Variations In Star Formation Rate Research Articles

Kpc-scale properties of dust temperature in terms of dust mass and star formation activity

ABSTRACT We investigate how dust temperature is affected by local environmental quantities, especially dust surface density (Σdust), dust-to-gas ratio (D/G), and interstellar radiation field. We compile multiwavelength observations in 46 nearby galaxies, uniformly processed with a common physical resolution of 2 kpc. A physical dust model is used to fit the infrared dust emission spectral energy distribution (SED) observed with WISE and Herschel. The star formation rate (SFR) is traced with GALEX ultraviolet data corrected by WISE infrared. We find that the dust temperature correlates well with the SFR surface density (ΣSFR), which traces the radiation from young stars. The dust temperature decreases with increasing D/G at fixed ΣSFR, as expected from stronger dust shielding at high D/G, when ΣSFR is higher than $\sim 2\times 10^{-3}~\rm M_\odot ~yr^{-1}~kpc^{-2}$. These measurements are in good agreement with the dust temperature predicted by our proposed analytical model. Below this range of ΣSFR, the observed dust temperature is higher than the model prediction and is only weakly dependent on D/G, possibly due to dust heating from an old stellar population or the variation of SFR within the past 1010 yr. Overall, the dust temperature as a function of ΣSFR and Σdust predicted by our analytical model is consistent with observations. We also notice that, at fixed gas surface density, ΣSFR tends to increase with D/G, i.e. we can modify the Kennicutt–Schmidt law empirically with a dependence on D/G to match observations better.

The importance of the way in which supernova energy is distributed around young stellar populations in simulations of galaxies

ABSTRACT Supernova (SN) feedback plays a crucial role in simulations of galaxy formation. Because blast waves from individual SNe occur on scales that remain unresolved in modern cosmological simulations, SN feedback must be implemented as a subgrid model. Differences in the manner in which SN energy is coupled to the local interstellar medium and in which excessive radiative losses are prevented have resulted in a zoo of models used by different groups. However, the importance of the selection of resolution elements around young stellar particles for SN feedback has largely been overlooked. In this work, we examine various selection methods using the smoothed particle hydrodynamics code swift. We run a suite of isolated disc galaxy simulations of a Milky Way-mass galaxy and small cosmological volumes, all with the thermal stochastic SN feedback model used in the eagle simulations. We complement the original mass-weighted neighbour selection with a novel algorithm guaranteeing that the SN energy distribution is as close to isotropic as possible. Additionally, we consider algorithms where the energy is injected into the closest, least dense, or most dense neighbour. We show that different neighbour-selection strategies cause significant variations in star formation rates, gas densities, wind mass-loading factors, and galaxy morphology. The isotropic method results in more efficient feedback than the conventional mass-weighted selection. We conclude that the manner in which the feedback energy is distributed among the resolution elements surrounding a feedback event is as important as changing the amount of energy by factors of a few.

Spiral Structure Boosts Star Formation in Disk Galaxies

Abstract We investigate the impact of spiral structure on global star formation using a sample of 2226 nearby bright disk galaxies. Examining the relationship between spiral arms, star formation rate (SFR), and stellar mass, we find that arm strength correlates well with the variation of SFR as a function of stellar mass. Arms are stronger above the star-forming galaxy main sequence (MS) and weaker below it: arm strength increases with higher log ( SFR / SFR MS ) , where SFRMS is the SFR along the MS. Likewise, stronger arms are associated with higher specific SFR. We confirm this trend using the optical colors of a larger sample of 4378 disk galaxies, whose position on the blue cloud also depends systematically on spiral arm strength. This link is independent of other galaxy structural parameters. For the subset of galaxies with cold gas measurements, arm strength positively correlates with H i and H2 mass fraction, even after removing the mutual dependence on log ( SFR / SFR MS ) , consistent with the notion that spiral arms are maintained by dynamical cooling provided by gas damping. For a given gas fraction, stronger arms lead to higher log ( SFR / SFR MS ) , resulting in a trend of increasing arm strength with shorter gas depletion time. We suggest a physical picture in which the dissipation process provided by gas damping maintains spiral structure, which, in turn, boosts the star formation efficiency of the gas reservoir.

