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We use a large mosaic of HST WFPC2 images to measure the colors and morphologies of 194 spectroscopically confirmed members of the rich galaxy cluster CL1358+62 at z=0.33. We study the color-magnitude (CM) relation as a function of radius in the cluster. The intrinsic scatter in the restframe B-V CM relation of the elliptical galaxies is very small: ~0.022 magnitudes. The CM relation of the ellipticals does not depend significantly on the distance from the cluster center. In contrast, the CM relation for the S0 galaxies does depend on radius: the S0s in the core follow a CM relation similar to the ellipticals, but at large radii (R>0.7Mpc) the S0s are systematically bluer and the scatter in the CM relation approximately doubles to ~0.043 magnitudes. The blueing of the S0s is significant at the 95% confidence level. These results imply that the S0 galaxies in the outer parts of the cluster have formed stars more recently than the S0s in the inner parts. A likely explanation is that clusters at z=0.33 continue to accrete galaxies and groups from the field and that infall extinguishes star formation. The apparent homogeneity of the elliptical galaxy population implies that star formation in recently accreted ellipticals was terminated well before accretion occurred. We have constructed models to explore the constraints that these observations place on the star formation history of cluster galaxies. We conclude that the population of S0s in clusters is likely to evolve as star forming galaxies are converted into passively evolving galaxies. Assuming a constant accretion rate after z=0.33, we estimate ~15% of the present day early-type galaxy population in rich clusters was accreted between z=0.33 and z=0. The ellipticals (and the brightest S0s) are probably a more stable population, at least since z=0.6., Accepted for publication in the ApJ. 20 pages, 12 figures. Full version and plates available at http://www.astro.rug.nl/~dokkum/papers.html

We present Very Large Array ({\sc vla}) and Westerbork Synthesis Radio Telescope ({\sc wsrt}) 21-cm H{\sc i} observations of 19 late-type low surface brightness (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low H{\sc i} surface densities, about a factor of $\sim 3$ lower than in normal late-type galaxies. We show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surface brightness counterparts. The rotation curves of LSB galaxies rise more slowly than those of HSB galaxies of the same luminosity, with amplitudes between 50 and 120~km~s$^{-1}$, and are often still increasing at the outermost measured point. The shape of the rotation curves suggests that LSB galaxies have low matter surface densities. We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB galaxies are among the least compact, least evolved galaxies. We show that both $M_{\rm HI}/L_B$ and $M_{\rm dyn}/L_B$ depend strongly on central surface brightness, consistent with the surface brightness--mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surface density systems in all respects., Accepted for MNRAS. Needs mn.sty. 20 pages, including figures. Figure 2 not included: available upon request from blok@astro.rug.nl or at http://www.astro.rug.nl/~blok/lsb.html

We present results on the Fundamental Plane of early-type galaxies in the rich cluster CL 0024+16 at z=0.391. The galaxies satisfy a tight Fundamental Plane relation which is similar to that at low redshift. The scatter is 15 % in log re. The data show that massive early-type galaxies existed at z=0.4. The evolution of the M/L ratio is derived from the Fundamental Plane, by a comparison with Coma. The M/L ratio increases by 31 +- 12 % between z=0.391, and z=0.023. The evolution is low when compared to models for stellar populations. The expected evolution depends on the IMF, q0, and the formation redshift of the galaxies. The data are in agreement with high formation redshifts. The most serious bias in the modeling may be the progenitor bias: if the progenitors of some current day early-types are spirals at z=0.4, they would not be included in the sample, and the sample would be biased towards the oldest galaxies. More data are needed to measure the evolution of the Fundamental Plane more precisely, and its scatter. There is a hint that the form of the Fundamental Plane changes with redshift, and this needs to be determined better., Comment: 18 pages, uuencoded compressed postscript file. Accepted for publication in Monthly Notices, March 1996. Figure 3 available at ftp://kapteyn.astro.rug.nl/preprints/200.fig3.ps.gz

We present a spectroscopic survey of the rich, X-ray selected, galaxy cluster CL 1358+6245 at z=0.328. When our 173 new multi-slit spectra of cluster galaxies are combined with data from the literature, we produce a catalog of 232 cluster members in a region 10'x11' (3.5 Mpc x 3.8 Mpc) surrounding the brightest cluster galaxy. These data are used to study the structure and dynamics of the cluster and to examine the radial and velocity distributions as a function of spectral type. We classify the spectral types of the cluster members according to the strengths of the Balmer absorption lines (Hdelta, Hgamma, and Hbeta) and the [OII] 3727 Ang emission line., 29 pages, 14 figures, uses aas2pp4, Accepted for publication in ApJ

