Caltech Submillimeter Observatory Research Articles

658 GHz vibrationally-excited water masers with the Submillimeter Array

AbstractDiscovered in 1995 at the Caltech Submillimeter Observatory (CSO), the vibrationally-excited water maser line at 658 GHz (455 micron) is seen in oxygen-rich giant and supergiant stars. Because this maser can be so strong (up to thousands of Janskys), it was very helpful during the commissioning phase of the highest frequency band (620-700 GHz) of the Submillimeter Array (SMA) interferometer. From late 2002 to early 2006, brief attempts were made to search for emission from additional sources beyond the original CSO survey. These efforts have expanded the source count from 10 to 16. The maser emission appears to be quite compact spatially, as expected from theoretical considerations; thus these objects can potentially be used as atmospheric phase calibrators. Many of these objects also exhibit maser emission in the vibrationally-excited SiO maser at 215 GHz. Because both maser lines likely originate from a similar physical region, these objects can be used to test techniques of phase transfer calibration between millimeter and submillimeter bands. The 658 GHz masers will be important beacons to assess the performance of the Atacama Large Millimeter Array (ALMA) in this challenging high-frequency band.

Star Formation Feedback on the ISM Properties

We observed nearby (D < 10 Mpc) galaxies (Henize 2-10, IC 10, IC 342, M 83, NGC 253 and NGC 6946) in the rotational lines of carbon monoxide CO and in the fine-structure transitions of atomic carbon [CI] with the Caltech Submillimeter Observatory (CSO). Using our new data, those from the literature and the LVG and PDR models, we derived the physical properties of the warm and dense molecular gas (kinetic temperature, UV radiation field, gas density...) in each nucleus. We predicted lines intensities for the 12CO(5-4), 12CO(7-6) and 12CO(8-7) lines which are, even now, still difficult or impossible to detect from the ground but will become available in few years with ALMA and HIFI or PACS, on board the Herschel satellite. Studying the ISM of nearby galaxies, we provide a first step in the understanding of the ISM processes in more distant galaxies.

Massive Quiescent Cores in Orion. II. Core Mass Function

We have surveyed submillimeter continuum emission from relatively quiescent regions in the Orion molecular cloud to determine how the core mass function in a high-mass star-forming region compares to the stellar initial mass function. Such studies are important for understanding the evolution of cores to stars, and for comparison to formation processes in high- and low-mass star-forming regions. We used the SHARC II camera on the Caltech Submillimeter Observatory telescope to obtain 350 μm data having angular resolution of about 9'', which corresponds to 0.02 pc at the distance of Orion. Further data processing using a deconvolution routine enhances the resolution to about 3''. Such high angular resolution allows a rare look into individually resolved dense structures in a massive star-forming region. Our analysis combining dust continuum and spectral line data defines a sample of 51 Orion molecular cores with masses ranging from 0.1 to 46 M☉ and a mean mass of 9.8 M☉, which is 1 order of magnitude higher than the value found in typical low-mass star-forming regions, such as Taurus. The majority of these cores cannot be supported by thermal pressure or turbulence, and are probably supercritical. They are thus likely precursors of protostars. The core mass function for the Orion quiescent cores can be fitted by a power law with an index equal to -0.85 ± 0.21. This is significantly flatter than the Salpeter initial mass function and is also flatter than the core mass function found in low and intermediate star-forming regions. When compared with other massive star-forming regions such as NGC 7538, this slope is flatter than the index derived for samples of cores with masses up to thousands of M☉. Closer inspection, however, indicates slopes in those regions similar to our result if only cores in a similar mass range are considered. Based on the comparison between the mass function of the Orion quiescent cores and those of cores in other regions, we find that the core mass function is flatter in an environment affected by ongoing high-mass star formation. Thus, it is likely that environmental processes play a role in shaping the stellar IMF later in the evolution of dense cores and the formation of stars in such regions.

