Far-infrared Sources Research Articles

Constraints on Far-Infrared Source Counts in the Lockman Hole using a Power Spectrum Analysis

We present the characteristics of far-infrared (FIR) brightness fluctuations at 90 μm and 170 μm in the Lockman Hole, which were surveyed with the ISOPHOT instrument aboard the Infrared Space Observatory (ISO), and give constraints on the galaxy number counts down to 30 mJy at 90 μm and 50 mJy at 170 μm. The fluctuation power spectra of the FIR images are not dominated by IR cirrus, and are instead most likely due to star-forming galaxies. This analysis indicates the existence of strong evolution in the counts. Especially at 90 μm, the source density is much larger than that expected from the currently available galaxy count models. The galaxies responsible for the fluctuations also significantly contribute to the cosmic infrared background radiation recently derived from an analysis of the COBE data.

The Structure of Infrared‐luminous Galaxies at 100 Microns

We have observed 22 galaxies at 100 μm with the Kuiper Airborne Observatory in order to determine the angular size of their FIR-emitting regions. This one-dimensional array data constitutes the highest spatial resolution ever achieved on luminous galaxies in the far-infrared. Most of these galaxies are very luminous far-infrared sources, with LFIR > 1011 L⊙. We clearly resolved six of these galaxies at 100 μm and have some evidence for extension in seven others. Those galaxies that we have resolved can have little of their 100 μm flux directly emitted by a pointlike active galactic nucleus. Dust heated to ~40 K by recent bursts of nonnuclear star formation provides the best explanation for their extreme FIR luminosity. In a few cases, heating of an extended region by a compact central source is also a plausible option.

IR study of exclusion-accumulation effects enhanced bythe geometrical factor

A detailed experimental and theoretical study ofexclusion and accumulation at the l-h contact in cylindricalsamples (with a current along the radius) has been carried out.Two types of the second contact have been used: either the samel-h contact or a recombination type ohmic contact. It has beenshown that both the inhomogeneous electric field in a cylinder andthe high generation-recombination capability of the second ohmiccontact at the larger radius enhance the exclusion andaccumulation effects near the small-radius contact. This is manifestin lowering the bias voltage needed for a high-level depletionor accumulation, the latter becoming competitive with injectionby the excess carrier concentration level. Experimentalinvestigations of non-equilibrium charge carrier distributionin these samples have been implemented by two methods. Alongwith the well known IR probing technique in the spectral rangeof free carrier absorption, the thermovision technique,enabling us to visualize the concentration distribution, has beenused for the first time. The obtained results are useful forimprovement of the far-infrared source and modulatorparameters.

Far-infrared Detection of C[TINF]3[/TINF] in Sagittarius B2 and IRC +10216

We report on the detection of nine lines of the nu2 bending mode of triatomic carbon, C3, in the direction of Sagittarius B2. The R(4) and R(2) lines of C3 have been also detected in the carbon-rich star IRC +10216. The abundances of C3 in the direction of Sgr B2 and IRC +10216 are approximately 3x10-8 and approximately 10-6, respectively. In Sgr B2 we have also detected the 23-12 line of NH with an abundance of a few times 10-9. Polyatomic molecules will have a weak contribution from their pure rotational spectrum to the emission/absorption in the far-infrared. We suggest, however, that they could be, through their low-lying vibrational bending modes, the dominant carriers of emission/absorption in the spectrum of bright far-infrared sources.

Application of the reflection–absorption spectroscopy to the semiconductor thin films

The study of bulk phonons in thin films is limited due to the low energy of far-infrared sources. The infrared reflection–absorption spectroscopy (IRAS) technique has, therefore, been developed. In the case of incidence deviating from normal two states of polarization of incident light should be taken into consideration. The application of approximation of ultra thin films enables to obtain the simple relations ΔR/R0 between the change of reflectivity of a metal plate covered with a film and that of the metal substrate. ΔR/R0 has been obtained for two polarizations, namely: s and p. The present research has demonstrated that this method proved to be particularly usable for the multimode crystals where the oscillator strength is distributed on several modes. The IRAS technique has been applied to the ZnxCdyHg1−x−yTe (ZMCT) thin films. The frequencies of LO and TO phonons were determined by comparing the theoretical and experimental spectra.

