We present the detection of a low surface-brightness stellar counterpart to an enormous (190 kpc) ring of neutral hydrogen (HI) gas that surrounds the nearby radio galaxy B2 0648+27. This system is currently in an evolutionary stage between major merger and (radio-loud) early-type galaxy. In a previous paper we investigated in detail the timescales between merger, starburst and AGN activity in B2 0648+27, based on its unusual multi-wavelength properties (large-scale HI ring, dominating post-starburst stellar population and infra-red luminosity). In this Research Note we present deep optical B- and V-band imaging that provides further evidence for the merger origin of B2 0648+27. The host galaxy shows a distorted optical morphology and a broad tidal arm is clearly present. A low surface-brightness stellar tail or partial ring curls around more than half the host galaxy at a distance of up to 55 kpc from the centre of the galaxy, following the large-scale, ring-like HI structure that we detected previously around this system. The gas and stars in this ring are most likely tidally expelled material that slowly fell back onto the host galaxy after the merger event. There also appear to be sites of star formation in the HI ring that may have formed within the gaseous tidal debris after the merger. We argue that the observed properties of the gas and stars in B2 0648+27, as well as the apparent time-delay between the merger and the starburst event, may be the logical result of a merger between two gas-rich disk galaxies with a prominent bulge, or of a merger between an elliptical and a gas-rich spiral galaxy. There also appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity.
We present preliminary results of an HST/NICMOS program to image merger remnants in the J, H and K bands. The nuclear brightness profiles for most sample galaxies are similar to those typical for elliptical galaxies, but some (including the well-studied NGC 3921 and 7252) have an unusually high luminosity density at small radii. This is consistent with the prediction of N-body simulations that gas flows to the center during a merger and forms new stars.
We studied the bulge and the disk kinematic of the giant Low Surface Brightness (LSB) galaxy ESO 323-G064 in order to investigate its dynamics and its Dark Matter (DM) content. We observed the galaxy with the integral field spectroscopy (VLT/VIMOS, in IFU configuration). Results for the gaseous kinematics (bulge and disk) and stellar kinematics (bulge) are presented, together with a Jeans model for the stellar bulge kinematics.
Giant low surface brightness galaxies (GLSBs) have flat discs extending up to ~100 kpc. Their formation is a puzzle for cosmological simulations in the cold dark matter scenario. We suggest that GLSBs might be the final product of the evolution of collisional ring galaxies. In fact, our simulations show that, approximately 0.5-1.5 Gyr after the collision which led to the formation of the ring galaxy, the ring keeps expanding and fades, while the disc becomes very large (~100 kpc) and flat. At this stage, our simulated galaxies match many properties of GLSBs (surface brightness profile, morphol…
We present the results of a search for gravitationally-lensed giant arcs conducted on a sample of 825 SDSS galaxy clusters. Both a visual inspection of the images and an automated search were performed and no arcs were found. This result is used to set an upper limit on the arc probability per cluster. We present selection functions for our survey, in the form of arc detection efficiency curves plotted as functions of arc parameters, both for the visual inspection and the automated search. The selection function is such that we are sensitive only to long, high surface brightness arcs with g-ba…
The LMC and SMC are rich in binary star clusters, and some mergers are expected. It is important to characterize single clusters, binary clusters and candidates to mergers. We selected a sample of star clusters in each Cloud with this aim. Surface photometry of 25 SMC and 22 LMC star clusters was carried with the ESO Danish 1.54 m telescope. 23 clusters were observed for the first time for these purposes. We fitted Elson, Fall and Freeman (1987, EFF) profiles to the data, deriving structural parameters, luminosities and masses. We also use isophotal maps to constrain candidates to cluster inte…