Images of the Vega Dust Ring at 350 and 450 μm: New Clues to the Trappingof Multiple-Sized Dust Particles in Planetary Resonances

We have used the SHARC II camera at Caltech Submillimeter Observatory to make 350 and 450 μm images of the Vega dust disk at spatial resolutions (FWHM) of 9.7 and 11.1, respectively. The images show a ringlike morphology (radius ~100 AU) with inhomogeneous structure that is qualitatively different from that previously reported at 850 μm and longer wavelengths. We attribute the 350/450 μm emission to a grain population whose characteristic size (~1 mm) is intermediate between that of the centimeter-sized grains responsible for emission longward of 850 μm and the much smaller grains (≲18 μm) in the extensive halo, visible at 70 μm, discussed by Su et al. We have combined our submillimeter images with Spitzer data at 70 μm to produce two-dimensional of line-of-sight optical depth (relative column density). These maps suggest that the millimeter-sized grains are located preferentially in three symmetrically located concentrations. If so, then this structure could be understood in terms of the Wyatt model in which planetesimals are trapped in the mean motion resonances of a Neptune-mass planet at 65 AU, provided allowance is made for the spatial distribution of dust grains to differ from that of the parent planetesimals. The peaks of the tau are, in fact, located near the expected positions corresponding to the 4 : 3 resonance. If this identification is confirmed by future observations, it would resolve an ambiguity with regard to the location of the planet.

Submillimeter imaging spectroscopy of the Horsehead nebula

We present ~15 arcsecond resolution single-dish imaging of the Horsehead nebula in the CI (1-0) and CO (4-3) lines, carried out using the CHAMP array at the Caltech Submillimeter Observatory (CSO). The data are used together with supporting observations of the (2-1) transitions of the CO isotopologues to determine the physical conditions in the atomic and molecular gas via Photon Dominated Region (PDR) modeling. The CO (4-3)/(2-1) line ratio, which is an excellent tracer of the direction of the incoming UV photons, increases at the western and northern edges of the nebula, confirming that the illumination is provided mostly by the stars σ and گ Orionis. The observed line intensities are consistent with PDR models with an H nuclei volume density of ~3- 7 x 10^4 cm^(-3). The models predict a kinetic temperature of ~12 K and a C^(18)O fractional abundance with respect to H atoms of 2.4 x 10^(-7) in the shielded region, which in turn imply a total molecular mass of ~24 M_☉ in the C^(18)O filament. The outer halo, devoid of C^(18)O, but traced by the CI emission has a comparable density and contributes additional ~13 M_☉ of material, resulting in an upper limit of ~37 M_☉ for the total molecular mass of the nebula.

Bolocam Survey for 1.1 mm Dust Continuum Emission in the c2d Legacy Clouds. II. Ophiuchus

We present a large-scale millimeter continuum map of the Ophiuchus molecular cloud. Nearly 11 deg2, including all of the area in the cloud with AV ≥ 3 mag, was mapped at 1.1 mm with Bolocam on the Caltech Submillimeter Observatory (CSO). By design, the map also covers the region mapped in the infrared with the Spitzer Space Telescope. We detect 44 definite sources, and a few likely sources are also seen along a filament in the eastern streamer. The map indicates that dense cores in Ophiuchus are very clustered and often found in filaments within the cloud. Most sources are round, as measured at the half-power point, but elongated when measured at lower contour levels, suggesting spherical sources lying within filaments. The masses, for an assumed dust temperature of 10 K, range from 0.24 to 3.9 M☉, with a mean value of 0.96 M☉. The total mass in distinct cores is 42 M☉, 0.5%-2% of the total cloud mass, and the total mass above 4 σ is about 80 M☉. The mean densities in the cores are quite high, with an average of 1.6 × 106 cm-3, suggesting short free-fall times. The core mass distribution can be fitted with a power law with slope α = 2.1 ± 0.3 for M > 0.5 M☉, similar to that found in other regions, but slightly shallower than that of some determinations of the local IMF. In agreement with previous studies, our survey shows that dense cores account for a very small fraction of the cloud volume and total mass. They are nearly all confined to regions with AV ≥ 9 mag, a lower threshold than found previously.

