Kieron J. Leech

Disappearance of stellar debris disks around main-sequence stars after 400 million years

Almost 5 billion years ago, the Sun formed in a local contraction of a cloud of molecular gas. A rotating disk of gas and dust is believed to have fed material onto the proto-Sun for the first few million years of its life, and to have formed the planets, comets and other Solar System objects. Similar disks, but with less mass, have been observed around a few main-sequence stars such as Vega. The dust particles orbiting stars like Vega will be removed on timescales of the order of 1 Myr (Vega is about 350 Myr old), and therefore must be resupplied, at least for a time. But earlier surveys lacked the sensitivity to determine how many nearby stars have dust disks, and to investigate how long such disks survive. Here we report infrared observations indicating that most stars younger than 300 Myr have dust disks, while most older than 400 Myr do not: ninety per cent of the disks disappear when the star is between 300 and 400 Myr old. Several events that are related to the ‘clean up’ of debris in the early history of our Solar System have a similar timescale.

New Wavelength Determinations of Mid-Infrared Fine-structure Lines by Infrared Space Observatory Short Wavelength Spectrometer

We report accurate new wavelengths for 29 mid-infrared ionic fine-structure lines, based on observations with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Our results originate from observations of NGC 7027, NGC 6543, NGC 6302, the Circinus galaxy, Sgr A West, and W51 IRS 2. The obtained accuracies (λ/Δλ) range from 3 × 104 to 1 × 105, depending on instrumental mode and uncertainty in radial velocities.

Dust Temperatures in the Infrared Space Observatory Atlas of Bright Spiral Galaxies

We examine far-infrared and submillimeter spectral energy distributions for galaxies in the Infrared Space Observatory Atlas of Bright Spiral Galaxies. For the 71 galaxies where we had complete 60–180 lm data, we fitted blackbodies with � � 1 emissivities and average temperatures of 31 K or � � 2 emissivities and average temperatures of 22 K. Except for high temperatures determined in some early-type galaxies, the temperatures show no dependence on any galaxy characteristic. For the 60–850 lm range in eight galaxies, we fitted black

An Infrared Space Observatory Atlas of Bright Spiral Galaxies

In this first paper in a series we present an atlas of infrared images and photometry from 1.2 to 180 μm for a sample of bright spiral galaxies. The atlas galaxies are an optically selected, magnitude-limited sample of 77 spiral and S0 galaxies chosen from the Revised Shapley-Ames Catalog (RSA). The sample is a representative sample of spiral galaxies and includes Seyfert galaxies, LINERs, interacting galaxies, and peculiar galaxies. Using the Infrared Space Observatory (ISO), we have obtained 12 μm images and photometry at 60, 100, and 180 μm for the galaxies. In addition to its imaging capabilities, ISO provides substantially better angular resolution than is available in the IRAS survey, and this permits discrimination between infrared activity in the central regions and global infrared emission in the disks of these galaxies. These ISO data have been supplemented with JHK imaging using ground-based telescopes. The atlas includes 2 and 12 μm images. Following an analysis of the properties of the galaxies, we have compared the mid-infrared and far-infrared ISO photometry with IRAS photometry. The systematic differences we find between the IRAS Faint Source Catalog and ISO measurements are directly related to the spatial extent of the ISO fluxes, and we discuss the reliability of IRAS Faint Source Catalog total flux densities and flux ratios for nearby galaxies. In our analysis of the 12 μm morphological features we find that most but not all galaxies have bright nuclear emission. We find 12 μm structures such as rings, spiral arm fragments, knotted spiral arms, and bright sources in the disks that are sometimes brighter than the nuclei at mid-infrared wavelengths. These features, which are presumably associated with extranuclear star formation, are common in the disks of Sb and later galaxies but are relatively unimportant in S0–Sab galaxies.

Gas cooling within the diffuse ISM of late-type galaxies

We combine observations of spiral galaxies in the (CII) line at 158 m, made with the Long Wavelength Spectrometer aboard ISO, with previous data from the Kuiper Airborne Observatory to study the origin of this line, which is the main coolant of the interstellar medium at relatively low temperatures. We also use HI and CO(1 0) observations of these galaxies and estimate the respective line fluxes in the same beam as the (CII) observations. We conrm the existence of a linear relation between the (CII) line intensity and the CO(1 0) line intensity, that we extend to intrinsically fainter galaxies. The dispersion around this relation is signicant and due to variations in the far-UV flux, thus in the star formation rate. We nd that for the least active galaxies of our sample, in terms of star formation, the rate of (CII) line emission per interstellar hydrogen atom is similar to that in the Solar neighbourhood. For those galaxies, most of the (CII) line emission comes probably from the diuse cold atomic medium. In more active galaxies, considered globally, the average (CII) line emission is dominated by dense photodissociation regions and to some extent by the warm ionized diuse medium. This is true in the central regions of many spiral galaxies, and probably even in the interarm regions of the most actively star{forming ones.