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z similar to 0.5) clusters observed with WFPC2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressier for low-redshift clusters. Taken collectively, the ''morphology-density'' relationship, T-Sigma, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the T-Sigma relation is strong in centrally concentrated ''regular'' clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low-redshift clusters, where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the SO fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters and occurs instead in the loose-group phase or even earlier, but SO's are generated in large numbers only after cluster virialization.

The cluster CL 1358+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 from the knot. The effective radius of the bright knot is 0''.022 or 130 h(50)(-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 3 x 10(11) L.. This can be produced by a star formation rate of 36 h(50)(-2) M. yr(-1) (q(0) = 0.5) or by an instantaneous starburst of 3 x 10(8) M.. The spectral lines show velocity variations on the order of 300 km s(-1) along the arc. The Si II line is blueshifted 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(-1) different than that of the arc. The serendipitous discovery of these two galaxies suggests that systematic searches may uncover galaxies at even higher redshifts.

We present numerical simulations of isolated disk galaxies including gas dynamics and star formation. The gas is allowed to cool to 10 K, while heating of the gas is provided by the far-ultraviolet flux of all stars. Stars are allowed to form from the gas according to a Jeans instability criterion: gas is unstable when the Jeans mass is smaller than a critical mass, and stars form as soon as the gas remains unstable longer than the collapse time. With these ingredients we are able to create a two-phase interstellar medium and our model gives realistic star formation rates (SFRs). We investigate the influence of free parameters on the star formation. In order of decreasing importance these are: ionization fraction of the gas (determines the cooling properties), initial mass function (controls the heat input for the gas), collapse time for molecular clouds, and star formation efficiency. In the simulations the star formation quickly settles a kind of thermal equilibrium of the ISM. This result strongly favours the self-regulating mechanism for star formation. The model yields a Schmidt law power dependence of the SFR on gas density (SFR proportional to p(n)) with index n approximate to 1.3, in good agreement with observations. The simulations show that star formation can only occur in the mid-plane of the galaxy, where the gas is dense enough to cool below 100 K. The gas above the plane and outside approximately 6 radial scale lengths is always warm (T > 8000 K), heated by the stellar photons. It suggests that radial truncation of stellar disks is a thermal rather than a dynamical process. Flocculent spiral structure is generally found in the cold gas and consequently also in the young stellar population. It suggests that flocculent spirals are due to the dissipational nature of gas.

Submitted to: Astrophys. J. Suppl. Ser. Abstract: We present here the results of applying a new chemo-evolutionary stellar population model developed by ourselves in a previous paper (Vazdekis et al. 1996) to new high quality observational data of the nuclear regions of two representative elliptical galaxies and the bulge of the Sombrero galaxy. Here we fit in detail about 20 absorption lines and 6 optical and near-infrared colors following two approaches: fitting a single-age single-metallicity model and fitting our full chemical evolutionary model. We find that all of the iron lines are weaker than the best fitting models predict, indicating that the iron-abundance is anomalous and deficient. We also find that the Ca_I index at 4227 A is much lower than predicted by the models. We can obtain good fits for all the other lines and observed colors with models of old and metal-rich stellar populations, and can show that the observed radial gradients are due to metallicity decreasing outward. We find that good fits are obtained both with fully evolutionary models and with single-age single-metallicity models. This is due to the fact that in the evolutionary model more than 80% of stars form with in 1.5 Gyr after the formation of the galaxies. The fact that slightly better fits are obtained with evolutionary models indicates these galaxies contain a small spread in metallicity.

We show that the gas mass fraction of spiral galaxies is strongly correlated with luminosity and surface brightness. It is not correlated with linear size. Gas fraction varies with luminosity and surface brightness at the same rate, indicating evolution at fixed size. Dim galaxies are clearly less evolved than bright ones, having consumed only similar to 1/2 of their gas. This resolves the gas consumption paradox, since there exist many galaxies with large gas reservoirs. These gas-rich galaxies must have formed the bulk of their stellar populations in the last half of a Hubble time. The existence of such immature galaxies at z = 0 indicates that either galaxy formation is a lengthy or even ongoing process, or the onset of significant star formation can be delayed for arbitrary periods in tenuous gas disks.