We have used the Nancay Radio Telescope to obtain new global HI data for 16 giant low surface brightness (LSB) galaxies. Our targets have optical luminosities and disk scale lengths at the high end for spiral galaxies (L_B~10^10 Lsun and h_r>~6 kpc for H_0=75 km/s/Mpc), but they have diffuse stellar disks, with mean disk surface brightnesses ~1 magnitude or more fainter than normal giant spirals. Thirteen of the galaxies previously had been detected in HI by other workers, but the published HI observations were either confused, resolved by the telescope beam, of low signal-to-noise, or showed …
We have used a 50.4 ksec Chandra obsffervation to examine the galaxy cluster Abell 115 (z=0.1971). The X-ray surface brightness distribution shows a merger of 2 sub-clusters with substantial off-axis motion. The northern sub-cluster moves toward the southwest and exhibits a strong X-ray surface brightness gradient toward this direction as well as a "fan" of X-ray bright Intra-Cluster Medium (ICM) toward the northeast. The southern sub-cluster shows a pronounced northeast-southwest elongation, probably as a consequence of the ongoing merger. The temperature of the ICM is highly non-uniform acro…
We simulate the collisional formation of a ring galaxy and we integrate its evolution up to 1.5 Gyr after the interaction. About 100-200 Myr after the collision, the simulated galaxy is very similar to observed ring galaxies (e.g. Cartwheel). After this stage, the ring keeps expanding and fades. Approximately 0.5-1 Gyr after the interaction, the disc becomes very large (~100 kpc) and flat. Such extended discs have been observed only in giant low surface brightness galaxies (GLSBs). We compare various properties of our simulated galaxies (surface brightness profile, morphology, HI spectrum and …
With a peak luminosity of ~10^47 erg/s, the December 27th 2004 giant flare from SGR1806-20 would have been visible by BATSE (the Burst and Transient Source Experiment) out to ~50 Mpc. It is thus plausible that some fraction of the short duration Gamma-Ray Bursts (sGRBs) in the BATSE catalogue were due to extragalactic magnetar giant flares. According to the most widely accepted current models, the remaining BATSE sGRBs were most likely produced by compact object (neutron star-neutron star or neutron star-black hole) mergers with intrinsically higher luminosities. Previously, by examining corre…
We present 0.55 x 10^6 particle simulations of the accretion of high-density dwarf galaxies by low-density giant galaxies, using models that contain both power-law central density cusps and point masses representing supermassive black holes. The cusp of the dwarf galaxy is disrupted during the merger, producing a remnant with a central density that is only slightly higher than that of the giant galaxy initially. Removing the black hole from the giant galaxy allows the dwarf galaxy to remain intact and leads to a remnant with a high central density, contrary to what is observed. Our results sup…
There is increasing evidence of a local population of short duration Gamma-ray Bursts (sGRB), but it remains to be seen whether this is a separate population to higher redshift bursts. Here we choose plausible Luminosity Functions (LF) for both neutron star binary mergers and giant flares from Soft Gamma Repeaters (SGR), and combined with theoretical and observed Galactic intrinsic rates we examine whether a single progenitor model can reproduce both the overall BATSE sGRB number counts and a local population, or whether a dual progenitor population is required. Though there are large uncertai…
Using the surface brightness fluctuations in HST WFPC-2 images, we determine that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC 4476 lies in the background at ~21 Mpc. We also examine the globular clusters of M87 using archived HST fields. We detect the bimodal color distribution, and find that the amplitude of the red peak relative to the blue peak is greatest near the center. This feature is in good agreement with the merger model of elliptical galaxy formation, where some of the clusters originated in progenitor galaxies while other formed during mergers.
