubic light-year). Most astronomers now
believe that the power for AGN comes from accretion of matter onto the
central supermassive black hole located at the centre of every galaxy
with a central, spheroidal stellar component. A major goal in the
study of AGN has been to understand how the accretion onto the black
hole is triggered, and why such activity in the nuclear region of
galaxies is associated with strong bursts of star formation
in much extended regions. Numerical simulations have
shown that interactions between galaxies can bring gas from the disc
to the central regions, leading to enhanced star formation and AGN
activity. On the observational side, it has long been known that
galaxy-galaxy interactions are associated with enhanced star
formation. However, there has been little clear evidence in support
of a similar interaction-induced enhancement of nuclear activity.
Galaxies are not isolated. In the generally accepted picture, most of
them have undergone some form of gravitational interaction, or even
merger, with another galaxy during the lifetime of the Universe (see
Fig. 1 for an example of interacting galaxies). Indeed,
it is believed that galaxy-galaxy interactions/mergers play a central
role in at least four of the most important processes in galaxy
formation and evolution. First, in the currently popular
"hierarchical formation paradigm", galaxies are formed through
merging of smaller galaxies. Second, gas-rich spiral galaxies
transform to gas-poor elliptical galaxies through mergers, as also shown by
numerical simulations
( Research Highlight Februrary 2005).
Third, interacting galaxies
often exhibit high rates of star formation. This is explained by the
fact that, tidal forces induced by interactions casue the gas in the
galaxies to lose its angular momentum, and to fall to the centres of
the galaxies, where it may ignite a firestorm of star birth. Finally,
some of this gas may be accreted onto the central supermassive black
hole, and this will trigger activity in the nucleus of the galaxy.
(Such galaxies are called active galaxies and their nuclei are termed
as Active Galactic Nuclei, AGN.) This conjecture sounds plausible
given the observed tight correlation between the mass of the black
hole and the total mass of the stars in the central, spheroidal
component of galaxies (called bulge)
(Research Highlight April 2005,
Research Highlight August 2007),
and
has motivated many theoretical models in which accretion onto black
holes and AGN activity are assumed to be closely linked to galaxy
interactions and mergers
(Research Highlight Februrary 2005).
However, on the
observational side, there has been little direct, clear evidence in
support of this hyphothesis.
The large redshift surveys assembled in recent years, in particular
the Sloan Digital Sky Survey
( SDSS),
have provided angular
positions and redshifts for hundreds of thousounds of galaxies and
have allowed astronomers to make detailed maps of how galaxies are
distributed in space
(Research Highlight Februrary 2006).
The survey also
provides an optical spectrum for each galaxy
which allows to determine a variety of physical
properties such as the total mass of their stars (stellar mass), the
rate at which they form stars (star formation rate), and the rate at
which matter is accreting on to the central supermassive black hole
(accretion rate)
(Research Highlight January 2003).
The new data allows astronomers to
obtain a much clearer picture of the connections between star
formation, AGN activity and galaxy-galaxy interactions.
A team of astronomers at the Max Planck Institute for Astrophysics
(Cheng Li, Guinevere Kauffmann and Simon White), in collaboration with
colleagues from Johns Hopkins University (Timothy Heckman) and
Shanghai Astronomical Observatory (Yipeng Jing), have been exploring
these connections by applying a variety of statistics to SDSS data.
It has been shown that all galaxies, regardless of whether they are
active or inactive, form new stars at higher rates if they have close
neighbours. In contrast, the accretion rate onto the black hole is not
influenced by the presence or absence of close neighbours. Previous
work has shown that strong AGN activity is often associated with
strong bursts of recent star formation in the host galaxy, known as
the starburst-AGN connection
(Research Highlight January 2003).
This leads the team to
conclude that star formation induced by a close neighbour and star
formation associated with black hole accretion are distinct
events. However, these events may be part of the same long-term
physical process, for example a merger, provided that they are
separated in time. In this case, accretion on to the black hole and
its associated star formation would occur only after the two
interacting galaxies have merged.
Cheng Li
Publications
Cheng Li, Guinevere Kauffmann, Timothy M. Heckman, Simon D. M. White
and Y. P. Jing,
"Interactions, star formation and AGN activity",
2008, Monthly Notices of Royal Astronomical Society, in press
arXiv:0712.0383
Cheng Li, Guinevere Kauffmann, Timothy M. Heckman, Y. P. Jing
and Simon D. M. White,
"Interaction-induced star formation in a
complete sample of 105 nearby star-forming galaxies",
2008,
Monthly Notices of Royal Astronomical Society, in press
arXiv:0711.3792
Cheng Li, Guinevere Kauffmann, Lan Wang, Simon D. M. White,
Timothy M. Heckman and Y. P. Jing,
"The clustering of narrow-line AGN in the local Universe",
2006, Monthly Notices of Royal Astronomical Society, 373, 457-468
astro-ph/0607492
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