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Showing papers on "Star formation published in 1995"


Journal ArticleDOI
TL;DR: Galaxy harassment as discussed by the authors drives the morphological transformation of galaxies in clusters, provides fuel for quasars in subluminous hosts and leaves detectable debris arcs, and simulated images of harassed galaxies are strikingly similar to the distorted spirals in clusters at $z \sim 0.4$ observed by the Hubble Space Telescope.
Abstract: Disturbed spiral galaxies with high rates of star formation pervaded clusters of galaxies just a few billion years ago, but nearby clusters exclude spirals in favor of ellipticals. ``Galaxy harassment" (frequent high speed galaxy encounters) drives the morphological transformation of galaxies in clusters, provides fuel for quasars in subluminous hosts and leaves detectable debris arcs. Simulated images of harassed galaxies are strikingly similar to the distorted spirals in clusters at $z \sim 0.4$ observed by the Hubble Space Telescope.

1,226 citations


Journal ArticleDOI
TL;DR: In this article, the chemical evolution of 76 stable isotopes, from hydrogen to zinc, is calculated using the output from a grid of 60 Type II supernova models of varying mass (11 approx. less than M/solar mass) and metallicity.
Abstract: Using the output from a grid of 60 Type II supernova models (Woosley & Weaver 1995) of varying mass (11 approx. less than (M/solar mass) approx. less than 40) and metallicity (0, 10(exp -4), 0.01, and 1 solar metallicity), the chemical evolution of 76 stable isotopes, from hydrogen to zinc, is calculated. The chemical evolution calculation employs a simple dynamical model for the Galaxy (infall with a 4 Gyr e-folding timescale onto a exponential dsk and 1/r(exp 2) bulge), and standard evolution parameters, such as a Salpeter initial mass function and a quadratic Schmidt star formation rate. The theoretical results are compared in detail with observed stellar abundances in stars with metallicities in the range -3.0 approx. less than (Fe/H) approx. less than 0.0 dex. While our discussion focuses on the solar neighborhood where there are the most observations, the supernova rates, an intrinsically Galactic quality, are also discussed.

925 citations


Journal ArticleDOI
TL;DR: In this article, the results of an extensive grid of evolutionary synthesis models for populations of massive stars were presented, and the parameter space was chosen to correspond to conditions typically found in objects like giant H II regions, H II galaxies, blue compact dwarf galaxies, nuclear starbursts, and infrared luminous starburst galaxies.
Abstract: We present the results of an extensive grid of evolutionary synthesis models for populations of massive stars. The parameter space has been chosen to correspond to conditions typically found in objects like giant H II regions, H II galaxies, blue compact dwarf galaxies, nuclear starbursts, and infrared luminous starburst galaxies. The models are based on the most up-to-date input physics for the theory of stellar atmospheres, stellar winds, and stellar evolution. A population of massive stars is not only important in terms of its output of radiation but also via its deposition of mechanical energy. The output of radiative and mechanical luminosity is compared at various starburst epochs. In a supernova dominated instantaneous starburst, the mechanical luminosity can be as large as almost 10% of the total radiative luminosity. This occurs when most massive O stars have disappeared, and the synthetic spectrum in the optical and near-ultraviolet is dominated by B and A stars. During this epoch, the output of ionizing radiation below 912 A becomes very small, as indicated by a very large Lyman discontinuity and a very small ratio of ionizing over mechanical luminosity. We discuss the relevance of these results for the interpretation of starburst galaxies, active galactic nuclei, and the energetics of the interstellar medium.

678 citations


Journal ArticleDOI
TL;DR: In this article, high-resolution images of the disks of NGC 4038/4039 obtained with the Wide Field Camera of the Hubble Space Telescope (HST) are presented.
Abstract: New, high-resolution images of the disks of NGC 4038/4039 obtained with the Wide Field Camera of the Hubble Space Telescope (HST) are presented NGC 4038/4039, nicknamed The Antennae, is a prototypical example of a pair of colliding galaxies believed to be at an early stage of a merger Down to the limiting magnitude of V approximately 23 mag, the HST images reveal a population of over 700 blue pointlike objects within the disks The mean absolute magnitude of these objects is M(sub V) = -11 mag, with the brightest objects reaching M(sub V) approximately -15 Their mean apparent color indices ar U - V = -07 mag and V - 1 = 08 mag on the Johnson UVI passband system, while their mean indices corrected for internal reddening are (u - v)(sub 0) = -10 mag and (V - I(sub 0) = 05 Their mean effective radius, determined from slightly resolved images, is 18 pc (for H(sub 0) = 50 km/s /Mpc) Based on their luminosities and resolution, most of these objects cannot be individual stars, but are likely young compact star clusters The brighter ones are similar to the objects found in NGC 1275 and NGC 7252, which appear to be young globular clusters formed during recent galazy mergers Based on their U - V and V - I colors, the brightest, bluest clusters of NGC 4038/4039 appear to be less than 10 Myr old Most of these bright clusters are relatively tightly clustered themselves, with typically a dozen individual clusters belonging to a complex identified as a giant H II region from ground-based observations The cluster luminosity function (LF) is approximately a power law, phi(L)dL proportional to L(exp -178+/-005)dL, with no hint of a turnover at fainter magnitudes This power-law shape agrees with the LF of Magellanic Cloud clusters and Galactic open clusters, but differs from the LF of old globular cluster systems that is typically Gaussian with a Full Width at Half Maximum (FWHM) of approximately 3 mag Besides the blue clusters, we also find about a dozen extremely red objects with V - I greater than 30 The highest number density of these red objects is found in the SE quadrant, where star formation appears to be most recent We propose that these objects may be very young star clusters still embedded in their placental dust cocoons