Far-infrared induced current in a ballistic channel–potential barrier structure

We consider electron transport in a ballistic multi-mode channel structure in the presence of a transversely polarized far-infrared (FIR) field. The channel structure consists of a long resonance region connected to an adiabatic widening with a potential barrier at the end. At frequencies that match the mode energy separation in the resonance region we find distinct peaks in the photocurrent, caused by Rabi oscillations in the mode population. For an experimental situation in which the width of the channel is tunable via gates, we propose a method for reconstructing the spectrum of propagating modes, without having to use a tunable FIR source. With this method the change in the spectrum as the gate voltage is varied can be monitored.

A Study of the Molecular Cloud toward the H II Regions S 35 and S 37 with NANTEN

We have made new observations of a large field of ∼ 10 square degrees toward the two southern H II regions S35 and S37. Observations were made in the 12CO, 13CO, and C18O J= 1−0 emission with the NANTEN telescope at Las Campanas, Chile. These observations have revealed a giant molecular cloud of ∼ 1.3 × 105M⊙ in 12CO, highly elongated with a size of ∼ 80 pc × ∼ 20 pc, where a previously known active site of star formations, including GGD 27IR and HH 80/81, is located. The cloud appears lifted up to z ∼ 100 pc from the galactic plane at an inclination angle of ∼ 70°, exhibiting indications of strong interactions with the H II regions. Star formation has been found to be very active in the cloud. We have discovered four molecular outflows driven by luminous far-infrared sources of ∼ 102−104L⊙ in addition to that previously known toward GGD 27 IRS. The activity is also demonstrated by the existence of several dense C18O clumps whose mass ranges from ∼ 3 × 102M⊙ to ∼ 4 × 103M⊙ . The five molecular outflows are embedded in these C18O clumps. In addition, a comparison with H I shows that the CO cloud is located at the edge of an H I hole of ∼ 55 pc radius. Since the H I gas surrounding the hole shows a signature of expansion at several km s-1, we suggest that the H I hole represents a supershell created by some explosive events, like supernova explosions, during the last ∼ 4 × 106 yr. This supershell may also provide an explanation for the origin of the molecular distribution.

The hyperluminous infrared quasar 3C 318 and its implications for interpreting submm detections of high-redshift radio galaxies

We present near-infrared spectroscopy and imaging of the compact steep-spectrum radio source 3C 318 which shows it to be a quasar at redshift z = 1.574 (the z = 0.752 value previously reported is incorrect). 3C 318 is an IRAS, ISO and SCUBA source so its new redshift makes it the most intrinsically luminous far-infrared (FIR) source in the 3C catalogue (there is no evidence of strong gravitational lensing effects). Its bolometric luminosity greatly exceeds the 10(exp 13) solar luminosity level above which an object is said to be hyperluminous. Its spectral energy distribution (SED) requires that the quasar heats the dust responsible for the FIR flux, as is believed to be the case in other hyperluminous galaxies, and contributes (at the greater than 10% level) to the heating of the CIA dust responsible for the sub-mm emission. We cannot determine whether a starburst makes an important contribution to the heating of the coolest dust, so evidence for a high star-formation rate is circumstantial being based on the high dust, and hence gas, C-1 mass required by its sub-mm detection. We show that the current sub-mm and FIR data available for the highest-redshift radio galaxies are consistent with SEDs similar to that of 3C 318. This indicates that at least some of this population may be detected in the sub-mm because of dust heated by the quasar nucleus, and that interpreting sub-mm detection as evidence for very high (approx. less than 1000 solar mass/yr) star-formation rates may not always be valid. We show that the 3C318 quasar is slightly reddened (A(sub v) approx. = 0.5), the most likely cause of which is SMC-type dust in the host galaxy. If very distant radio galaxies are reddened in a similar way then we show that only slightly greater amounts of dust could obscure the quasars in these sources. We speculate that the low fraction of quasars amongst the very high redshift (z approx. greater than 3) objects in low-frequency radio-selected samples is the result of such obscuration. The highest-z objects might be preferentially obscured because like 3C318 they are inevitably observed very shortly after the jet-triggering event, or because their host galaxies are richer in dust and gas at earlier cosmic epochs, or because of some combination of these two effects.