Galaxy And Mass Assembly (GAMA): the interplay between galaxy mass, SFR, and heavy element abundance in paired galaxy sets

ABSTRACT We study the star formation rate (SFR), stellar mass (M⋆), and the gas metallicity (Z) for 4636 galaxy pairs using the Galaxy And Mass Assembly (GAMA) survey. Our galaxy pairs lie in a redshift range of 0 < z < 0.35, mass range of 7.5 < log(M⋆/M⊙) < 11.5 and ΔV < 1000 km s−1. We explore variations in SFR and Z from three point of views: multiplicity, pair separation, and dynamics. We define multiplicity as the number of galaxies paired with a single galaxy, and analyzed for the first time variations in SFR and Z for both, single pairs and pairs with higher multiplicity. For the latter, we find SFR enhancements from 0.025–0.15 dex, that would shift the M–SFR relation of single pairs by 27${{\ \rm per\ cent}}$ to higher SFRs. The effect of Z, on the other hand, is of only 4${{\ \rm per\ cent}}$. We analyse the most and least massive galaxy of major/minor pairs as a function of the pair separation. We define major pairs those with mass ratios of 0.5 < M1/M2 < 2, while pairs with more discrepant mass ratios are classified as minor pairs. We find SFR enhancements of up to two and four times with respect to their control sample, for major and minor pairs. For the case of Z, we find decrements of up to 0.08 dex for the closest pairs. When we focus on dynamics, Z enhancements are found for minor pairs with high-velocity dispersion $(\sigma _p \gt 250 \,\, \mathrm{km\, s ^{-1}})$ and high multiplicity.

The ALMaQUEST survey – III. Scatter in the resolved star-forming main sequence is primarily due to variations in star formation efficiency

ABSTRACT Using a sample of 11 478 spaxels in 34 galaxies with molecular gas, star formation, and stellar maps taken from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we investigate the parameters that correlate with variations in star formation rates on kpc scales. We use a combination of correlation statistics and an artificial neural network to quantify the parameters that drive both the absolute star formation rate surface density (ΣSFR), as well as its scatter around the resolved star-forming main sequence (ΔΣSFR). We find that ΣSFR is primarily regulated by molecular gas surface density ($\Sigma _{\rm H_2}$) with a secondary dependence on stellar mass surface density (Σ⋆), as expected from an ‘extended Kennicutt–Schmidt relation’. However, ΔΣSFR is driven primarily by changes in star formation efficiency (SFE), with variations in gas fraction playing a secondary role. Taken together, our results demonstrate that whilst the absolute rate of star formation is primarily set by the amount of molecular gas, the variation of star formation rate above and below the resolved star-forming main sequence (on kpc scales) is primarily due to changes in SFE.

The initial mass function in the extended ultraviolet disc of M83

ABSTRACT Using Hubble Space Telescope ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations of four fields within the extended ultraviolet disc (XUV disc) of M83. These observations show a clumpy distribution of main-sequence stars and a mostly smooth distribution of red giant branch stars. We constrain the upper end of the initial mass function (IMF) in the outer disc using the detected population of main-sequence stars and an assumed constant star formation rate (SFR) over the last 300 Myr. By comparing the observed main-sequence luminosity function to simulations, we determine the best-fitting IMF to have a power-law slope α = −2.35 ± 0.3 and an upper mass limit $M_{\rm u}=25_{-3}^{+17} \, \mathrm{M}_\odot$. This IMF is consistent with the observed H $\rm \alpha$ emission, which we use to provide additional constraints on the IMF. We explore the influence of deviations from the constant SFR assumption, finding that our IMF conclusions are robust against all but strong recent variations in SFR, but these are excluded by causality arguments. These results, along with our similar studies of other nearby galaxies, indicate that some XUV discs are deficient in high-mass stars compared to a Kroupa IMF. There are over one hundred galaxies within 5 Mpc, many already observed with HST, thus allowing a more comprehensive investigation of the IMF, and how it varies, using the techniques developed here.