350 μm Dust Emission from High‐Redshift Quasars

We report detections of six high-redshift (1.8 ≤ z ≤ 6.4), optically luminous, radio-quiet quasars at 350 μm, using the SHARC II bolometer camera at the Caltech Submillimeter Observatory. Our observations double the number of high-redshift quasars for which 350 μm photometry is available. By combining the 350 μm measurements with observations at other submillimeter/millimeter wavelengths, for each source we have determined the temperature of the emitting dust (ranging from 40 to 60 K) and the far-infrared luminosity [(0.6-2.2) × 10^(13) L⊙]. The combined mean spectral energy distribution of all high-redshift quasars with two or more rest-frame far-infrared photometric measurements is best fit with a graybody with temperature of 47 ± 3 K and a dust emissivity power-law spectral index of β = 1.6 ± 0.1. This warm dust component is a good tracer of the starburst activity of the quasar host galaxy. The ratio of the far-infrared to radio luminosities of infrared-luminous, radio-quiet high-redshift quasars is consistent with that found for local star-forming galaxies.

350 µm sharc‐ii imaging of luminous high‐redshift radio galaxies

AbstractUsing the filled bolometer array SHARC‐II on the Caltech Submillimeter Observatory (CSO), we have obtained deep (σ rms ∼ 15 mJy beam–1), high‐quality 350 µm maps of five of the most luminous high‐z radio galaxies known. In all cases the central radio galaxy is detected at the ≳3σ level, and in some cases the high resolution of SHARC‐II (FWHM= 9″) allows us to confirm the spatially extended nature of the dust emission. In PKS 1138–262 (z = 2.156), 8C 1909+722 (z = 3.538) and 4C 41.17 (z = 3.792), additional sources – first discovered by SCUBA at 850 µm and believed to be dusty, merging systems associated with the central radio galaxy – are detected at 350 µm. Furthermore, in PKS 1138 and 4C41.17 additional SHARC‐II sources are seen which were not detected at 850 µm, although the reality of these sources will have to be confirmed by independent submm observations. Thus, our observations seem to support the notion of extended star formation taking place in radio galaxies at high redshifts, and that these systems sit at the centers of overdense regions harbouring a complexity of dusty and vigorously star forming systems. At the redshift of the radio galaxies, the 350 µm observations sample very close to the rest‐frame dust peak (typically at ∼100 µm), and they therefore contribute a particularly important point to the spectral energy distributions of these sources, which we use this in conjunction with existing (sub)millimeter data to derive FIR luminosities, dust temperatures and spectral emissivities of the central radio galaxies. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Ground State Rotational Lines of Doubly Deuterated Ammonia as Tracers of the Physical Conditions and Chemistry of Cold Interstellar Medium

We report the first detection of the N = 111 → 000 and 110 → 000 ground state rotational lines of o-ND2H at 335.5 and 388.7 GHz, obtained in the Lynds 1689N, Barnard 1, and Lynds 1544 molecular clouds using the Caltech Submillimeter Observatory (CSO). The submillimeter ND2H lines have moderate opacities and simple hyperfine patterns, which allow accurate determination of the excitation temperature, H2 volume density, and molecular column density. Both transitions have high critical densities. The 389 GHz line, in particular, traces molecular material with densities above a few × 106 cm-3. The strong 389 GHz ND2H emission in LDN 1689N implies a high fraction of dense gas in this source, ~30%, as compared to ~15% in B1 and LDN 1544. All these regions are sites of strong molecular depletion and heavy deuteration. Nonaccreting molecules, H and its isotopologues, are difficult to study, but in the sources studied here it appears that ammonia and its isotopologues are not completely frozen out, even in the high density gas. In the well-studied case of LDN 1544, the volume probed by the ND2H emission has densities of ~106-107 cm-3, within the range where the complete freezeout has been predicted to occur. The critical density of the 389 GHz ND2H line is close to that of the 309 GHz ND3 line. Observations of these two transitions thus provide an accurate measure of the [ND3]/[ND2H] fractionation ratio in the very dense gas. The [ND3]/[ND2H] ratio in LDN 1689N (~3%) appears lower than the values measured in B1 and LDN 1544 (~7%-10%), indicating that different chemical processes may be at work in these environments. The submillimeter lines of deuteroammonia are relatively strong and detectable from good sites, such as Mauna Kea or Chajnantor. Interferometric observations of these lines with the Submillimeter Array (SMA), and subsequently the Atacama Large Millimeter Array (ALMA), will provide new opportunities to study the physics and chemistry of cold, dense ISM, where most molecules are depleted onto dust grains.

