The redshift (Z) and Early Universe Spectrometer (ZEUS) is a direct detection, submillimeter echelle grating spectrometer designed to study star formation in the Universe from about 2 billion years after the Big Bang to the present through spectroscopy of distant star forming galaxies. The sensitivity of ZEUS enables spectroscopic studies in many of the most important gas cooling lines. From local (z<0.1) systems, these lines include the CO(J=8-7, 7-6, and 6-5) and 13CO(6-5) rotational lines, and the 370 and 610 um [CI] fine-structure lines which probe the neutral gas and far-UV radiation fields. From distant (z> 1) galaxies, we are using the redshifted fine-structure lines of [CII] 158 um, [OIII] 88 um, [OI] 63 um, and [NII] 122 and 205 um which probe stellar radiation fields and its effects on both the neutral and ionized gas components of the interstellar medium.
ZEUS is diffraction limited, attains sensitivity very close to the background limit on large submillimeter telescopes, and
has a resolving power ~ 1000. It therefore has unsurpassed sensitivity (corresponding to single-side-band receiver
temperatures < 40 K) for detecting broad lines from faint, point sources, enabling exciting new science portions of which we detail
ZEUS is designed for use in the 350, 450, and 610 um telluric windows on large submillimeter telescopes such as the 10.4 m Caltech Submillimeter Observatory (CSO), the 15 m James Clerk Maxwell Telescope (JCMT), and the 12 m Atacama Pathfinder EXperiment (APEX) telescope. The ZEUS grating is a 35 cm long echelle with a 5th order blaze wavelength of 355 um operated in near Littrow mode. By tilting the grating we access the spectral range between 333 and 381 microns, and 416 to 477 in 5th and 4th orders of the grating providing near complete coverage of the 350 and 450 micron telluric windows. It is straightforward to access the 610 um telluric window (555 to 636 um coverage in 3rd order of the echelle), but we have not done this to date.
Distant galaxies are essentially point sources compared with the 7-10" diffraction limited beams delivered by 10 m class submillimeter telescopes in the 350 and 450 micron windows. For instance, at the distance of the nearest ULIRG galaxy (Arp 220, d = 72 Mpc) a 7" beam corresponds to 2.4 kpc linear extent. To maximize sensitivity for point source detection given background limited operation, we operate ZEUS with a near diffraction limited slit width. On the CSO we have used 8.7" and 10.8" slits which corresponds to 1.1 and 1.37 lambda/D at the middle of our wavelength coverage (400 um). ZEUS achieves resolving powers between 565 and 1600 depending on the wavelength and entrance slit width over the 350 and 450 um bands. This resolving power is well matched to extragalactic lines widths (~ 300 km/sec), optimizing sensitivity for detection of weak lines. The resolving power is ~ 1200 at 372 microns, sufficient to well resolve the astrophysically important CO(7-6) and 370 [CI] lines that are spaced by only 1000 km/s.
The current ZEUS detector array is a 1 x 32 pixel neutron-transmutation doped silcon bolometer array kept at 220 mK with a dual stage 3He refrigerator. The array was manufactured by S.H. Moseley's group at Goddard Space Flight Center. The 1 mm square pixels are well matched to the slit width so that most of the line flux from a monochromatic point source will fall on a single pixel. The 1 x 32 pixel format yeilds an instantaneous spectrum of 32 spectral elements (instantaneous coverage up to 3% bandwidth) on a single beam on the sky. At present, we have our final bandpass filters located directly in front of the array, so that the 32 element spectrum is split -- one 16 pixel half operates in 4th order, while the other half operates in 5th order of the grating. This eliminates the complexity of a milli-Kelvin filter wheel in the system, and enables simultaneous observations in both telluric windows. A line co-incidence is 12CO(6-5) in the 450 um window can be observed simultaneously with 13CO(8-7) in the 350 um window.
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).
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.
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 Lsolar, 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 Msolar/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.