We present new results on the fundamental plane of galaxies in two rich clusters, Cl 1358+62 at z = 0.33 and MS 2053-04 at z = 0.58, based on Keck and Hubble Space Telescope observations. Our new data triple the sample of galaxies with measured fundamental plane parameters at intermediate redshift. The early-type galaxies in these clusters define very clear fundamental plane relations, confirming an earlier result for Cl 0024+16, at z = 0.39. This large sample allows us to estimate the scatter reliably. We find it to be low, at +/-0.067 in log r(e), or 17% in r(e), similar to that observed in comparable low-redshift clusters. This suggests that the structure of the older galaxies has changed little since z = 0.58. The M/L(V) ratios of early-type galaxies clearly evolve with redshift; the evolution is consistent with Delta log (M/L(V)) similar to -0.3z. The M/L(V) ratios of two E+A galaxies in Cl 1358+62 are also lower by a factor of similar to 3, consistent with the hypothesis that they underwent a starburst 1 Gyr previously. We conclude that the fundamental plane can therefore be used as a sensitive diagnostic of the evolutionary history of galaxies. Our data, when compared with the predictions of simple stellar population models, imply that the oldest cluster galaxies formed at high redshift (z > 2). We infer a different evolutionary history for the E+A galaxies, in which a large fraction of stars formed at z

The behaviour of the 4000 Angstrom break index and of the equivalent width of the main Balmer lines is investigated a) for a single star as a function of effective temperature, gravity and metallicity and b) for a single stellar population as a function of age and metallicity. Consequences for the interpretation of integrated spectra are presented., 5 pages, needs A&A l-aa.sty, accepted for publication in Astronomy & Astrophysics, complete preprint with figures available on request at bianca@ast.cam.ac.uk

Using analytic stability criteria, we demonstrate that, due to their low surface mass density and large dark matter content, LSB disks are quite stable against the growth of global nonaxisymmetric modes such as bars. However, depending on their (poorly constrained) stellar velocity dispersions, they may be only marginally stable against local instabilities. We simulate a collision between an LSB and HSB galaxy and find that, while the HSB galaxy forms a strong bar, the response of the LSB disk is milder, manifesting weaker rings and spiral features. The lack of sufficient disk self-gravity to amplify dynamical instabilities naturally explains the rarity of bars in LSB disks. The stability of LSB disks may also inhibit interaction-driven gas inflow and starburst activity in these galaxies., 13 pages, 3 figures, LaTeX using AASTeX macros 4.0, accepted for publication in the Astrophysical Journal Letters

The stellar and dust content of spiral galaxies as function of radius has been investigated using near-infrared and optical broadband surface photometry of 86 face-on spiral galaxies. Colors of galaxies correlate with the azimuthally averaged local surface brightness both within and among galaxies, with the lower surface brightness regions being bluer. The colors formed from different passband combinations correlate strongly indicating that they probably arise from the same physical process. A 3D radiative transfer model was developed to calculate the effect of dust absorption and scattering on the luminosity and color profiles of galaxies. Stellar synthesis models were used to investigate the effects of the star formation history and the metallicity on the broadband color profiles. Combining all optical and near-infrared data shows that the color gradients in this sample of face-on galaxies are best explained by a combined stellar age and metallicity gradient across the disk, with the outer regions being on average younger and of lower metallicity. Dust reddening probably plays only a minor role, as the dust models cannot produce reddening profiles that are compatible with the observations. The observed color differences implicate substantial M/L(lambda) differences, both within galaxies and among galaxies. The variations are such that the ''missing light'' problem derived from rotation fitting becomes even worse. Late-type galaxies (T greater than or equal to 6) have lower metallicities and are often of younger average age than earlier types and have therefore an entirely different M/L(lambda) in most passbands. The near-infrared passbands are recommended for studies where the M/L(lambda) ratios should not vary too much.