A simple non-equilibrium feedback model for galaxy-scale star formation: delayed feedback and SFR scatter

ABSTRACT We explore a class of simple non-equilibrium star formation models within the framework of a feedback-regulated model of the ISM, applicable to kiloparsec-scale resolved star formation relations (e.g. Kennicutt–Schmidt). Combining a Toomre-Q-dependent local star formation efficiency per free-fall time with a model for delayed feedback, we are able to match the normalization and scatter of resolved star formation scaling relations. In particular, this simple model suggests that large (∼dex) variations in star formation rates (SFRs) on kiloparsec scales may be due to the fact that supernova feedback is not instantaneous following star formation. The scatter in SFRs at constant gas surface density in a galaxy then depends on the properties of feedback and when we observe its star-forming regions at various points throughout their collapse/star formation ‘cycles’. This has the following important observational consequences: (1) the scatter and normalization of the Kennicutt–Schmidt relation are relatively insensitive to the local (small-scale) star formation efficiency; (2) but gas depletion times and velocity dispersions are; (3) the scatter in and normalization of the Kennicutt–Schmidt relation is a sensitive probe of the feedback time-scale and strength; (4) even in a model where $\tilde{Q}_{\rm gas}$ deterministically dictates star formation locally, time evolution, variation in local conditions (e.g. gas fractions and dynamical times), and variations between galaxies can destroy much of the observable correlation between SFR and $\tilde{Q}_{\rm gas}$ in resolved galaxy surveys. Additionally, this model exhibits large scatter in SFRs at low gas surface densities, in agreement with observations of flat outer H i disc velocity dispersion profiles.

Dissecting star formation in the “Atoms-for-Peace” galaxy

Context. The tidal tails of post-merger galaxies exhibit ongoing star formation far from their disks. The study of such systems can be useful for our understanding of gas condensation in diverse environments. Aims. The ongoing star formation in the tidal tails of post-merger galaxies can be directly studied from ultraviolet (UV) imaging observations. Methods. The post merger galaxy NGC7252 (“Atoms-for-Peace” galaxy) is observed with the Astrosat UV imaging telescope (UVIT) in broadband NUV and FUV filters to isolate the star-forming regions in the tidal tails and study the spatial variation in star formation rates. Results. Based on ultraviolet imaging observations, we discuss star-forming regions of ages <200 Myr in the tidal tails. We measure star formation rates in these regions and in the main body of the galaxy. The integrated star formation rate (SFR) of NGC7252 (i.e., that in the galaxy and tidal tails combined) without correcting for extinction is found to be 0.81 ± 0.01 M⊙ yr−1. We show that the integrated SFR can change by an order of magnitude if the extinction correction used in SFR derived from other proxies are taken into consideration. The star formation rates in the associated tidal dwarf galaxies (NGC7252E, SFR = 0.02 M⊙ yr−1 and NGC7252NW, SFR = 0.03 M⊙ yr−1) are typical of dwarf galaxies in the local Universe. The spatial resolution of the UV images reveals a gradient in star formation within the tidal dwarf galaxy. The star formation rates show a dependence on the distance from the centre of the galaxy. This can be due to the different initial conditions responsible for the triggering of star formation in the gas reservoir that was expelled during the recent merger in NGC7252.

Relaxation leptogenesis, isocurvature perturbations, and the cosmic infrared background

Observations of cosmic infrared background (CIB) radiation exhibit significant fluctuations on small angular scales. A number of explanations have been put forth, but there is currently no consensus on the origin of these large fluctuations. We consider the possibility that small-scale fluctuations in matter-antimatter asymmetry could lead to variations in star formation rates which are responsible for the CIB fluctuations. We show that the recently proposed Higgs relaxation leptogenesis mechanism can produce such small-scale baryonic isocurvature perturbations which can explain the observed excess in the CIB fluctuations.

