Showing papers on "Star formation published in 1985"

Thermal infrared and nonthermal radio: remarkable correlation in disks of galaxies

Abstract: A tight, linear correlation is established between the far-infrared flux measured by IRAS and the nonthermal radio flux density (at 1.4 GHz) from disks of spiral galaxies. This correlation defines a ratio of infrared to radio fluxes that is characteristic of star formation activity. Galaxies with nuclear starbursts seem to follow the correlation. If the far-infrared is reradiated luminosity from young massive stars, then the supernova remnants alone account for less than 10 percent of the radio emission. These results indicate a close coupling between dust heating and cosmic-ray generation and confinement in a wide range of conditions.

Protostars and planets II

Abstract: Various papers on the formation of stars and planets are presented. The general topics addressed include: molecular clouds and star formation, young stellar objects and circumstellar disks, chemistry and grains in molecular clouds and the solar nebula, formation of giant planets, and models of the solar nebula and other planetary systems. Individual papers address: molecular cloud cores, physical conditions in isolated dark globules, rotation in dark clouds, and turbulence in molecular clouds. Also discussed are: fragmentation and hierarchical structure in the interstellar medium, formation of bound stellar clusters, ambient radiation field of young solar systems, and magnetic fields.

Molecular gas at high galactic latitudes

Abstract: The present quantitative results of a CO survey of high latitude molecular gas shows 57 clouds in 35 complexes at absolute values of b not smaller than 25 deg. Maps of 33 clouds are presented, together with C-13O and CO(J = 2-1) observations. The clouds, 70 percent of which are associated with optical emission, are noted to be distributed asymmetrically with respect to b = 0 deg; this is consistent with a displacement of the sun of 30 pc above the midplane. Since the mean distance to the clouds is about 100 pc, they are the nearest clouds to the sun and may contain the nearest regions of star formation. 51 references.

The effects of interactions on spiral galaxies. I - Nuclear activity and star formation

Abstract: We present results of spectrophotometry of the nuclei of 161 (mostly spiral) galaxies with bright companions, and emission-line imaging of 63 galaxies. The program includes a complete sample, unbiased as to morphological disturbance, and an Arp-atlas sample selected for visual evidence of tidal distortion. When compared to a similarly observed sample of isolated-spiral nuclei, both samples of interacting galaxies show significant excesses of nuclear emission. Both nonthermal (Seyfert) activity and nuclear star formation are enhanced, while low-ionization (LINER) nuclei are uncommon. The rate of nuclear star formation is significantly above average even in systems without noticeable tidal distortion in the outer disks, suggesting that the near-nuclear gas is only marginally stable in isolated galaxies. There is evidence that Seyfert nuclei are more frequent in very close pairs. Very disrupted systems possess significantly fewer Seyfert nuclei. These effects become more pronounced with increasing galaxy luminosity. Sizes of nuclear emission regions appear to be unchanged by interactions. The enhancement in nuclear star formation depends only weakly on pair separation over the range we have sampled. Our results suggest that nuclear phenomena are triggered by tidally induced influx of gas from the disk into the nuclear regions, rather than gas transfer between themore » galaxies.« less

Recent star formation in interacting galaxies - II. Super starbursts in merging galaxies.

Abstract: Contrairement au cas des Seyfert, les proprietes radio, IR et optiques des objets resultant du fusionnement ne revelent pas de source centrale compacte non thermique, et peuvent etre interpretees grâce a un sursaut de formation stellaire d'intensite et d'etendue spatiale extraordinaires: ce sont les «super sursauts stellaires». On suggere qu'ils se produisent au cours de la plupart des fusionnements

Chemical evolution of the galactic disk with radial gas flows.

Abstract: Models are presented for the chemical evolution of the galactic disc with radial inflows of the gas at a velocity constant with time, but that may vary with the galactocentric radius. The models include the infall of metal-free gas from outside the disc at a rate decaying exponentially with time, a star formation rate that varies as the surface density of the gas layer to the power of m, with m = 1 or 2, and in some cases an outer edge to the stellar disc at the star formation cutoff radius. The models are compared with the observed age-metallicity relation and metallicity distribution for stars in the solar neighborhood and with the observed radial variations of the metallicity, the star formation rate, and the surface densities of the gas and stars. Inflow velocities of approximately -(0.5-1.0) km/s are expected.

