Star Burst Research Articles

CO Observations of High-z Objects

We have made a CO(J=2-1) observations using the Nobeyama 45m telescope aimed at examining the physical properties of the molecular gas in this object. The upper limit obtained is 1.8 mK (3σ) at a velocity resolution of 100 km s−1, which leads to an upper limit on the molecular gas mass of 5.3 × 1011M⊙, if we assume a line width of 250 km s−1 obtained in the CO(J = 5 - 4) line (rest-frame) and the Galactic CO-to-H2 conversion factor of 4.5 (M⊙ K km s−1 pc2). The line ratio between the 2–1 line and the 5–4 line as well as those from the 7–6 and the 4–3 lines (Omont et al. 1996, Nature, 382, 428) imply that the mean gas density is as high as 103–5 cm−3, which is comparable to that in nearby star burst galaxies (e.g., Solomon et al. 1992, ApJ, 387, L55).

Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities?

Aperiodic optical variability is a common property of active galactic nuclei (AGNs), though its physical origin is still open to question. To study the origin of the optical-ultraviolet variability in AGNs, we compare light curves of two models to observations of quasar 0957+561 in terms of a structure function analysis. In the starburst (SB) model, random superposition of supernovae in the nuclear starburst region produces aperiodic luminosity variations, while in the disk-instability (DI) model, variability is caused by instabilities in the accretion disk around a supermassive black hole. We calculate fluctuating light curves and structure functions, V(τ), by simple Monte Carlo simulations on the basis of the two models. Each resultant V(τ) possesses a power-law portion, [V(τ)]1/2 ∝ τβ, at short time lags (τ). The two models can be distinguished by the logarithmic slope β; β ~ 0.74-0.90 in the SB model and β ~ 0.41-0.49 in the DI model, while the observed light curves exhibit β ~ 0.35. Therefore, we conclude that the DI model is favored over the SB model in explaining the slopes of the observational structure function in the case of 0957+561, though this object is a radio-loud object and thus is not really a fair test for the SB model. In addition, we examine the time asymmetry of the light curves by calculating V(τ) separately for the brightening and the decaying phases. The two models exhibit opposite trends of time asymmetry to some extent, although the present observation is not long enough to test this prediction.

Pre-enriched, not primordial ellipticals

We follow the chemical evolution of a galaxy through star formation and its feedback into the inter-stellar medium (ISM), starting from primordial gas and allowing for gas to inflow into the region being modelled. We attempt to reproduce observed spectral line-strenghts for early-type galaxies to constrain their star formation histories (SFH). The efficiencies and times of star formation are varied as wellas the amount and duration of inflow. We evaluate the chemical enrichment and the mass of stars made with time. Single stellar population (SSP) data are then used to predict line-strengths for composite stellar populations. The results are compared with observed line-strengths in ten ellipticals, including some features which help to break the problem of agemetallicity degeneracy in old stellar populations. We find that the elliptical galaxies modelled require high metallicity SSPs (> 3 Z⊙) at later times. In addition the strong lines observed cannot be produced by an initial starburst in primordial gas, even if a large amount of inflow is allowed for during the first few ×10 8 years. This is because some pre-enrichment is required for lines in the bulk of the stars to approach the observed line-strengths in ellipticals. These strong lines are better modelled by a system with a delayed burst of star formation, following an early SFH which can be a burst or more steady star formation. Such a model is representative of star formation in normal ellipticals or spirals respectively, followed by a starburst and gas inflow during a merger or strong interaction with a gas-rich galaxy. Alternatively, a single initial burst of normal stars with a Salpeter initial mass function could produce the observed strong lines if it followed some pre-enrichment process which did not form long-lived stars (e.g. population III stars).