THE STAR FORMATION MAIN SEQUENCE: THE DEPENDENCE OF SPECIFIC STAR FORMATION RATE AND ITS DISPERSION ON GALAXY STELLAR MASS

The dispersion of the star-formation main sequence (SFMS) reflects the diversity of star formation histories and variation in star formation rates (SFRs) in star-forming galaxies (SFGs) with similar stellar masses ($M^\ast$). We examine the dispersion of local SFMS using a complete sample of Sloan Digital Sky Survey galaxies at 0.01$<z<$0.03 with $\log(M^\ast/M_\odot)>$8.8. The SFRs are estimated from H$\alpha$ in combination with 22$\mu m$ observation from WISE. The catalog of bulge+disk decomposition from Simard et al. (2011) is available for the sample galaxies. We measure the dispersion of specific SFR (SSFR) as a function of $M^*$. We confirm that the dispersion increases with $M^*$ from 0.37$\pm0.01$dex at $\log(M^\ast/M_\odot)<$9.6 to 0.51$\pm0.02$dex at $\log(M^\ast/M_\odot)>$10.2. Despite star formation is mostly associated with disks, the dispersion of disk SSFR still increases with $M^*$. We conclude that the presence of bulges/bars is likely responsible for the large dispersion of SSFR in massive SFGs while low-mass SFGs are mostly disk-dominated and thus with small dispersion. Our results suggest that star formation on galactic scales is dramatically affected by central dense structures through both enhancing and/or quenching processes; while lower-mass SFGs tend to have less bursty star formation histories. However, the dispersion of SSFR becomes significantly smaller and remains constant when only disk-dominated SFGs are counted. This finding implies that the impact of stochastic stellar feedback on star formation is likely to follow the same pattern in all disk galaxies, showing no correlation with halo potential.

CALIBRATING UV STAR FORMATION RATES FOR DWARF GALAXIES FROM STARBIRDS

Integrating our knowledge of star formation traced by observations at different wavelengths is essential for correctly interpreting and comparing star formation activity in a variety of systems and environments. This study compares extinction corrected integrated ultraviolet (UV) emission from resolved galaxies with color-magnitude diagram (CMD) based star formation rates (SFRs) derived from resolved stellar populations and CMD fitting techniques in 19 nearby starburst and post-starburst dwarf galaxies. The datasets are from the panchromatic STARBurst IRregular Dwarf Survey (STARBIRDS) and include deep legacy GALEX UV imaging, HST optical imaging, and Spitzer MIPS imaging. For the majority of the sample, the integrated near UV fluxes predicted from the CMD-based SFRs - using four different models - agree with the measured, extinction corrected, integrated near UV fluxes from GALEX images, but the far UV predicted fluxes do not. Further, we find a systematic deviation between the SFRs based on integrated far UV luminosities and existing scaling relations, and the SFRs based on the resolved stellar populations. This offset is not driven by different star formation timescales, variations in SFRs, UV attenuation, nor stochastic effects. This first comparison between CMD-based SFRs and an integrated FUV emission SFR indicator suggests that the most likely cause of the discrepancy is the theoretical FUV-SFR calibration from stellar evolutionary libraries and/or stellar atmospheric models. We present an empirical calibration of the FUV-based SFR relation for dwarf galaxies, with uncertainties, which is ~53% larger than previous relations.

Quantitative constraints on starburst cycles in galaxies with stellar masses in the range 108-1010 M

We have used 4000 u break and HA indices in combination with SFR/M∗ derived from emission line flux measurements, to constrain the recent star formation histories of galaxies with stellar masses in the range 10 8 − 10 10 M⊙. The fraction of the total SFR density in galaxies with ongoing bursts is a strong function of stellar mass, de- clining from 0.85 at a stellar mass of 10 8 M⊙ to 0.25 for galaxies with M∗ ∼ 10 10 M⊙. Low mass galaxies are not all young. The distribution of half mass formation times for galaxies with stellar masses less than 10 9 M⊙ is broad, spanning the range 1-10 Gyr. The peak-to-trough variation in star formation rate among the bursting population ranges lies in the range 10-25. In low mass galaxies, the average duration of the burst bursts is comparable to the dynamical time of the galaxy. Galaxy structure is corre- lated with estimated burst mass fraction, but in different ways in low and high mass galaxies. High mass galaxies with large burst mass fractions are more centrally concen- trated, indicating that bulge formation is at work. In low mass galaxies, stellar surface densities µ∗ decrease as a function of Fburst. These results are in good agreement with the observational predictions of Teyssier et al (2013) and lend further credence to the idea that the cuspy halo problem can be solved by energy input from multiple star- bursts over the lifetime of the galaxy. We note that there is no compelling evidence for IMF variations in the population of star-forming galaxies in the local Universe.

