Transcript The galaxies that host powerful radio sources

Dusty star formation at high redshift
Chris Willott, HIA/NRC
1. Introductory cosmology
2. Obscured galaxy formation: the view
with current facilities, e.g. JCMT, IRAM
3. The ALMA perspective
The Universe we see today is
composed of billions of galaxies,
each containing billions of stars.
Some galaxies are in
clusters, some are in
groups and some are
all alone.
How did these
galaxies form from
the Big Bang?
Two main types of galaxy:
elliptical and spiral.
Redder. Older stars. Little dust
and gas. Massive galaxies (some).
Bluer. Ongoing star formation.
More dust and gas.
Redshift, the Doppler effect and the Expanding Universe
Hubble’s law:
V = H0 D
Optical spectra of a star and galaxies
Hubble’s law explained by
Doppler effect.
Redshift z
= dl / l = v / c
View of the Universe at the epoch of recombination z=1000
Galaxies form as gas collapses under gravity and cools.
Within dense pockets of gas, the first stars are formed.
2D evolution of gas component under its own self-gravity
Hierarchical galaxy formation - smallest galaxies form first
The cosmic energy density spectrum – note that infrared
background has similar energy density to optical background.
The Hubble Deep Field – the deepest ever optical image.
Optical
Sub-mm (850 mm)
The brightest source in this JCMT image is not detected
in this optical image!!! Total luminosity of the 5 detected
sub-mm sources exceeds that of all the optical galaxies!!!
At what epoch did most of the star formation in the Universe occur?
Z=
t(Gyr)= 13.5
6
3
2
1.5
1.1
0.9
Why is observing the high-redshift Universe
at millimeter wavelengths so easy?
IR
Optical
What are the sources being discovered in submillimeter surveys?
Very difficult to find
secure optical
identifications for
SCUBA sources because:
• Poor resolution
(14 arcsec FWHM).
• Optically faint (R>25).
• Faint at radio and IR
wavelengths.
These facts suggest they
are distant and dusty.
Back to the brightest SCUBA source in the HDF…
Near-infrared image from Subaru 8m
Need high resolution millimeter
or radio data to confirm position.
What do we know about these sources:
Sub-mm flux combined with redshift gives very high farinfrared luminosity – 1000 times that of our galaxy.
Making basic assumptions about the source properties one gets
estimates for the mass of dust of ~100 million solar masses.
Assuming powered by star formation, rate is ~1000 solar masses
per year.
At this rate can form giant
elliptical galaxies in a Gyr.
Cumulative redshift
distribution of bright
SCUBA sources.
Star formation rate density as a function of cosmic epoch:
Latest data shows star formation in dusty obscured systems
at a similar level to that observed in the optical – but note
little overlap in the populations. Still very uncertain.
What do we not know about these galaxies?
• Optical counterparts uncertain for most.
• Redshifts – crucial for luminosities and star formation history
• Masses of the galaxies
• Timescale of star formation
• Morphologies of the galaxies – mergers?
• Spatial distribution of dust and gas
• Connection with active nuclei – fuelled black holes
• What about less luminous high redshift galaxies ?
Need a much bigger telescope with vastly superior
sensitivity and resolution to answer these questions
ALMA – unprecedented sensitivity, resolution and bandwidth
ALMA continuum
sensitivity:
100 times better
than JCMT / IRAM
No longer limited to tip of the iceberg, rare objects like the
current surveys.
Can detect a galaxy like ours at z=3.
Angular resolution – comparable to the best optical imaging.
No more “blobs at high redshift” – will be able to map the
distribution of gas and dust in forming galaxies.
JCMT
Maybe even
discriminate
between AGN and
starburst heating
PdB
Typical size of high
redshift galaxy
So far just talked about dust continuum emission, but more
information comes from spectral observations of molecular gas
CO is the most easily
observed molecule in
galaxies.
Note how CO emission
(contours) comes from
physically distinct
region to optical
emission in this
interacting galaxy.
Dust obscuration.
CO lines can be detected with current technology from a
few objects at very high redshifts, but takes a long time…
IRAM PdB observations of redshift 4.7 quasar
From CO luminosity, convert to total mass of molecular
gas – this is the gas reservoir from which stars are made
Measure observed frequencies of millimeter emission lines
gives source redshifts.
Optical identification and spectroscopy no longer necessary!
Small field-of-view of ALMA means
it is not optimized for large surveys
A major use will be follow-up observations of sources
found in other surveys with SCUBA(2), LMT, SIRTF,
HERSCHEL, NGST, CHANDRA, XMM-NEWTON.
ALMA will be able to resolve the dust and gas in such
sources, measure molecular gas masses and
temperatures, galaxy total masses via rotation curves
What do we not know about SCUBA galaxies?
ALMA
• Optical counterparts uncertain for most.
Yes
• Redshifts
Yes
• Masses of the galaxies
Yes
• Timescale of star formation
??
• Morphologies of the galaxies – mergers?
Yes
• Spatial distribution of dust and gas
Yes
• Connection with active nuclei – black holes
Yes
• Less luminous high redshift galaxies ?
Yes
ALMA will vastly improve our knowledge in all these
areas and many more we haven’t even thought of yet