We perform a large parameter survey of 112 collisionless N-body simulations of binary mergers of disk-galaxies with mass ratios of 1:1, 2:1, 3:1, and 4:1 using the special purpose hardware GRAPE. The basic properties of the remnants correlate with the mass ratio of the progenitor disks. We find that about 80% of the equal-mass merger simulations lead to slowly rotating remnants with significant minor-axis rotation. One half of all projected 1:1 remnants shows boxy and the other half shows disky isophotes. Remnants with mass ratios of 3:1 and 4:1 all rotate fast and show a small amount of minor…
Recent observations of globular clusters in intermediate-age (2-4 Gyr old) merger remnants have provided the hitherto `missing link' between between young merger remnants and normal elliptical galaxies. The luminosity functions (LFs) of the `blue' globular clusters exhibit a Gaussian shape centered on M_V ~ -7.2 as found for giant ellipticals, while the `red' clusters exhibit LFs that are more extended towards both the bright and faint ends, as expected for a cluster population of intermediate age. Specific frequencies for these systems have been calculated by evolving both the LF of the `red'…
We present results on the spectroscopic study of the ionized gas in the high redshift radio galaxy USS0828+193 at z=2.57. Thanks to the high S/N of the emission lines in the Keck spectrum, we have been able to perform a detailed kinematic study by means of the spectral decomposition of the emission line profiles. This study reveals the existence of two types of material in this object: a) a low surface brightness component with apparent quiescent kinematics consistent with gravitational motions and b) a perturbed component with rather extreme kinematics. The quiescent halo extends across the e…
NGC5018, one of the weakest UV emitters among giant ellipticals (gE) observed with IUE, appears to consist of an optical stellar population very similar to that of the compact, dwarf elliptical M32, which is several magnitudes fainter in luminosity than NGC5018 and whose stellar population is know to be ~3 Gyr old. Here we show that the mid-UV spectra of these two galaxies are also very similar down to an angular scale hundreds times smaller than the IUE large aperture (as probed by HST/FOS UV spectra obtained through 0.86 arcsec apertures). This implies a reasonably close match of the populat…
The visual surface brightness of K/M giants and dwarfs with near-solar metallicity differ slightly in agreement with the gravity effects predicted by recent theoretical models. We show that M-dwarfs display also a metallicity dependence of the surface brightness in the infrared $K$-band in agreement with theory. Based on these results, we present improved Barnes-Evans type relations and estimate the radii of 60 single or presumed M and K-dwarfs
We estimate the relativistic binaries merger rate and redshift distribution population synthesis of an ensemble of evolving close binaries. Results of such simulations definitely depend on the cosmic star formation rate history, which is different in galaxies of different types. This leads to a difference in merger rate in spiral and elliptical galaxies, which in principle can be an observational test for GRB models. Also a fit of BATSE long bursts \lns is performed, showing that only a wide (2-3 orders of magnitude) luminosity function provides a good fit.
A popular formation scenario for giant elliptical galaxies proposes that they might have formed from binary mergers of disk galaxies. Difficulties with the scenario that emerged from earlier studies included providing the necessary stellar mass and metallicity, maintaining the tight color-magnitude relation and avoiding phase space limits. In this paper we revisit the issue and put constraints on the binary disc merger scenario based on the stellar populations of disc galaxies. We draw the following conclusions: Low redshift collisionless or gaseous mergers of present day Milky Way like disc g…
We present the discovery a giant low surface-brightness arc, of length luminous matrix with surface-brightness mu_B < 26.5 mag arcsec^{-2} and a number of embedded condensations. It is not associated with any giant galaxy in Coma in particular; neither does it have the properties of a gravitational arc. We argue that a fast interaction between the nearby barred S0 galaxy IC 4026 and either IC 4041 or RB 110 is the most natural explanation for the origin of the arc.