The “entropy floor” is porous – remarks on the coexistence of star formation and kinetic AGN feedback

AbstractWe discuss the morphology of star forming clouds and filaments in the central (≲ 50 kpc) regions of 16 low redshift (z&lt;0.3) cool core brightest cluster galaxies (BCGs). The sample spans decades-wide ranges of X-ray mass deposition and star formation rates as well as active galactic nucleus (AGN) mechanical power, encompassing both high and low extremes of the supposed intracluster medium (ICM) cooling and AGN heating feedback cycle. Amid evidence that the gas fueling both star formation and AGN activity has condensed from the hot atmosphere, we present new and archival Hubble Space Telescope (HST) images of far ultraviolet (FUV) continuum emission directly associated with young stars, acting as a calorimeter for the degree to which the suppression of star formation by AGN mechanical feedback may be spatially or temporally inefficient. We discuss evidence for temporal and possibly cyclical variation in star formation rate, wherein elevated cooling episodes are permitted when AGN feedback is in a low-power state, and vice-versa. Several sources exhibit strong morphological evidence that low levels of star formation can survive and may indeed be triggered by the passage of a propagating radio source. We conclude by discussing the apparent coexistence of feedback and star formation. If AGN mechanical power does establish an “entropy floor”, this floor must be porous, or raise and lower as the AGN varies in power.

Connecting stellar mass and star-formation rate to dark matter halo mass out to z ∼ 2

We have constructed an extended halo model (EHM) which relates the total stellar mass and star-formation rate (SFR) to halo mass (M_h). An empirical relation between the distribution functions of total stellar mass of galaxies and host halo mass, tuned to match the spatial density of galaxies over 0<z<2 and the clustering properties at z~0, is extended to include two different scenarios describing the variation of SFR on M_h. We also present new measurements of the redshift evolution of the average SFR for star-forming galaxies of different stellar mass up to z=2, using data from the Herschel Multi-tiered Extragalactic Survey (HerMES) for infrared-bright galaxies. Combining the EHM with the halo accretion histories from numerical simulations, we trace the stellar mass growth and star-formation history in halos spanning a range of masses. We find that: (1) The intensity of the star-forming activity in halos in the probed mass range has steadily decreased from z~2 to 0; (2) At a given epoch, halos in the mass range between a few times 10^{11} M_Sun and a few times 10^{12} M_Sun are the most efficient at hosting star formation; (3) The peak of SFR density shifts to lower mass halos over time; (4) Galaxies that are forming stars most actively at z~2 evolve into quiescent galaxies in today's group environments, strongly supporting previous claims that the most powerful starbursts at z~2 are progenitors of today's elliptical galaxies.

Spectrophotometry with Hectospec, the MMT's Fiber-Fed Spectrograph

ABSTRACTThe spectrophotometric calibration of surveys is a significant, but often neglected, issue when measuring the history of star formation by combining spectroscopic surveys conducted with different instruments. We describe techniques for photometric calibration of optical spectra obtained with the MMT’s fiber-fed spectrograph, Hectospec. The atmospheric dispersion compensation prisms built into the MMT’s f/5 wide-field corrector effectively eliminate errors due to differential refraction and simplify the calibration procedure. The procedures that we describe here are applicable to all 220,000+ spectra obtained to date with Hectospec because the instrument response is stable. We estimate the internal error in the Hectospec measurements by comparing duplicate measurements of ∼1500 galaxies. For a sample of 400 galaxies in the Smithsonian Hectospec Lensing Survey (SHELS) with a median z = 0.10, we compare line and continuum fluxes measured by Hectospec through a 1.5′′ diameter optical fiber with those measured by the Sloan Digital Sky Survey (SDSS) through a 3′′ diameter optical fiber. Agreement of the [O II] and Hα SHELS and SDSS line fluxes, after scaling by the R-band flux in the different apertures, suggests that the spatial variation in star-formation rates over a 1.5 to 3 kpc radial scale is small. The median ratio of the Hectospec and SDSS spectra, smoothed over 100 Å scales, is remarkably constant to ∼5% over the range of 3850 to 8000 Å. Offsets in the ratio of the median [O II] and Hα fluxes, the equivalent width of Hδ and the continuum index d4000 are a few percent, small compared with other sources of scatter. We also explore the impact of atmospheric absorption. Observing redward of 6500 Å, it is impossible to remove the effects of atmospheric absorption perfectly because the variation of absorption with wavelength is not resolved by a moderate dispersion spectrograph. Thus measurements of spectral line fluxes including Hα, and derived physical quantities including star-formation rates, may have sizable systematic errors where the redshifted spectral features land on strong atmospheric absorption troughs.

