Submillimeter High Angular Resolution Camera Research Articles

An Improved Radio Pointing Model for SHARC II of the Caltech Submillimeter Observatory Telescope

In this research, we develop a new method of upgrading the radio pointing model of an important instrument of the Caltech Submillimeter Observatory (CSO) telescope, i.e., the Submillimeter High Angular Resolution Camera II (SHARC II), by making three types of structural reconstruction of its existing model. First, the axial displacement of the secondary reflector of the telescope is introduced to the radio pointing model for SHARC II. Second, the multi-layer perceptron is applied for better describing higher-order terms in the radio pointing model, which are hard to be mathematically formulated. Third, a receding horizon modeling method is proposed to replace the time-dependent term in the existing model, for better reducing the negative impact of the time drift on the model’s accuracy. Results of numerical experiments and statistical significance analysis based on the real pointing data of SHARC II show that the reconstructed radio pointing model can improve the accuracy of estimating the pointing error, and the proposed method of upgrading the radio pointing model is effective. Considering that the CSO telescope will be moved from the old site at Maunakea, Hawaii to the new site at the Chajnantor Plateau in Chile, the proposed methods of upgrading the radio pointing model are expected to be employed for pointing correction after the telescope is refurbished at the new site.

Wide-field mid-infrared and millimetre imaging of the high-redshift radio galaxy, 4C 41.17

We present deep 350- and 1200-μm imaging of the region around 4C 41.17 – one of the most-distant (z=3.792) and luminous known radio galaxies – obtained with the Submillimeter High Angular Resolution Camera (SHARC-II) and the Max Planck Millimeter Bolometer Array (MAMBO). The radio galaxy is robustly detected at 350 and 1200 μm, as are two nearby 850-μm-selected galaxies; a third 850-μm source is detected at 350 μm and coincides with a ∼2σ feature in the 1200-μm map. Farther away from the radio galaxy additional nine sources are detected at 1200 μm, bringing the total number of detected (sub)millimetre-selected galaxies (SMGs) in this field to 14. Using radio images from the Very Large Array and Spitzer midinfrared data, we find statistically robust radio and/or 24-μm counterparts to eight out of the 14 SMGs in the field around 4C 41.17. Follow-up spectroscopy with Keck/Low-Resolution and Imaging Spectrograph (LRIS) has yielded redshifts for three out of the eight robustly identified SMGs, placing them in the redshift range 0.5 ≾ z ≾ 2.7 that is well below that of 4C 41.17. We infer photometric redshifts for a further four sources using their 1.6-μm (rest-frame) stellar feature as probed by the IRAC bands; only one of them is likely to be at the same redshift as 4C 41.17. Thus at least four, and as many as seven, of the SMGs within the 4C 41.17 field are physically unrelated to the radio galaxy. With the redshift information at hand, we are able to constrain the observed overdensities of SMGs within radial bins stretching to R = 50 and 100 arcsec (∼0.4 and ∼0.8 Mpc at z ≃ 3.8) from the radio galaxy to approximately five times and two times that of the field, dropping off to the background value at R = 150 arcsec. We thus confirm that 4C 41.17 resides in an overdense region of the Universe, but we have only been able to identify SMGs along the line of sight to the radio galaxy, typical of the blank-field SMG population. Finally, we report on the discovery of an extremely extended (∼110 kpc) Lyα blob at z = 2.672 associated with the brightest 1200-μm source in the field.

First Constraints on Source Counts at 350 μm

We have imaged a ~6 arcmin2 region in the Bootes Deep Field using the 350 μm-optimized second-generation Submillimeter High Angular Resolution Camera (SHARC II), achieving a peak 1 σ sensitivity of ~5 mJy. We detect three sources above 3 σ, and determine a spurious source detection rate of 1.09 in our maps. In the absence of 5 σ detections, we rely on deep 24 μm and 20 cm imaging to deduce which sources are most likely to be genuine, giving two real sources. From this we derive an integral source count of 0.84 sources arcmin-2 at S > 13 mJy, which is consistent with 350 μm source count models that have an IR-luminous galaxy population evolving with redshift. We use these constraints to consider the future for ground-based short-submillimeter surveys.

