Showing papers by "Stuart D. Ryder published in 2000"

An Extragalactic H I Cloud with No Optical Counterpart

Abstract: We report the discovery, from the H I Parkes All-Sky Survey (HIPASS), of an isolated cloud of neutral hydrogen, which we believe to be extragalactic. The H I mass of the cloud (HIPASS J1712-64) is very low, 1.7 x 10(7) M-circle dot, using an estimated distance of similar to 3.2 Mpc. Most significantly, we have found no optical companion to this object to very faint limits [mu(B) similar to 27 mag arcsec(-2)]. HIPASS J1712-64 appears to be a binary system similar to, but much less massive than, H I 1225 + 01 (the Virgo H. I cloud) and has a size of at least 15 kpc. The mean velocity dispersion measured with the Australia Telescope Compact Array (ATCA) is only 4 km s(-1) for the main component and, because of the weak or nonexistent star formation, possibly reflects the thermal line width (T < 2000 K) rather than bulk motion or turbulence. The peak column density for HIPASS J1712-64, from the combined Parkes and ATCA data, is only 3.5 x 1019 cm(-2), which is estimated to be a factor of 2 below the critical threshold for star formation. Apart from its significantly higher velocity, the properties of HIPASS J1712-64 are similar to the recently recognized class of compact high-velocity clouds. We therefore consider the evidence for a Local Group or Galactic origin, although a more plausible alternative is that HIPASS J1712-64 was ejected from the interacting Magellanic Cloud-Galaxy system at perigalacticon similar to 2 x 10(8) yr ago.

An Extragalactic HI Cloud with No Optical Counterpart

Abstract: We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of an isolated cloud of neutral hydrogen which we believe to be extragalactic. The HI mass of the cloud (HIPASS J1712-64) is very low, 1.7 x 10^7 Msun, using an estimated distance of ~3.2 Mpc. Most significantly, we have found no optical companion to this object to very faint limits (mu(B)~ 27 mag arcsec^-2). HIPASS J1712-64 appears to be a binary system similar to, but much less massive than, HI 1225+01 (the Virgo HI Cloud) and has a size of at least 15 kpc. The mean velocity dispersion, measured with the Australia Telescope Compact Array (ATCA), is only 4 km/s for the main component and because of the weak or non-existent star-formation, possibly reflects the thermal linewidth (T<2000 K) rather than bulk motion or turbulence. The peak column density for HIPASS J1712-64, from the combined Parkes and ATCA data, is only 3.5 x 10^19 cm^-2, which is estimated to be a factor of two below the critical threshold for star formation. Apart from its significantly higher velocity, the properties of HIPASS J1712-64 are similar to the recently recognised class of Compact High Velocity Clouds. We therefore consider the evidence for a Local Group or Galactic origin, although a more plausible alternative is that HIPASS J1712-64 was ejected from the interacting Magellanic Cloud/Galaxy system at perigalacticon ~ 2 x 10^8 yr ago.

Nuclear star formation in the hot-spot galaxy NGC2903

Abstract: We present high-resolution near-infrared imaging obtained using adaptive optics and HST/NICMOS, and ground-based spectroscopy of the hot-spot galaxy NGC2903. Our near-infrared resolution imaging enables us to resolve the infrared hot spots into individual young stellar clusters or groups of these. The spatial distribution of the stellar clusters is not coincident with that of the bright HII regions, as revealed by the HST/NICMOS Pa_alpha image. Overall, the circumnuclear star formation in NGC2903 shows a ring-like morphology with an approximate diameter of 625pc. The SF properties of the stellar clusters and HII regions have been studied using the photometric and spectroscopic information in conjunction with evolutionary synthesis models. The population of bright stellar clusters shows a very narrow range of ages, 4-7 X 10^6 yr after the peak of star formation, or absolute ages 6.5-9.5 X 10^6yr (for the assumed short-duration Gaussian bursts), and luminosities similar to the clusters found in the Antennae interacting galaxy. This population of young stellar clusters accounts for some 7-12% of the total stellar mass in the central 625pc of NGC2903. The HII regions in the ring of star formation have luminosities close to that of the super-giant HII region 30 Doradus, they are younger than the stellar clusters, and will probably evolve into bright infrared stellar clusters similar to those observed today. We find that the star formation efficiency in the central regions of NGC2903 is higher than in normal galaxies, approaching the lower end of infrared luminous galaxies.

