NGC 7318B in Stephan's Quintet has two optical arms (toward N and S) emanating from the eastern part of the main body. Since these arms are similar morphologically to the tidal tails of merging galaxies such as NGC 4038/9, it is considered that NGC 7318B itself is a major merger with a retrograde orbit. In order to study the emission-line activity in the tidal arms of NGC 7318B, we took CCD narrow-band (Hα ON and OFF) images and then found a large-scale arc in Hα emission which traces closely the arms. This Hα arc resembles both the radio and the soft X-ray arcs morphologically (van der Hulst & Rots 1981; Pietsch et al. 1997), suggesting that a single physical mechanism is responsible for all of these kinds of emission. Our optical spectroscopic observations of the shell-like feature at the southern tip of the arc reveal both broad Hα emission and stronger-than-normal [NII] and [SII] emission lines, which are typical of supernova remnants (SNRs). The required number of SNRs is estimated to be as much as ~ 106.

We assume that there are Kc subclusters and Kf fields (foreground or background) in a cluster region. Then, the distribution of all galaxies in this region can be described as follow:

where, nc and nf are normalized numbers of subcluster members and field galaxies. φc, φf, are their normalized distributions in radial velocity space. Both of them can be assumed as Gaussian. μc and μf are normalized distributions in the projected surface of the celestial sphere. For field galaxies, it's uniform, and for subcluster members, usually we use the King's approximate formulae. Distribution parameters and their uncertainties can be found by using the standard maximum likelihood method. And membership probabilities of the ith galaxy belonging to the cth subcluster can be calculated as Pc(i) = φc(i)/φp(i).

In order to study environmental effects on the nuclear activity in galaxies, we have been conducting a spectroscopic study of Hickson Compact Groups of galaxies (HCGs, Hickson 1982) which are the densest agglomeration of galaxies. We obtained nuclear spectra of 62 galaxies in 29 HCGs in the spectral range 6200–7000Å with the 188cm telescope at Okayama Astrophysical Observatory. These spectra were classified into the three types by using the emission line ratio [NII]λ6583/Hα; (1) AGN: [NII]λ6583/Hα >0.6, (2) HII nuclei: [NII]λ6583/Hα <0.6, and (3) Absorption: no emission line. We compared the nuclear activity of galaxies in HCGs with that of nearby galaxies (Ho 1996; Ho, Filippenko & Sargent 1997) which provides a representative sample of field galaxies. In early-type spirals (Sa-Sbc), the fraction of HII nuclei in HCGs is smaller than that in the field galaxies, while the fraction of absorption in HCGs is larger than that in field galaxies. On the other hand, in early-type galaxies (E-S0a) and late-type spirals (Sc-P), we found little difference in the nuclear activity between HCGs and field galaxies.

Motivation: In the hierarchical clustering scenario, objects with smaller masses collapse and virialize earlier. I am interested in the effects of the existence of virialized subsystems on the structures of a collapsing larger mass object, and the fate of subsystems.

The ultraluminous IRAS galaxy Mrk 273 is a Seyfert 2, multi-merger system with a triple-nucleus and a 20 kpc (H0 = 100 km s−1 Mpc−1) jet to the South. Also, a plume and faint tail extends ~50 kpc Northwest and there are more than 10 dwarf galaxies within 100 kpc of Mrk 273 as determined from the POSS II R and J film copy. Therefore, the precursor of Mrk 273 is probably connected with a group of galaxies.

In principle, observations of the high redshift population of field galaxies should provide powerful probes of interactions and mergers. As initial examples of such explorations, we highlight recent results from the DEEP program that take advantage of Keck and HST, including median redshifts for samples fainter than I ~ 22, morphologies and colors to z ~ 1, and especially the nature of z ~ 3 galaxies in the Hubble Deep Field. Although tantalizing, the results will need much larger samples to be confirmed and better quantified.