350 μm Observations of Local Luminous Infrared Galaxies and the Temperature Dependence of the Emissivity Index

We report 350 μm observations of 18 nearby luminous infrared galaxies (LIRGs) using the Submillimeter High Angular Resolution Camera II (SHARC-II) mounted on the Caltech Submillimeter Observatory (CSO) 10.4 m telescope. Combining our 350 μm flux measurements with the existing far-infrared (FIR) and submillimeter data, we fit a single-temperature model to the spectral energy distributions (SEDs) and find the dust temperatures, emissivity indices, and FIR luminosities having sample medians of Td = 39.4 ± 7.9 K, β = 1.6 ± 0.3, and LFIR = 1011.2±0.6 L☉. An empirical inverse Td-β correlation, best described by Td = (9.86 × 109)1/(4.63 + β), is established for the local LIRG sample, which we argue can be explained by the intrinsic interdependence between the dust temperature and grain emissivity index as physical parameters, as well as variations in grain properties in the interstellar medium (ISM).

350 μm Observations of Ultraluminous Infrared Galaxies at Intermediate Redshifts

We present 350 μm observations of 36 ultraluminous infrared galaxies (ULIRGs) at intermediate redshifts (0.089 ≤ z ≤ 0.926) using the Submillimeter High Angular Resolution Camera II (SHARC-II) on the Caltech Submillimeter Observatory (CSO). In total, 28 sources are detected at S/N ≥ 3, providing the first flux measurements longward of 100 μm for a statistically significant sample of ULIRGs in the redshift range 0.1 ≲ z ≲ 1.0. Combining our 350 μm flux measurements with the existing IRAS 60 and 100 μm data, we fit a single-temperature model to the spectral energy distribution (SED) and thereby estimate dust temperatures and far-IR luminosities. Assuming an emissivity index of β = 1.5, we find a median dust temperature and far-IR luminosity of Td = 42.8 ± 7.1 K and log(LFIR/L☉) = 12.2 ± 0.5, respectively. The far-IR-radio correlation observed in local star-forming galaxies is found to hold for ULIRGs in the redshift range 0.1 ≲ z ≲ 0.5, suggesting that the dust in these sources is predominantly heated by starbursts. We compare the far-IR luminosities and dust temperatures derived for dusty galaxy samples at low and high redshifts with our sample of ULIRGs at intermediate redshift. A general LFIR-Td relation is observed, albeit with significant scatter due to differing selection effects and variations in dust mass and grain properties. The relatively high dust temperatures observed for our sample compared to that of high-z submillimeter-selected starbursts with similar far-IR luminosities suggest that the dominant star formation in ULIRGs at moderate redshifts takes place on smaller spatial scales than is found at higher redshifts.

SHARC-II Mapping ofSpitzerc2d Small Clouds and Cores

We present the results of a submillimeter survey of 53 low-mass dense cores with the Submillimeter High Angular Resolution Camera II (SHARC-II). The survey is a follow-up project to the Spitzer Legacy Program ``From Molecular Cores to Planet-Forming Disks'', with the purpose being to create a complete data set of nearby low-mass dense cores from the infrared to the millimeter. We present maps of 52 cores at 350 microns and three cores at 450 microns, two of which were observed at both wavelengths. Of these 52 cores, 41 were detected by SHARC-II: 32 contained one submillimeter source while 9 contained multiple sources. For each submillimeter source detected, we report various source properties including source position, fluxes in various apertures, size, aspect ratio, and position angle. For the 12 cores that were not detected we present upper limits. The sources detected by SHARC-II have, on average, smaller sizes at the 2sigma contours than those derived from longer-wavelength bolometer observations. We conclude that this is not caused by a failure to integrate long enough to detect the full extent of the core; instead it arises primarily from the fact that the observations presented in this survey are insensitive to smoothly varying extended emission. We find that SHARC-II observations of low-mass cores are much better suited to distinguishing between starless and protostellar cores than observations at longer wavelengths. Very Low Luminosity Objects, a new class of objects being discovered by the Spitzer Space Telescope in cores previously classified as starless, look very similar at 350 microns to other cores with more luminous protostars.