The Las Campanas/Anglo-Australian Telescope Rich Cluster Survey — III. Spectroscopic studies of X-ray bright galaxy clusters at z~ 0.1

We present the analysis of the spectroscopic and photometric catalogues of 11 X-ray luminous clusters at 0.07 < z < 0.16 from the Las Campanas/Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic data set consists of over 1600 galaxy cluster members, of which two-thirds are outside r(200). These spectra allow us to assign cluster membership using a detailed mass model and expand on our previous work on the cluster colour-magnitude relation ( CMR) where membership was inferred statistically. We confirm that the modal colours of galaxies on the CMR become progressively bluer with increasing radius d( B - R)/dr(p) = - 0.011 +/- 0.003 and with decreasing local galaxy density d( B - R)/dlog ( Sigma)= - 0.062 +/- 0.009. Interpreted as an age effect, we hypothesize that these trends in galaxy colour should be reflected in mean H delta equivalent width. We confirm that passive galaxies in the cluster increase in Hd line strength as dH delta/dr(p) = 0.35 +/- 0.06. Therefore, those galaxies in the cluster outskirts may have younger luminosity-weighted stellar populations; up to 3 Gyr younger than those in the cluster centre assuming d( B - R)/dt = 0.03 mag per Gyr. A variation of star formation rate, as measured by [ O II]lambda 3727 angstrom, with increasing local density of the environment is discernible and is shown to be in broad agreement with previous studies from the 2dF Galaxy Redshift Survey and the Sloan Digital Sky Survey. We divide our spectra into a variety of types based upon the MORPHs classification scheme. We find that clusters at z similar to 0.1 are less active than their higher-redshift analogues: about 60 per cent of the cluster galaxy population is non-star forming, with a further 20 per cent in the post-starburst class and 20 per cent in the currently active class, demonstrating that evolution is visible within the past 2 - 3 Gyr. We also investigate unusual populations of blue and very red non-star forming galaxies and we suggest that the former are likely to be the progenitors of galaxies which will lie on the CMR, while the colours of the latter possibly reflect dust reddening. We show that the cluster galaxies at large radii consist of both backsplash ones and those that are infalling to the cluster for the first time. We make a comparison to the field population at z similar to 0.1 and examine the broad differences between the two populations. Individually, the clusters show significant variation in their galaxy populations which we suggest reflects their recent infall histories.

The Effect of Star Formation History on the Inferred Stellar Initial Mass Function

Peaks and lulls in the star formation rate (SFR) over the history of the Galaxy produce plateaus and declines in the present-day mass function (PDMF) where the main-sequence lifetime overlaps the age and duration of the SFR variation. These PDMF features can be misinterpreted as the form of the intrinsic stellar initial mass function (IMF) if the star formation rate is assumed to be constant or slowly varying with time. This effect applies to all regions that have formed stars for longer than the age of the most massive stars, including OB associations, star complexes, and especially galactic field stars. Related problems may apply to embedded clusters. Evidence is summarized for temporal SFR variations from parsec scales to entire galaxies, all of which should contribute to inferred IMF distortions. We give examples of various star formation histories to demonstrate the types of false IMF structures that might be seen. These include short-duration bursts, stochastic histories with lognormal amplitude distributions, and oscillating histories with various periods and phases. The inferred IMF should appear steeper than the intrinsic IMF over mass ranges where the stellar lifetimes correspond to times of decreasing SFRs; shallow portions of the inferred IMF correspond to times of increasing SFRs. If field regions are populated by dispersed clusters and defined by their low current SFRs, then they should have steeper inferred IMFs than the clusters. The SFRs required to give the steep field IMFs in the LMC and SMC are determined. Structure observed in several determinations of the Milky Way field star IMF can be accounted for by a stochastic and bursty star formation history.