First astronomical images obtained with an array of multiplexed superconducting bolometers

We present multicolor images of Jupiter observed in the 350 μ m band with the first deployed astronomical instrument to use multiplexed superconducting bolometers. The Fabry–Perot Interferometer Bolometer Research Experiment (FIBRE) is a broadband submillimeter spectrometer that made these images in July 2004 at the Caltech Submillimeter Observatory (CSO). FIBREs detectors are superconducting bilayer transition edge sensor (TES) bolometers read out by a SQUID multiplexer. An order-sorted Fabry–Perot provides illumination of a 16-element linear bolometer array, resulting in five orders at a spectral resolution R of 1200 covering a band of 1 7 of the observed wavelength. The optics permit these orders to be scanned to cover the entirety of either the 350 or 450 μ m bands.

Z-Spec's first light at the Caltech Submillimeter Observatory

We report the success of our first engineering run with Z-Spec, a new millimeter spectrograph on the 10 m Caltech Submillimeter Telescope during the first week of June 2005. The bolometric instrument is indeed the first of its kind, covering the atmospheric window from 187 to 310 GHz at the spectral resolution of 250–400, and will provide a critical follow-up study for discoveries by SCUBA and Bolocam. We present the spectrum of an ultra-luminous infrared galaxy NGG 6240 obtained by Z-Spec. Z-Spec serves as a prototype for future far-IR spectrometers on a cold space telescope where large gains in sensitivity are expected.

MIPS J142824.0+352619: A Hyperluminous Starburst Galaxy atz= 1.325

Using the SHARC-II camera at the Caltech Submillimeter Observatory to obtain 350 μm images of sources detected with the MIPS instrument on Spitzer, we have discovered a remarkable object at z = 1.325 ± 0.002 with an apparent far-infrared luminosity of 3.2(±0.7) × 1013 L☉. Unlike other z > 1 sources of comparable luminosity selected from mid-IR surveys, MIPS J142824.0+352619 lacks any trace of AGN activity, and is likely a luminous analog of galaxies selected locally by IRAS, or at high redshift in the submillimeter. This source appears to be lensed by a foreground elliptical galaxy at z = 1.034, although the amplification is likely modest (≲10). We argue that the contribution to the observed optical/near-IR emission from the foreground galaxy is small, and hence are able to present the rest-frame UV through radio spectral energy distribution of this galaxy. Due to its unusually high luminosity, MIPS J142824.0+352619 presents a unique chance to study a high-redshift dusty starburst galaxy in great detail.

Dissociative Shocks in the Neighborhood of Orion IRc2 Traced with Atomic Carbon

We present Caltech Submillimeter Observatory 10'' resolution maps of the 3P2 → 3P1 line of neutral atomic carbon (809.3435 GHz) and the J = 7-6 line of CO (806.6518 GHz) covering a region of 2' × 45 around Orion IRc2 in the BN/KL Nebula. The two lines were observed with the same receiver simultaneously, and therefore the two maps have zero relative pointing error. The atomic carbon (C I) emission does not peak toward the IRc2 position and displays a very different spatial distribution with respect to most molecular species. Moderately strong red wings are detected in C I around the region defined by the high-velocity CO gas with a spatial distribution that delineates a shell of ~20'' radius. We propose that CO dissociation in the J-type shocks produced by the interaction of the IRc2 outflows with the surrounding quiescent gas could play a significant role in this enhancement of atomic carbon in that shell.