Dynamical evolution in galactic disks

Abstract: The dynamical structure of a galactic disk is changed by fluctuations in its gravitational potential field. The orbits of stars are deflected, changing the velocity dispersion and angular-momentum distribution. We derive an expression for the induced changes in the phase-space distribution function, finding them to be second order in the perturbing potential, and of the form of a diffusion in phase space. We evaluate our result for a model transient spiral. The radial-velocity dispersion of the stars increases in response to a steady sequence of perturbations as t/sup 1/2/, for low velocity stars, declining towards t/sup 1/5/ as the radial excursions of stars become large in comparison to the spatial wavelength of the spirals. The waves also transport angular momentum, allowing the disk to become more centrally concentrated. In equilibrium the wave amplitude is controlled by the rate of dynamical cooling in the disk, and, inasmuch as the gas also gives rise to star formation, the heating time scale may be similar to the galactic star-formation time scale. Spiral waves may then rearrange the structure of the disk, as stars form out of the gas. Our predictions are in good agreement with the age/velocity-dispersion relation observed in our N-body experiments,more » and we conclude that recurrent transient spirals naturally account for that observed for the solar neighborhood stars.« less

CO detections and IRAS observations of bright radio spiral galaxies at cz equal or less than 9000 kilometers per second

Abstract: CO emission has been detected from 20 of 21 bright radio spirals with strong extended nuclear sources, including the most distant (NGC 7674) and the most luminous (IC 4553 = Arp 220, NGC 6240) galaxies yet detected in CO. All of these galaxies are rich in molecular gas, with M total(H2) = 3 x 10 to the 8th - 2 x 10 to the 10th solar masses. IRAS observations show that they have a strong far-infrared (FIR) excess, with L(FIR)/L(B) approximately equal to 1-35 and L(FIR) (40-400 microns) approximately equal to 10 to the 10th - 10 to the 12th L solar masses. The primary luminosity source for these radio cores appears to be star formation in molecular clouds. A strong correlation is found between the FIR and extended 21 cm continuum flux, implying that the fraction of massive stars formed is independent of the star formation rate. The ratio L(FIR)/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. At these rates their molecular gas will be depleted in about 10 to the 8th yr.

Shapley constellation III - a region of self-propagating star formation

Abstract: The H I dynamics and stellar content of the Shapley III/supergiant loop LMC 4 region are studied. There are three components, one associated with the disk H I with a mean velocity of 285 km/s with respect to the local standard of rest, and the other two with shells of gas ejected above and below the plane of the LMC with an expansion velocity of 36 km/s. The disk H I shows a ringlike structure, 1.8 kpc in diameter with a very pronounced central hole, and the H II regions form a ring 1.4 kpc in diameter. Star formation appears to have started approximately 15 Myr ago near the center and has propagated outwards at a constant speed of 36 km/s. Newly formed stars generate a local overpressure in the ISM which drives a shock in the intercloud medium, sweeping over and around dense H I clouds and so initiating new episodes of star formation. 54 references.

Star-forming properties and histories of dwarf irregular galaxies: down but not out

Abstract: Star formation processes and their relationships to other galactic properties are investigated in a sample of low optical surface brightness dwarf irregular galaxies. New observations made for this study include B and R images taken with a CCD detector; flux-calibrated, H..cap alpha.., narrow-band digital imagery; large-aperture spectrophotometry; and spectrophotometry of individual H II regions. These data provide the basis for several measurements of star-forming characteristics of dwarf irregular galaxies: (1) Total current star formation rates are deduced from H..cap alpha.. luminosities. (2) Properties of typical star-forming centers are derived from the sizes, luminosities, and surface brightness of H II regions. (3) Star formation histories are estimated from the ratio of H..cap alpha.. to blue luminosities and from the stellar content models. (40 The metallicity of the gas comes from oxygen emission lines. (5) Images provide information on the optical light distributions.