Near-Infrared Line Imaging of the Circumnuclear Starburst Ring in NGC 7771

Abstract: We present high spatial resolution near-infrared broadband JHK images and, for the first time, Brγ 2.1661 μm and H2 1-0 S(1) 2.122 μm emission-line images of the circumnuclear star-forming ring (major axis diameter 7'' = 2 kpc) in the starburst galaxy NGC 7771. These data are used to investigate the morphology and extinction of the starburst ring and to study its star-forming properties and history by comparing the observed quantities with an evolutionary population synthesis model. The clumpy morphology of NGC 7771 varies strongly with wavelength, as a result of the combination of extinction (for which we derive an average value of AV = 2.8), emission from hot dust and red supergiants, and several stellar generations in the ring. Also, the ellipticity and the position angle of the ring depend on the wavelength. The starburst ring in NGC 7771 exhibits small Brγ equivalent widths. Assuming a constant star formation model with Mu = 100 M☉ results in very long lifetimes of the star-forming regions (up to 1 Gyr), in disagreement with the clumpy near-infrared morphology and the observed radio spectral index of NGC 7771. This situation is only slightly remedied by assuming a reduced upper mass cutoff (Mu = 30 M☉), resulting in ages between 8 and 180 Myr. We prefer an instantaneous star formation model with Mu = 100 M☉, which can explain the derived Brγ equivalent widths if a single starburst occurred 6-7 Myr ago. The main excitation mechanism of the molecular gas, based on the observed S(1)/Brγ ratio, appears to be excitation by UV radiation from hot young stars. We derive M 1900 M☉ for the mass of the excited H2.

Near-infrared line imaging of the circumnuclear starburst rings in the active galaxies NGC 1097 and NGC 6574

Abstract: We present high spatial resolution near-infrared broad-band JHKand Br 2.166 m and H2 1-0 S(1) 2.121 m emission line images of the circumnuclear star formation rings in the LINER/Seyfert 1 galaxy NGC 1097 and in the Seyfert 2 galaxy NGC 6574. We investigate the morphology, extinction, and the star formation properties and history of the rings, by comparing the observed properties with an evolutionary popu- lation synthesis model. The clumpy morphology in both galaxies varies strongly with wavelength, due to a combination of extinction, emission from hot dust and red supergiants, and the age of the stellar populations in the rings. The near-infrared and radio morpholo- gies are in general agreement, although there are differences in the detailed morphology. From the comparison of Br and H fluxes, we derive average extinctions toward the hot spots A V = 1.3 for NGC 1097 and AV = 2.1 for NGC 6574. The observed H2/Br ratios indicate that in both rings the main excitation mechanism of the molecular gas is UV radiation from hot young stars, while shocks can contribute only in a few regions. The starburst rings in both galaxies exhibit small Br equiv- alent widths. Assuming a constant star formation rate with Mu = 100 M results in extremely long ages (up to 1 Gyr), in dis- agreement with the morphology and the radio spectral index of the galaxies. This situation is only slightly remedied by a reduced upper mass cutoff (Mu =3 0 M ). We prefer a model of an instantaneous burst of star formation with Mu = 100 M occurring 6-7 Myr ago. Gaseous nuclear bars parallel to the stellar nuclear bar were detected in both galaxies, and we derive M 100 M for the mass of the excited nuclear H2 emission. Finally, we briefly discuss the connection between the rings, bars and the fuelling of nuclear activity.

Near-infrared line imaging of the starburst galaxies NGC 520, NGC 1614 and NGC 7714

Abstract: We present high spatial resolution (0.6 arcsec) near-infrared broad-band JHK images and Br_gamma 2.1661 micron and H_2 1-0 S(1) 2.122 micron emission line images of the nuclear regions in the interacting starburst galaxies NGC 520, NGC 1614 and NGC 7714. The near-infrared emission line and radio morphologies are in general agreement, although there are differences in details. In NGC 1614, we detect a nuclear double structure in Br_gamma, in agreement with the radio double structure. We derive average extinctions of A(K) = 0.41 and A(K) = 0.18 toward the nuclear regions of NGC 1614 and NGC 7714, respectively. For NGC 520, the extinction is much higher, A(K) = 1.2 - 1.6. The observed H_2/Br_gamma ratios indicate that the main excitation mechanism of the molecular gas is fluorescence by intense UV radiation from clusters of hot young stars, while shock excitation can be ruled out. The starburst regions in all galaxies exhibit small Br_gamma equivalent widths. Assuming a constant star formation model, even with a lowered upper mass cutoff of M_u = 30 M_o, results in rather old ages (10 - 40 Myr), in disagreement with the clumpy near-infrared morphologies. We prefer a model of an instantaneous burst of star formation with M_u = 100 M_o, occurring 6 - 7 Myr ago, in agreement with previous determinations and with the detection of W-R features in NGC 1614 and NGC 7714. Finally, we note a possible systematic difference in the amount of hot molecular gas between starburst and Seyfert galaxies.

Near-Infrared Line Imaging of Circumnuclear Star Formation in Nearby Active Galaxies

Abstract: We present J H K, Brγ and H2 1–0 S(1) imaging of the circumnuclear star forming rings in the active galaxies NGC 1097 and NGC 7771 Their clumpy morphology varies with wavelength, due to extinction, emission from hot dust and red supergiants, and several stellar generations The H2/Brγ ratios in the rings indicate the main excitation mechanism of the molecular gas to be UV radiation from hot young stars Both galaxies exhibit similar small Brγ equivalent widths To explain the observations, we prefer a model of an instantaneous starburst occurring 6–7 Myr ago, as opposed to a constant star forming rate model A gaseous nuclear bar aligned parallel to the stellar nuclear bar has been detected in NGC 1097, and we discuss the connection between the rings, bars and the fueling of nuclear activity