A Pair of Lensed Galaxies at [CLC][ITAL]z[/ITAL][/CLC] = 4.92 in the Field of CL 1358+62

The cluster CL1358+62 displays a prominent red arc in WFPC2 images obtained with the Hubble Space Telescope. Keck spectra of the arc show Ly alpha emission at 7204 Angstrom, a continuum drop blueward of the line, and several absorption lines to the red. We identify the arc as a gravitationally lensed galaxy at a redshift of z=4.92. It is the highest redshift object currently known. A gravitational lens model was used to reconstruct images of the high-redshift galaxy. The reconstructed image is asymmetric, containing a bright knot and a patch of extended emission 0.4 arcsec from the knot. The effective radius of the bright knot is 0.022 arcsec or 130 h^-1 pc. The extended patch is partially resolved into compact regions of star formation. The reconstructed galaxy has I_AB= 24, giving a bolometric luminosity of about 3e11 Lsun. This can be produced by a star formation rate of 36 h^-2 Msun /yr (q0=0.5), or by an instantaneous star burst of 3e8 Msun. The spectral lines show velocity variations on the order of 300 km/s along the arc. The Si II line is blue shifted with respect to the Ly alpha emission, and the Ly alpha emission line is asymmetric with a red tail. These spectral features are naturally explained by an outflow model, in which the blue side of the Ly alpha line has been absorbed by outflowing neutral H I. Evidence from other sources indicates that outflows are common in starburst galaxies at high and low redshift. We have discovered a companion galaxy with a radial velocity only 450 km/s different than the arc's. The serendipitous discovery of these two galaxies suggests that systematic searches may uncover galaxies at even higher redshifts.

A NEAR INFRARED STUDY OF BLUE DWARF GALAXIES

ABSTRACT Blue Dwarf Galaxies (BDG) are objects of low luminosity (MVge-18) and very blue visible colors. Both the colors and the presence of narrow emission lines in their spectra are usually interpreted to result from intense episodes of star formation. The metallicity of the BDG is usually low with values in the range 10-2-1~Zodot. Wolf-Rayet (WR) features are detected in many cases in the optical spectra. This is an unexpected characteristic since according to stellar evolution, the number of WR stars in a star forming region should increase with the metallicity while WR features are only rarely detected in solar metallicity galaxies. Different morphologies are found among BDG including compact elliptical, irregular, magellanic, interacting. Due to the low luminosity in most of these objects the starburst (SB) episode dominates the emitted radiation. Therefore starbursts can be studied in more detail in dwarf galaxies than in giant spiral galaxies where the presence of a quiescent stellar population is important in defining the observed characteristics. Moreover BDG are a unique laboratory to study the star formation process in low metallicity environments. We have used new imaging and spectroscopy in the near infrared to study both the star formation history and the excitation mechanism of the near infrared lines in BDG. The observations were taken with the Steward Observatory telescopes. We concentrated on 8 objects selected to be a representative but bright sample of BDG, they are: II Zw 40, He2-10, NGC 3077, NGC 4214, NGC 4449, NGC 1569, NGC 4861 and NGC 5253. The data were used to constrain a starburst model. For II Zw 40 we were able to fit the observations using a gaussian star formation rate with FWHM=5 My and a solar neiborhood IMF. The star formation episode appears to have occured about 4 Myr ago with aproximately one tenth of the total mass of the galaxy involved. Evidence for an evolved population was found in the outer parts of II Zw 40. We also studied the possible periodicity of the starbursting process ruling out episodes comparable to the present one for the last billion years. For all the other galaxies we confirm the tendency of the burst in BDG to be very short, no more than a few million years. The short duration of the burst deduced from our modelling is consistent with the detection of WR features, since short bursts also produce a WR/O ratio higher than a long duration or continous burst. The presence of WR stars is well correlated with the age of the burst. We found a very good correlation between the [FeII] line at 1.64 microns and the age of the burst for a sample of SB galaxies, well explained in terms of a model where all the [FeII] emissin originates from supernova remnants. All the BDG, with the exception of He2-10, show a deficiency in [FeII], in agreement with the low metal content of these objects. Using observations of M 82 we calibrated the [FeII] emission for solar metallicity SB, obtaining good agreement with the supernova remnants observed in the Galaxy. The correlation between molecular hydrogen emission (H2) at 2.12 microns and age is weaker. In this case there is a minimum emission level associated with supernovae and an additional contribution from different excitation mechanisms. No deviation from the general trend is detected for BDG when compared to normal starburst in agreement with an independency of H2 on metallicity. Indications for fluorescently excitated H2 were found in a few cases. We briefly analyzed the FIR-radio relationship finding that BDG follow the trend defined by IR galaxies, despite the low metallicity of the BDG. Finally we use the HeI line at 1.70 microns as an indicator for the presence of high mass stars. Our results show a tendency of commonly used [OIII]/Hβ parameter to overestimate the stellar temperature. The correlation between the 1.70 micron line and the age of the burst is very good. We believe this line is a very reliable indicator of stellar temperature being less affected by the extinction and the electron temperature than the temperature indicators in the visible.