549 citations


Journal ArticleDOI
TL;DR: In this article, the super star clusters (SSC) were found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area.
Abstract: Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies reveal them to be highly irregular, even after excluding compact sources (clusters and resolved stars). Most (7/9) are found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The brightest clusters, or super star clusters (SSC), are preferentially found at the very heart of starbursts. The size of the nearest SSCs are consistent with those of Galactic globular clusters. The luminosity function of SSCs is well represented by a power law with a slope alpha ~ -2. There is a strong correlation between the far infrared excess and the UV spectral slope. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust.

494 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the massive star content of Milky Way clusters and OB associations in order to answer three questions: (1) how coeval is star formation, (2) How constant is the initial mass function (IMF), and (3) What is the progenitor mass of Wolf-Rayet stars.
Abstract: We investigate the massive star content of Milky Way clusters and OB associations in order to answer three questions: (1) How coeval is star formation? (2) How constant is the initial mass function (IMF)? (3) What is the progenitor mass of Wolf-Rayet stars? Our sample includes NGC 6823/Vul OB1, NGC 6871/Cyg OB3, Berkeley 86/Cyg OB1, NGC 6983/Cyg OB1, NGC 7235, NGC 7380/Cep OB1, Cep OB5, IC 1805/Gas OB6, NGC 1893/Aug OB2, and NGC 2244/Mon OB2. Large-field CCD imaging and multiobject, fiber spectroscopy has resulted in UBV photometry for >14000 stars and new spectral types for approximate to 200 stars. These data are used to redetermine distances and reddenings for these regions and to help exclude probable nonmembers in constructing the H-R diagrams. We reanalyze comparable data previously published on Cyg OB2, Tr 14/16, and NGC 6611 and use all of these to paint a picture of star formation and to measure the IMFs. We find the following: (1) Most of the massive stars are born during a period Delta tau 7 M. A comparison with similarly studied OB associations in the Magellanic Clouds reveals no difference in IMF slope, and hence we conclude that starformation of massive stars in clusters proceeds independently of metallicity, at least between z = 0.02 and z = 0.002. The masses of the highest mass stars are approximately equal in the Milky Way, LMC, and SMC associations, contrary to the expectation that this value should vary by a factor of 3 over this metallicity range. We conclude that radiation pressure on grains must not limit the mass of the highest mass star that can form, in accord with the suggestion of Wolfire & Cassinelli that the mere existence of massive stars suggests that shocks or other mechanisms have disrupted grains in star-forming events. (3) The four Wolf-Rayet stars in our sample have come from stars more massive than 40 M.; one WC star and one late-type WN star each appear to have come from very massive (approximate to 100 M.) progenitors.

489 citations



Journal ArticleDOI
TL;DR: In this article, the super star clusters (SSC) were found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area.
Abstract: Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies reveal them to be highly irregular, even after excluding compact sources (clusters and resolved stars). Most (7/9) are found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The brightest clusters, or super star clusters (SSC), are preferentially found at the very heart of starbursts. The size of the nearest SSCs are consistent with those of Galactic globular clusters. The luminosity function of SSCs is well represented by a power law with a slope alpha ~ -2. There is a strong correlation between the far infrared excess and the UV spectral slope. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust.