In order to explain the spectroscopic observations of most of the galaxies in intermediate redshift clusters, bursts of star formation superimposed to the traditional scenario of galactic evolution are needed. The analysis of spectral lines and colours by means of an evolutionary synthesis model, including both the stellar contribution and the emission of the ionized gas, allows in most of the cases the determination of the time elapsed since the end of the burst and the fraction of galactic mass involved in it. In the four clusters considered (AC103, AC114, AC118 at $\rm z=0.31$ and Cl1358+6245 at $\rm z=0.33$), the theoretical analysis demonstrates that the bursts affect substantial galactic mass fractions, typically 30 \% or more. The observations can be equally well reproduced by either elliptical+burst models or by spiral+burst models in which the star formation is truncated at the end of the burst. A way to determine the galactic original type is suggested., Comment: 15 pages, accepted for publication in Astronomy & Astrophysics, Latex version using l-aa style macro, preprint with figures available on request to: bianca@astro.rug.nl

Low surface brightness (LSB) galaxies are galaxies dominated by an exponential disc whose central surface brightness is much fainter than the value of mu(B)(0) = 21.65 +/- 0.30 mag arcsec(-2) found by Freeman. In this paper we present broadband photometry of a sample of 21 late-type LSB galaxies. The median central surface brightness of the sample is mu(B)(0) = 23.2 mag arcsec(-2) and the median scale length is 3.2 kpc, showing that LSB galaxies are normal-sized galaxies. We find colours that are comparable to or bluer than those of the more widely studied 'normal' high surface brightness (HSB) galaxies. LSB galaxies are therefore not faded discs that have no current star formation. The colours cannot on the other hand be ascribed entirely to metallicity effects, but we can explain them by assuming a sporadic star formation rate scenario. LSB galaxies hence appear to be unevolved and quiescent objects, which give us an insight into the evolution of galaxies in an unperturbed environment.

There are strong indications from recent papers (e.g. Worthey et al. 1992) that the abundance ratio of Mg/Fe, and consequently also O/Fe in giant elliptical galaxies is not solar. The line strengths of two Fe lines at 5270 and 5335 Angstrom are weaker than one expects from the strength of the Mg b line if [Mg/Fe] = 0. We have synthesized absorption line indices to derive the Mg and Fe abundances of these galaxies. For these models we have calculated new evolutionary tracks of high metallicity stars with a range of Mg/Fe abundances. This is the first time that such tracks have been generated. Integrating along isochrones to synthesize metal line strengths we find that for a typical bright giant elliptical [Mg/Fe] should be between +0.3 and +0.7. We show that this result is independent of other parameters such as age, total metal content and mixing length parameter. The total metal content is super-solar, but the iron abundance of elliptical galaxies not necessarily should be larger than solar. For the formation of elliptical galaxies our result on the Mg and Fe abundances has the implication that most of the enrichment of the gas has to come from SNe II, which have more massive progenitors and as such produce relatively more O and Mg than Fe. It means that most of the stars have to be formed within a period of 3 x 10(8) years, so that there can only be one major collapse phase of the galaxy.

New optical and NIR K-band images of the inner 3 kpc region of the nearby Virgo spiral M100 (NGC 4321) display remarkable morphological changes with wavelength. While in the optical the light is dominated by a circumnuclear zone of enhanced star formation, the features in the 2.2\mum image correspond to a newly discovered kpc-size stellar bar, and a pair of leading arms situated inside an ovally-shaped region. Only 5% of the K flux is emitted in antisymmetric structures. This indicates that the morphology seen in the NIR is dominated by a global density wave. Making a first-order correction for the presence of localized dust extinction in K using the I-K image, we find that the observed leading arm morphology is slightly hidden by dust. Possible mechanisms responsible for the optical and NIR morphology are discussed, and tests are proposed to discriminate between them. Conclusions are supported with an evolutionary stellar population model reproducing the optical and NIR colors in a number of star forming zones. We argue that this morphology is compatible with the presence of a pair of inner Lindblad resonances in the region, and show this explicitly in an accompanying paper by detailed numerical modeling. The observed phenomena may provide insight into physical processes leading to central activity in galaxies., 8 pages: postscript,compressed,uuencoded. 8 compressed postscript figures available at ftp://asta.pa.uky.edu/shlosman/lett100/, mget *.eps.Z To appear in ApJ. Letters

