FIR Continuum Research Articles

Gas Content of Markarian Starburst Galaxies

The main results of this study can be summarized as follows: (a) The HI and CO linewidths are well correlated. Interaction between galaxies has little influence on the HI and CO line broadening. A rapidly rotating nuclear disk in the galaxy could lead to CO line broadening, while the HI line is less affected by the rotating disk. Molecular gas in Markarian galaxies is centrally concentrated. (b) For past and present star formation activity both HI and H2 components of the gas are important. The atomic and molecular gas surface densities are well correlated with blue, FIR, and radio continuum surface brightnesses, but the H2 surface density is better correlated than that of the HI. The two gas phases are also connected. (c) In general, galaxies with UV-excess (Markarian galaxies) are not distinguished by star formation properties from non-UV galaxies, however some second order differences may exist, like the relation between atomic surface density and radio continuum surface brightness.

Observations of [Cii] 158 Micron Line and Far‐Infrared Continuum Emission toward the High‐Latitude Molecular Clouds in Ursa Major

We report the results of a rocket-borne observation of the [C II] 158 μm line and far-infrared continuum emission at 152.5 μm toward the high-latitude molecular clouds in Ursa Major. We also present the results of a follow-up observation of the millimeter 12CO J = 1 → 0 line over a selected region observed by the rocket-borne experiment. We have discovered three small CO cloudlets from the follow-up 12CO observations. We show that these molecular cloudlets, as well as the MBM clouds (MBM 27, 28, 29, and 30), are not gravitationally bound. Magnetic pressure and turbulent pressure dominate the dynamic balance of the clouds. After removing the H I-correlated and background contributions, we find that the [C II] emission peak is displaced from the 152.5 μm and CO peaks, while the 152.5 μm continuum emission is spatially correlated with the CO emission. We interpret this behavior by attributing the origin of the [C II] emission to the photodissociation regions around the molecular clouds illuminated by the local UV radiation field. We also find that the ratio of the molecular hydrogen column density to the velocity-integrated CO intensity is 1.19 ± 0.29 × 1020 cm-2 (K km s-1)-1, from the FIR continuum and the CO data. The average [C II]/FIR intensity ratio over the MBM clouds is 0.0071, which is close to the all-sky average of 0.0082 reported by FIRAS on the COBE satellite. The average [C II]/CO ratio over the same regions is 420, significantly lower than in molecular clouds in the Galactic plane.

Star Formation Histories of Spiral Galaxies from a Multi-Wavelength Study

We study the star formation histories of a sample of 113 nearby spiral galaxies using their radio continuum (20cm), FIR (40-120μm), H (1.65μm) and B (4400Å) luminosities. The first two are used as indicators of star formation rate over the past ∼ 108 years, as suggested by the tight and nearly universal FIR/radio correlation (Xu et al. 1994). Compared to other indicators of recent star formation rate such as Hα and UV, FIR and radio continuum have the advantage of being insensitive to extinction. The B luminosity is taken as star formation indicator for the time scale of 3 109 years, and the H luminosity for the time scale of 1010 years. We find: 1.The long-term star-formation history (from a few billion years to the entire Hubble time), as indicated by the B-to-H luminosity ratio, depends strongly on the Hubble type.2.The recent star-formation history in the last a few billion years, as indicated by the radio-to-B luminosity ratio and the FIR-to-B luminosity ratio, does not depend on the Hubble type.3.Galaxies of a given Hubble type have similar long-term star-formation histories. On the other hand, their recent star-formation histories in the last a few billion years can be much different: the ratio between the star-formation rate averaged over the past 108 years to that over the past 3 109 years can be different by two orders of magnitude, as indicated by the scattering of the radio-to-B luminosity ratio and that of the FIR-to-B luminosity ratio. This is not likely to be due to the extinction on the B luminosity, because the scattering of the B-to-H luminosity ratio for a given type is much smaller.

Protostellar condensations in the core of NGC 2024

The J = 4-3 transition of HCO(+) was observed in the core of NGC 2024. The findings indicate a possibly expanding torus centered around the FIR 6 continuum source and an E-W ridge below the ionization front. The presence of HCO(+) J = 4-3 emission from the torus and E-W ridge suggests relatively high gas densities in these regions. The relatively strong intensity of the observed HCO(+) J = 4-3 emission toward the FIR source suggests that the emission originates from relatively warm and dense gas. Continuum radiation transport modeling of the core shows that submillimeter and millimeter continuum observations of the cores can be reproduced under the assumption that the cores are externally heated objects without internal heating sources. This requires a factor of about 250 enhancement of the external radiation field intensity compared to the standard Galactic radiation field. This enhancement is consistent with the stellar luminosities required to explain the observed FIR continuum emission from the region.