393 citations


Journal ArticleDOI
TL;DR: In this article, an IR lunar occultation and direct imaging search for companions in the Ophiuchus star-forming region and update a similar search of the Taurus region is presented.
Abstract: We present an IR lunar occultation and direct imaging search for companions in the Ophiuchus star-forming region and update a similar search of the Taurus region. The search is sensitive to companions in the angular separation range 0.005-10 sec. In Ophiuchus, we surveyed 35 young star targets; this sample contains at least 10 binaries, two triples, and one quadruple. Ten of the companion stars are newly discovered. In Taurus, the survey now includes 47 systems among which there are at least 22 binaries and four triples. Only two companion stars are newly identified because there is strong overlap with prior work. All the triples and quadruple are hierarchical. The observed binary frequency in Ophiuchus, in the 3-1400 AU range of separations, is at least 1.1 +/- 0.3 that of the nearby solar-like stars. This value is a lower bound because we make no corrections for incompleteness. In Taurus, in the same range of separations, the observed binary frequency is at least 1.6 +/- 0.3 that of the nearby solar-like stars. This value extends Ghez et al.'s (1993) and Leinert's et al.'s (1993) determination of an excess binary frequency to 3 AU separation. We used the weak-line T Tauri star/T Tauri star (WT/TT) type and the K-L color index to distinguish between systems with and without inner disks. We find no convincing difference in the binary frequency or distribution of separations of the systems with and without inner disks. The 1.3 mm continuum emission of the single systems exceeds that of the multiples suggesting that their extensive outer disks are more massive. The specific angular momenta of the binaries overlap those of molecular cloud cores measured by Goodman et al. (1993).

383 citations


Journal ArticleDOI
TL;DR: In this paper, high-quality aperture synthesis observations of the neutral hydrogen distribution in a sample of five galactic systems believed to be involved in progressive stages of merging are presented, including Arp295, NGC4676, NCL4, NCC4, and NGC7252.
Abstract: We present high-quality aperture synthesis observations of the neutral hydrogen distribution in a sample of five galactic systems believed to be involved in progressive stages of merging: Arp295, NGC4676, NGC520, NGC3921, and NGC7252. These data are supplemented by wide-field images taken through a narrow band Halpha filter, and by deep R-band surface photometry. In the early stages, large amounts of HI exist within the galactic disks and star formation is widespread. The ionized gas emission often takes on the appearance of plumes and arcs emanating from the nuclear regions, which are presumably the sites of interaction induced starbursts. In the final stages there is little if any HI within the remnant bodies, and tidal material is seen moving inward. This suggests that these remnants will evolve into elliptical galaxies in their atomic gas contents as well as their photometric properties. However the observations of NGC520 reveal an extensive rotating gaseous disk, suggesting that perhaps some mergers will not destroy the atomic gas disks of the progenitors. Overall, large quantities of both gas and starlight are seen at large radii. Since this material evolves on very long time scales, it may leave observable signatures for many Gyr.

368 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reported the first results of an extensive new study of the Galactic center stellar cluster, which strongly supported the existence of a central dark mass of 3 × 106 M☉ (density ≥ 108.5 M≉ pc-3, M/L ≥ 10 M⌉/L ⌉) within 0.14 pc of the dynamic center.
Abstract: We report the first results of an extensive new study of the Galactic center stellar cluster. The central parsec is powered by a cluster of about two dozen luminous and helium-rich blue supergiants/Wolf-Rayet stars (Teff ~ 20,000-30,000 K) with ZAMS masses up to ~100 M☉. The most likely scenario for the formation of the massive stars is a small star formation burst between 3 × 106 and 7 × 106 years ago. In this scenario the Galactic center is presently in a short-lived, post-main-sequence "wind phase." In addition, there is evidence for another star formation event about 108 years ago, as well as for recently formed massive stars that may have been transported into the central core along with orbiting gas streamers. The radial velocity dispersion of 35 early- and late-type stars with distances of 1''-12'' from Sgr A* is 154 ± 19 km s-1. Our new results strongly favor the existence of a central dark mass of ~3 × 106 M☉ (density ≥ 108.5 M☉ pc-3, M/L ≥ 10 M☉/L☉) within 0.14 pc of the dynamic center.