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.

Resolving IRAS 09111–1007 at 350 µm: a different path to ULIRG formation?

Abstract We have resolved the ultraluminous infrared galaxy (ULIRG) IRAS 09111–1007 with the new 350-µm-optimized Second Generation Submillimeter High Angular Resolution Camera (SHARC II), and present the first submillimetre fluxes and images for the system. IRAS 09111–1007 comprises two interacting luminous infrared galaxies (LIRGs) with a projected nuclear separation of 39 h71−1 kpc. The western galaxy is roughly four times more luminous in the submillimetre than its eastern counterpart. It is an extremely bright LIRG with an active galactic nucleus (AGN). The classification of the eastern source is uncertain: it could be a Seyfert 2 galaxy or a LINER. We highlight IRAS 09111–1007 as a system that necessitates further study: a double AGN ULIRG whose molecular gas content differs from that of other widely separated pairs, and whose ULIRG phase might not be explained by current multiple-merger and/or final-stage ULIRG scenarios.

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.

Astronomical results from SHARC-II

The Submillimeter High Angular Resolution Camera (SHARC-II) is a facility instrument for far-infrared (350μm) imaging at the Caltech Submillimeter Observatory (CSO). With 384 pixels, SHARC-II uses the world's largest bolometer array for astronomical observations. SHARC-II is most efficiently utilized for observations of extended sources and for deep sky surveys. The low 1/f detector noise allows total power measurements without the need to observe an emission free “off position”. This is possible because the sky emission can be distinguished from the celestial emission when the array scans over the sky at sufficient speed. Here we present a representative set of SHARC-II observations, which highlight the capabilities of the instrument. The observations show the submillimeter continuum emission from our own Galactic center, the nearby galaxy M51, and the gravitationally lensed high-z Cloverleaf galaxy H1413+1143.

Development of low-noise high value chromium silicide resistors for cryogenic detector applications

Extremely high sensitivity detectors, such as silicon bolometers are required in many NASA missions for detection of photons from the X-ray to the far infrared regions. Typically, these detectors are cooled to well below the liquid helium (LHe) temperature of 4.2 K to achieve the maximum detection performance. As photodetectors, they are generally operated with a load resistor and a pre-set bias voltage, which is coupled to the input gate of a source-follower field effect transistor (FET) circuit. It is imperative that the detector system signal-to-noise performance be limited by the noise of the detector and not by the noise of the external components. The load resistor value is selected to optimize the detector performance. These two criteria tend to be contradictory in that these detectors require load resistors in the hundreds of megaohms, which leads to a higher Johnson noise. Additionally, the physical size of the resistor must be small for device integration as required by such missions as the NASA High Resolution Airborne Wide-Band Camera instrument and the Submillimeter High-Angular Resolution Camera for the Caltech Submillimeter Observatory, both of which employ 384 detectors and resistors. We have designed, fabricated and characterized thin film resistors using a CrSi/TiW/Al metal system on optical quality quartz substrates. The resistor values range from 75 M Ω to over 650 M Ω and are Johnson noise limited to below LHe temperatures. The resistor film is sputtered with a sheet resistance ranging from 300 to 1600 Ω/□ and the processing sequence developed for these devices allows for chemically fine-tuning the sheet resistance in situ. The wafer fabrication process was of sufficiently high yield (>80%) providing clusters of good resistors for integrated multiple detector channels, a very important feature in the assembly of these two instruments.