Evolution of the Luminosity Density in the Universe: Implications for the Nonzero Cosmological Constant

We show that evolution of the luminosity density of galaxies in the universe provides a powerful test for the geometry of the universe. Using reasonable galaxy evolution models of population synthesis which reproduce the colors of local galaxies of various morphological types, we have calculated the luminosity density of galaxies as a function of redshift $z$. Comparison of the result with recent measurements by the Canada-France Redshift Survey in three wavebands of 2800{\AA}, 4400{\AA}, and 1 micron at z<1 indicates that the \Lambda-dominated flat universe with \lambda_0 \sim 0.8 is favored, and the lower limit on \lambda_0 yields 0.37 (99% C.L.) or 0.53 (95% C.L.) if \Omega_0+\lambda_0=1. The Einstein-de Sitter universe with (\Omega_0, \lambda_0)=(1, 0) and the low-density open universe with (0.2, 0) are however ruled out with 99.86% C.L. and 98.6% C.L., respectively. The confidence levels quoted apply unless the standard assumptions on galaxy evolution are drastically violated. We have also calculated a global star formation rate in the universe to be compared with the observed rate beyond z \sim 2. We find from this comparison that spiral galaxies are formed from material accretion over an extended period of a few Gyrs, while elliptical galaxies are formed from initial star burst at z >~ 5 supplying enough amount of metals and ionizing photons in the intergalactic medium.

The universality of the stellar initial mass function

We propose that the stellar initial mass function (IMF) is universal in the sense that its functional form arises as a consequence of the statistics of random supersonic flows. A model is developed for the origin of the stellar IMF, that contains a dependence on the average physical parameters (temperature, density, velocity dispersion) of the large scale site of star formation. The model is based on recent numerical experiments of highly supersonic random flows that have a strong observational counterpart. It is shown that a Miller-Scalo like IMF is naturally produced by the model for the typical physical conditions in molecular clouds. A more ``massive'' IMF in star bursts is also predicted.

The Evolution of Dwarf Galaxies with Star Formation in an Outward-propagating Supershell

We simulate the dynamical and chemical evolution of a dwarf galaxy embedded in a dark matter halo, using a three-dimensional N-body/SPH simulation code combined with stellar population synthesis. The initial condition is adopted in accord with a $10^{10}M_\odot$ virialized sphere in a 1-sigma CDM perturbation which contains 10% baryonic mass. A supersonic spherical outflow is driven by the first star burst near the center of the galaxy and produces an expanding super shell in which stars are subsequently formed. Consecutive formation of stars in the expanding shell makes the stellar system settled with the exponential brightness profile, the positive metallicity gradient, and the inverse color gradient in agreement with observed features of dwarf galaxies. We therefore propose that the energy feedback via stellar winds and supernovae is a decisive mechanism for formation of less compact, small systems like dwarf galaxies.

Spectral index evolution of young shell supernova remnants in the milky way and other galaxies

Abstract We show that the radio spectral index of young shell supernova remnants evolvs differently in galaxies with and without a star burst (M82, NGC253 and the Galaxy, M31, M33, LMC, SMC). This is explained by the difference in gas density, which determines the dynamic evolution of shell supernova remnants in these galaxies (ρM82/ρGal ≈ 150). The spectral index – diameter dependence is suggested to be used for the estimation of a typical initial mass, M 0, of a shell supernova remnant. For shells in M82, it is estimated as M 0 = 2.3 ± 1.1M[odot]. This is the first known estimate of this kind for the remnants in M82, as well as the kinetic energy of the shell 41.9+58, 1051 ergs. We conclude that the unusually high radio brightness of supernova remnants in M82 is a consequence of only the interaction with ambient medium.

Evolution of Galaxy Luminosity in the CDM Model

we analyze the evolution of the luminosity function of galaxies using the CDM model in numerical simulations. There is an observational excess in the number counts of galaxies per square degree in the blur band Nb(i.e. APM, Maddoxet al.1990). Several authors have tried to reproduce it trying to fit the observations. Our model assumes an instantaneous star formation rate (SFR) proportional to a power of the local density. A ‘single star burst’ is produced each time step and we follow the evolution of the luminosity and colour of each ‘stellar group’. The galaxies are identified with a density criterium. We compute U,V,B,K colours and Nband our results agree quite well with observations.

