Warps are ubiquitously present in galactic disks. While their amplitude is larger in denser environments, frequently symmetric warps of large amplitude are found in isolated systems. This suggests that symmetric warps are quite long-lived excitations of disk and Dark Matter halo. Recent attempts to explain the frequency and the common shape of warps invoke the infall of intergalactic material as an agent. According to these theories, warps are generated in response to accreted (dark) material orbiting the galaxy at large distances, or the warped disk itself is proposed to consist of accreted intergalactic gas. Since usually the bending sets in at large radii, they are best observed in the HI emission line tracing the most extended constituent of disk galaxies. We carry out a program to parametrize the structure of selected warped galaxies employing observations in the HI emission line. We introduce our project, present current results and compare them with theories for warp formation and their maintenance.

The extreme outer Galaxy (EOG) has a quite different environment from that in the solar neighbourhood, with low metallicity (less than -0.5 dex), much lower gas density, and small or no perturbation from spiral arms. We suggest that EOG is an excellent laboratory for the study of the star formation processes that happened during the formation period of the Galaxy. In particular, the study of the EOG may shed light on the origin and role of the thick disk, whose metallicity range matches well with that of the EOG. In this poster paper, we show an example of a molecular cloud in EOG (Digel's “Cloud 2”), which is located at R_g~20 kpc beyond the Outer arm. Based on our NIR and ¹²CO data as well as HI, radio continuum, and IRAS data in the archives, we studied the detailed star formation processes in this unique environment, especially the supernova triggered star formation, which should have been the major star formation mode during the halo and thick disk formation.

While traditional theories of disk galaxy formation successfully predict stellar disks with exponential density profiles, it has been acknowledged for some time that the outermost regions of disks exhibit more complicated behavior. Evidence exists that a significant fraction of disks show a second steeper exponential following a short transition region from the inner exponential. Since they are common, disk breaks must not be considered a detail but instead an integral component of spiral galaxies - indeed the outer disk should contain a fossil signature of the formation history, revealing clues about the nature of processes vital to galaxy assembly and evolution such as star formation and its dependence on feedback and gas dynamics, redistribution of stellar mass by resonant interactions, and the nature of gas accretion. To investigate this phenomenon, we have performed a suite of high mass and force resolution N-body+SPH simulations of galaxy formation through dissipational collapse, representative of the onset of disk formation after the last major merger. Our isolated galaxy experiments are unique in that we allow the disk to form completely from the collapse of the gas halo without any a-priori assumptions about its structure. As a result, we can follow in detail the growth and evolution of the disk and the break that develops. Our simulated breaks agree well with break parameters derived from observations of nearby galaxies. The simulations include star formation and feedback prescriptions making them suitable for testing different theories of disk break formation, and most importantly, making observational predictions regarding stellar populations across the break region. These predictions along with their implications for the evolution of galactic disks in general will be discussed.

High-resolution 2D-MHD numerical simulations have been carried out to investigate the effects of continuing infall of clumpy gas in extended HI galactic disks. Given a certain accretion rate, the response of the disk depends on its surface gas density and temperature. For Galactic conditions at a galactocentric distance of ~20 kpc, and for mass accretion rates consistent with current empirical and theoretical determinations in the Milky Way, the rain of compact high velocity clouds (HVC) onto the disk can maintain transonic turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only mechanism of driving turbulence. Some statistical properties of the resulting forcing flow are shown in this work.

In this work we present a model for warped disks based on geometrical parameters observed in edge-on galaxies and on the tilted-ring model to describe the effect that warps can cause on the observed velocity field of galaxies. Supposing that the aspect of a warped disk is a consequence of how successive thin rings (larger in radius and systematically inclined from its mean plane) are seen on the sky plane, we were able to get the main parameters to construct a three-dimensional model of a warped disk from the researches made about edge-on warped galaxies. Since each ring has its own projection on the sky and its own velocity field, assuming axial symmetry along each ring, the overall projection of the galaxy on sky and its velocity field projected on the line of sight must be reinterpreted to take into account the effect that warped disks cause on the gradient of observed velocities and consequently on the rotation curves extracted by the usual way. To check how warps affect the observed rotation curve we simulated the velocity field of a typical warped galaxy with predetermined rotation curve as it would be observed from random line of sights. Comparing the rotation curve inserted with that one extracted for each projection condition we evaluated the warp contribution on the observed velocity field in function of the parameters of projection (inclination, position angle and warp position angle). We verified that warps, as we described, can systematically affect the gradients on the velocity field in such way that their contribution becomes dominant for low inclination galaxies (<30º). In these cases warps can favor the observation of larger velocities toward the external parts of the rotation curves, depending on how the regions affected by the warp are projected on the sky.