Possible Young Stellar Objects without Detectable CO Emission

Young stellar objects (YSOs) usually appear in molecular clouds as infrared objects associated with a molecular envelope. Wouterloot and Brand (1989, AAA 50.133.012) searched 1302 IRAS point sources with reliable fluxes at 25, 60, and 100 μm near to the galactic plane for 12CO (J = 1–0) emission; 1077 sources were detected. Among their far-infrared sources without detectable CO emission, we found that at least 18 objects are invisible at optical and near-infrared wavelengths. The infrared spectral indices between 2.2 μm and 25 μm correspond to those of class I YSOs, and the IRAS colors are similar to those of the usual YSOs. These peculiar far-infrared objects are highly concentrated around the galactic plane and the distances are estimated to be ∼1 kpc. Although their distribution is away from molecular clouds, some of them seem to be associated with large dark clouds or weak radio sources. These objects are possible YSOs with low CO abundance in the envelopes.

Radio Emission from Galaxies in the Las Campanas Redshift Survey

To increase the redshift range and look-back time over which the radio luminosity function can be measured directly, we identified 1157 galaxies in the Las Campanas Redshift Survey (LCRS) having isophotal (red) magnitudes miso ≤ 18.0 with radio sources brighter than 2.5 mJy beam-1 in the 1.4 GHz NRAO VLA Sky Survey (NVSS). Since the NVSS has 45'' FWHM angular resolution, these radio and optical limits include nearly all LCRS galaxies with 1.4 GHz luminosities L ≥ 1022.4 W Hz-1 at z ≈ 0.05 to L ≥ 1023.6 W Hz-1 at z ≈ 0.2. The mean redshift z ≈ 0.14 of the radio-detected galaxies is higher than the mean redshift z ≈ 0.10 of the optical sample. This indicates that, statistically, the radio emission was detected from galaxies with the highest optical luminosities. Of the 1157 galaxies, 261 were also identified with far-infrared (FIR) sources in the IRAS Point Source Catalog and Faint Source Catalog. The principal radio energy sources in all identified galaxies were classified as either starburst or AGN on the basis of their FIR-radio flux ratios, FIR spectral indices, and radio-optical flux ratios. We show that the radio-optical flux ratio can be effectively used to classify the dominant energy source for the radio emission even if FIR fluxes and radio morphological data are not available.

Monsters and babies from the first/IRAS survey

Radio continuum emission at cm wavelengths is relatively little affected by extinction. When combined with far-infrared (FIR) surveys this provides for a convenient and unbiased method to select (radio-loud) AGN and starbursts deeply embedded in gas and dust-rich galaxies. Such radio-selected FIR samples are useful for detailed investigations of the complex relationships between (radio) galaxy and starburst activity, and to determine whether ULIRGs are powered by hidden quasars (monsters) or young stars (babies). We present the results of a large program to obtain identifications and spectra of radio-sleected, optically faint IRAS/FSC objects using the FIRST/VLA 20 cm survey (Becker, White and Helfand 1995). These objects are all radio-'quiet' in the sense that their radio power/FIR luminosities follow the well-known radio/FIR relationship for star forming galaxies. We compare these results to a previous study by our group of a sample of radio-'loud' IRAS/FSC ULIRGs selected from the Texas 365 MHz survey (Douglas et al. 1996). Many of these objects also show evidence for dominant, A-type stellar populations, as well as high ionization lines usually associated with AGN. These radio-loud ULIRGs have properties intermediate between those of starbursts and quasars, suggesting a possibile evolutionary connection. Deep Keck spectroscopic observations of three ULIRGsmore » from these samples are presented, including high signal-to-noise spectropolarimetry. The polarimetry observations failed to show evidence of a hidden quasar in polarized (scattered) light in the two systems in which the stellar light was dominated by A-type stars. Although observations of a larger sample would be needed to allow a general conclusion, our current data suggest that a large fraction of ULIRGs may be powered by luminous starbursts, not by hidden, luminous AGN (quasars). While we used radio-selected FIR sources to search for evidence of a causal AGN/starburst connection, we conclude our presentation with a dramatic example of an AGN/starburst object from an entirely unrelated quasar survey selected at the opposite, blue end of the spectrum.« less