Differential population synthesis of early-type galaxies. III. Synthesis results

Abstract: Synthesis results are presented for 12 elliptical and five lenticular galaxies which cover nearly a 6 mag range of absolute magnitude in the Fornax cluster (V = 1430 km s ). The results of age, metallicity, and mass-to-light ratio (M/L) determinations are presented differentially as functions of galactic luminosity and morphology. Bright elliptical galaxies are well represented by main-sequence turnoff colors of (V-R)/sub c/roughly-equal0.37 and B-Vroughly-equal0.70, in good agreement with O'Connell's results for bright Virgo ellipticals. The relatively young main-sequence turnoff ages, of 6-10 Gyr for ellipticals of all luminosities, indicate that substantial star formation activity occurred in these galaxies for a period of 6dagger10 Gyr after the epoch of globular cluster formation. There is strong evidence for small amounts of current star formation in at least the brightest ellipticals of all luminosities, indicate that substantial star formation in at least the brightest ellipticals, variation in the amount of which may account for significant dispersion in the cluster U-V versus V color-magnitude relation. A metallicity gradient in (Fe/H) of at least 0.16 dex per absolute magnitude is derived, with the brightest ellipticals being 2 to 3 times more metal-rich than solar. The mean metallicities and turnoff ages of the faintestmore » ellipticals are probably dependent on environment. Distance-independent upper limits to galaxy M/L ratios derived from the syntheses conform well with M/L ratios derived from velocity dispersion measurements and give no evidence for unseen mass in the nuclei of early type galaxies of any luminosity.« less

A large, cold, and unusual molecular cloud in Monoceros

Abstract: Observations of the J = 1 tends to 0 rotational transition of CO near the galactic plane in Monoceros (1 is approximately equal to 216 deg) reveal a molecular cloud with unusually low peak CO temperatures (less than 2 K) but wide lines (about 7 km/s) typical of much warmer clouds. At the assumed distance of 3 kpc, the cloud is large (250 x 100 pc), has a mass of 7-11 x 10 to the 5th solar masses, and is well removed from the galactic midplane (130 pc). Except for a possible H II region, all the signs of star formation usually shown by clouds of comparable mass are missing. The cloud, unlike cloud complexes of similar size, is a single, continuous object that apparently has not been torn apart by star formation. Clouds with such properties are rare in the Galaxy; only one or two similar objects have been found. The possibility that the cloud is young and not yet forming stars but will evolve into a typical cloud complex once star formation begins is discussed.

The inner disk of NGC 253

Abstract: The stellar and interstellar matter distributions in the luminous Sc galaxy NGC 253 are analyzed using maps of the near-infrared continuum (λ = 1-2 μm) and the millimeter CO emission line. The stellar disk traced out in the near-infrared exhibits two components: a nuclear peak of diameter ~20" (~300 pc) and an extended, inner disk of diameter ~360" (~6 kpc). Similar components are also evident in the molecular gas distribution. The 2 μm brightness distribution mapped over the full extent of the inner disk is dominated by a barlike feature at position angle 17° east of the major axis extending to ± 120" on opposite sides of the nucleus. The CO distribution exhibits a weak secondary peak just outside the bar. The derived abundance of molecular gas in the inner disk exceeds that of H I by a factor of 10 and exhibits a much steeper falloff with radius. The total mass of H_2 inside R = 4 kpc is ~2 x 10^9 M_☉, which is ~7% of the dynamical mass at the same radius. If the observed far-infrared luminosity in the central R ≤ 500 pc is produced by young stars, the minimum rate of OBA star formation is 2.3 M_☉ yr^(-1), and the observed molecular clouds will be cycled into stars in ≾3 x 10^8 yr. It is speculated that the bar, detected in the near-infrared map, could feed fresh molecular gas from the outer disk to the active, star-forming nucleus.

On the formation rate of galactic clusters in clouds of various masses

Abstract: The formation rate of galactic clusters is estimated to be 2.5 +- 1 x 10/sup -7/ kpc/sup -2/ yr/sup -1/ from published tables of cluster positions and ages. OB associations form at about the same rate, so most clusters could form in giant molecular clouds, along with the associations. This result leads to a serious problem with the fate of star formation in low-mass clouds. The space density of low-mass clouds is so high, and the cluster formation rate is so low, that low-mass clouds must seldom form bound clusters even though they have enough mass to do so. The formation of a cluster depends on the efficiency of star formation in the cloud, and this efficiency depends on the relative mass of stars that have formed by the time a sufficiently luminous star destroys the cloud. Thus the formation of a bound cluster depends on the formation order of stars of various masses. We assume that this formation order is random and estimate the cluster formation probability for clouds of various masses with Monte Carlo simulations. Critical efficiencies for bound cluster formation are also calculated, including cloud support from magnetic fields. The results suggest that at least 10% ofmore » low-mass clouds should form bound clusters. Consequently, either the cluster formation time per low-mass cloud exceeds 10/sup 8/ yr, or such clouds avoid forming clusters even when their star formation efficiencies are high. If the formation order of stars is not random, but high-mass stars form after low-mass stars, then the probability of forming a bound cluster, and the mean formation time per low-mass cloud, are even larger. Implications of these results are discussed; in particular, the observed cluster luminosity function can be reproduced by the model.« less