We present the results of N-body simulations of the accretion of high-density dwarf galaxies by low-density giant galaxies. Both galaxies contain power-law central density cusps and point masses representing supermassive black holes; the ratio of galaxy masses is 3:1. The cusp of the dwarf galaxy is always disrupted during the merger, leading to a remnant with a weak power law in the intrinsic density and a ``core'' in the projected density. Removing both black holes from the giant and dwarf galaxies allows the dwarf galaxy to remain intact and leads to a remnant with a high central density, c…
Giant radio pulses of the millisecond pulsar B1937+21 were recorded with the S2 VLBI system at 1.65 GHz with NASA/JPL's 70-m radio telescope at Tidbinbilla, Australia. These pulses have been observed as strong as 65000 Jy with widths <= 15 ns, corresponding to a brightness temperature T_b >= 5 x 10^39 K, the highest observed in the universe. The vast majority of these pulses occur in a 5.8 mcs and 8.2 mcs window at the very trailing edges of the regular main pulse and interpulse profiles, respectively. Giant pulses occur in general with a single spike. Only in one case out of 309 was the struc…
The bimodal globular cluster (GC) metallicity distributions of many giant elliptical galaxies are often cited as evidence for the formation of such galaxies through mergers involving gas-rich spirals. In such models, the metal- rich GCs are assumed to have formed during the merger process. We explore an alternative possibility: that these metal-rich clusters represent the galaxy's intrinsic GC population and that the metal-poor component of the observed GC metallicity distribution arises from the capture of GCs from other galaxies, either through mergers or through tidal stripping. Starting wi…
Abridged: We present analysis of Spitzer Space Telescope observations of the three low surface brightness (LSB) optical giant galaxies Malin 1, UGC 6614 and UGC 9024. Mid- and far-infrared morphology, spectral energy distributions, and integrated colors are used to derive the dust mass, dust-to-gas mass ratio, total infrared luminosity, and star formation rate (SFR). The 8 micron images indicate that polycyclic aromatic hydrocarbon molecules are present in the central regions of all three metal-poor LSB galaxies. The diffuse optical disks of Malin 1 and UGC 9024 remain undetected at mid- and f…
Aims:We have studied the bulge and the disk kinematics of the giant low surface brightness galaxy ESO 323-G064 in order to investigate its dynamical properties and the radial mass profile of the dark matter (DM) halo. Methods:We observed the galaxy with integral field spectroscopy (VLT/VIMOS, in IFU configuration), measured the positions of the ionized gas by fitting Gaussian functions to the O[III] and Hbeta emission lines, and fit stellar templates to the galaxy spectra to determine velocity and velocity dispersions. We modeled the stellar kinematics in the bulge with spherical isotropic Jea…
Recent, and older accurate, data on (limb-darkened) angular diameters is compiled for 221 stars, as well as BVRIJK[12][25] magnitudes for those objects, when available. Nine stars (all M-giants or supergiants) showing excess in the [12-25] colour are excluded in the analysis as this may indicate the present of dust influencing the optical and near-infrared colours as well. Based on this large sample, Baade-Wesselink surface-brightness (SB) relations are presented for dwarfs, giants, supergiants and dwarfs in the optical and near-infrared. M-giants are found to follow different SB-relations fro…
Anomalously high K-band surface brightness fluctuations (SBF) have been reported in NGC 4489 by Pahre & Mould (1994), Jensen et al. (1996) and Jensen et al. (1998). However, these conclusions were uncertain because of relatively low signal-to-noise data. New high signal-to-noise data for NGC 4489 have been obtained at the NOAO/KPNO 2.1m and the ESO/La Silla 3.5m NTT telescopes. Adopting the I-band SBF distance modulus determined by Tonry et al. (1990) and the M_I versus (V-I) calibration of Tonry et al. (2000), a value of M_K =-6.18 +/-0.09 mag was derived. Relative to the average empirical M_…
The giant merger remnant galaxy NGC 1316 (Fornax A) is an ideal probe for studying the long-term effects of a past major merger on star cluster systems, given its spectroscopically derived merger age of ~3 Gyr which we reported in a recent paper. Here we present new ground-based, large-area optical and near-IR imaging of star clusters in NGC 1316, complemented with deep HST/WFPC2 imaging. We find that the optical-near-IR colours and luminosities of the brightest ~10 clusters in NGC 1316 are consistent with those of intermediate-age (2-3 Gyr) populations. Unlike `normal' giant ellipticals, the …
