Dusty star formation at high Dusty star formation at high redshiftredshift

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?

Redder. Older stars. Little dust and gas. Massive galaxies (some).

Two main types of galaxy: elliptical and spiral.

Bluer. Ongoing star formation. More dust and gas.

Redshift, the Doppler effect and the Expanding Universe

Optical spectra of a star and galaxies

Hubble’s law:

V = H0 D

Hubble’s law explained by Doppler effect.

Redshift z = = v / c

Redshift

Z=infinity z=1000

z=10 z=1

z=0

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 m)

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?

t(Gyr)= 13.5 6 3 2 1.5 1.1 0.9

Z =

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…

Need high resolution millimeter or radio data to confirm position.

Near-infrared image from Subaru 8m

Cumulative redshift distribution of bright SCUBA sources.

What do we know about these sources:

Sub-mm flux combined with redshift gives very high far-infrared 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.

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

PdB

Typical size of high redshift galaxy

Maybe even discriminate between AGN and starburst heating

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