Here we present some results of our ongoing project on the search for disk cut-offs and tidal streams in nearby late type galaxies. In this poster we present first results on NGC300, NGC7793, M83 and M33. Most of the data was observed with the ESO/MPG 2.2 Wide-Field-Imager and reduced with the THELI-Pipeline. We carried out stellar photometry on all these images and searched for over-densities in the maps created by selecting the RGB stars based on the color-magnitude diagrams. In NGC 300 this search revealed the presence of a break in the RGB counts at a radius of about 8.5 kpc from the center. Interestingly blue plume stars are present over the whole field of view (FOV) (34x32 arcmin). The M83 images showed three low surface brightness streams, which were detected before only on photographic plates. Using the transformation to the standard magnitude system, derived from the stellar sources, we were able to calculate integral colors for these three streams to constrain the stellar content. Unfortunately our data are not deep enough (B=24.5) to detect substructure in the stellar counts. Using the M33 and NGC7793 datasets we started to test and apply a new algorithm to search for substructure. In addition to using an adaptive binning strategy to reveal over-densities in the RGB maps, we adopted 4-dimensional cluster-finding algorithms, used mainly in the field data-mining, to search for substructures in the photometric data cube. First results and a discussion of the strengths and limitations of this method will be presented.

We present HST imaging of the recent star formation complexes located in the extended UV disk (XUV-disk) component of NGC 5236 (M83), NGC 5055 (M63), and NGC 2090. ACS and WFPC2 observations obtained in FUV (F150LP) and visible (F435W, F606W, F814W) bands permit us to constrain the spectral type of resolved stars and effective age of clusters, in addition to extinction. SED fitting has been utilized for this analysis. Color-magnitude diagrams for the field star population outside of UV-bright complexes yield the SF history over a longer temporal baseline. Our HST observations probe a diverse set of regions (selected in terms of local environment, stellar content, and host galaxy disk type) and thereby aid our understanding of the XUV-disk phenomenon in general.

We are conducting a deep near-infrared (NIR) imaging survey of young embedded clusters in the extreme outer Galaxy (hereafter EOG) with the Galactic radius (R_g) of more than 18 kpc. EOG has the following major characteristics: (i) low metallicity (-1.5<[Fe/H]<-0.5), (ii) low density (less than ~1/100 of the solar neighborhood), (iii) little or no perturbation from the spiral arms, which is the major trigger of the star formation of the inner region of the Galaxy. These characteristics are similar to those of nearby irregular dwarf galaxies (Im) and damped Lyα systems (DLAs). Therefore, EOG is an excellent laboratory to study the detail of IMF in the low-metallicity environments with a great advantage of the proximity compared to nearby galaxies (e.g., D~10 kpc for EOG, while D~50 kpc for LMC). As a first step, we obtained deep NIR (JHK) images of Digel Cloud 2 clusters, which are located at R_g~19 kpc (D~12 kpc), using the Subaru 8.2-m telescope and the MOIRCS wide-field NIR imager. As a result, we detected cluster members down to ~0.05 M(Sun). This is the first deep NIR imaging of the EOG region of this sensitivity. The observed K-band luminosity function suggests that the underlying IMF of the cluster down to ~0.1 M(Sun) is not significantly different from the typical IMFs in the field and in the nearby star clusters (Yasui et al. 2006), while the IMF in the substellar mass regime is still unknown.

Deep optical/near-IR surface photometry of galaxies outside the Local Group have revealed the existence of faint and very red halos around objects as diverse as spirals and blue compact galaxies. The colours of these structures are much too extreme to be reconciled with stellar populations similar to those seen in the halos of the Milky Way or M31, and alternative explanations like dust reddening, high metallicities or nebular emission are also disfavoured. A stellar population obeying an extremely bottom-heavy initial mass function, similar to that recently reported for the LMC field population, is on the other hand consistent with all available data. Because of its high mass-to-light ratio, such a population would effectively behave as baryonic dark matter and could account for some of the baryons still missing from local inventories. A number of potential tests of this hypothesis are discussed, including the constraints set by the observed luminosity function of subdwarfs in the stellar halo of the Milky Way. We also report on the progress of ongoing searches for red halos around low surface brightness disks in stacked SDSS data.

Surface photometry of galaxies at very faint surface brightness levels is largely limited by problems in subtracting the night sky with sufficient accuracy. While most of the sky flux comes from regions inbetween the observer and the target object (i.e. airglow, zodiacal light, unresolved stars in the Milky Way), a small fraction - the extragalactic background light - comes from behind. Here, we point out that since this part of the sky flux can be subject to extinction by small amounts of dust present in the target galaxy, standard reduction procedures may lead to a systematic oversubtraction of this component. This introduces spurious features in the luminosity profiles of galaxies at a surface brightness level comparable to that of the extragalactic background itself. Using the best current estimates for the extragalactic background flux in various optical and near-IR filters, we predict the characteristics of this effect under various assumptions concerning the distribution of dust in galaxies.