High‐Resolution Far‐Infrared Studies of Intermediate‐Mass Pre–Main‐Sequence Objects

We have obtained high-resolution far-infrared maps of nine regions with 10 Herbig Ae/Be stars (intermediate-mass pre-main-sequence stars). Similar maps were obtained for 10 embedded IRAS sources with Sν(100 μm) > Sν(60 μm) and L ~ 200 L☉, which are possible evolutionary precursors of Herbig Ae/Be stars. Single far-infrared sources were found in most maps. The embedded sources have positions in agreement with those of the IRAS PSC, but some of the Herbig Ae/Be stars are offset significantly from the position of peak far-infrared emission. For all objects where it was possible to obtain 100 μm flux densities, they are consistent with those observed by IRAS, but derived 50 μm flux densities are larger than expected. The far-infrared maps reveal that objects in at least 17 of 19 emission regions are significantly extended at the 30''-40'' resolution of the Kuiper Airborne Observatory at 100 μm. Only sources associated with AB Aur and possibly IRAS 05338-0624 have unresolved far-infrared emission. Detailed analyses of the flux densities and positions from our maps suggest the far-infrared emission in regions with Herbig Ae/Be stars may not immediately surround these stars in all cases. Instead, far-infrared emission from these objects may originate from dust heated externally by the Herbig stars, or from dust heated internally by other sources. For other objects arguably surrounded by far-infrared emission, the Herbig stars or embedded IRAS objects have similar mean deconvolved sizes (i.e., 0.10-0.15 pc), but possibly have different mean deconvolved shapes (i.e., aspect ratios). Thus, far-infrared emission here may originate from flattened dust envelopes; the appearance of a far-infrared object as either a Herbig Ae/Be star or an embedded IRAS source may be merely a matter of viewing orientation.

Far-Infrared Laser Sideband Spectroscopy of the NH2Radical

We have constructed a far-infrared (FIR) laser sideband spectrometer for carrying out pure rotational spectroscopy of fundamental radicals and ions in the FIR or “terahertz” frequency region. The tunable FIR source is obtained by sideband generation of a carbon dioxide laser-pumped FIR laser with a GaAs Schottky barrier diode. The minimum detectable absorption coefficient of the spectrometer achieved so far is 1 × 10−6cm−1at 1600 GHz. Observation of the pure rotational spectrum of the NH2radical in the2B1ground electronic state was demonstrated as the first example. Spectra were measured by utilizing the double modulation technique;i.e.,the Zeeman effect was used in addition to the normal frequency modulation technique in order to flatten the baseline and to appreciate full system sensitivity. The radical was generated by a DC glow discharge of a mixture of ammonia and helium. Nine Q- and R-branch rotational transitions were measured in the frequency range between 1300 and 2600 GHz. Zero-field transition frequencies were obtained with an accuracy of around 1 MHz. The line frequencies were analyzed with previous microwave spectroscopic results, and the rotational and spin–rotation constants with their centrifugal correction terms were revised. With the aid of the revised rotational constants, it becomes possible to predict the line frequency of certain low N transitions of the radical to within about 1 MHz for the first time.

