A team of astronomers has conducted infrared observations of luminous,
gas-rich, merging galaxies with the Subaru Telescope to study active,
mass-accreting supermassive black holes (SMBHs). They found that at
least one SMBH almost always becomes active and luminous by accreting a
large amount of material (Figure 1).
However, only a small fraction of the observed merging galaxies show
multiple, active SMBHs. These results suggest that local physical
conditions near SMBHs rather than general properties of galaxies
primarily determine the activation of SMBHs.
Figure 1: Artist's rendition of an active, mass-accreting black hole in a luminous, gas-rich merging galaxy. (Credit: NAOJ)
In this Universe, dark matter has a much higher mass
than luminous matter, and it dominates the formation of galaxies and
their large-scale structures. The widely accepted, cold-dark-matter
based galaxy formation scenario posits that collisions and mergers of
small gas-rich galaxies result in the formation of massive galaxies seen
in the current Universe. Recent observations show that SMBHs with more
than one-million solar masses ubiquitously exist in the center of
galaxies. The merger of gas-rich galaxies with SMBHs in their centers
not only causes active star formation but also stimulates mass accretion
onto the existing SMBHs. When material accretes onto a supermassive
black hole (SMBH), the accretion disk surrounding the black hole becomes
very hot from the release of gravitational energy, and it becomes very
luminous. This process is referred to as active galactic nucleus (AGN)
activity; it is different from the energy generation activity by nuclear
fusion reactions within stars. Understanding the difference between
these kinds of activities is crucial for clarifying the physical
processes of galaxy formation. However, observation of these processes
is challenging, because dust and gas shroud both star-formation and AGN
activities in merging galaxies. Infrared observations are indispensable
for this type of research, because they substantially reduce the effects
of dust extinction.
To better understand these activities, a team of
astronomers at the National Astronomical Observatory of Japan (NAOJ),
led by Dr. Masatoshi Imanishi, used Subaru Telescope’s Infrared Camera
and Spectrograph (IRCS) and its adaptive optics system to observe
infrared luminous merging galaxies at the infrared K-band (a wavelength
of 2.2 micrometers) and L’-band (a wavelength of 3.8 micrometers). They
used imaging data at these wavelengths to establish a method to
differentiate the activities of deeply buried, active SMBHs from those
of star formation. The radiative energy-generation efficiency from
active, mass-accreting SMBHs is much higher than that of the nuclear
fusion reactions inside stars. An active SMBH generates a large amount
of hot dust (several 100 Kelvins), which produces strong infrared
L’-band radiation; the relative strengths of the infrared K- and L’-band
emission distinguish the active SMBH from star-forming activity. Since
dust extinction effects are small at these infrared wavelengths, the
method can detect even deeply buried, active SMBHs, which are elusive in
optical wavelengths. Subaru Telescope’s adaptive optics system enabled
the team to obtain high spatial resolution images that allowed them to
effectively investigate emission that originates in active SMBHs in the
nuclear regions of galaxies by minimizing emission contamination from
galaxy-wide, star-forming activity.
The team observed 29 infrared luminous gas-rich
merging galaxies. Based on the relative strength of the infrared K- and
L’-band emission at galaxy nuclei, they confirmed that at least one
active SMBH occurs in every galaxy but one (Figure 2).
This indicates that in gas-rich, merging galaxies, a large amount of
material can accrete onto SMBHs, and many such SMBHs can show AGN
activity.
Figure 2: Examples of infrared K-band images of
luminous, gas-rich, merging galaxies. The image size is 10 arc seconds.
North is up, and east is to the left. The individual images clearly show
aspects of the merging process, such as interacting double galaxy
nuclei and extended/bridging faint emission structure. (Credit: NAOJ)
However, only four merging galaxies display multiple, active SMBHs (Figure 3).
If both of the original merged galaxies had SMBHs, then we would expect
that multiple SMBHs would occur in many merging galaxies. To observe
these SMBHs as luminous AGN activity, the SMBHs must actively accrete
material. The team’s results mean that not all SMBHs in gas-rich merging
galaxies are actively mass accreting, and that multiple SMBHs may have
considerably different mass accretion rates onto SMBHs. Quantitative
measurement of the degree of mass accretion rates of SMBHs is usually
based on the brightness of AGNs per unit SMBH mass (Figure 4).
Comparison of SMBH-mass-normalized AGN luminosity (=AGN luminosity
divided by SMBH mass) among multiple nuclei confirms the scenario of
different mass accretion rates onto multiple SMBHs in infrared-luminous,
gas-rich merging galaxies.
Figure 3: Infrared K-band and L’-band images of four
luminous, gas-rich, merging galaxies that display multiple, active
SMBHs. The image size is 10 arc seconds. North is up, and east is to
the left. They show emission from multiple galaxy nuclei. The infrared
K-band to L’-band emission strength ratios characterize emission of
AGN-heated hot dust, not a star-formation-related one. (Credit: NAOJ)
Figure 4: The vertical axis is the comparison of
SMBH-mass-normalized AGN luminosity (= AGN luminosity divided by SMBH
mass) between multiple nuclei. The horizontal axis is the apparent
separation of galaxy nuclei. 1 kilo-parsec corresponds to 30000 trillion
kilometers (19000 trillion miles). The supermassive black-hole (SMBH)
masses are derived from stellar emission luminosity at individual galaxy
nuclei, because SMBH mass and galaxy stellar emission luminosity are
found to correlate in nearby galaxies. If both SMBHs have the same mass
accretion rate, when normalized to the SMBH mass, then such objects are
distributed around the horizontal solid line, at the value of unity in
the vertical axis. Objects above the horizontal solid line are SMBHs
with larger mass and show more active mass accretion, while those below
have a smaller mass and show less active mass accretion.(Credit: NAOJ)
The findings demonstrate that local conditions around
SMBHs rather than general properties of galaxies dominate the mass
accretion process onto SMBHs. Since the size scale of mass accretion
onto SMBHs is very small compared to the galaxy scale, such phenomena
are difficult to predict based on computer simulations of galaxy
mergers. Actual observations are crucially important for best
understanding the mass accretion process onto SMBHs that occurs during
galaxy mergers.
Reference:
Imanishi, M. & Saito, Y. 2014 “Subaru
Adaptive-optics High-spatial-resolution Infrared K- and L’-band Imaging
Search for Deeply Buried Dual AGNs in Merging Galaxies”, Astrophysical
Journal, Volume 780, article id. 106.
Source: Subaru Telescope