Journal ArticleDOI
TL;DR: In this article, a model for the origin of nuclear activity in a quiescent disk galaxy is presented. But the model does not include star formation and, therefore, cannot determine the ultimate fate of the gas.
Abstract: Mergers between gas--rich disks and less--massive dwarf galaxies are studied using numerical simulation. As the orbit of a satellite decays through dynamical friction, the primary disk develops large-amplitude spirals in response to its tidal forcing. While these features arise in both the stars and the gas in the disk, the non--axisymmetric structures in the gas differ slightly from those in the stars. In particular, as a consequence of the formation of strong shocks in the gas and the effects of radiative cooling, the gas response tends to lead the stellar response, enabling the stars to strongly torque the gas. These torques deprive the gas of its angular momentum, forcing a significant fraction of it into the inner regions of the disk. The radial inflows induced by these mergers accumulate large quantities of interstellar gas in the nuclear regions of the host disks. In some cases, nearly half of all the gas initially distributed throughout the disk winds up in a dense ``cloud'' several hundred parsecs in extent. The models reported here do not include star formation and, so, we cannot determine the ultimate fate of the gas. Nevertheless, given the high densities in the nuclear gas, it is plausible to identify these concentrations of dense gas in the remnants with those accompanying intense starbursts in some active galaxies. Therefore, the calculations here provide a framework for interpreting the origin of nuclear activity in otherwise quiescent disk galaxies. To the extent that galaxy formation is a chaotic process in which large structures are built up by the accretion of smaller fragments, our models may also be relevant to starbursts and the onset of nuclear activity in proto--galaxies at high redshifts.

Journal ArticleDOI
TL;DR: In this paper, the bolometric luminosity-temperature (BLT) diagram was constructed for 128 young stellar objects (YSOs) in Taurus, 74 in the Ophiuchus 'core', and 33 in the off-core' region, and the authors found that T(sub bol) correlates reasonably well with the age inferred from the evolutionary models of pre-main-sequence stars and protostars for embedded 'protostars' and weak-line T Tauri stars.
Abstract: We calculated bolometric temperature (T(sub bol)) and luminosity (L(sub bol)) for 128 young stellar objects (YSOs) in Taurus, 74 in the Ophiuchus 'core', and 33 in the Ophiuchus 'off-core' region. We have constructed the bolometric luminosity-temperature (BLT) diagram, the log-log plot of L(sub bol) versus T(sub bol), for the three samples. T(sub bol) is defined as the temperature of a blackbody having the same frequency as the observed continuum spectrum. It measures the redness (or coldness) of an astronomical source. The BLT diagram is analogous to the H-R diagram and allows for a direct and quantitative comparison of YSOs at a wide variety of evolutionary states, ranging from the most deeply embedded stars to T Tauri stars nearly on the main sequence. We found (1) T(sub bol) increases monotonically from embedded sources (approximately 60-500 K) to classical T Tauri stars (approximately 1000-3000 K) to weak-line T Tauri stars (approximately 2000-5000 K); (2) T(sub bol) correlates reasonably well with the age inferred from the evolutionary models of pre-main-sequence stars and protostars for embedded 'protostars' and weak-line T Tauri stars. There is no significant correlation for the classical T Tauri stars. These results can be understood in terms of dissipation of circumstellar dust envelope and disk during the early stages of stellar evolution. Sources in the three regions have different distributions in the BLT diagram. The Ophiuchus core has the highest fraction of cold sources among the three regions. These cold sources are also more luminous than the YSOs in the other regions. The Ophiuchus off-core sample is dominated by the more evolved pre-main-sequence stars. The Taurus sources have distributions intermediate in L(sub bol), T(sub bol), and age between the Ophiuchus core and off-core distributions. These may suggest differences in the star formation history, and possibly in the stellar masses and mass accretion rates in these star-forming regions.

Journal ArticleDOI
TL;DR: In this article, a Schechter function was used to estimate the Hα luminosity function for the star-forming galaxies in the surveyed volume of the universe, which is the first observational determination of this quantity, which will provide a direct test of current galaxy formation and evolution models.
Abstract: The Universidad Complutense de Madrid (UCM) survey is a long-term project aimed at finding and analyzing star-forming galaxies detected by their Hα emission in Schmidt objective-prism plates The instrumental setup limits the volume of the universe surveyed to a redshift z ≲ 0045 So far we have discovered several hundred emission-line galaxies (ELGs) undergoing star formation at a wide range of levels Analyzing a complete sample of ELGs from the UCM survey, we have computed the Hα luminosity function for the star-forming galaxies in the surveyed volume of the universe A Schechter function provides a good fit to the H_α luminosity function with the following parameters: α = -13 ± 02, L^*(H_α) = 10^4215±008 ergs s^-1, and φ^* = 10^-32±02 Mpc^3 for H_0 = 50 km s^-1 Mpc^-1 Integrating over the full range of luminosities, we obtain an H_α luminosity density of 10^391±02 ergs s^-1 Mpc^-3 Using the H_α emission as a star formation rate (SFR) estimator, this translates into a SFR density for the local universe of 0013_−0005^+0007 M_☉ yr^-1 Mpc^-3 in star-forming galaxies with EW(H_α + [N_II]) > 10 A and z lesssim 0045 for a Scalo initial mass function This is the first observational determination of this quantity, which will provide a direct test of current galaxy formation and evolution models