350 Micron Dust Emission from High-Redshift Objects

We report observations of a sample of high-redshift sources ( ), mainly radio-quiet quasars, at 1.8 & z & 4.7 350 mm using the bolometer camera SHARC (Submillimeter High Angular Resolution Camera) at the Caltech Submillimeter Observatory. Nine sources were detected (*4 j), and upper limits were obtained for 11 with 350 mm flux density limits (3 j) in the range 30‐125 mJy. Combining published results at other far-infrared and millimeter wavelengths with the present data, we are able to estimate the temperature of the dust, finding relatively low values, averaging 50 K. From the spectral energy distribution, we derive dust masses of a few times 10 8 M, and luminosities of L, (uncorrected for any magnification), which imply sub2 21 2 2 2 h (4‐33) # 10 h 100 100 stantial star formation activity. Thus, both the temperature and dust masses are not very different from those of local ultraluminous infrared galaxies. For this redshift range, the 350 mm observations trace the 60‐100 mm restframe emission and are thus directly comparable with IRAS studies of low-redshift galaxies.

350 Micron Continuum Imaging of the Orion A Molecular Cloud with the Submillimeter High Angular Resolution Camera

We have used the Submillimeter High Angular Resolution Camera (SHARC) bolometer camera at the Caltech Submillimeter Observatory to map the distribution of the broadband 350 ?m continuum emission toward the Orion A molecular cloud. A comparison of the 350 ?m flux densities in OMC-1 with previous 1100 ?m measurements indicates a strong spatial variation of the grain emissivity exponent, ?. The lowest value of the exponent (? 1.75) is found toward the Orion Bar photon-dominated region (PDR), while the highest value (? 2.5) is found toward the Orion Ridge north of IRc2. This variation is consistent with the destruction of grain mantles by the UV photons from the Trapezium cluster. The observed spatial variation of ? in OMC-1 suggests that the long-wavelength grain emissivity may also vary significantly in GMC cores on small linear scales (0.5 pc), affecting H2 column density and mass estimates. The 350 ?m continuum emission in the Orion Bar region correlates well with the CO (6-5) peak brightness temperature and is shifted by ~10'' from the molecular component traced by the 13CO (6-5) emission. This indicates that the 350 ?m dust emission in this region originates predominantly in the outer high-temperature PDR layers. Several filamentary structures previously detected in molecular tracers are also seen in our map at flux levels comparable to those seen in the Bar. Over 30 compact dust sources are detected in the OMC-2 and OMC-3 clouds, including a dozen sources not previously known. The average 350/1300 ?m flux ratio based on our data and previous observations of this region (63 ? 19) indicates low dust temperatures (17 ? 4 K, assuming ? = 2) for most of the sources. The brightest 350 ?m source in OMC-3 has a low 350/1300 ?m ratio (~23), indicating a very low dust temperature (~10 K), or a significant opacity at 350 ?m (~2.5). This source appears to be a deeply embedded and cold young protostar. A comparison of the mass estimate for the OMC-2/3 filament based on the 350 ?m continuum emission with previous C18O mass estimates indicates a relatively high grain emissivity, Q(350) = 4 ? 10-4, in this region.