On the Discovery of the First Galaxy Selected at 350 Microns

We report the detection of a 3.6 σ source selected at 350 μm in the Bootes Deep Field. The source, the first short-wavelength submillimeter-selected galaxy (SSG 1), was discovered as part of a blank-field extragalactic survey using the 350 μm-optimized Submillimeter High Angular Resolution Camera (SHARC II) at the Caltech Submillimeter Observatory. With multiwavelength photometry from the NOAO Deep Wide-Field Survey (R and I band), FLAMEX (J and Ks), Spitzer (IRAC and MIPS), and the Westerbork 1.4 GHz deep survey (radio upper limit), we are able to constrain the photometric redshift using different methods, all of which suggest a redshift of ~1. In the absence of long-wavelength submillimeter data, we use SED templates to infer that this source is an ultraluminous infrared galaxy with a dust temperature of 30 ± 5 K, occupying a region of luminosity-temperature space shared by moderate-redshift ISO-selected ULIRGs (rather than high-redshift SCUBA-selected submillimeter galaxies). SHARC II can thus select galaxies with moderately warm dust that might be missed in submillimeter surveys at longer wavelengths.

Measured telluric continuum-like opacity beyond 1 THz

An extensive study of the atmospheric transmission at millimeter and submillimeter wavelengths has been performed since the early 1990s with a Fourier Transform Spectrometer (FTS) mounted on the Caltech Submillimeter Observatory (CSO) atop Mauna Kea (Hawaii), 4100 m above sea level. The goal of these observations is to compile a data base of accurately calibrated atmospheric transmission spectra for use in refining atmospheric models. In this context, the definition of the “quasi-continuum” opacity component is paramount. While our earlier work extending up to 1.0 THz has allowed the separation of the “wet” and “dry” quasi-continua components, with both shown to be following ν 2 laws in this regime, here we report on the extension of these observations to 1.6 THz. In the higher frequency regime, our preliminary results indicate that the ν 2 description may begin to fail due to proximity to the FIR band centers. Because the opacities in the potentially interesting atmospheric windows at 1.02, 1.35 and 1.5 THz have a large contribution from the quasi-continuum terms, accurate predictions for the transmissions in these windows require refinements in lower frequency models. Comparisons of our data below 1 THz with extant models give a remarkable agreement.

Broadband submillimeter measurements of the full Moon center brightness temperature and application to a lunar eclipse

We report on observations of the full Moon brightness temperature covering the frequency range of 300–950 GHz, and also on observations of the lunar eclipse of July 16, 2000, though only covering the frequency range of 165–365 GHz due to poor atmospheric transmission at higher frequencies. All observations were performed from the summit of Mauna Kea (HI) using a Fourier Transform Spectrometer mounted on the Caltech Submillimeter Observatory and supplemented by measurements of the atmospheric opacity using a 183 GHz Water Vapor Monitor. The telescope was pointed to the center of the lunar disk (with a footprint of ∼45–15 km on the Moon at 300 through 900 GHz). In order to obtain the correct values of the Moon brightness temperatures at all frequencies we carefully corrected for the atmospheric absorption, which varies across the submillimeter domain. This correction is fully described. The measured pre-eclipse brightness temperature is around 337 K in the 165–365 GHz range. This temperature slightly increases with frequency to reach ∼353 K at 950 GHz, according to previous broader band data. The magnitude of the temperature drop observed during the eclipse at 265 GHz (central frequency of the band covered) was about ∼70 K, in very good agreement with previous millimeter-wave measurements of other lunar eclipses. We detected, in addition, a clear frequency trend in the temperature drop that has been compared to a thermal and microwave emission model of the lunar regolith, with the result of a good match of the relative flux drop at different frequencies between model and measurements.