Journal ArticleDOI
TL;DR: In this article, the authors investigate the massive star population of the Magellanic Clouds with an emphasis on the field population, which they define as stars located further from any OB association than massive stars are likely to travel during their short lifetimes.
Abstract: We investigate the massive star population of the Magellanic Clouds with an emphasis on the field population, which we define as stars located further from any OB association than massive stars are likely to travel during their short lifetimes. The field stars must have been born as part of more modest star-forming events than those that have populated the large OB associations found throughout the Clouds. We use new and existing data to answer the following questions: Does the field produce stars as massive as those found in associations? Is the initial mass function (IMF) of these field massive stars the same as those of large OB complexes? How well do the Geneva low-metallicity evolutionary models reproduce what is seen in the field population, with its mixed ages? To address these issues we begin by updating existing catalogs of LMC and SMC members with our own new spectral types and derive H-R diagrams (HRDs) of 1584 LMC and 512 SMC stars. We use new photometry and spectroscopy of selected regions in order to determine the incompleteness corrections of the catalogs as a function of mass and find that we can reliably correct the number of stars in our HRDs down to 25 M.. Using these data, we derive distance moduli for the Clouds via spectroscopic parallax, finding values of 18.4 +/- 0.1 and 19.1 +/- 0.3 for the LMC and SMC. The average reddening of the field stars is small: E(B - V) = 0.13 (LMC) and 0.09 (SMC), with little spread. We find that the field does produce stars as massive as any found in associations, with stars as massive as 85 M. present in the HRD even when safeguards against the inclusion of runaway stars are included. However, such massive stars are much less likely to be produced in the field (relative to lower mass stars) than in large OB complexes: the slope of the IMF of the field stars is very steep, GAMMA = -4.1 +/- 0.2 (LMC) and GAMMA = -3.7 +/- 0.5 (SMC). These may be compared with GAMMA = -1.3 +/- 0.3, which we rederive for the Magellanic Cloud associations. (We compare our association IMFs with the somewhat different results recently derived by Hill et al. and demonstrate that the latter suffer from systematic effects due to the lack of spectroscopy.) Our reanalysis of the Garmany et al. data reveals that the Galactic field population has a similarly steep slope, with GAMMA = -3.4 +/- 1.3, compared to GAMMA = -1.5 +/- 0.2 for the entire Galactic sample. We do not see any difference in the IMFs of associations in the Milky Way, LMC, and SMC. We find that the low metallicity evolutionary tracks and isochrones do an excellent job of reproducing the distribution of stars in the HRD at higher masses, and in particular match the width of the main-sequence well. There may or may not be an absence of massive stars with ages less than 2 Myr in the Magellanic Clouds, as others have found for Galactic stars; our reddening data renders unlikely the suggestion that such an absence (if real) would be due to the length of time it takes for a massive star to emerge. There is an increasing discrepancy between the theoretical ZAMS and the blue edge of the main-sequence at lower luminosities; this may reflect a metallicity dependence for the intrinsic colors of stars of early B and later beyond that predicted by model atmospheres, or it may be that the low metallicity ZAMS is misplaced to higher temperatures. Finally, we use the relative number of field main-sequence and Wolf-Rayet stars to provide a selection-free determination of what mass progenitors become WR stars in the Magellanic Clouds. Our data suggest that stars with initial masses > 30 M. evolve to a WR phase in the LMC; while the statistics are considerably less certain for the SMC, they are consistent with this limit being modestly higher there, possibly 50 M., in qualitative agreement with modern evolutionary calculations.

Journal ArticleDOI
TL;DR: In this paper, the authors investigate the hypothesis that stars form in aggregates of binary systems and that the dynamical evolution of these aggregates leads to the observed properties of binary stars in the Galactic field.
Abstract: We investigate the hypothesis that stars form in aggregates of binary systems and that the dynamical evolution of these aggregates leads to the observed properties of binary stars in the Galactic field We assume that the initial distribution of periods is flat in logP, where P is the orbital period in days, and 3 75 After the cluster with R=08pc disintegrates we obtain a population which consists of about 60 per cent binary systems with a period distribution for logP>4 as is observed and in which the G-dwarf binaries have a mass ratio distribution which agrees with the observed distribution This result indicates that the majority of Galactic field stars may originate from a clustered star formation mode We invert the orbit depletion function and obtain an approximation to the initial binary star period distribution for star formation in the dominant mode cluster Comparison with the measured distribution of orbits for pre-main sequence stars suggests that the initial distribution may not depend on the star formation environment Inverse dynamical population synthesis suggests that the Galactic field stellar mass function may be related to the stellar density at birth in the most common, or dominant, mode of star formation