(Aims) We present a physical model of the formation of J033239.72-275154.7, a galaxy observed at z=0.41 and characterized by a big young bar of size 6 kpc. The study of this system is particularly interesting for understanding the connection between mergers and bars as well as the properties and fate of this system as it relates to disk galaxy formation. (Methods) We compare the morphological and kinematic properties of J033239.72-275154.7, the latter obtained by the GIRAFFE spectrograph, to those derived from the merger of two spiral galaxies described by idealized N-body simulations includin…
LSB galaxies have low metallicities, diffuse stellar disks, and massive HI disks. We have detected molecular gas in two giant LSB galaxies, UGC 6614 and F568-6. A millimeter continuum source has been detected in UGC 6614 as well. At centimeter wavelengths we have detected and mapped the continuum emission from the giant LSB galaxy 1300+0144. The emission is extended about the nucleus and is most likely originating from the AGN in the galaxy. The HI gas distribution and velocity field in 1300+0144 was also mapped. The HI disk extends well beyond the optical disk and appears lopsided in the inte…
We quantify an evolutionary channel for single sdB stars based on mergers of binaries containing a red giant star and a lower mass main sequence or brown dwarf companion in our Galaxy. Population synthesis calculations that follow mergers during the common envelope phase of evolution of such systems reveal a population of rapidly rotating horizontal branch stars with a distribution of core masses between 0.32 Mo - 0.7 Mo that is strongly peaked between 0.47 Mo - 0.54 Mo. The high rotation rates in these stars are a natural consequence of the orbital angular momentum deposition during the merge…
The early-R stars are carbon-rich K-type giants. They are enhanced in C12, C13 and N14, have approximately solar oxygen, magnesium isotopes, s-process and iron abundances, have the luminosity of core-helium burning stars, are not rapid rotators, are members of the Galactic thick disk and, most peculiarly of all, are all single stars. Conventional single-star stellar evolutionary models cannot explain such stars, but mergers in binary systems have been proposed to explain their origin. We have synthesized binary star populations to calculate the number of merged stars with helium cores which co…
Using a combination of numerical simulation and synthesized Hubble Space Telescope Wide Field Camera 2 (HST WFC2) images, we follow the detectability of morphological signatures of disk galaxy mergers at intermediate redshifts z=0.4 and z=1.0. Rapid evolution in the surface brightness of tidal tails makes their use as an interaction signature limited; long tidal tails will only be detectable relatively early in the encounter, before the galaxies actually merge. Violent relaxation rapidly smooths isophotal irregularities after the galaxies merge, leaving the very low surface brightness tidal de…
We report high quality near-infrared spectroscopy of 12 ultra-luminous infrared galaxy mergers (ULIRGs). Our new VLT and Keck data provide ~0.5" resolution, stellar and gas kinematics of these galaxies most of which are compact systems in the last merger stages. We confirm that ULIRG mergers are 'ellipticals-in-formation'. Random motions dominate their stellar dynamics, but significant rotation is common. Gas and stellar dynamics are decoupled in most systems. ULIRGs fall on or near the fundamental plane of hot stellar systems, and especially on its less evolution sensitive, r(eff)-sigma proje…
We present preliminary results of the XMM-Newton 50 ksec observation of the Perseus cluster. The global east/west asymmetry of the gas temperature and surface brightness distributions, approximately aligned with the chain of bright galaxies, suggests an ongoing merger, although the modest degree of the observed asymmetry certainly excludes a major merger interpretation. The chain of galaxies probably traces the filament along which accretion has started some time ago and is continuing at the present time. A cold and dense (low entropy) cluster core like Perseus is probably well "protected" aga…
We use recent results in binary stellar evolution to argue that binaries with at least one black hole dominate the rate of compact-object mergers. Two phenomena generally attributable to such mergers, gamma-ray bursts and gravity-wave bursts, are therefore likely to originate from near the event horizon of a black hole. In addition to sheer numbers, black holes have an added advantage over neutron stars in both phenomena. For gamma-ray bursts, the presence of an event horizon eases the baryon pollution problem, because energy can be stored into rotation until most baryons have been swallowed, …