Application of the Limit-Cycle Model to Star Formation Histories in Spiral Galaxies: Variation among Morphological Types

We propose a limit-cycle scenario of star formation history for any morphological type of spiral galaxy. It is known observationally that an early-type spiral sample has a wider range of present star formation rate (SFR) than a late-type sample. This tendency is understood in the framework of the limit-cycle model of the interstellar medium (ISM), in which the SFR changes cyclically in accordance with the temporal variation of the mass fraction of the three ISM components. When the limit-cycle model of the ISM is applied, the amplitude of variation of the SFR is expected to change with the supernova (SN) rate. Observational evidence indicates that early-type spiral galaxies should show smaller rates of present SNe than late-type ones. Combining this evidence with the limit-cycle model of the ISM, we predict that the early-type spiral galaxies will show larger amplitudes in their SFR variation than the late types. Indeed, this prediction is consistent with the observed wider range of the SFR in the early-type sample than in the late-type sample. Thus, in the framework of the limit-cycle model of the ISM, we are able to interpret the difference in the amplitude of SFR variation among the morphological classes of spiral galaxies.

CO Kinematics, Star Formation and Dynamical Evolution of the Lensed Starburst NGC 4102

The circumnuclear (inner kiloparsec) regions of spiral galaxies are time-dependent systems whose morphology and dynamics can change significantly over less than a Hubble time. To develop an insight into this evolution it is important to study the fate of gas driven towards the central parts of a galaxy especially near the dynamical resonances. The panoply of CO morphologies displayed by the circumnuclear regions of starburst and nonstarburst galaxies such as NGC 3504, NGC 3351, NGC 7479, NGC 6951, etc, (Kenney, these proceedings) is not yet clearly understood. Another unsettled issue concerns the criteria for star formation in the circumnuclear region where, in contrast to the outer disk, the rotation curve is nearly solid-body and rises rapidly. These issues need to be resolved; the gas mass fraction, the radial variation of SFR (star formation rate) and its interplay with the gas kinematics can shed light not only on the temporal evolution of the circumnuclear CO morphology but also on secular evolution through the destruction and creation of new stellar components. For instance, stellar bars can be destroyed by an increase in the central gas mass concentration of a few % of the total galaxy mass while mechanisms to build or enhance a bulge by vertical scattering of the stars in a compact circumnuclear stellar disk have been proposed (Sellwood 1994, Pfenniger &amp; Norman 1990). We have carried out a study of the circumnuclear region of NGC 4102 with these important issues in mind. NGC 4102, a LINER/HII, SABb spiral galaxy at a distance of 17 Mpc with a bar and/or lens feature qualifies as one of the most luminous nearby starbursts (Devereux 1989) and seems to be in a very early phase of evolution. The outflow timescale of the central starburst wind in NGC 4102, determined from optical spectroscopy of the ionized swept-up gas, is 106 years (Boer 1994), suggesting it is at an earlier evolutionary phase than M82 and NGC 253 whose outflow timescales are significantly larger.

Colors and the evolution of amorphous galaxies

UBVRI and H-alpha photometric observations are presented for 16 amorphous galaxies and a comparison sample of Magellanic irregular (Im) and Sc spiral galaxies. These data are analyzed in terms of star-formation rates and histories in amorphous galaxies. Amorphous galaxies have mean global colors and star-formation rates per unit area that are similar to those in giant Im systems, despite differences in spatial distributions of star-forming centers in these two galactic structural classes. Amorphous galaxies differ from giant Im systems in having somewhat wider scatter in relationships between B - V and U - B colors, and between U - B and L(H-alpha)/L(B). This scatter is interpreted as resulting from rapid variations in star-formation rates during the recent past, which could be a natural consequence of the concentration of star-forming activity into centrally located, supergiant young stellar complexes in many amorphous galaxies. While the unusual spatial distribution and intensity of star formation in some amorphous galaxies is due to interactions with other galaxies, several amorphous galaxies are relatively isolated and thus the processes must be internal. The ultimate evolutionary fate of rapidly evolving amorphous galaxies remains unknown. 77 references.