A Submillimeter Imaging Survey of Ultracompact H ii Regions

This research explores the process of massive star formation in the Galaxy through submillimeter continuum and CO spectral line observations of ultracompact H ii (UC H ii) regions. First, I describe the design and operation of the Submillimeter High Angular Resolution Camera (SHARC)—a 24 pixel bolometer array camera for broadband continuum imaging at 350 and 450 mm at the Caltech Submillimeter Observatory (CSO). Detailed information is included on the reflective off-axis optical design and the instrument control software interface (Hunter, T. R., Benford, D. J., & Serabyn, E. 1996, PASP, 108, 1042). Second, I present 100–120 resolution SHARC images of 350 and 450 mm continuum emission from a sample of 17 UC H ii regions with different radio morphologies. The sample includes G19.6120.23, G29.9620.02, G34.2610.14, G45.1210.13, G45.0710.13, K3-50A, K3-50C, G75.8410.34, G75.7810.34, W3(OH), G138.29511.555, G139.90910.197, Monoceros R2, GGD12-15, S255 FIR2, G192.58420.041, and G240.3110.07. Although the dust emission typically peaks at or near the UC H ii region, additional sources are often present, sometimes coincident with the position of H2O or CH3OH masers. The most notable discovery so far is the proto-B star G34.2410.13MM (Hunter, T. R., Neugebauer, G., Benford, D. J., Matthews, K., Lis, D. C., Serabyn, E., & Phillips, T. G. 1998, ApJ, 493, L97). The combination of submillimeter, millimeter, and IRAS far-infrared flux densities forms the basis of modified graybody models of the spectral energy distributions. The average dust temperature is K, and the average 40 5 10 grain emissivity index (b) is . Using a radiative 2.00 5 0.25 transfer program that solves for the dust temperature as a function of radius, the distribution of dust around UC H ii regions is modeled with a power-law spherical density profile to match the observed radial flux density profiles. By fixing the source boundary at the outer limit of the submillimeter emission, the resulting density profiles can be classified into four 2p n(r) ∝ r categories: three regions exhibit (isothermal sphere), four p 5 2 exhibit (dynamical collapse), two exhibit in the p 5 1.5 p 5 2 outer regions and in the inner regions, and six exhibit p 5 1.5 (logatropic). Although these simplified models may not p 5 1 be unique, a good correlation between the dust luminosity-tomass ratio and the temperature indicates that the more centrally condensed sources exhibit higher star formation rates. Third, I present 200–300 resolution CO maps that reveal bipolar outflows from 15 out of 17 UC H ii regions. The outflow mechanical luminosities and mass ejection rates follow the scaling relations with bolometric luminosity established for less luminous pre–main-sequence stars. However, in contrast to lower luminosity sources, the momentum from stellar radiation pressure is comparable to that required to drive the outflows. Many regions show evidence of separate, overlapping outflows. In a final detailed study, 20 resolution images obtained with the Owens Valley Millimeter Array reveal multiple outflows emanating from the molecular core containing the UC H ii region G45.1210.13, while simultaneous outflow and infall motion is seen in CS toward the neighboring, less evolved J 5 2 r 1 core containing G45.0710.13 (Hunter, T. R., Phillips, T. G., & Menten, K. M. 1997, ApJ, 478, 283).

G34.24+0.13MM: A Deeply Embedded Proto–B Star

By means of millimeter and submillimeter imaging, we have identified a massive protostellar object that coincides with a methanol maser and is not detectable in the continuum at centimeter wavelengths. Located 84 (1.5 pc) southeast of the ultracompact H II (UCHII) region G34.26+0.15, the new object G34.24+0.13MM was discovered in a wide-field 350 μm continuum image obtained with the Submillimeter High Angular Resolution Camera (SHARC) at the Caltech Submillimeter Observatory (CSO). Interferometric imaging at 225.7 and 110.7 GHz continuum has determined more precisely the position and angular diameter (2.0, or 7600 AU) of the object. No source was detected at that position in 1.2-3.7 μm imaging or 10 and 20 μm photometry. Our observations are consistent with a cool dust core with temperature ~50 K, total gas mass 100 M_☉, and total luminosity in the range of 1600-6300 L_☉. Considering the high luminosity and lack of compact radio continuum emission, we conclude that this core probably contains a deeply embedded proto-B star.

OPTICAL DESIGN OF THE SUBMILLIMETER HIGH ANGULAR RESOLUTION CAMERA (SHARC)

ABSTRACT The optical and mechanical design and performance of the Submillimeter High Angular Resolution Camera (SHARC) is described. The camera currently operates with a monolithic 24-pixel linear bolometer array in the 350 and 450 micron atmospheric windows at the Caltech Submillimeter Observatory (CSO). The design extends the techniques of geometric optics employed in optical and near-infrared cameras to submillimeter wavelengths. Using an off-axis ellipsoidal mirror and cold stops, excellent imaging (Strehl ratio > 0.95) is achieved across a 2' by 2' focal plane field even with secondary throws of up to 4'. The camera's symmetric mechanical assembly provides fixed, machined alignment of the optical elements. We demonstrate the imaging capabilities of the system with 350 micron observations of a point source at the telescope. The optical design can easily accommodate future planned upgrades to two-dimensional bolometer arrays.