The Submillimeter Astrophysics group

A more detailed design description is found elsewhere on this page (see link) and in our publication list (see link).

ZEUS can accomodate a much larger format array -- about 54 pixels spectrally, and 20 spatially, so that a 54 spectral element spectrum can be delivered for 20 beams on the sky along a long slit simultaneously. We are creating a multi-color, multi-beam version of ZEUS, ZEUS-2 (see link).

ZEUS Science

Since late 2006, we have enjoyed five very successful runs with ZEUS on the 10.4 m CSO. We detected the CO(6-5), CO(7-6), and [CI] 370 um lines from about two dozen nearby starforming galaxies and ultraluminous galaxies. We also detected the CO(8-7) line from five ULIRGs, ¹³CO(6-5) from two galaxies, and the redshifted [CII] line emission from six galaxies at redshifts between 1.1 and 1.8. These lines trace the excitation of the neutral interstellar medium and are important coolants, enabling cloud collapse to form stars.

First Detection of ¹³CO(6-5) from an External Galaxy

Our detection of the ¹³CO(6-5) line from the nucleus of NGC 253 is the first detection of the ¹³CO(6-5) line from an external galaxy, and the first detection of any ¹³CO transition greater than J=3-2 from a source beyond the Magellanic clouds. We detected the line from the nuclear regions of the starburst galaxy, NGC 253, where at a distance of 2.5 pc, our 11" beam subtends 275 kpc. We observed the ¹³CO(6-5) the ¹²CO(6-5), the ¹³CO(6-5), and [CI] 370 um lines from NGC 253, mapping the last three lines (right). The ¹³CO(6-5) line is bright, at ~ 7% of the line flux in the ¹²CO(6-5) line indicating optically thick emission in the latter. We model the observed run of CO and ¹³CO line emission with J using a large velocity gradient (LVG) method, and find that 35 to 60% of the molecular gas mass in the nuclear regions is both warm (T ~ 110 K), and dense (~ 10 ⁴cm ^-3) in support of our prior work based on the detection of the ¹²CO(7-6) line (Bradford et al. 2003). We conclude that the gas is heated by either the cosmic rays from the nuclear starburst, or by the decay of turbulence within molecular clouds, presumably also driven by the formation of stars. The heating of the molecular ISM by the starburst therefore is inhibiting further episodes of star formation, so that for NGC 253 the starburst is self-limiting (Hailey-Dunsheath et al. 2008).

First Detections of the [CII] 158 um line from the Epoch of Enhanced Star Formation in the Universe

Detailed studies in the infrared to submillimeter continuum Over the past decade have shown that the rate of star formation per unit co-moving volume peaked when the Universe was only 15-45% of its present age (redshifts 1 to 3) at values 30 times the present rate. We have begun a program to study star formation in this epoch using the bright far-IR fine-structure lines as they are redshifted into the submm windows as probes. These lines are important coolants for most of the important phases of the interstellar medium, so that their measure yields important information on the star formation process on galactic scales.

The brightest of the far-IR lines (and indeed, it can be the brightest single line from a star forming galaxy) is the 158 um [CII] line which cools the cold neutral medium, the warm neutral medium, diffuse ionized gas and the photo-dissociation regions (PDRs) formed on the surfaces of molecular clouds exposed to the far-UV starlight from nearby OB stars and/or the general interstellar radiation field. Most of the [CII] line arises in PDRs (see link) where the gas is predominantly heated through the photo-electic ejection of electrons from grains, and cooled through its [CII] line radiation. About 1% of the far-UV energy flux heats the gas in this way, while most of the remainer heats the dust which cools via its far-IR continuum radiation. The [CII] line to far-IR continuum luminosity ratio, R is therefore a measure of the efficiency of the gas heating via the photo-electric effect. This efficiency is a strong function of the strength of the ambient interstellar radiation field, so to measure R is to measure those fields, i.e. the concentration of the starburst.

We have made the first detections of the [CII] 158 um line from star forming galaxies at redshifts between 1 and 2 - so that we can probe the physical parameters of the newly formed stars and their environs in the epoch of enhanced star formation in the Universe. At present, we have strongly detected emission from five systems including two submillmeter galaxies (SMGs), and two quasars. Many of the submillimeter galaxies are quite massive, and appear to be forming more than 1000 stars/year so that it could be these are the progenitors of modern day giant elliptical galaxies. We find the [CII] line is very luminous in all systems, and exceptionally bright in three of the galaxies, where the line flux amounts to more than 0.1% of the total far-IR continuum luminosity. CO rotational line emission has been detected from two of the three exceptionally bright [CII] line systems so that we can build a reasonably constrained model for the line emission regions. In a PDR scenario, the combination of [CII] and CO lines, together with the far-IR continuum constrain both the strength of the ambient interstellar radiation fields and the gas density. For MIPS J142824.0+352619 we find the far-UV radiation fields to be ~ 1000 times the local interstellar radiation field, and gas densities ~ 10⁴cm^-3 very similar to the conditions in the nearby starburst galaxy M82 (Hailey Dunsheath et al. 2009). The starburst in MIPS J14218 has similar physical conditions to that of M82, but is more than ~ 1000 times the luminosity! Combining our measure of the stellar radiation field strength with the total luminosity of the systems we can estimate the physical size of the starbursting regions. We find the starburst in MIPS J142824 is greatly extended, occupying a 4 kpc diameter region. This is in contrast to the local, lower luminosity ULIRG galaxes where the star forming regions are often spatially confined to regions ~ few hundred pc on a side. Something stimulates galaxy-wide starbursts in the epoch of maximum star formation for the Universe.

For the two quasars in our sample PKS0215+015 and PG1206+495, the [CII] line is relatively weak at only ~ 0.05% of the far-IR continuum luminosity. This lowered ratio is expected, since a significant fraction of the far-IR continuum may arise from regions near the active nucleus where UV radiation fields are exceptionally strong. The [CII] to far-IR continuum ratio is inversely proportional to the strength of the ambient interstellar radiation field since strong fields will charge grains resulting in lessened efficiency for photo-electric heating of the gas. Higher UV fields means less efficient gas heating, hence relatively lower [CII] cooling. The far-IR continuum luminosity is not affected by the strength of the UV fields (essentially all of the far-UV goes into heating the dust), so that the net effect is a smaller [CII] to far-IR continuum luminosity ratio.

First Detections of the CO(7-6), [CI] and CO(8-7) Line Emission from ULIRG Galaxies

Ultraluminous infrared galaxies (ULIRGs) are galaxies with luminosities in excess of 10¹² L_solar, whose luminosity is dominated by the far-IR emission from dust. Since their discovery, the source of their prodigious luminosity has been hotly debated. Are they powered by super-starbursts (> 100 M_solar/yr) or an active galactic nucleus (AGN)? We detect very bright mid-J CO line emission from ULIRG galaxies. Strong mid-J CO line emission is only produced in starbursts, so our results support starburst powered scenarios. Furthermore, we find a negative correlation between the luminosity of the CO(6-5) line and the far-IR continuum in the sense that ULIRG galaxies have smaller mid-J CO line to far-IR continuum ratios than starbursters. The observed fall-off can be explained through increased far-UV field strengths and cloud densities in ULIRG galaxies. Therefore, we find that star formation is both much more compact and much more vigorous in ULIRG galaxies than in lower luminosity systems.

For more information about the Caltech Submillimeter Observatory, visit the CSO Homepage.