1,3-Dihydroxyacetone in Sagittarius B2(N-LMH): The First Interstellar Ketose

We present here the results of an initial search for 1,3-dihydroxyacetone [or DHA, CO(CH2OH)2], the simplest ketose monosaccharide, toward the Sagittarius B2(N-LMH) hot core. Nine possible DHA emission lines have been observed with the Caltech Submillimeter Observatory, from which a rotational excitation temperature of 220 ± 65 K and a beam-averaged column density of (4.9 ± 2.2) × 1015 cm-2 are derived. If confirmed by additional observations, DHA would be among the most abundant complex molecules yet detected. DHA is not considered in any existing grain mantle or molecular cloud chemical model, highlighting the need for a more detailed investigation of the formation of prebiotic compounds in hot cores.

An upper limit to [C II] emission in a z = 5 galaxy

Low-ionization-state far-infrared (FIR) emission lines may be useful diagnostics of star formation activity in young galaxies, and at high redshift may be detectable from the ground. In practice, however, very little is known concerning how strong such line emission might be in the early Universe. We attempted to detect the 158-μm [CII] line from a lensed galaxy at z = 4.926 using the Caltech Submillimeter Observatory. This source is an ordinary galaxy, in the sense that it shows high but not extreme star formation, but lensing makes it visible. Our analysis includes a careful consideration of the calibrations and weighting of the individual scans. We find only a modest improvement over the simpler reduction methods, however, and the final spectrum remains dominated by systematic baseline ripple effects. We obtain a 95 per cent confidence upper limit of 33 mJy for a 200 km s -1 full width at half maximum line, corresponding to an unlensed luminosity of 1 x 10 9 L ○. for a standard cosmology. Combining this with a marginal detection of the continuum emission using the James Clerk Maxwell Telescope, we derive an upper limit of 0.4 percent for the ratio of L [CII] /L FIR in this object.

A Molecular Line Survey of Orion KL in the 350 Micron Band

With the Caltech Submillimeter Observatory, we have carried out an unbiased spectral line survey of Orion KL throughout the 350 μm band (from 795 to 903 GHz). This is the first systematic study of molecular radiation in this frequency range. A total of 541 features, resulting from 929 transitions from a total of 26 species, have been detected. High-excitation transitions from CH3OH, CH3CN, H2CO, HNCO, and C2H5CN indicate the presence of a very hot (~250 K) component at the systemic velocity characteristic of the Hot Core. Physical parameters (column density and rotational temperature) relative to a number of species have been estimated by fitting, in the LTE approximation, the whole 100 GHz spectrum at once, thus taking line blending and optical depth effects properly into account. We also report the tentative detection, for the first time outside the Galactic center region, of the radical NH2, one of the building blocks of the chemistry of ammonia.

Image of Fomalhaut Dust Ring at 350 Microns: The Relative Column Density Map Shows Pericenter-Apocenter Asymmetry

We have imaged the circumstellar disk of Fomalhaut at 350 mm wavelength, using SHARC II (Submillimeter High Angular Resolution Camera II) at the Caltech Submillimeter Observatory. The spatial resolution of the raw images (9) has been enhanced by a factor of 3 using the HiRes deconvolution procedure. We find that at this wavelength and signal-to-noise ratio (approx.12), the observed morphology is that of a simple inclined ring (i approx. 70 deg), with little or no other apparent structure--this is the first observation that shows clearly the ring morphology of the disk. We have combined our 350 mm data with Spitzer Space Telescope images at 24, 70, and 160 mm in order to estimate the two-dimensional spatial variation of relative column density (tau map) using our DISKFIT procedure. The tau map is based on the following physical assumptions: (1) the wavelength variation of opacity is the same throughout the disk, (2) the radial variation of dust temperature is dictated by the energy balance of individual grains in the stellar radiation field, and (3) the vertical scale height of the disk follows a power-law radial variation. The results confirm the ringlike morphology but also show that the geometric center is displaced from the star by about 8 AU and that the ring has an apocentric enhancement of approximately 14% in integrated column density. If we interpret the displacement in terms of elliptical orbital motion due to gravitational perturbation by an unseen planet, then the implied forced eccentricity is ~0.06; dynamical modeling then predicts an apocentric density enhancement consistent with that inferred from the tau map.