Compact H II regions: hydrogen recombination and OH maser lines

Abstract: On a cartographie les raies de recombinaison de l'hydrogene a 15 et 22 GHz et l'emission maser OH a 1,6 GHz en direction de 9 regions HII compactes, a l'aide du VLA. On a trouve que tous les masers OH ont des vitesses radiales qui sont decalees vers le rouge par rapport aux vitesses des regions HII deduites des raies de recombinaison de l'hydrogene. Les masers OH doivent faire partie d'un reste d'enveloppe qui continue de s'effondrer vers l'etoile nouvellement formee

A law of star formation in disk galaxies: evidence for self-regulating feedback

Abstract: The assumption that the pressure in the interstellar medium is derived from energetic processes associated with star formation and with the older stellar population is shown to lead to a law of star formation which is applicable to all disk galaxies. This states that the rate of star formation per unit total mass is linearly related to the ratio of gas to total surface densities. The star formation rate decreases exponentially in a given galaxy, but the gas content declines to a finite limit. The solution is characterized by two natural time scales, a gas depletion time scale, tau sub 0, and an equipartition time scale, tau sub 1, which is the inverse of the specific rate of star formation at which the pressure produced by young stars in the interstellar medium matches the pressure generated by the older population. Real galaxies appear to obey this law of star formation and it is found that tau sub 0 = (2-4) x 10 to the 9th yr, tau sub 1/tau sub 0 = 0.005 (+0.002, -0.003) gives the best fit to the data. In galaxies in which the rate of star formation determines the pressure of the disk medium, it ismore » also shown that the rate of star formation is directly proportional to the product of the total mass and the mean surface density of gas. 43 references.« less

The molecular bar and star formation in the nucleus of NGC 6946.

Abstract: The bright, nearby galaxy NGC 6946 in the 2.6 mm line of CO has been observed with the Owens Valley Millimeter-Wave Interferometer, synthesizing a beam 7.6 arc sec by 5.4 arc sec. The molecular gas in the central 1.5 kpc of the galaxy is confined to a bar of extent of approximately 2700 pc by 550 pc, which is centered on the nucleus and connects two dust lanes in the innermost spiral arms. The molecular bar is the likely source of the star formation activity suggested by optical and infrared studies in the nucleus of NGC 6946. 34 references.

A very bright water vapour maser source in the galaxy NGC3079

Abstract: The 616–523 transition of water vapour at 22.235080 GHz has been found in our Galaxy to be associated with regions of young star formation1,2. H2O maser emission was detected initially in an external galaxy by Churchwell et al.3. To extend the range of detected properties of ‘star burst’ galaxies (galaxies showing an enhanced star formation from their optical spectra) we undertook an initial survey for H2O maser emission in this type of galaxy, but with negative results. Here we report on the subsequent extension of this survey to spiral galaxies displaying continuum radio emission, which is also an indication of star formation activity. We have detected the most luminous H2O maser source yet reported, ∼500 L0, in the galaxy NGC3079. OH absorption was also detected in this galaxy and we suggest that maser amplification of the nuclear radio source causes the observable H2O emission.