We present the B band Tully-Fisher relation for Low Surface Brightness (LSB) galaxies. These LSB galaxies follow the same Tully-Fisher relation as normal spiral galaxies. This implies that the mass-to-light ratio (M/L) of LSB galaxies is typically a factor of 2 larger than that of normal galaxies of the same total luminosity and morphological type. Since the dynamical mass of a galaxy is related to the rotation velocity and scale length via M \propto V^2 h, at fixed linewidth LSB galaxies must be twice as large as normal galaxies. This is confirmed by examining the relation between scale length and linewidth for LSB and normal galaxies. The universal nature of the Tully-Fisher relation can be understood if LSB galaxies are galaxies with low mass surface density, \sigma. The mass surface density apparently controls the luminosity evolution of a galaxy such as to keep the product \sigma M/L constant., Comment: 9 pages, PostScript. Accepted for publication in Monthly Notices of the Royal Astronomical Society

Author(s): Mathews, GJ; Boyd, RN; Fuller, GM | Abstract: We study several models for the origin and chemical evolution of compact irregular galaxies in order to determine the primordial 4He abundance, Yp, from the zero metallicity intercept of the observed Y versus Z correlations. This study confirms the suggestion that a straight-line fit to the observations does not necessarily give the correct primordial 4He abundance. This is especially true for the extrapolation of the Y versus N/H data which depends upon the relative contributions from secondary and primary nitrogen in low metallicity stars. The extrapolation of the O/H data is also slightly nonlinear even for primary oxygen in a closed-box model with instantaneous recycling due to the time dependence of the hydrogen mass fraction, the breakdown of the instantaneous recycling approximation, the production of helium, nitrogen, and oxygen in stars of different mass, and ejection of metal-poor material from low-mass stars. Our best fits to the data, even after excluding possible contamination of H II regions from neighboring Wolf-Rayet stars, gives Yp = 0.228 ± 0.005 for O/H and Yp = 0.223 ± 0.006 for N/H. These primordial helium abundances are as much as 2 σ below the minimum helium abundance which can be produced in the standard homogeneous big bang model with three light neutrino flavors. This discrepancy may be due to shortcomings of the chemical evolution models, additional systematic errors in the determination of the helium and/or metal abundances in extragalactic H II regions, or effects of nonstandard primordial nucleosynthesis.

We present the discovery of a weak radio galaxy from the Leiden Berkeley Deep Survey at a redshift of 2.390, the faint optical and IR counterpart of the steep-spectrum, compact radio source 53W002. Its lambda-dependent optical continuum morphology is compact with linear size approximately 10-35 kpc (H0 = 50, q0 = 0). In redshifted Ly-alpha, the galaxy is somewhat more extended (less-than-or-similar-to 67 kpc x 40 kpc). Its radio source is 7 times smaller than, and confined by, the Ly-alpha gas. It shows alignment with the Ly-alpha gas and the best seeing optical continuum images. We present nine-band photometry (Ly-alpha U BgriJHK) for the galaxy as well as surrounding objects. The source 53W002 is not variable on time scales of years, in either radio or optical. We compare its rest-frame UV continuum with IUE spectra of various nearby galaxies with relatively recent starbursts, and nearby active galactic nuclei (AGNs). The C IV/Ly-alpha and N v/Ly-alpha ratios suggest that 53W002 has a Seyfert 1-like AGN, and constrain the nonthermal component to be approximately 35% of the observed UV continuum. Several independent age estimates yield a consistent value of 0.25-0.32 Gyr: (1) its small 4000 angstrom break or UV-visual continuum amplitude compared with nearby galaxies; (2) a best model fit to the downturn of its UV spectrum below approximately 2000 angstrom; (3) limits from the lack of detected stellar absorption features; (4) its total stellar mass (from its V- and K-band luminosity) compared with its SFR [from W-lambda (Ly-alpha, z = 0)]. These parameters together suggest that at z = 2.390, 53W002 had processed only a few times 10(11) M. into stars, significantly less than most powerful radio galaxies have converted into stars at z approximately 2-3.8. Hence, star formation in 53W002 has proceeded slower, and started at a later epoch, than in the most powerful high-redshift radio galaxies. The available data are consistent with 53W002 being a genuinely young (radio) galaxy seen at z = 2.390 during its first major starburst. It likely started forming most of its current stars at redshifts 2.5-3.0 (for H0 = 50-100, q0 = 0.0-0.5). This suggests that (radio) galaxies do not form the bulk of their stars coevally, but start doing so over a wide range of cosmic time.