Engineering topics
Publications and source records attributed to Thuan, T. X..
We have observed the Blue Compact Dwarf Galaxy VII Zw 403 with the Position Sensitive Proportional Counter (PSPC) camera onboard ROSAT. We found a total X-ray luminosity of 1.94 x 10(exp 38) erg/s distributed in a central core to which are connected three elongated structures. We interpret this X-ray morphology as the result of a hot gas outflow from the core of the dwarf galaxy powered by the present starburst.
We have obtained an 8.4 GHz Very Large Array (VLA) A-array map of the starburst galaxy M82 with a resolution Full Width at Half Maximum (FWHM) approximately 0.182 sec. About 50 compact radio sources in the central region of M82 were detected with a peak surface brightness approximately greater than 10(exp -17) W/Hz/sq m/sr. Comparison with previous observations shows that most sources are declining in flux. Three previously visible sources have faded into the background of our map (approximately less than 0.2 mJy/beam), while a few sources, including the second and third brightest radio sources in M82, may have increased slightly in flux over the last decade. No new radio supernova was found. The birth rate of the compact radio sources is estimated to be 0.11 + or - 0.05/yr. We attribute the population of such bright, small supernova remnants (SNRs) in M82 to the high pressure in the central region that can truncate the mass loss during a red supergiant phase or allow dense ionized clouds to be present. The compact radio sources obey a Sigma(radio surface brightness) - D(diameter) relation which is remarkably similar to that followed by supernova remnants in the Galaxy and the Magellanic Clouds and by two of the strongest known extragalactic radio supernovae: SN 1986J and SN 1979C. A least-squares fit to the SNR data gives: Sigma(sub 8.4 GHz) (W/Hz/sq m/sr) = 4.4 x 10(exp -16) D(sub pc)(exp -3.5 +/- 0.1) covering seven orders of magnitude in Sigma. Possible selection effects are discussed and a theoretical discussion of the correlation is presented.
IRAS flux densities are presented for all galaxies in the Center for Astrophysics magnitude-limited sample (mB not greater than 14.5) detected in the IRAS Faint Source Survey (FSS), a total of 1544 galaxies. The detection rate in the FSS is slightly larger than in the PSC for the long-wavelength 60- and 100-micron bands, but improves by a factor of about 3 or more for the short wavelength 12- and 25-micron bands. This optically selected sample consists of galaxies which are, on average, much less IR-active than galaxies in IR-selected samples. It possesses accurate and complete redshift, morphological, and magnitude information, along with observations at other wavelengths.
The 40 ultraluminous galaxies in the IRAS Bright Galaxy Sample of sources stronger than S = 5.24 Jy at lambda = 60 microns were mapped with approximately 0.25 arcsec resolution at 8.44 GHz. Twenty-five contain diffuse radio sources obeying the FIR-radio correlation; these are almost certainly starburst galaxies. Fourteen other galaxies have nearly blackbody FIR spectra with color temperatures between 60 and 80 K so their (unmeasured) FIR angular sizes must exceed approximately 0.25 arcsec, yet they contain compact (but usually resolved) radio sources smaller than this limit. The unique radio and FIR properties of these galaxies can be modeled by ultraluminous nuclear starbursts so dense that they 67 are optically thick to free-free absorption at about 1.49 GHz and dust absorption at about 25 microns. Only one galaxy (UGC 08058 = Mrk 231) is a dominated by a variable radio source too compact to be an ultraluminous starburst; it must be powered by a 'monster'.
Infrared Astronomical Satellite (IRAS) maps of the Magellanic Clouds smoothed to a 5-min angular resolution and young star catalogs are used to study the effects of stellar age and metallicity on the infrared emission of these galaxies. The construction of the stellar catalog is described, along with the conversion from the observational to the theoretical temperature-luminosity diagram and the derivation of stellar ages. The infrared color-color diagram of the Clouds is investigated, and the effect of stellar age on their infrared emission is investigated. The effects of the UV radiation field and of the metallicity on the infrared emission are also considered with emphasis on the influence of such parameters as gas-to-dust ratio on the infrared colors. It is shown that the far-infrared fluxes of individual pixels in the Clouds' maps follow the same behavior as the integrated emission of normal galaxies in the IRAS color-color diagram and that the precise position of a particular pixel depends on the age of the youngest star in that pixel.