Masers in Massive Star-forming Regions Associated with the Brightest Steep-Spectrum [ITAL]IRAS[/ITAL] Point Sources

Twenty-four bright, steep-spectrum far-infrared IRAS point sources, which were originally discussed by Hughes & MacLeod, have been searched for hydroxyl, water, and methanol masers and for formaldehyde absorption features. Distances and spectral types for the stars exciting these masers were estimated using radio continuum, IRAS spectra, and maser velocity data. It is found that 21 of the 24 sources are regions of massive star formation containing stars earlier than type B2. We report the discovery of five new OH masers, one new 6.7 GHz CH3OH maser, and two new 12.2 GHz CH3OH masers toward these IRAS sources. By contrast, no 4.765 GHz excited-OH masers were found toward these sources. This sample contains only star-forming regions; there are no planetary or reflection nebulae or extragalactic sources present, as seen in samples with less extreme IRAS colors.

The distribution of far-infrared emission from edge-on galaxies

We use resolution-enhanced IRAS images (HiRes) to study the distribution of far-infrared (FIR) emission in 20 edge-on, disc galaxies. Half of our sample are selected as infrared-bright galaxies which emit most of their energy at wavelengths longer than a few mum. The remaining 10 objects are `normal' (quiescent) galaxies. The distribution of FIR emission along the major axis of the infrared-bright galaxies tends to be far more centrally concentrated than that detected in quiescent discs. Indeed, many of the infrared-bright objects are dominated by a luminous, compact or unresolved source (<3 kpc) located in the nuclear region. It is known from emission-line studies carried out in the past (Hα imaging and long-slit spectroscopy) that most of the galaxies that we have selected as infrared-bright possess an energetic outflow or `superwind', which is powered by enhanced, circumnuclear star formation. We believe, therefore, that the compact FIR emission that we have detected arises from dust enclosing the central starburst. Low-resolution radio continuum maps for the same galaxies show a striking morphological similarity to the IRAS images, confirming the presence of enhanced, recent star formation at the centre of the infrared-bright galaxies. The wider availability of higher resolution FIR images, such as HiRes, also invites an investigation into whether edge-on discs are resolved along the minor axis. In particular, we explore the possibility that the starburst-driven superwinds, present within many of our infrared-bright galaxies, are transporting dust grains out of the disc. None of the objects that we have examined (infrared-bright or quiescent) exhibits structure that can be reliably interpreted as dust outflow. Various artefacts of the HiRes data, specifically those which produce faint, spurious structure in the vicinity of bright FIR sources, ultimately constrain the sensitivity of our images to about 1 MJy sr^-1 at 60 mum. Thus, if the grains entrained in the outflow are too cold (<=15 K), we cannot expect to detect them with the present study.

Infrared emission and H-R diagram of bright far-infrared sources at the stage of star formation

JHK observations were made for a selection of strong far-infrared sources containing large-mass YSOs using the 1.26 Beijing-Xinglong telescope. Three models, blackbody photosphere and envelope, double-shell dust emission, and detailed radiative transfer, were applied to these and other published infrared data to derive the temperature of the central star, T op, the circumstellar extinction and the thermal structure of the dust shell. An extended H-R diagram based on T op and L ∗ derived from the infrared fluxes, was constructed for the YSOs. The infrared colour index S[16–60] was found to increase with age along given evolutionary tracks, and with mass for given isochrones.

Star Formation in the Cepheus Flare Molecular Clouds. I. Distance Determination and the Young Stellar Object Candidates

