X-ray observations have led to the conclusion that many galaxy clusters are hosting cooling flows. The brightest cluster galaxies could have accreted masses of the order of 1011–1012M⊙, but is still uncertain what the final fate of the accreted gas may be.

The total mass and total extent of galaxies (including their dark halos) are fundamental parameters that are completely unknown for all galaxies. The best estimates we have for spiral and dwarf irregular galaxies come from detailed mass models using extended HI rotation curves. But, in every galaxy studied so far, such analysis only succeeded to derive lower limits of the total mass and total extent of their dark halo out to the last measured velocity point of the rotation curves which are still flat or even rising, implying that more dark mass is present at larger radii.

The age–metallicity relation (AMR) from the Old Open Clusters population (Carraro & Chiosi, 1994; Friel & Janes, 1993) is compared with the disc stellar populations obtained from a recently developed model of the Milky Way by Ng 1994. A picture for the chemical evolution of the disc is presented in which the presence of a newly discovered Bar population (t = 8–9 Gyr, Z = 0.005–0.030) is taken into account. We suggest that the past history of the Galactic Disc has been significantly influenced by infall of metal poor gas from the halo and accretion events. The results are shown in Fig. 1.

We investigate the effects of the observed UV background radiation on galaxy formation. The biasing mechanism by photoionization is examined in detail, and its implication for galaxy number counts is discussed. Photoionization by UV radiation, J, decreases the cooling rate of the gas in halos, so that objects with only large density contrasts can self-shield against the radiation. In the context of the CDM model, we use the criterion that self-shielding is essential for star formation to calculate the mass function of galaxies (see Chiba & Nath 1994, ApJ, 436, 618 for details). We find that the cooling rate in the big (low-density) system, into which smaller objects are merged, is reduced by photoionization. This means that, in a merging dominated region, where the number density at the low-mass end (Mb ≤ 1010M⊙) is usually expected to decrease with time, the trend is reversed (the number of low-mass galaxies is more at lower redshifts z) due to the decreasing UV flux with time after z ∼ 2 (see Figure 1).

We present a simple recipe to derive the metallicity distribution of galaxies as a function of their integrals of the motion. Elliptical galaxies are known to possess metallicity gradients that frequently show variations of a factor of 2 from the center to the effective radius (Peletier, 1989). The observed gradients are the result of projection and orbital mixing. Orbital mixing arises because stars at a given radius may have their apocenters spanning a wide range of radii. Thus, the mean metallicity at any given point inside the galaxy is the result of the metallicity distribution in phase space weighted by the galaxian distribution function. We have proposed (Ciotti, Stiavelli & Braccesi, 1995) a simple inversion procedure allowing one to derive the dependence of metallicity on the integrals of motion for a spherical galaxy. The ideas of this paper can be generalized to oblate two-integrals models following the Hunter & Qian (1993) technique.

Ultraluminous IRAS Galaxies (ULIRGs) are unusual galaxies with luminosities > 1012L⊙ (H0 = 75 kms−1 Mpc−1), comparable to optically selected quasars, with up to 90% of this emitted in the 8-1000 μm region. Initial work on nearby ULIRGs suggested that all of these objects were in interacting or merging systems. Larger samples (e.g. Leech et al 1994) suggested that this might only be true for 2/3 of the objects. The triggering of ULIRG-type activity in the non-merging galaxies would then be difficult to understand.

The population and evolutionary synthesis of the integrated light of clusters and galaxies requires a good knowledge of the underlying stellar spectra. Libraries of observed stellar spectra can be used, but they have several disadvantages, e.g. uneven sampling — which causes problems in the integration phase. Furthermore, no comprehensive library of low– or high–metallicity stars does exist, which would be required to model chemical evolution. Libraries of synthetic spectra could — and should — solve these problems. Using Kurucz and more recent libraries dedicated to cool stars, we constituted a library covering the whole range of Teff: 50 000 – 2500 K.

We have been working on multiband surface photometry, spectrophotometry and Hi synthesis data for 20 Low Surface Brightness (LSB) galaxies. LSB galaxies are well described by disks with an average central surface brightness of ∼ 23.4B-mag arcsec–2. They have scale lengths typical for high surface brightness (HSB) galaxies, though a large range of sizes is present. Their colours are blue, especially at the red side of the spectrum, where they are significantly bluer than HSB galaxies (de Blok et al. 1995a). Modelling and measurements of gas abundances (McGaugh 1994) suggests a low, stochastic star formation rate, and a lack of a large old population. The Hi surface densities are a factor of three lower than those in HSB galaxies (de Blok et al 1995b). However the difference is not as large as in the optical. The Hi disks are considerably larger, relative to the optical disks, than in HSB galaxies. The gas mass fraction is higher, indicating slow evolution. Star formation is inhibited by the low surface densities which are typically below the critical treshold as stipulated by Toomre's gravitational instability criterion. The rotation curves rise gradually, and are observed to flatten out only in a few cases. Often they still rise at the last measured point, or remain solid body through-out. Preliminary mass models suggest extended low density dark matter halos, with baryon dominated inner regions. The inferred evolution for LSB galaxies shows mass and density are fundamental parameters in determining a galaxy's evolutionary fate.