1200-3200 A IUE spectra of the nucleus of NGC 4552 (M89) were obtained in order to investigate the nature of the strong 10 micron source in this Galaxy. There is a strong, extended UV source in NGC 4552 which has a spatial distribution nearly identical with that at optical wavelengths and is undoubedly stellar in origin. Its properties are consistent with the correlation between UV source strength and metallicity pointed out by Faber (1983). There is no evidence for a nonthermal point source in the UV. It appears unlikely that the 10 micron emission is from heated dust grains. Instead, it is believed the 10 micron radiation is nonthermal in origin, implying a remarkably small size of only 0.1 AU for this source.
B and R CCD and H I observations of the Virgo dwarf galaxy IC 3475 were obtained. The galaxy is very large, having a diameter of about 10 kpc; its hydrogen content is at least 60 times smaller than that of a normal dwarf irregular galaxy; and the galaxy shows a central bar structure with knots and inclusions scattered throughout its body but with the majority in the bar region. The results support the hypothesis that IC 3475 is a dwarf irregular galaxy that has been stripped of its neutral gas.
Studies of three different types of dwarf galaxies have been conducted, taking into account low surface brightness (LSB) dwarf irregular (dI) galaxies, dwarf elliptical (dE) galaxies, and blue compact dwarf galaxies (BCDGs). Near-infrared observations for a large sample of BCDGs have been reported by Thuan (1983). The present paper is concerned with complementary near-infrared observations for a large sample of LSB dI and dE galaxies, and, in addition, with a few additional BCDG observations. In a discussion of dwarf elliptical galaxies, attention is given to an infrared color-color diagram, optical-infrared colors and burst ages, and the UVK color plane. Low surface brightness dwarf irregular galaxies and blue compact dwarf galaxies are considered along with the possible evolutionary scenarios which may link LSB dIs, BCDGs, and dEs.
The high excitation emission line galaxies NGC 1667, NGC 5135, and IC 5135 show activity which, while similar to that of Seyfert 2 galaxies, is 1-2 orders of magnitude smaller. The line and continuum emission is in all three cases more extended than would correspond to a point source. In addition to showing such Seyfert-like characteristics as broad, low contrast wings in the Lyman-alpha profiles and unusual strengths in the emission lines of such high ionization species as He II, NGC 5135 and IC 5135 show starburst characteristics in their UV spectra, with P Cygni profiles that furnish direct evidence of the presence of very massive and hot O stars in their nuclei. It is suggested that they may be galaxies undergoing the transition from the starburst stage to the mini-Seyfert stage.
Einstein Observatory Imaging Proportional Counter X-ray observations of fields in SA 57, SA 68, and U Mi that possess deep optical and radio coverage have been conducted with a sensitivity comparable to that of the Einstein Observatory's medium sensitivity survey of serendipitous X-ray sources. The number of X-ray sources detected is consistent with a log N(greater than S) - log S slope of about -1.5. The fraction of active galactic nuclei in the X-ray sample is less than about 50 percent, with the rest being composed of clusters and normal galaxies, consistent with the medium sensitivity survey.
Radio, near IR, optical, and UV observations of I Zw 36 = Mrk 209 = Haro 29 are reported. The H I distribution shows a core-halo structure, the core containing half of the mass and showing systematic motions; the halo is diffuse and contains several H I clumps. The visible star formation region is associated with the core but is shifted slightly with respect to the H I peak column density; and the virial mass is 5 to 7 times the H I mass. Star formation models with an initial mass function of slope 1.5 (the Salpeter value being 1.35) and a burst age or duration of a few million years fit well the optical spectrophotometric measurements. The data also suggest that the column density of molecular hydrogen in I Zw 36 is 6 + or - 3 times that of the neutral hydrogen, about the right amount to account for the virial mass.
An IR photometric survey was performed of 36 blue compact dwarf galaxies (BCDG) where intense bursts of star formation have been observed. The survey covered the J, H, and K lines, with all readings taken at the level of a few mJy. Although the near-IR fluxes observed in the galaxies are due to K and M giants, the bursts have calculated ages of less than 50 million yr. However, the BCDG galaxies surveyed are not young, with the least chemically evolved galaxy observed, I Zw 18, featuring 50 pct of its stars formed prior to its last burst, but with a missing mass that is not accounted for by H I interferometric observations. It is concluded that the old stars must be more spatially extended than the young stars, and a mixture of OB stars with the K and M giants is projected as capable of displaying the colors observed. The star formation processes in the BCDG galaxies is defined as dependent on the total mass of the galaxies, with low mass galaxies having a high ratio of star formation, compared to their previous rates.