We analyze Chandra temperature maps for a sample of clusters with high quality radio halo data, to study the origin of the radio halos. The sample includes A520, A665, A754, A773, A1914, A2163, A2218, A2319, and 1E0657-56. We present new temperature maps for all but two of them (A520 and A754). All these clusters exhibit distorted X-ray morphology and strong gas temperature variations indicating ongoing mergers. Some clusters, e.g., A520, A665, 1E0657-56, exhibit the previously reported spatial correlation between the radio halo brightness and the hot gas regions. However, it is not a general …
Mergers between red galaxies are observed to be common in the nearby Universe, and are thought to be the dominant mechanism by which massive galaxies grow their mass at late times. These ``dry'' mergers can be readily identified in very deep ground based images, thanks to their extended low surface brightness tidal features. However, ground-based images lack the required resolution to determine the morphologies of the merging galaxies, and to measure the amount of dust and associated gas. We present HST/ACS and WFPC2 observations of a sample of 31 bulge-dominated red-sequence galaxies at z~0.1…
We discuss various models and scenarios proposed to explain the nature of the V838 Mon type eruptions. In this class of eruptive objects we include: M31 RV (erupted in 1988), V4332 Sgr (erupted in 1994) and V838 Mon (erupted in 2002). We concentrate on three models: (i) thermonuclear runaway on an accreting white dwarf (nova-like event); (ii) He-shell flash in a post asymptotic giant branch star (born-again AGB); and (iii) merger of stars. We show that models (i) and (ii) cannot account for the majority of the observed properties of the objects. Most significantly, in both nuclear burning type…
We present BIMA and IRAM CO(1--0) observations of seven low surface brightness (LSB) galaxies, including three large spirals with faint disks but prominent bulges, and four relatively small LSB galaxies with irregular disks. The giant LSB galaxies are UGC 5709, UGC 6614 and F568-6 (Malin2). The smaller LSB galaxies are NGC 5585, UGC 4115, UGC 5209 and F583-1. The galaxies were selected based on their relatively high metallicity and apparent signs of star formation in their disks. The BIMA maps suggested the presence of molecular gas in 2 of the giant LSB galaxies, F568-6 and UGC 6614. Using th…
The processes that form and shape galaxy clusters, such as infall, mergers and dynamical relaxation, tend to generate distinguishable differences between the distributions of a cluster's giant and dwarf galaxies. Thus the dynamics of dwarf galaxies in a cluster can provide valuable insights into its dynamical history. With this in mind, we look for differences between the spatial and velocity distributions of giant (b<18) and dwarf (b>18) galaxies in the Coma cluster. Our redshift sample contains new measurements from the 2dF and WYFFOS spectrographs, making it more complete at faint magnitude…
We discuss the near-infrared properties of the nuclei in the 11 merging galaxies of the Toomre sequence, based on high spatial resolution J, H, and K imaging data using NICMOS onboard the Hubble Space Telescope (HST). The observations are less affected by dust extinction than our previous HST/WFPC2 observations and offer higher spatial resolution than existing ground-based near-IR data. We see a marginal trend for the nuclei to become bluer with advancing merger stage, which we attribute to a dispersal of dust at late times in the merging process. Our data also indicate a statistically signifi…
We present near-IR K-band imaging and spectroscopy of a sample of galaxy mergers, which we use to derive light profile indices, absolute magnitudes and central velocity dispersions. It is found that the light distributions of mergers more nearly resemble those of ellipticals than of bulges, but that the mergers lie well away from the Fundamental Plane defined by the ellipticals. This is interpreted as being due to enhancement of the K-band surface brightness of the mergers by a significant population of supergiant stars, and independent evidence for such a population is inferred from measureme…
An unexpected characteristic of low surface brightness galaxies (LSBGs) is that a significant number are massive and possess substantial amounts of atomic gas. We present preliminary results of an ongoing program to obtain BVRIJHK imaging, along with some nuclear spectroscopy, of a well-defined sample of LSBGs which are gas-rich and of similar size to giant, high surface brightness spiral galaxies (HSBGs). These LSBGs span the entire range of Hubble disk morphologies. While their disks are bluer, on average, than comparable HSBGs, the optical morphology of massive LSBGs indicates that many of …