The influence of environment on gas and dust in S0 galaxies

Consideration is given to the effect of local environemnt on the gas and dust content of S0 galaxies relative to other spiral systems in the Local Supercluster. The optical properties, H I content, and FIR continuum properties of a complete sample of Local Supercluster S0 galaxies are analyzed. The characteristics of S0 galaxies are compared with those of Sa and Sc galaxies. It is concluded that, like other spiral galaxies, S0's undergo continuing gas removal when they pass through high-density regions.

An isolated, well-defined infrared cirrus cloud

High spatial resolution H I measurements of a high-latitude FIR cirrus cloud (l = 9 deg, b = 51 deg) were obtained with the Arecibo 310 m telescope. The cloud is well isolated spatially and has a velocity well-separated from other H I clouds, providing an ideal laboratory for studying diffuse interstellar clouds. The Iv(100)/N(H) and Iv(60)/Iv(100) ratios are consistent with the Draine and Anderson model which includes a distribution of grain sizes down to 3 A. The small grains undergo transient heating and affect the 60 micron FIR continuum emission. Current models, however, underpredict Iv(12)/Iv(100) and Iv(25)/Iv(100) by more than a factor of 2. The effects small grains have on the thermal equilibrium of diffuse interstellar clouds are discussed.

Silicon Bearing Molecules in Molecular Clouds

We have used the IRAM 30-m telescope to make high angular resolution observations of the silicon-bearing molecule, SiO and SiS in region of massive star formation. We have mapped the J=2-l and the J=5-4 lines of SiO with angular resolutions of 26 and 12” respectively. For all the sources mapped, the SiO emission is more extended that the beam. The extreme cases are found toward NGC7538 and W49N where the SiO emission has sizes of 1x0.5 pc and 2.8x1 pc respectively. This result is in contrast with the size of the SiO region in Orion-IRc2 which is of the order 0.03 pc. The maxima of the SiO emission are found at the position of the H2O masers. This indicates, as previously thought, that SiO emission probes the high temperature regions and/or the shocked gas and dust surrounding young stars. In the DR21(OH) region we have found the unique SiO source which is not associated to H20 maser emission. This SiO condensation is 40” south of DR21(OH) and it was first detected as a source of 350µm emission. The SiO emission is elongated in the east-west direction. From an analysis of the FIR continuum emission this object appears to be cool (Tk < 30) and it has been proposed that this source represents a pre-stellar condensation which may be evolving toward the star formation stage. The detection of SiO, howevwer, casts some doubts about the evolutionary stage of this source. Futher high angular resolution observations are necessary to establish the nature of this source.

CO Observations of Bright IRAS Galaxies

CO emission has been detected from 75 bright infrared galaxies with CZ = 2 000 – 16 000 km/s. These include the most distant and the most luminous galaxies (Arp 55, IR 1713+63) yet detected in CO. All of these galaxies are rich in molecular gas with Mtotal (H2) = 2 × 109 −6x1010 M⊙, and they have a strong far-infrared excess, with LFIR/LB = 2-40 and LFIR (40-400μ) = 1010 – 3 × 1012 L⊙. The primary luminosity source appears to be star formation in molecular clouds. A strong correlation is found between the FIR and 21-cm continuum flux, implying that the IMF is independent of the star formation rate. The ratio LFIR/M(H2) provides a measure of the current rate of star-formation, which is found to be a factor 3-20 larger in these galaxies than for the ensemble of molecular clouds in the Milky Way. VLA maps plus a few high resolution (14″-30″) CO (1-0) and CO (2-1) maps suggest that most of the luminosity comes from core regions 1-3 kpc in size. The abnormal concentration of molecular gas in these galactic cores is presumably the result of a collision or strong interaction with a nearby companion.

Observations of the 157.7 micron (C II) emission from the galactic H II regions W3 and W51

The 157.7 micron ground-state fine-structure transition of singly ionized carbon was detected from three positions, each in galactic H II region W3 and W51. These findings show that this emission goes beyond the H II regions and the carbon recombination region in each source. In addition these observations establish a lower limit to the luminosity in the 157.7 micron (C II) line of 360 solar luminosity from W3 and 41 solar luminosity from W51. Mechanisms for 157.7 micron-line excitation are discussed, and it is noted that a substantial part of the observed emission originates in the thin carbon recombination line-emitting region which borders on the H II regions and the warm atomic and molecular gas at the surface of neighboring molecular clouds. It is proposed that excitation by electrons present in the diffuse gas surrounding W51 contributes to the extended 157.7 micron (C II) line emission from this region. It was found that the (C II)FIR continuum ratios for W3 and W51 are in agreement with those of other galactic H II regions.