FEATURE

Kavli IPMU Associate Professor

John Silverman
Research Area： Astronomy

Cosmic Fireworks‘花火’in Colliding Galaxies

For much of their history, galaxies grow byforming new stars out of their gas reservoirs ata steady pace over billions of years. On occasion,an episode of intense star formation occurs,, that is usually the result of
namely a‘starburst’
a collision between two massive galaxies (Figure1) with an eventual merger in many cases. These
spectacular events are relatively short-lived (~100million years), somewhat analogous to the fleeting
桜‘Cherry blossom’season in Japan. In addition togenerating new stars, galaxy mergers are effectiveat channeling gas to their centers to grow asupermassive black hole and a central concentrationof stars (namely a bulge) that is a ubiquitous featureof massive galaxies in the nearby Universe.
Astrophysicists, both theorists and observers,
view starbursts as an important laboratory tostudy the more extreme physical conditions (i.e.,density, temperature and pressure) of the coolgas component of the interstellar medium. To
understand why certain galaxies form stars atdifferent rates, it is important to measure theproperties of the molecular gas out of whichstars form and over a range of scales from giant
molecular clouds within galaxies to galaxy-widequantities.

4In the nearby Universe (out to a distance of ～50 Megaparsecs or 163 million light years), we
have a clear understanding of the impact of galaxy
interactions and mergers on the rate at whichgalaxies form their stars. In the more distant andyounger Universe, the situation may not be thesame since galaxies are richer in molecular gas andhence may be able to form stars rapidly without
any help from a merger. Until recently, it has beenchallenging to study the molecular gas properties ofgalaxies at great distances.

A new window on distant starburst
galaxies with ALMAThe Atacama Large Millimeter/submillimeter
Array (ALMA), a telescope located in Chile on theChajnantor plateau, is now enabling investigationsof the molecular gas (and dust) properties ofgalaxies out to the largest distances and on smaller
physical scales than previously possible. This is dueto the large collecting area of 66 antennas thatwork in unison, as an interferometer, and can be
spread over wide separations that reach up to 16kilometers (Figure 2) thus providing the highest
resolution images possible over a wavelength(frequency) range of 9.6 to 0. 3 millimeters (31 toKavli IPMU News No. 42 June 2018

Figure 1: Hubble Space Telescope image of the interacting galaxy pair, Arp 87 (courtesy STSCI).

1000 GHz).
ALMA is an international observatory with Japan
contributing as a major partner including Taiwan
and South Korea that together constitute the East
Asian participation group. In Japan, ALMA offers
remarkable capabilities to fully explore interesting
objects, such as distant quasars and galaxies that
reside in the first emerging large-scale structures,
currently being discovered by the wide and deep
surveys with Subaru Telescope.
With respect to distant starburst galaxies, pastinterferometric investigations were restricted to themost luminous examples at the expense of having
limited spatial information. With ALMA , we can
now study the spatial and velocity distribution ofgas within distant galaxies with ease.

Finding bona-fide distant starbursts
To know exactly where in the sky to point ALMA ,we need to identify which galaxies are undergoinga starburst phase. Such an effort requires a multi-wavelength observational approach to make key
measurements that shed light on their current stateand history of formation. With limited observational
resources and manpower to compile suchobservational data sets, multi-national collaborations
have been effectively pooling resources for such
purpose. Here, we describe our own effort in theCOSMOS field, a contiguous region of the sky,about nine times the size of the moon, that has
been surveyed by numerous telescopes on theground (e.g., VLT, Subaru, and Keck) and in space(e.g., Hubble, Spitzer, and Herschel Space Telescopes)that cover most of the electromagnetic spectrum.
Japan is an important participant in the COSMOS
survey and is now leading in the next generationof wide-field optical surveys with Subaru including
the imager Hyper Suprime Cam and Prime-Focus
Spectrograph to follow in a few years.
As a first step, we need to assess the rate at which
new stars are being formed, usually in terms of themass in stars per year. Since stars are born in regionsof dense molecular gas, they are enshrouded by5Featuredust thus making it difficult for optical or ultraviolet
emission to escape without being absorbed andreradiated at longer wavelengths. To account for
dust-obscured star formation, the brightness of agalaxy at infrared wavelengths is used as a measureof the rate that stars are being produced. Dueto warm thermal‘infrared’emission from theEarth, space-based telescopes (i.e., Spitzer, Akari,and Herschel) are required for their detection atwavelengths greater than a few microns. Over thelast decade or more, we have learned that the rates
that galaxies form new stars can be off by an orderof-magnitude, if restricted to optical observations
alone, especially in the case of starburst galaxies.
To measure the intrinsic star formation rate, weneed to know the distance to the galaxy since
the detector on Herschel or Spitzer only provides
a brightness for distant galaxies, not its intrinsic
luminosity. Furthermore, an accurate measure of thedistance is needed to tune ALMA to the appropriate
frequency. This requires observations with aspectrograph to disperse the optical or near-infrared
light to detect spectral features that provide ameasure of a galaxy's redshift, a consequence of theexpansion of the Universe, that is directly related toits distance given our current cosmological model.

6In our case, we spent 60 nights at Subaru
Telescope observing over 3500 star-forming galaxies
with the Fiber Multi-Object Spectrograph (FMOS).
As a result, we identified close to 1500 galaxies
with a detection of emission lines such as Hα fromionized hydrogen gas that provided a measure of
their redshift between 1.4 and 1.7. This project
was a collaboration between researchers in Japan,
Europe, and the United States. For some perspective
on future technological advancement, the same
effort could be accomplished with only a few nights
using Subaru's Prime-Focus Spectrograph. Evenso, we identified 150 galaxies (from the 1500 withredshift measurements) detected at far-infrared
wavelengths with the Herschel Space Observatory.
Therefore, we are able to estimate their starformation rate and single out the starburst galaxies
that are forming stars at remarkable rates, well
above the typical population.