Red post-starburst galaxies

Observations of nearby star-forming regions suggest that the initial mass function is deficient in low-mass stars when the ambient star formation rate is high. We investigate the consequences of a truncated initial mass function on the photometric evolution of star burst galaxies using new models of stellar population synthesis. A major prediction with respect to the case of a standard initial mass function is that the spectra of the galaxies become unusually red when the turnoff mass reaches the cutoff of the initial mass function. This red phase can last for more than 109 yr if the lower cutoff is less than 2M ⊙. The amplitude of the reddening depends only weakly on the lower cutoff but increases with the fraction of the galaxy mass involved in the starburst. Such unusually red (V — K > 3.5) post-starburst galaxies can be distinguished from galaxies reddened by dust or with abnormally high metallicities by their unusually strong 4000 A break and the presence of stellar absorption features typical of late-type giants.

The Nucleus of Our Home Galaxy: A Remnant of an Active or a Starburst Galaxy?

To resolve the above dilemma, two essentially different approaches are undertaken: First, a new, detailed analysis of the entire radiation spectrum of Sgr A∗, from radio band up to gamma-rays, is reviewed, which enables us to put substantial constraints on the mass of a putative black hole. The derived upper limit turns out to be too small to allow the black hole to serve as an ‘engine’ for a Seyfert galaxy. Second, analyses of recent data on the 10 KeV gas in the central 200 pc and on star formation history at the Galactic center both make a star burst the likely episode in a recent past. Taken together, the two approaches seem to indicate that the history of the central part of our Galaxy can be better described as that of a starburst, rather than a Seyfert, galaxy.

OB ASSOCIATIONS: MASSIVE STARS IN CONTEXT

ABSTRACT Massive stars are important in a variety of contexts: individually they are examples of stars that undergo significant mass loss which is driven by line radiation pressure; as denizens of the Galaxy they play a significant role in the galactic recycling effort; a ensembles of stars they contain clues to massive star evolution in general and the question of supernova progenitors in particular. Finally, under extreme conditions of pathological star formation massive stars participate in the star burst phenomena seen in some galaxies. If we wish to understand starbursts, we must first understand locally observed variations in the mas spectrum. But all of these diverse areas of study rely on certain fundamental parameters of the stars themselves--here we include temperature, luminosity, mass, and chemical composition. And central to accurate determinations of these parameters are the distances to the stars. This review will focus on what is known about hte loose clustering of massive stars termed OB associations. These associations ontain many, but byno means all, of the stars with masses greater than about 15 solar masses. Most of our knowledge of the stellar content of the OB associations in our own Galaxy is surprisingly sketchy due to their large angular size, but large-field CCD detectors are changing this. Magellanic Cloud associations can now be compared with OB associations in our own Galaxy, and the initial mass function shows suprising variations. The physical cause of these variations is still unknown. Another development is the study of embedded star clusters: infrared array detectors have been probing the buried stars in molecular clouds and finding an unexpectedly large population of these objects. Both these young IR clusters, and a springling of evolved red supergiants, suggest that the star formation history of OB associations may be more complicated than we have supposed.

ENHANCED INHIBITORY EFFECT OF POLYMERIC STAR BURST FORMED YIGSR PEPTIDE ON NEWLY ESTABLISHED NUDE-MOUSE SPONTANEOUS ADENOCARCINOMA METASTATIC MODEL

A highly metastatic nude mouse adenocarcinoma cell line, XK-4, was established, and the inhibitory effect of the polymeric star burst YIGSR peptide on XK-4 metastasis was examined. XK-4 adenocarcinoma arose spontaneously in a nude mouse after the subucutaneous transplantation of a human colon cancer. XK-4 cells were first cultured in February, 1990 after serial transplantation of the tumor in nude mice since October, 1985. The XK-4 cell line produces blood borne lung or liver metastasis 2 weeks after injection into the tail vein or anterior mesenteric vein, respectively. Chromosomal analysis of the XK-4 cell line revealed only murine chromosomes, and isozyme analysis showed a murine pattern. Transplantation of XK-4 cells into a thymus-intact litter mate was rejected. These results suggest that the XK-4 adenocarcinoma grew spontaneously in the nude mouse due to some kind of human cancer stimulation. A peptide consisting of the active domain of laminin, YIGSR, was chemically synthesized as a polymer with a star burst pattern. The inhibitory effect of polymeric star burst YIGSR on XK-4 metastasis was enhanced, compared with monomeric YIGSR. These effects appear to be due to the prolonged half-life of polymeric star burst YIGSR in vivo, and increased binding of polymeric star burst YIGSR to the YIGSR receptor. In this study we established a new metastatic model, and confirmed the enhanced metastatic inhibitory effect of a polymeric form of the star burst YIGSR peptide.