Journal ArticleDOI
TL;DR: In this article, the authors review the observations regarding the angular momentum of molecular cloud cores, of proto stars, and of the disks, binary orbits, and spin of young stars.
Abstract: The effects of angular momentum play an important role in star formation. However, when a young star emerges on the Hertzsprung-Russell diagram it is generally rotating slowly. This article reviews the observations regarding the angular momentum of molecular cloud cores, of proto stars, and of the disks, binary orbits, and spin of young stars. Various physical processes for transfer of angular momentum during the star formation process are discussed, including magnetic braking during the molecular cloud stage; fragmentation during the protostar collapse; viscosity, magnetic torques, and gravitational torques in disks; and magnetically driven mass loss in a star-disk system. Important problems for future study are highlighted.

Journal ArticleDOI
TL;DR: In this article, high-resolution observations of the central region in the late-type spiral galaxy M100 (NGC 4321) supplemented by three-dimensional numerical modeling of stellar and gas dynamics, including star formation, are presented.
Abstract: We present new high-resolution observations of the central region in the late-type spiral galaxy M100 (NGC 4321) supplemented by three-dimensional numerical modeling of stellar and gas dynamics, including star formation (SF). Near-infrared imaging has revealed a small bulge of 4'' effective diameter; a 60'' radial length stellar bar of moderate strength, previously inferred from optical and 21 cm observations; and an ovally shaped, ringlike structure in the plane of the disk between 10''-22'' from the center, whose major axis makes a large angle with the bar. The K isophotes become progressively elongated and skewed to the position angle of the bar both outside and inside the ''ring,'' forming an inner barlike region. The galaxy exhibits a mild circumnuclear starburst concentrated in the inner part of the K ''ring.'' This SF is prominent in Ha and the U, B, and V bands, forming an incomplete ring. In addition, two strong local maxima of K emission have been observed to lie remarkably symmetrically with respect to the galactic nucleus and equidistant from it at 7.''5, slightly leading the stellar bar. CO molecular emission is peaked in the dust lanes seen in the I - K color index image. We interpret the twists in K isophotes and the swinging of spiral arms through similar to 360 degrees inside the corotation radius as being indicative of the presence of a double inner Lindblad resonance (ILR) and test this hypothesis by modeling the gas flow in a self-consistent gas+stars disk embedded in a halo, with an overall NGC 4321-like mass distribution in the system. Both ILRs have been verified using nonlinear orbit analysis by determining the spatial extent of the family of orbits oriented along the minor axis of the bar. We have reproduced the basic morphology of the region inside corotation, including (1) the similar to 60 degrees bar; (2) the large-scale trailing shocks representing the offset dust lanes in the bar; (3) two symmetric K peaks corresponding to gas compression maxima which lie at the caustic formed by the interaction of a pair of trailing and leading shocks in the vicinity of the inner ILR, both peaks being sites of SF; and (4) two additional zones of SF corresponding to gas compression maxima at the bar's minor axis, where the large-scale shocks start to curl and which are referred in the literature as ''twin peaks.'' We argue further that the twisting of K isophotes in the neighborhood of the resonance region requires a population of red stars which are dynamically young and follow gas rather than stellar orbits, i.e., red supergiants. At the same time, a substantial contribution from OB stars to the K light is expected within the inner kiloparsec and especially in the symmetric K peaks. We also conclude that NGC 4321 hosts a single stellar bar which fuels the starburst activity within the circumnuclear ''ring'' by channeling gas there at the median rate of similar to 0.1-1 Mo yr(-1).


Journal ArticleDOI
TL;DR: In this paper, a detailed multi-wave-band search for supernova-driven mass outflows in galaxies with extensive recent or ongoing centrally concentrated star formation was conducted, and the first results of this search were reported in the present paper.
Abstract: Dwarf galaxies play a crucial role in our understanding of the formation and evolution of galaxies, and the concept of supernova-driven mass outflows is a vital ingredient in theories of the structure and evolution of dwarf galaxies. Despite the theoretical importance of these outflows, there is a very limited amount of direct observational evidence for their existence. We have therefore begun a detailed multi-wave-band search for outflows in dwarf (M(sub B) greater than or = -18) galaxies with extensive recent or ongoing centrally concentrated star formation. We report the first results of this search in the present paper. Observations of the ionized gas in dwarf amorphous galaxies with centrally concentrated populations of massive stars provide evidence for the large-scale expansion of their expansion of their ionized interstellar media. Fabry-Perot H alpha images reveal the presence of kiloparsec-scale 'superbubbles' and filaments which tend to be oriented along the galaxy minor axis. These structures are comparable in size to the chracteristic optical sizes of the galaxies, and dominate the morphology of the galaxies at low surface brightness in H alpha. Since expanding structure of this size and velocity are not observed in all low-mass galaxies with recent or ongoing star formation, we suggest that we are witnessing transient events that likely have a relatively low 'duty cycle' in such galaxies. That is, we argue that the particular galaxies in the present paper have had significantly elevated star formation rates over the past 10(exp 7)-10(exp 8) yr (i.e., these are starburst or young poststarburst systems). This interpretation is consistent with the optical colors and emission-line properties of these galaxies.