Results of an objective prism Schmidt survey are combined with IRAS survey data in order to assess the star-forming activity in the Cepheus Flare, a nearby giant molecular cloud complex at ~+15° above the Galactic equator. The distribution of absorbing matter along the line of sight was also studied. The Wolf diagrams, displaying the cumulative distribution of field star distance moduli, show that the interstellar matter in this region is concentrated at three characteristic distances: 200, 300, and 450 pc. The three components, though partly overlapping, can be separated along the Galactic latitude. Within the area of the Cepheus Flare, distances are determined for 14 Lynds dark clouds and for some other clouds. In order to compile a sample of young stellar object (YSO) candidates, three types of star formation signposts were searched for the following: 1. Prestellar cores in the IRAS 100 μm optical depth image of the region, 2. Far-infrared sources representing embedded YSOs and optically visible pre-main-sequence star candidates using IRAS Point Source Catalog, Faint Source Catalog, and calibrated IRAS detector scans, 3. Hα emission stars appearing on low-dispersion objective prism Schmidt plates as candidate pre-main-sequence stars. The IRAS 100 μm optical depth image of the region revealed 107 dense cores. Most of them are probably starless cores and, as such, potential sites of future star formation. One hundred twenty-two IRAS point sources were selected as probable YSOs at various evolutionary stages. An objective prism search for Hα emission stars covering an area of about 150 deg2 resulted in the detection of 142 Hα emission stars. Infrared fluxes taken from the IRAS catalogs or determined from the detector scans are listed for 95 of them. This sample is expected to consist of mostly T Tauri stars associated with the cloud complex. Finding charts and catalogs of the young stellar object candidates are also given. These results may serve as a basis for further dedicated studies of the region and will be useful for comparison when star-forming activity in other cloud complexes is investigated.

IRAS 20050+2720: An Embedded Young Cluster Associated with a Multipolar Outflow

We obtained near-IR images at 1-5 μm of IRAS 20050+2720, a very red far-infrared source with a multipolar outflow (as noted by Bachiller et al.). Of the 212 sources we detected, half of them are probably members of an embedded cluster around the IRAS source. More than 50% of the cluster members show near-IR excesses, suggesting that they are pre-main-sequence stars with circumstellar emission. The luminosity functions of the cluster show turnovers and slopes consistent with pre-main-sequence model predictions for an average cluster age of ~1 Myr. Three subclusters are identified within the cluster. One of them (subcluster A) appears to be associated with denser molecular gas and higher extinction than the other two subclusters. It also coincides with the IRAS source, a compact millimeter continuum source, and the center of the multipolar CO outflow. We identified in subcluster A several deeply embedded sources with sharply rising spectral energy distributions. These results support strongly the suggestion that the multipolar outflow is the result of simultaneous star formation within 0.1 pc of the IRAS source. We suggest that the cluster was formed in several episodes of star formation over the last ~1 Myr and that subcluster A represents the most recent episode that formed multiple young stars within ~0.1 Myr.

CO detection in the quasar BR1202-0725 at z = 4.69

The number of identified high-redshift far-infrared hyperluminous sources (LFIR ≥ 1013L⊙) is now approaching ~20 (see e.g. Rowan-Robinson 1996). In about half of them, at z &gt; 3.5, evidence for strong far-infrared emission has been obtained from its redshifted detection in the mm continuum. Most of the latter are bright radio-quiet QSOs (Omont et al. 1996b). The most prominent one, BR1202-0725 at z = 4.7, was first detected at 1.25mm with the IRAM 30m telescope (McMahon et al. 1994) with a flux of ∼ 10-15 mJy. Its submm detection at JCMT (Isaak et al. 1994) supports dust emission because of a submm-mm spectral index ≥ 3.

J = 1–0 HCN towards bright far-infrared sources: Observational data and results of modelling

Abstract Results of J = 1–0 HCN observations of 17 bright far-infrared sources are reported and analyzed. These objects have been observed earlier in the J = 1–0 CO line. HCN column densities and abundances have been calculated for two objects. The correlations between peak HCN and CO temperatures, HCN linewidths and FIR luminosities, HCN linewidths and masses of the objects are revealed. HCN and CO intergrated intensities do not correlate with FIR luminosities. The R 12 intensity ratios of the J = 1–0 HCN hyperfine structure for several objects are less than expected for optically thin case while the HCN linewidths are much greater than thermal ones for possible kinetic temperatures of the dense gas. This fact cannot be explained in the framework of the model with constant turbulent velocity over the cloud. The possible cloud model for these objects should implement inhomogeneous turbulent velocity and density structure.