The most luminous galaxies in the present Universe are found at the centers of the most massive dark matter haloes, rich galaxy clusters. In the LCDM cosmology, such massive halo cores are present at redshift z=6 with a comoving number density (as a function of mass interior to ~10 kpc) that is comparable to today's value. The identity of the matter in these central regions is, however, predicted to change as major mergers bring together stars and dark matter from initially well separated sub-units. We use N-body simulations to investigate how these mergers push pre-existing matter outwards in…
We report the discovery of a giant (~160 kpc) knotty extended emission line nebula associated with the radio galaxy PKS1932-46 at z=0.23. The 2-d long slit spectra, obtained with VLT-FORS2 at a large angle (~63 degr) to the radio source axis, shows that the nebula extends well beyond the radio structure and the ionization cones of the active nucleus. This is one of the largest ionized nebulae yet detected around a radio galaxy at any redshift. The analysis of the ionization, morphological and kinematic properties of the knots suggests that these are star forming objects, probably compact HII g…
We present an analysis of a Chandra observation of the massive, nearby galaxy cluster Abell 2319. A sharp surface brightness discontinuity--suggested by previous, lower angular resolution X-ray imaging--is clearly visible in the ACIS image. This roughly 300kpc feature suggests that a major merger is taking place with a significant velocity component perpendicular to the line of sight. The cluster emission-weighted mean temperature is 11.8+/-0.6kev, somewhat higher than previous temperature measurements. The Chandra temperature map of A2319 reveals substructure resembling that anticipated based…
CCD images and spectra of ionized hydrogen in the merging system NGC3256 were acquired as part of a kinematic study to investigate the formation of globular clusters (GC) during the interactions and mergers of disk galaxies. This paper focuses on the proposition that giant H II regions, with an H_alpha luminosity > 1.5 x 10^{40} erg s^{-1}, are birthplaces of young populous clusters (YPC). We estimate 900 30-Doradus-like H II regions formed during the galaxy-galaxy interaction which created NGC 3256. A specific frequency argument for forming a sufficient number of GC, assuming that the NGC3256…
Mergers of gas-rich galaxies lead to gravitationally driven increases in gas pressure that can trigger intense bursts of star and cluster formation. Although star formation itself is clustered, most newborn stellar aggregates are unbound associations and disperse. Gravitationally bound star clusters that survive for at least 10-20 internal crossing times (~20-40 Myr) are relatively rare and seem to contain <10% of all stars formed in the starbursts. The most massive young globular clusters formed in present-day mergers exceed omega Cen by an order of magnitude in mass, yet appear to have norma…
We present the first VI CCD photometry for the Sculptor group galaxy ESO 540-032 obtained at the Very Large Telescope UT1+FORS1. The (I, V-I) colour-magnitude diagram indicates that this intermediate-type dwarf galaxy is dominated by old, metal-poor ([Fe/H]~-1.7 dex) stars, with a small population of slightly more metal-rich ([Fe/H]~-1.3 dex), young (age 150-500 Myr) stars. A discontinuity in the I-band luminosity function is detected at I_0 = 23.44 \pm 0.09 mag. Interpreting this feature as the tip of the red giant branch and adopting M_I=-4.20\pm 0.10 mag for its absolute magnitude, we have …
The structural properties of a sample of 50 dEs in the Virgo cluster are here derived from Near InfraRed (NIR, H-band 1.65 micron) surface photometry and analysed. One-dimensional surface brightness (SB) profiles are extracted using elliptical isophote fitting. They are characterised by means of structural parameters, namely the half light radius R_e, the average surface brightness within R_e (mu_e), and a concentration index (c_31). We show that typical dEs have close-to-exponential NIR SB distributions. The relations between dEs and giant ellipticals (Es) are investigated by comparing the NI…
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ~440 kpc away. This jet emanates from an extremely massive black hole (>10^9 solar mass) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counterjet, even though it is similar to the main jet in brightness up to a scale of tens of kiloparsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in…
New multi-wavelength (radio, optical & X-rays) observational evidences are presented which show that the nearby (z=0.046), rich cluster of galaxies Abell 3376 is experiencing a major event of binary subcluster merger. The key evidence is the discovery of a pair of large, optically unidentified diffuse radio sources (`arcs'), symmetrically located about 2.6 h_{50}^{-1} Mpc apart at the opposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. It is argued that the gas-dynamical shock-waves, which occur naturally during cluster formation, are accelerating charged particles (cosmic ra…