CONSTRAINING GALAXY EVOLUTION AND COSMOLOGY FROM GALAXY KINEMATICS: FIRST OBSERVATIONS AT Z=0.18

ABSTRACT There is significant evidence that galaxy evolution is a complex process, in which mass accretion, mergers, and star bursts play an important role. This complex evolution adds significant freedom to the modeling of observations of high redshift galaxies. Additional constraints are necessary. Such constraints may come from measurements of circular velocities, or velocity dispersions of galaxies at intermediate redshift. The first observations of central velocity dispersions are presented for galaxies in the rich cluster Abell 665 at z=0.18. Full curves of radial velocity dispersion of cluster members are presented. The measurements result in a well defined Faber Jackson relation for the red cluster members. A blue galaxy is lying 2.2 mag off the relation, and has an "E+A" spectrum. This "Butcher Oemler" galaxy is rotating rapidly, which is indicative of a strong disk. The observations show that measurements of central velocity dispersions are possible with current telescope capabilities. Observaitons at higher redshifts will lead to new constraints on both galaxy evolution and cosmology.

Digital photographic photometry of NGC 5253 and NGC 2915

The surface digital photometry for NGC 5253 and NGC 2915 is given. The luminosity profiles and colour gradients are discussed and compared. It is concluded that NGC 2915 differs from NGC 5253, both in the extension and intensity of the star burst event because of the kinematical behaviour of the captured gas.

Oblique Orientation of Incisions in Four-Incision Radial Keratotomy

ABSTRACT We recommend that the incisions in four-incision radial keratotomy cases be oriented obliquely as a means of avoiding glare or star burst.

Effect of a QSO on the interstellar medium of its host galaxy

view Abstract Citations References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Effect of a QSO on the Interstellar Medium of Its Host Galaxy Shanbhag, Smita Abstract We consider the hydrodynamic evolution of gas in the interstellar medium of the host galaxy of a quasar due to Compton heating by the QSO radiation. We show that a Lagrangean formulation of the problem is necessary. It is found that the "hydrodynamic time scale" becomes important compared to the Compton heating time scale. We also relax the "single fluid" approximation by considering the existence of clouds and taking into account the mass loss from stars. The results predict star burst activity, and thus we explain the blue colors of the active galaxies. Publication: The Astrophysical Journal Pub Date: February 1991 DOI: 10.1086/169642 Bibcode: 1991ApJ...367..462S Keywords: Active Galactic Nuclei; Interstellar Gas; Quasars; Compton Effect; Hydrogen; Molecular Clouds; Radio Galaxies; Star Formation; Starburst Galaxies; Astrophysics; GALAXIES: NUCLEI; GALAXIES: STELLAR CONTENT; QUASARS; STARS: FORMATION full text sources ADS |

Surface photometry of the dwarf galaxy VIIZw009

AbstractPhotographic photometry reveals the dwarf galaxy VIIZw009 as an outstanding object with respect to the large colour excess of its central star burst region. It seems to be a similarity to the system IC 10, which also contains at least one star burst region with large internal extinction. The results are supported by the identification of the IRAS source 3550+6657 with VIIZw009.

Optical Search of Faint IRAS Point Sources in the Virgo Cluster Region

The infrared emission in galaxies is strongly connected to star burst processes, while emission from the intergalactic medium results mainly from dust heated by other luminosity sources. The Virgo cluster provides an extensive region in which the properties of such infrared emission can be studied. Galaxies in the Virgo cluster with dust temperatures of 40 K and masses of dust as small as about 5 × 104M⊙ are detectable at the limit of the IRAS survey (Soifer et al., 1987). The IRAS observations of 196 optically selected galaxies in the Virgo cluster have been studied by Helou et al. (Helou et al., 1988; see also Leggett et al., 1988).