Journal ArticleDOI
TL;DR: It is suggested that grain-grain collisions induced by velocity shear zones surrounding the outflow axes transiently desorb the grain mantles, resulting in large abundance enhancements of selected species.
Abstract: Molecular line surveys and fully sampled spectral line maps at 1.3 and 0.87 mm are used to examine the physical and chemical characteristics of the extreme Class I sources IRAS 4A and 4B in the L1450/NGC 1333 molecular cloud complex. A very well collimated, jetlike molecular outflow emanates from IRAS 4A, with a dynamical age of a few thousand years. Symmetric, clumpy structure along the outflow lobes suggests that there is considerable variability in the mass-loss rate or wind velocity even at this young age. Molecular emission lines toward IRAS 4A and 4B are observed to be weak in the velocity range corresponding to quiescent material surrounding the young stellar objects (YSOs). Depletion factors of 10-20 are observed for αll molecules, including CO, even for very conservative mass estimates from the measured millimeter and submillimeter dust continuum. However, abundances scaled with respect to CO are similar to other dark molecular cloud cores. Such depletions could be mimicked by high dust optical depths or increased grain emissivities at the observing frequencies of 230 and 345 GHz, but the millimeter and submillimeter spectral energy distributions suggest that this is unlikely over the single-dish size scales of 5000-10,000 AU. Dense, outflowing gas is found to be kinematically, but not spatially, distinct from the quiescent material on these size scales. If CO is used as a chemical standard for the high-velocity gas, we find substantial enhancements in the abundances of several molecules in outflowing material, most notably CS, SiO, and CH_30H. The SiO emission is kinematically well displaced from the bulk cloud velocity and likely arises from directly shocked material. As is the case for CO, however, the outflow features from more volatile species are centered near the cloud velocity and are often characterized by quite low rotational temperatures. We suggest that grain-grain collisions induced by velocity shear zones surrounding the outflow axes transiently desorb the grain mantles, resulting in large abundance enhancements of selected species. Similar results have recently been obtained in several other low-mass YSOs, where the outflowing gas is often both kinematically and spatially distinct, and are illustrative of the ability of accretion and outflow processes to simultaneously modify the composition of the gas and dust surrounding young stars.

Journal ArticleDOI
TL;DR: In this article, the authors present high-resolution observations of the center of the late-type spiral M100 (NGC 4321) supplemented by 3D numerical modeling of stellar and gas dynamics, including star formation.
Abstract: We present new high-resolution observations of the center of the late-type spiral M100 (NGC 4321) supplemented by 3D numerical modeling of stellar and gas dynamics, including star formation (SF). NIR imaging has revealed a stellar bar, previously inferred from optical and 21 cm observations, and an ovally-shaped ring-like structure in the plane of the disk. The K isophotes become progressively elongated and skewed to the position angle of the bar (outside and inside the `ring') forming an inner bar-like region. The galaxy exhibits a circumnuclear starburst in the inner part of the K `ring'. Two maxima of the K emission have been observed to lie symmetrically with respect to the nucleus and equidistant from it slightly leading the stellar bar. We interpret the twists in the K isophotes as being indicative of the presence of a double inner Lindblad resonance (ILR) and test this hypothesis by modeling the gas flow in a self-consistent gas + stars disk embedded in a halo, with an overall NGC4321-like mass distribution. We have reproduced the basic morphology of the region (the bar, the large scale trailing shocks, two symmetric K peaks corresponding to gas compression maxima which lie at the caustic formed by the interaction of a pair of trailing and leading shocks in the vicinity of the inner ILR, both peaks being sites of SF, and two additional zones of SF corresponding to the gas compression maxima, referred usually as `twin peaks').