Observing molecular gas as traced bycarbon monoxide
Armed with a sample of bona-fide starburst
galaxies at high redshift, we can now observewith ALMA at the appropriate frequency to detect
an emission feature given off from regions ofKavli IPMU News No. 42 June 2018

Figure 2: Atacama Large Millimeter/submillimeter Array (ALMA; credit: ESO).

molecular gas where stars are forming. Traditionally,
carbon monoxide (CO) has been used as a tracerof the total molecular gas content of galaxies thatis primarily composed of molecular hydrogen (H2).
Emission from a CO molecule is emitted duringtransitions from a higher to a lower rotational state,
initially excited due to collisions with H2 molecules.
The level of excitation is indicative of the presenceof gas at different temperatures and densities. With
the spatial resolution of a telescope dependent on
the wavelength (or frequency) of light, higher ordertransitions can produce maps of the gas distribution
at greater spatial resolution due to their higher
observed frequencies. The aim here is to measure
the total CO luminosity with ALMA of a given
molecular transition (e.g., J = 2 to 1; rest frequency= 230.54 Gigahertz) and convert the luminosity tothe mass in molecular gas. The relation between CO

luminosity and gas mass has been calibrated basedon molecular clouds in our Milky Way and galaxies
both at low and high redshift. The latter require ameasure of the gas mass in galaxies, independent ofCO luminosity. There is considerable ongoing debate
about the accuracy of such relations including thatfor starbursts that may differ substantially from
more typical star-forming galaxies.
FeatureA quintessential starburst event
induced by a collision of two galaxies
at high-z
At Kavli IPMU, we have been using ALMA todetect the total CO(2-1) emission from 12 starburst
galaxies at redshifts z ~ 1.6 (an epoch when theUniverse was 30% of its current age), selected ashaving star formation rates ≥ 4 times higher thanmore typical galaxies. Their star formation rates

7

Figure 3: Left - ALMA map of CO(5-4) emission from two galaxies in PACS-787. Right – An infrared image taken with the Hubble
Space Telescope using the Wide-Field Camera 3 with CO indicated by the blue contours.

range from ~100 to 700 solar masses per year.
CO emission is clearly detected in all but one thatindicates the presence of abundant gas reservoirs inthese starburst galaxies.
Unsurprisingly, the brightest galaxy in CO is thegalaxy (PACS-787) with the highest level of starformation at 720 solar masses per year. We werecurious as to what factors could contribute to agalaxy having such a high star formation rate whensimulations of gas-rich galaxies in mergers, similar to
those in the high redshift Universe, typically produce
only modest enhancements in star formation, notenough to reproduce the rates seen in PACS-787.
To answer this question, we re-observed PACS787 at a higher resolution by detecting the CO(5-4)
emission line at a rest frequency of 576. 27
Gigahertz (Figure 3 left panel). To our astonishment,
the CO emission was now resolved into two
distinct galaxies with a separation between thegalaxies being 8.6 kiloparsecs. With such a highstar formation rate, we expected the galaxies to beclose to final coalescence as seen in simulations, but8instead the two galaxies are still in the initial stagesof an interaction. With a recent infrared image ofPACS-787 taken with the Hubble Space Telescope(Figure 3 right panel), we now see clear evidence
for an ongoing collision between the two galaxies
since there are signs that some stars have beentidally stripped and form a bridge between the twogalaxies that is also evident in CO emission.
Based on these observations, we conclude thatthe high star-formation rates in PACS-787 arebased on a set of optimal factors including theinteraction of two gas-rich galaxies and an orbital
configuration conducive to drive gas to the nuclear
regions thus powering a central starburst in eachgalaxy. We further find that the CO emission in eachgalaxy is rotating as a disk that provides further
diagnostics on the amount of molecular gas present,independent of the CO luminosity.

A fuel-efficient engine in distant
starbursts
There is much interest in determining if the star

Kavli IPMU News No. 42 June 2018

Figure 4: Time (tdepl = Mgas / SFR) to deplete the full gas reservoir of a galaxy
at a given star formation rate (SFR). Starburst galaxies are shown as large
colored symbols including our ALMA sample in red. More typical star-forming
galaxies are indicated by smaller circles and a best-fit linear relation.

formation rates in distant starbursts are due toan elevated gas mass, a more efficient conversionof gas to stars, or a combination of the two. To
distinguish between these scenarios, we convert theCO luminosity to molecular gas mass for all twelve
starbursts in our sample using the appropriate
relation. Regarding the efficiency, we measure atime scale for a galaxy to consume all of its gas bydividing the gas mass by the star formation rate. Agalaxy that is efficiently forming stars will have ashort gas depletion time.
As shown in Figure 4, the depletion times forour starbursts are between 50 – 150 million years,
substantially shorter than more typical star-forming
galaxies with depletion times around 1 billion years.
This difference in times scales to form stars indicatesthat starbursts are likely triggered by a mechanism(e.g., galaxy merger) that induces conditions morefavorable for star formation to proceed at such highrates. The debate is not completely settled since
there is remaining uncertainty on the appropriate
value of the factor to convert CO luminosity togas mass for high-z starbursts (and more typicalgalaxies).
Further progress with ALMA will be affordedby higher resolution observations to determine
the physical characteristics of star-forming regions
within galaxies down to ~100 parsec resolution.
With new samples of high redshift galaxies tocome from Subaru surveys, our understanding ofstar formation will vastly improve over the years tocome.

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