In order to investigate the stability properties of galaxy models with central density cusps, TV-body simulations of oblate models with density ρ ∝ m–1 (m+a)–3 where m2=R2+[z/q]2 and distribution functions f(E, Lz) (computed as in Dehnen, 1995) have been performed with the following results.

1. 1. An E7 model with identical amounts of stars of either sense of rotation was stable over 30 tdyn(r=a). This is interesting for the bending instability has been argued to set in at about this flattening and be responsible for the absence of flatter elliptical galaxies (Merritt & Sellwood, 1994).

2. 2. Rapidly rotating E≳E5 models quickly form weak bars inside the cusp, which are stronger for the more flattened, faster rotating initial configurations. The bars grow in a self similar fashion from inside out: the pattern speed decreases with increasing bar length and time. This process is initiated at the origin, where, because of finite AT, the actual density no longer follows the power law, and stops when the edge of the cusp is reached. A typical example is given in the figure showing the z-y-coordinates of particles with |z|<0.1a after ≃20tdyn(r=a) for an initially rapidly rotating E7-model. The bar has axis ratios of about 5:3:1, and extends almost to corotation. However, it has no sharp edge, but an inhomogenous density with a cusp steeper than the initial model. No sign of a buckling insta-bility has been observerd.

A sample of 86 galaxies was imaged in the B, V, R, I, H and K passbands to study their light and colour distribution as function of radius (de Jong & van der Kruit 1994). The radial colour gradients were compared with new dust models, which included both absorption and scattering, and with the stellar population synthesis models of Bruzual & Chariot (1993) and Worthey (1994). By requiring that the models had to fit all six passband photometry at the same time, the relative effects of dust, stellar age and stellar metallicity could be seperated (de Jong 1995a, 1995b). The main results from this investigation are:

1. – All galaxies become bluer with increasing radius. The colour at each radius correlates strongly with the average surface brightness at that radius, with Hubble type being an additional effect. Late type galaxies are bluer at the same surface brightness than early type galaxies.

2. – The reddening profiles predicted by the dust models are incompatible with the data when all colours have to be fitted at the same time. Dust cannot be the major cause of the colour gradients.

3. – The population synthesis models by Worthey (1994) indicate that the colour gradients cannot be caused by metallicity gradients alone.

4. – The best fit to the data is reached in a model where the colour gradients are mainly caused by an age gradient across the disk, with an additional metallicity gradient to explain the very red central colours. The colours of galaxies of type later than Sc indicate that they have in general a lower metallicity at all radii than the earlier types.

Normally elliptical galaxies are thought to be old, evolved systems, but recently a controversy has arisen over the age of ellipticals. Measurements by Gonzáles (1993, Ph.D. thesis, UCSC) show that the Hβ absorption indices of ellipticals span a range of values. Population synthesis models indicate that the Hβ index is a good age indicator and hence, contrary to normal perception, the ages of ellipticals seem to span a range of values.

Our aim is to study the generic phase-space structure of flattened ellipticals by investigating a few typical cases. Here we report on the E4 elliptical NGC 4697.

The construction of 2-integral and 3-integral distribution functions asks for photometry and kinematic data. In the course of the data analysis, we additionally detected a nuclear dust lane at 3.4″ or 0.4 kpc from the centre, which could be confirmed with HST data. This was put to good use to constrain the inclination.

We present preliminary results of the population synthesis of the Galactic Bulge using different techniques. The observations, taken with CTIO's 4m CS, cover the range 3600Å to 10000Å. The integrated spectrum includes light from stars in the line-of-sight. For the synthesis, we used first a program that we developed (Couture & Hardy (1993), and Hardy et al. (1994)) along with three libraries, one of composite population spectra, (Jablonka et al. (1990) and Bica (1988)), one of stellar spectra (Pickles (1985)), and one of equivalent widths measured on the integrated spectra of coeval populations (Bica and Alloin (1986a), (1986b) and (1987)). Results obtained with the spectra were inconclusive. With the equivalent widths, we found indication of the presence of a young and rich population (1 Myr and [Z/Z⊙]=+0.3), but no old population or poor globular cluster-like population. Further tests are needed to evaluate the extent of the contamination from disk giants and its impact on the synthesis results. We also used Bica and Alloin's optimization program (Bica (1988)) with their library of equivalent witdhs. The contributions are spread over many groups but mainly intermediate-age, above-solar metallicity groups and old and poor groups. In all cases, no old metal-rich population was detected.

The central, spheroidal component of spiral galaxies is frequently found to be “box- or peanut” (b/p) shaped. More than 30% of edge-on galaxies may show this phenomenon (Dettmar 1995). To explain this large frequency a very common process for the origin of this specific shape is required. N-body simulations have shown that this can be attributed to instabilities and resonances caused by bars in spiral disks (e. g., Combes et al. 1990, Raha et al. 1991).