Results are presented for photographic and photoelectric photometry of the edge-on Sb galaxy NGC 4565. Major-axis, minor-axis, and perpendicular surface brightness profiles are determined, along with color gradients parallel and perpendicular to the plane of the galaxy. It is found that the galaxy's light can be deconvolved naturally into five components: (1) a starlike nucleus located at the center of the bulge and the edge of the dust lane; (2) a thin disk containing the gas and dust, the young OB stars, and the spiral arms; (3) a thick disk that may be a locally isothermal sheet of old Population I metal-rich stars; (4) a bulge that merges smoothly into the thick disk at a radial distance of about 2.9 kpc; and (5) a corona whose light dominates the perpendicular profiles from about 2.9 to at least 8.2 kpc above the galactic plane.
Peak optical H I absorption depths of 10% are found in a neutral hydrogen emission-absorption study of the active-nucleus Irr II galaxies NGC 520, 5363, and 5506. It is suggested that the absorption is due to a few standard clouds within several hundred parsecs of the nucleus, in the narrow-line emission region. The width of the absorption features is interpreted as being due to random motions of the gas within the absorbing clouds, in contrast to the large widths of the emission features that are due to the overall rotation of the galaxy.
Detailed photographic surface photometry is presented of the nine first brightest galaxies in poor clusters suspected of being cD galaxies. The surface photometry is put on an absolute scale, using multiaperture photoelectric photometry. Oemler (1976) and de Vaucouleurs (1948) laws are fitted to the profiles to derive structural parameters, and the structural properties of the galaxies are compared with those of the cD galaxies in rich clusters studied by Oemler (1976) and those of normal ellipticals studied by Kormendy (1977). It is found that (1) the profiles of first brightest galaxies are well fitted by an r to the 1/4 power law over a range of more than nine magnitudes and down to the faintest light levels observed and (2) structural parameters characterizing only the central part of first brightest galaxies in poor and rich clusters, and excluding the envelope component in the case of rich clusters, smoothly extend properties exhibited by normal ellipticals. These observations are consistent with the idea that the central part of first brightest galaxies in both poor and rich clusters is made up by the mergers of smaller galaxies.
HI observations are reported for a sample of 115 blue compact dwarf (M sub B greater than approximately -18) galaxies or 'extragalactic H II regions' chosen mostly from the objective prism surveys of Markarian (1967-1974) and Haro (1956), with a few objects from Zwicky (1971) and other investigators. Ninety-three galaxies are detected. H I profiles, neutral hydrogen masses, total masses, and all available optical data are given for the 115 galaxies in a consistent and homogeneous system and in a useful format for statistical studies. The data are used in a companion paper to study the stochastic mode of star formation in galaxies.
Two-color aperture photometry of the brightest galaxies in a complete sample of nearby Abell clusters is presented. The results are used to anchor the bright end of the Hubble diagram; essentially the entire formal error for this method is then due to the sample of distant clusters used. New determinations of the systematic trend of galaxy absolute magnitude with the cluster properties of richness and Bautz-Morgan type are derived. When these new results are combined with the Gunn and Oke (1975) data on high-redshift clusters, a formal value (without accounting for any evolution) of q sub 0 = -0.55 + or - 0.45 (1 standard deviations) is found.
The spectrum and absolute energy distribution of the quasar 3C 48 are analyzed, yielding the following results. The forbidden lines are blueshifted with respect to the permitted lines by 600 km/s. The Balmer lines can be decomposed into a narrow and a broad component, with the narrow component having the redshift of the forbidden lines. The Mg II profile fits well the broad components of H-beta and H-gamma along with the Fe II profiles, and the forbidden O II profile fits well the profiles of forbidden O III and higher-ionization forbidden lines. The spatial coverage in the broad-line region is large, ranging from 0.1 to 0.4, and the optical depth at the Lyman limit is much greater than unity; the electron density is greater than 10 million per cu cm. The forbidden-line region has a lower density (less than 1 million per cu cm) and is subdivided into a low-excitation (forbidden O II) and high-excitation (forbidden O III) zone. The helium abundance appears to be normal.