Journal ArticleDOI
TL;DR: In this article, the internal structure and kinematics of disk galaxies formed in cosmologically motivated simulations were analyzed, including dark matter, gas dynamics, radiative cooling, star formation, supernova feedback and metal enrichment.
Abstract: We present results concerning the internal structure and kinematics of disk galaxies formed in cosmologically motivated simulations. The calculations include dark matter, gas dynamics, radiative cooling, star formation, supernova feedback and metal enrichment. The initial model is a rigidly rotating overdense sphere with a mass of about 8 10^11 Msol which is perturbed by small scale fluctuations according to a biased CDM power spectrum. Converging, Jeans unstable and rapidly cooling regions are allowed to form stars. Via supernovae, metal enriched gas is returned to the interstellar medium. {}From these initial conditions a galaxy forms which shows the main properties of spiral galaxies: a rotationally supported exponential disk which consists of young stars with about solar metallicity, a slowly rotating halo of old metal poor stars, a bulge of old metal rich stars and a slowly rotating extended halo of dark matter. Bulge, stellar and dark halo are supported by an anisotropic velocity dispersion and have a de Vaucouleurs surface density profile. The flattening of the dark and stellar halo is too large to be explained by rotation only. Whether the flattening of the bulge is caused by an anisotropic velocity dispersion or by its rotation cannot be answered, because of the limited numerical resolution due to gravitational softening. The velocity dispersion and the thickness of the stellar disk increase with the age of the stars. Considering only the young stellar component, the disk is cold (sigma=20 km/sec) and thin (z <1 kpc). The dynamical formation process ends after about 4\,Gyr, when


Journal ArticleDOI
TL;DR: In this article, the authors used a combination of infrared and blue photometry to identify a universal Seyfert nuclear continuum from near-to-far-infrared wavelengths and derive the first bolometric luminosities for a large sample of galaxies.
Abstract: Author(s): Spinoglio, L; Malkan, MA; Rush, B; Carrasco, L; Recillas-Cruz, E | Abstract: Aperture photometry from our own observations and the literature is presented for the 12 μm galaxies in the near-infrared J, H, and K bands and, in some cases, in the L band. These data are corrected to "total" near-infrared magnitudes (with a typical uncertainty of 0.3 mag) for a direct comparison with our IRAS fluxes which apply to the entire galaxy. The corrected data are used to derive integrated total near-infrared and far-infrared luminosities. We then combine these with blue photometry and an estimate of the flux contribution from cold dust at wavelengths longward of 100 μm to derive the first bolometric luminosities for a large sample of galaxies. The presence of nonstellar radiation at 2-3 μm correlates very well with nonstellar IRAS colors. This enables us to identify a universal Seyfert nuclear continuum from near- to far-infrared wavelengths. Thus, there is a sequence of infrared colors which runs from a pure "normal galaxy" to a pure Seyfert/quasar nucleus. Seyfert 2 galaxies fall close to this same sequence, although only a few extreme narrow-line Seyfert galaxies have quasar-like colors, and these show strong evidence of harboring an obscured broad-line region. A corollary is that the host galaxies of Seyfert nuclei have normal near- to far-infrared spectra on average. Starburst galaxies lie significantly off the sequence, having a relative excess of 60 μm emission probably as a result of stochastically heated dust grains. We use these correlations to identify several combinations of infrared colors which discriminate between Seyfert 1 and 2 galaxies, LINERs, and ultraluminous starbursts. In the infrared, Seyfert 2 galaxies are much more like Seyfert Is than they are like starbursts, presumably because both kinds of Seyferts are heated by a single central source, rather than a distributed region of star formation. Moreover, combining the [25-2.2 μm] color with the [60-12 μm] color, it appears that Seyfert 1 galaxies are segregated from Seyfert 2 galaxies and starburst galaxies in a well-defined region characterized by the hottest colors, corresponding to the flattest spectral slopes. Virtually no Seyfert 2 galaxy is present in such a region. To reconcile this with the "unified scheme" for Seyfert 1 and 2 galaxies would therefore require that the higher frequency radiation from the nuclei of Seyfert 2 galaxies to be absorbed by intervening dust and reemitted at lower frequencies. We find that bolometric luminosity is most closely proportional to 12 μm luminosity. The 60 and 25 μm luminosities rise faster than linearly with bolometric luminosity, while the optical flux rises less than linearly with bolometric luminosity. This result is a confirmation of the observation that more luminous disk galaxies have relatively more dust-enshrouded stars. Increases in the dust content shifts luminosity from the optical to 25-60 μm, while leaving a "pivot point" in the mid-IR essentially unchanged. Thus, 12 μm selection is the closest available approximation to selection by a limiting bolometric flux, which is approximately 14 times vLv at 12 μm for non-Seyfert galaxies. It follows that future deep surveys in the mid-infrared, at wavelengths of 8-12 μm, will simultaneously provide complete samples to different bolometric flux levels of normal and active galaxies, which will not suffer the strong selection effects present both in the optical-UV and far-infrared.