A large number of high redshift galaxies observed with the Hubble Space Telescope (HST) show anomalous morphology and photometric properties, which may be an indication of evolutionary process in young galaxies. We show here by means of numerical simulations that the copious interstellar gas existing in the disks of rapidly collapsing protogalaxies can bring about these peculiarities. Gravitational instability in a gas-rich disk leads to the formation of massive gas clumps with a typical mass of 109M⊙. These subgalactic clumps make disk galaxy evolution a dynamically energetic and chaotic process, and give a natural explanation for peculiar morphology of high redshift galaxies. Moreover, the present model provides a new picture on the causal relationship between the emergence of quasar activities and the dynamical evolution of host galaxies. The clump-driven evolution model is also capable of explaining the correlations observed among present-day galaxies. Namely, the relative bulge dominance, existence of a thick disk, and a mass of the super-massive black hole situated at the galactic center should all be correlated positively. In contrast to their vigorous evolution in isolated state, primeval disk galaxies do not show any dramatic enhancement of activity or remarkable dynamical response in interaction with another galaxies.

The Hubble Deep Field, henceforth HDF, (Williams et al 1996) is a unique data set for studies of faint galaxies. A small area was chosen well away from known local density enhancements and sources of obscuration. It was imaged for 30 complete orbits in each of four filters centered on wavelengths λ300, 450, 606, 814 nm. There are three, contiguous WFPC2 frames and a solid angle 15,500 arcsec2 is usable. This tiny region is teeming with galaxies. At least 2500 significant features can be identified down to R = 30, though it is not clear that these are all entire galaxies.

Galaxy evolution is probably a complex process. Mergers, infall, and starbursts may change galaxy properties systematically with time. As a result, the interpretation of the luminosity function is ambiguous, and information on the mass evolution of galaxies is needed. Such information can be retrieved from the evolution of the Tully-Fisher relation, Faber-Jackson relation, or the Fundamental Plane with redshift.

The traditional view in population synthesis and evolutionary synthesis models assumes that galaxies can be considered closed systems. Thus, the evolution of the stellar population in galaxies which do not interact with the environment (including other galaxies) is described by the so called Pure Luminosity Evolution (PLE) model. As we have heard several times during this conference (and before), galaxies do interact and very few galaxies are expected to evolve passively. Early-type galaxies seem to behave closer to the PLE model than later types, but E galaxies in the bright phase predicted by this model at the galaxy formation epoch have not been observed. Most likely E galaxies do not form all their stars in short lived initial bursts (several talks in this conference). Number, size, and luminosity evolution is required to understand late-type and Irr galaxies. The PLE model is unable to reproduce the number counts, and color and redshift distributions of the galaxies in the HDF (Pozzetti et al. 1997).

In this paper we wish to introduce the first results on two new projects aimed at detecting emission lines in galaxies at z > 1. There are two primary motivations for doing this: Firstly to try and measure the cosmic star-formation rate at these redshifts. The combination of z < 1 redshift surveys and the discovery of the z ~ 3 Hubble Deep Field ultraviolet dropout objects has led to a ‘first draft’ history of the cosmic SFR (Fig. 1). These results are based on UV continuum fluxes, it is highly desirably to confirm these studies with line diagnostics and extend the work to the redshift of the inferred peak (z ~ 1.5).

In this paper we review some of the evidence that the hosts of powerful high redshift (1 ≲ z ≲ 5) radio galaxies (HzRGs) are the progenitors of present day brightest cluster galaxies (BCGs). On the basis of HST imaging we argue that the scenario describing the formation of BCGs consists of at least two important stages. At z > 2 a significant fraction of the stellar mass of a BCG is formed during a massive burst of star-formation. By z ~ 1, well developed massive ellipticals are already observed and from then until the present epoch, the total mass in stars of the BCG will further grow by a factor of order 3, mainly through the accretion of cluster ellipticals.

A decade ago Sanders et al. (1988a, b) proposed a possible QSO formation scenario as a sequence of Galaxy Mergers → Ultraluminous IR Galaxies (ULIRGs) → Warm ULIRGs → QSOs. Since then, this proposal has become a paradigm against which all the subsequent observations will be compared. Fortunately for us, this paradigm makes two specific predictions: that is, 1) ULIRGs are powered by hidden AGNs, and 2) ULIRGs should be more abundant at z ~ 2, where the comoving density of optically-selected QSOs is sharply peaking up.

The most significant unknown in searches for high-z starburst galaxies is the role and importance of absorption by dust. Emission line searches have mostly targeted the Lyα line, which, due to resonant scattering, will be strongly suppressed even if only small amounts of dust are present. Indeed, most of the high redshift starburst galaxies identified by Steidel et al. (1996) have no Lyα emission. The U-band and V-band dropout techniques developed by these authors and by Madau et al. (1996) are likewise sensitive to dust absorption, both because of absorption of the observed ultraviolet (UV) radiation and because UV-obscured galaxies will not be accounted for in these dropout studies.