Infrared Space Observatory Observations of Hickson Compact Group 31 with the Central Wolf-Rayet Galaxy NGC 1741

Hickson Compact Group (HCG) 31, consisting of the Wolf-Rayet galaxy NGC 1741 and its irregular dwarf companions, was observed using the Infrared Space Observatory. The deconvolved ISOCAM maps of the galaxies using the 7.7 and 14.3 ?m (LW6 and LW3) filters are presented, along with ISOPHOT spectrometry of the central starburst region of NGC 1741 and the nucleus of galaxy HCG 31A. Strong mid-IR emission was detected from the central burst in NGC 1741, along with strong polycyclic aromatic hydrocarbon (PAH) features and a blend of features, including [S?IV] at 10.5 ?m. The 14.3/6.75 ?m flux ratio, for which the 6.75 ?m flux was synthesized from the PHT-S spectrum, and 14.3/7.7 ?m flux ratio suggest that the central burst within NGC 1741 may be moving toward the poststarburst phase. Diagnostic tools including the ratio of the integrated PAH luminosity to the 40-120 ?m infrared luminosity and the far-infrared colors reveal that despite the high surface brightness of the nucleus, the properties of NGC 1741 can be explained in terms of a starburst and do not require the presence of an active galactic nucleus. The Tycho catalog star Tyc 04758 466 1, with mV = 11.3 and spectral type F6, was detected at 7.7 and 14.3 ?m.

Erratum: “Far-Infrared Photometry of a Statistical Sample of Late-Type Virgo Cluster Galaxies” (ApJS, 139, 37 [2002])

The legends for Figures 9 and 10 were switched. The legend for Figure 9 should read: ‘‘ Uncorrected 2 values derived from fitting the Gaussian models to the maps vs. the corresponding flux densities at 60 lm obtained from integrating the model fit to infinity. The data were plotted as diamonds, except for those galaxies for which the model was an imperfect fit to the data, which were plotted as crosses. The solid line rising for F60 > 1:5 Jy represents the linear correlations between flux and 2, as a result of systematic errors due to the oversampling in cross-scan direction. For faint flux densities 2 is dominated by systematic errors, producing a clumpy distribution at the left-hand side of the plot, with a lower envelope, which we traced by a solid line to guide the eyes.’’ The legend for Figure 10 should read: ‘‘Model fits to the observed brightness profiles of all the Virgo galaxies from our sample (except for the interacting system VCC 1673/VCC 1676; see x 6.2.4). Statistical uncertainties for selected map pixels near the extremities and center of the scans are indicated by the 3 error bars. The legend is as in Fig. 8.’’ In addition, the references to Figures 9 and 10 in the main text should be interchanged. The Press sincerely regrets this error. The Astrophysical Journal Supplement Series, 140:609–609, 2002 June

Star Formation in the Infrared Space Observatory Atlas of Bright Spiral Galaxies

We investigate star formation along the Hubble sequence using the ISO Atlas of Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc - Scd) spirals than in early-type (Sa - Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later-type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in LINERs and transition objects compared to HII galaxies. The mean star formation rate in the sample is 1.4 Msun/yr based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 x 10^8 yr, which is ~5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt Law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.We investigate star formation along the Hubble sequence using the Infrared Space Observatory Atlas of Bright Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc–Scd) spirals than in early-type (Sa–Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in low ionization nuclear emission regions and transition objects compared with H II galaxies. The mean star formation rate in the sample is 1.4 M⊙ yr-1, based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 × 108 yr, which is ~5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.

Star Formation in the ISO Atlas of Spiral Galaxies

We investigate star formation along the Hubble sequence using the ISO Atlas of Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc - Scd) spirals than in early-type (Sa - Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later-type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in LINERs and transition objects compared to HII galaxies. The mean star formation rate in the sample is 1.4 Msun/yr based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 x 10^8 yr, which is ~5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt Law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.We investigate star formation along the Hubble sequence using the Infrared Space Observatory Atlas of Bright Spiral Galaxies. Using mid-infrared and far-infrared flux densities normalized by K-band flux densities as indicators of recent star formation, we find several trends. First, star formation activity is stronger in late-type (Sc–Scd) spirals than in early-type (Sa–Sab) spirals. This trend is seen both in nuclear and disk activity. These results confirm several previous optical studies of star formation along the Hubble sequence but conflict with the conclusions of most of the previous studies using IRAS data, and we discuss why this might be so. Second, star formation is significantly more extended in later type spirals than in early-type spirals. We suggest that these trends in star formation are a result of differences in the gas content and its distribution along the Hubble sequence, and it is these differences that promote star formation in late-type spiral galaxies. We also search for trends in nuclear star formation related to the presence of a bar or nuclear activity. The nuclear star formation activity is not significantly different between barred and unbarred galaxies. We do find that star formation activity appears to be inhibited in low ionization nuclear emission regions and transition objects compared with H II galaxies. The mean star formation rate in the sample is 1.4 M⊙ yr-1, based on global far-infrared fluxes. Combining these data with CO data gives a mean gas consumption time of 6.4 × 108 yr, which is ~5 times lower than the values found in other studies. Finally, we find excellent support for the Schmidt law in the correlation between molecular gas masses and recent star formation in this sample of spiral galaxies.

ISO observations of the dusty quasar BR 1202−0725

We present mid- and far-infrared photometry of the high-redshift (z = 4.69) dusty quasar BR 1202-0725. The quasar was detected in the near-infrared, at a flux level (0.7 ′ 0.2mJy) consistent with an average radio-quiet quasar at its redshift. Only upper limits for the emission were obtained in the far-infrared. These upper limits, when combined with data from ground-based telescopes, are the first direct evidence for a turnover in the far-infrared emission, and hence confirm that a blackbody dominates the spectral energy distribution at far-infrared wavelengths. This blackbody is most probably cool dust, constrained to have a temperature below 80 K, for a β of 1.5.