Previous statistics (Jarvis 1986, Shaw 1987, de Souza & dos Anjos 1987) are hampered by the low dynamic range of the photographic material used as well as by dust absorption in late type spirals and the difficulty to disentangle disk and bulge in such systems. We present K-band imaging of the very dusty edge-on galaxy NGC 4302 in the Virgo cluster supporting the presence of b/p bulges in late type spirals.

NIR Observations were obtained with the NICMOS-3 ARNICA camera at TIRGO1 in spring 1994. The combined image of NGC 4302 clearly shows a flat-top “boxy” structure of the spheroid and an additional very thin component appears on the major axis cut. This latter component could be identified with a bar. The example of NGC 4302 demonstrates that b/p bulges are indeed present in disks of late type spirals and that bars can account for a significant fraction of these structures. As a consequence, the bulge/disk ratio of late type spirals has to be interpreted in an evolutionary scenario rather than as a primordial property.

We have solved the radiative transfer equation taking into account both absorption and scattering into the line of sight to the observer to model the disc of spiral galaxies.

A dust model has been adopted, suitable for the diffuse interstellar medium of our Galaxy, to obtain, in aU the considered spectral range (1000 ÷ 10000 Å), consistent quantities to describe both the absorption and the scattering properties of the dust.

We present broad- and narrow-band (B, R, Hα+[NII] and Hα continuum) observations of M86 obtained with a focal reducer at the Calar Alto 1.23m telescope, to look for any signs of material cooling out from the hot X-ray emitting ISM. The above figures show a quotient image (13′ × 13′, north up) (Hα cont. - Hα) to the left, and a Hα image (right) from which the light of the stellar spheroid of M86 has been subtracted. Both are overlayed with ROSAT PSPC X-ray contours.

A deep Hα image of the nearly edge-on (i ∼ 86°; Tully, 1988) irregular galaxy NGC 2188 shows the presence of spectacular filaments of ionized gas extending from a large star-forming complex several hundred pc into the halo of the galaxy (Dettmar, 1994). The origin and ionization mechanism of such extraplanar diffuse ionized features is not yet clear. One promising idea is that the extraplanar DIG is due to large scale mass circulation, i.e. a galactic “fountain” (Shapiro & Field, 1976) or “chimney” (Norman & Ikeuchi, 1989). Hα filaments could then be the photoionized walls of such chimneys or fountains.

Recently obtained VLA HI data also show peculiar HI filaments. East of the center of NGC 2188 a giant superbubble with a radius of 550 pc and energy requirement 1.6×1053 erg is found. However, a correlation between HI structures and Hα filaments cannot be found.

The HI observations reveal large-scale deviations from symmetry which are present in both, the gas distribution and in the velocity field. These suggest that NGC 2188 is a perturbed system, although it is not obviously an interacting galaxy.

We investigate the properties of early-type galaxies as distance indicators by applying the Dn – σ, Fundamental Plane (FP), and log(m) – log re relations to a complete and fairly homogeneous sample of galaxies members of the Virgo and Fornax clusters. The relative mean distance of the two clusters can be derived with an accuracy up to ∼ 3% provided that the internal kinematics of the galaxies is taken into account and a correct statistical approach is used.

The residuals of the Dn – σ and FP relationships do not correlate with many structural and geometrical parameters of the galaxies: the mean effective surface brightness, the total luminosity, the average ellipticity and the Fourier coefficient a4 of the isophotes, and, the exponent m of the r1/m law which parametrizes the shape of the light profiles. On the other hand, the kinematics of the galaxies affect both relations producing residuals that correlate with the maximum rotation velocity Vmax and the (V/σ) ratio.

Once confirmed by future more accurate observations, this effect would introduce a sistematic bias in the distance determination of the clusters; a problem difficult to manage if the internal kinematics of the cluster members is unknown.

We investigate the utility of UV spectra of old stellar populations as diagnostics of galaxy properties. For λ < 3800 Å, the integrated light of old stellar populations is dominated by two components: the hot stars which give rise to the UV upturn phenomenon at far-UV wavelengths, and the stars (i.e. main sequence, and subgiants) that lie closest to the turnoff. The mid-UV radiation from the turnoff varies strongly with metallicity, and less so with age; mid-UV light must be correct for the effect of the independent far-UV upturn component, but this is straightforward. Population synthesis models that account for the flux from the UV upturn can therefore determine the characteristics of the underlying stellar content from the mid-UV spectral region. The age & metallicity dependence of the far-UV 1550 Å is not well understood (see Dorman, O'Connell & Rood 1995, ApJ 442,105) since the mass loss mechanism on the RGB that produces very blue HB stars has no physical model. In contrast mid-UV indicators derive from the turnoff population which best represents the quantities we wish to measure, and are in addition visible at favourable optical wavelengths for z ≲ 1.

We performed a small survey of the total and linearly polarized emission of six edge-on galaxies at λ2.8 cm. The selected galaxies show very different levels in their star forming activity, their environment (interacting or not), and many other properties. In the following we present the main results of these observations.