Our understanding of the Lyα forest has changed considerably following observations by HST and Keck. Lyα clouds at low redshifts (z < 1.7) observed by HST showed two unexpected features: Lanzetta et al. (1995) found that most luminous galaxies at such redshifts produce Lyα absorptions at mean impact parameter ~160h−1kpc, and established the association between Lyα clouds and galaxies. Ulmer (1996) pointed out the strong clustering of Lyα clouds in this redshift range. Motivated by the above, we propose a two-component protogalaxy model for the Lyα clouds based upon our previous work (Miyahata & Ikeuchi 1995). In our model, the Lyα clouds are stable cold clouds confined by the pressure of ambient hot gas in a galactic halo. We determine the properties of these cold clouds and hot gas on the basis of theoretical and observational constraints. We take into account the stability of a cold cloud in the galactic halo in addition to the general stability conditions in a two-component medium (e.g. Ikeuchi & Ostriker 1986), and compare the derived quantities of Lyα clouds in the galactic halo and in the intergalactic medium at both high and low redshifts. We conclude that the ciondition that a cloud is stable against both evaporation and tidal disruption by a hot galactic halo is very restrictive. In the most noteworthy example at z ~ 0.5, a pressure-confined, stable spherical Lyα cloud with NHI = 1014cm−2 cannot survive in the galactic halo, although much higher column density clouds of NHI = 1017cm−2 can. Miyahata & Ikeuchi (1997) discuss how these results constrain an alternative model for Lyα clouds associated with the galaxies observed by Lanzetta et al. (1995).

We have considered the metal enrichment of the intergalactic medium (IGM) and Lyα clouds at high redshifts. The CIV absorption lines associated with the Lyα forest have been observed (e.g. Songaila & Cowie 1996) and the metallicity is estimated as ~0.01Z⊙. Here, based on a galactic wind model, we have examined how the IGM is metal-enriched by the outflows driven by supernova explosions in primeval galaxies and minihalos.

We use 2D hydrodynamical calculations to examine the effect of the external medium on evolution of supernova-driven outflows from dwarf galaxies. Babul & Rees (1992) have suggested a high external pressure may be able to prevent the outflows from escaping beyond the galaxy and that this material, as it cools and falls back into the galaxy, would serve as fuel for a second epoch of star formation. When thermal pressure is dominant, such evolution of the outflows is seen in our simulations and the gas falls back into the galaxy. Babul & Rees, however, did not take into account the possibility that in high pressure environments such as clusters, galaxies are moving and therefore, subject to ram pressure. In our simulations, we find that ram pressure causes the mass shell associated with the outflow to fragment into clumps. These clumps remain in the vicinity of the galaxy for a few tens of million years before being swept away. The distribution of the clouds gives the galaxy in our simulations a characteristic “head-tail” appearance. If the clouds experience star formation during this epoch, we would expect that the light distribution would also show this “head-tail” feature. The tail-like structure is a transient feature that will eventually disappear. We speculate that galaxies observed by Dickinson (1996) in the z = 1.15 cluster around 3C324 are such galaxies.

The effects of spatial correlations of density fluctuations on merger histories of dark matter halos (so-called ‘merger trees’) are analyzed (Nagashima & Gouda 1997). We compare the mass functions of dark haloes derived by a new method for calculating merger trees, that proposed by Rodrigues & Thomas (1996), with those given by other methods such as the Block model, the Press-Schechter formula and our own formula in which the mass functions are analytically expressed in a way that takes into consideration the spatial correlations (Yano et al. 1996). It is found that the mass functions given by the new method are well fit by those given by our formula. We believe that the new method naturally and correctly takes into account the spatial correlations of the density fluctuations due to a calculated, grid-based realization of the density fluctuations, and so is very useful for estimating the merger tree accurately in a way that takes into consideration spatial correlations.

Roukema & Valls-Gabaud (1997) reinforce the conclusion of Colley et al. (1996, 1997) that the Hubble Deep Field (HDF) “galaxies” are probably star-forming regions, not “building-blocks”.