Star formation in the inner Galaxy - A far-infrared and radio study of two H II regions

Abstract: Far-infrared and radio continuum maps have been made of the central 6' of the inner-galaxy HII regions G30.8-0.0 (in the W43 complex) and G25.4-0.2, along with radio and molecular line measurements at selected positions. The purpose of this study is an effort to understand star formation in the molecular ring at 5 kpc in galactic radius. Measurements at several far infrared wavelengths allow the dust temperature structures and total far infrared fluxes to be determined. Comparison of the radio and infrared maps shows a close relationship between the ionized gas and the infrared-emitting material. There is evidence that parts of G30.8 are substantially affected by extinction, even at far-infrared wavelengths. Using radio recombination line and CO line data for G25.4-0.2, the distance ambiguity for this source is resolved. The large distance previously ascribed to the entire complex is found to apply to only one of the two main components. The confusion in distance determination is found to result from an extraordinary near-superposition of two bright HII regions. Using the revised distances of 4.3 kpc for G25.4SE and 12 kpc for G25.4NW, it is found that the latter, which is apparently the fainter of the two sources, is actually the more luminous. The ratio of total luminosity to ionizing luminosity is very similar to that of HII regions in the solar circle. Assuming a coeval population of ionizing stars, a normal initial mass function is indicated.

Enhanced star formation in cluster galaxies.

Abstract: We present two lines of evidence indicating that the interaction of late-type cluster galaxies with their environment results in enhanced star formation.

The W3 molecular cloud

Abstract: Extensive J = 1 to 0 (C-12)(O-16) and (C-13)(O-16) observations of the W3 molecular cloud and the surrounding region are presented and discussed. The velocity structure in the region is strongly suggestive of a model of large-scale, externally induced star formation. It is shown that star formation occurred in W3 and the nearby star-forming region W3(OH) after the gas within which they lie was swept up by the expanding W4 ionization front. Two condensations dominate the mass structure of the core of W3, one associated with IRS 4 and the other with IRS 5 and 1. A velocity difference between the two condensations is interpreted as indicating the two sources actually are discrete knots.

Galactic mergers, starburst galaxies, quasar activity and massive binary black holes

Abstract: Many quasar-like objects show evidence for massive binary black holes1. The recent discovery2 of a massive (5 × 106 M⊙) object in the centre of the local group dwarf elliptical M 32 greatly raises the probability of forming such binaries through galactic mergers. Here I argue that the enhancement of all kinds of activity (quasar-like activity and star formation) in galaxies with companions is not so much a consequence of tidal interaction between the massive galaxies as the result of collisions with their dwarf satellites.

Radio studies of the ionized gas in the nucleus of M82

Abstract: We report here VLA observations of the radio continuum and radio recombination lines from the nearby gas-rich galaxy M82. We have combined these data with other radio and IR data, both continuum and line, to gain further insight into the state of the interstellar gas and relativistic particles in M82. With the aid of a simple model for the distribution of the gas and synchrotron emissivity, we interpret a ''vertical'' spectral index gradient in M82 as a corresponding gradient in the age of the radiating particles. We infer that the relativistic particles are being transported out of the plane at a speed of about 2000 km s/sup -1/, possibly by a highly rarefied galactic wind. A detailed study of the radio recombination lines and the radio continuum shows that the ionized gas in M82 which is coextensive with the synchrotron emission in the central region has a maximum density of about 30 cm/sup -3/ and a volume filling factor near unity. The large filling factor indicates that the ionized gas occupies most of the volume within the nuclear region. The molecular gas in M82 is confined to small clouds which are in pressure equilibrium with the ionized gas. This configurationmore » could have resulted from the expansion of the numerous H II regions resulting from intense star formation in M82. A compact H II complex near the compact VLBI source 41.9+58 is evident in the VLA recombination line data. It is coincident in position and velocity with features seen in CO J = 1-0 and J = 2-1 lines, as well as in the 63 ..mu..m line of (O I). The possible nature of this complex is briefly discussed.« less

Active star formation in NGC 2264

Abstract: The region of NGC 2264 near the cone nebula is the site of active star formation in a rotating ring seen nearly edge on as a two lobed source. Allen's infrared source (IRS 1) surrounds a B3V star still embedded in the southern lobe of the cloud. The northern lobe, IRS 2, also probably contains young stars.

The young open cluster Stock 16: an example of star formation in an elephant trunk?

Abstract: On presente la photometrie UBV de 33 etoiles du jeune amas ouvert Stock 16 situe dans la region H II RCW 75; on en deduit de nouvelles valeurs du rougissement (E B−V =0,49±0,01) de la distance (1,90±0,08 kpc) et de l'âge (3-5×10 6 ans) de l'amas