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Aims. We present deep surface photometry of a volume-limited sample of 21 UM emission line galaxies in broadband optical UBVRI and near infrared (NIR) HKs filters. The sample comprises 19 blue compact galaxies (BCGs) and two spirals. Methods. We separated the burst and host populations for each galaxy and compared them to stellar evolutionary models with and without nebular emission contribution. We measured and analyzed the A180 asymmetry in all filters, the concentration index C, the scale length, and the central surface brightness of the host galaxy. Results. A shift in the average A180 asymmetry is detected from optical to NIR. This shift seems correlated with the morphological class of the BCGs. Using the color-asymmetry relation, we identify five BCGs in the sample as mergers, which is confirmed by their morphological class. Though clearly separated from normal galaxies in the concentration-asymmetry parameter space, we find that it is not possible to distinguish luminous starbursting BCGs from the merely star forming low luminosity BCGs. [ABSTRACT FROM AUTHOR]

ABSTRACT The stellar haloes of galaxies can currently be studied either through observations of resolved halo stars or through surface photometry. Curiously, the two methods appear to give conflicting results, as a number of surface photometry measurements have revealed integrated colours that are too red to be reconciled with the halo properties inferred from the study of resolved stars. Several explanations for this anomaly have been proposed - including dust photoluminescence, extinction of extragalactic background light and a bottom-heavy stellar initial mass function. A decisive test is, however, still lacking. Here, we explain how observations of the halo of a nearby galaxy, involving a combination of both surface photometry and bright star counts, can be used to distinguish between the proposed explanations. We derive the observational requirements for this endeavour and find that star counts in filters VI and surface photometry in filters VIJ appear to be the optimal strategy. Since the required halo star counts are already available for many nearby galaxies, the most challenging part of this test is likely to be the optical surface photometry, which requires several nights of exposure time on a 4-8 m telescope, and the near-infrared surface photometry, which is most readily carried out using the upcoming James Webb Space Telescope. [ABSTRACT FROM AUTHOR]

ABSTRACT The numerous and massive young star clusters in blue compact galaxies (BCGs) are used to investigate the properties of their hosts. We test whether BCGs follow claimed relations between cluster populations and their hosts, such as the fraction of the total luminosity contributed by the clusters as function of the mean star formation rate (SFR) density, the V-band luminosity of the brightest youngest cluster as related to the mean host SFR and the cluster formation efficiency (i.e. the fraction of star formation happening in star clusters) versus the density of the SFR. We find that BCGs follow the trends, supporting a scenario where cluster formation and environmental properties of the host are correlated. They occupy, in all the diagrams, the regions of higher SFRs, as expected by the extreme nature of the starbursts operating in these systems. We find that the star clusters contribute almost to the 20 per cent of the UV luminosity of the hosts. We suggest that the BCG starburst environment has most likely favoured the compression and collapse of the giant molecular clouds, enhancing the local star formation efficiency, so that massive clusters have been formed. The estimated cluster formation efficiency supports this scenario. BCGs have a cluster formation efficiency comparable to luminous IR galaxies and spiral starburst nuclei (the averaged value is ∼35 per cent) which is much higher than the 8-10 per cent reported for quiescent spirals and dwarf star-forming galaxies. [ABSTRACT FROM AUTHOR]

We have investigated the starburst properties of the luminous blue compact galaxy ESO 185−IG13. The galaxy has been imaged with the high-resolution cameras onboard the Hubble Space Telescope. From the ultraviolet (UV) to the infrared (IR), the data reveal a system shaped by hundreds of young star clusters, and fine structures, like a tidal stream and a shell. The presence of numerous clusters and the perturbed morphology indicate that the galaxy has been involved in a recent merger event. Using previous simulations of shell formation in galaxy mergers we constrain potential progenitors of ESO 185−IG13. The analysis of the star cluster population is used to investigate the properties of the present starburst and to date the final merger event, which has produced hundreds of clusters younger than 100 Myr. We have found a peak of cluster formation only 3.5 Myr old. A large fraction of these clusters will not survive after 10-20 Myr due to the 'infant mortality' caused by gas expulsion. However, this sample of clusters represents a unique chance to investigate the youngest phases of cluster evolution. As already observed in the analogue blue compact galaxy Haro 11, a fraction of young clusters are affected by a flux excess at wavelengths longer than 8000 Å. Ages, masses and extinctions of clusters with this near-IR (NIR) excess are estimated from UV and optical data. We discuss similarities and differences of the observed NIR excess in ESO 185−IG13 clusters with other cases in the literature. The cluster ages and masses are used to distinguish among the potential causes of the excess. We observe, as in Haro 11, that the use of the IR and the (commonly used) band data results in overestimates of age and mass in clusters affected by the NIR excess. This has important implications for a number of related studies of star clusters. [ABSTRACT FROM AUTHOR]

We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO 350−IG038) by means of its stellar cluster population. The central starburst region has been observed in eight different high-resolution Hubble Space Telescope ( HST) wavebands, sampling the stellar and gas components from UV to near-infrared. Photometric imaging of the galaxy was also carried out at 2.16 μm by NaCo AO instrument at the ESO Very Large Telescope. We constructed integrated spectral energy distributions (SEDs) for about 200 star clusters located in the active star-forming regions and compared them with single stellar population models (suitable for physical properties of very young cluster population) in order to derive ages, masses and extinctions of the star clusters. The cluster age distribution we recover confirms that the present starburst has lasted for 40 Myr, and shows a peak of cluster formation only 3.5 Myr old. With such an extremely young cluster population, Haro 11 represents a unique opportunity to investigate the youngest phase of the cluster formation process and evolution in starburst systems. We looked for possible relations between cluster ages, extinctions and masses. Extinction tends to diminish as a function of the cluster age, but the spread is large and reaches the highest dispersion for clusters in partial embedded phases (<5 Myr). A fraction of low-mass (below ), very young (1–3 Myr) clusters is missing, either because they are embedded in the parental molecular cloud and heavily extinguished, or because of blending with neighbouring clusters. The range of the cluster masses is wide; we observe that more than 30 per cent of the clusters have masses above , qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000 Å which cannot be explained by simple stellar evolutionary models. Fitting SED models over all wavebands leads to systematic overestimates of cluster ages and incorrect masses for the stellar population supplying the light in these clusters. We show that the red excess affects also the HST filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40 Myr; we discuss possible physical explanations for the phenomenon. Finally, we estimate that Haro 11 has produced bound clusters at a rate almost a factor of 10 higher than the massive and regular spirals, like the Milky Way. The present cluster formation efficiency is ∼38 per cent of the galactic star formation rate. [ABSTRACT FROM AUTHOR]

The faint regions of galaxies, groups and clusters hold important clues about how these objects formed, and surface photometry at optical and near-infrared wavelengths represents a powerful tool for studying such structures. Here, we identify a hitherto unrecognized problem with this technique, related to how the night sky flux is typically measured and subtracted from astronomical images. While most of the sky flux comes from regions between the observer and the target object, a small fraction – the extragalactic background light (EBL) – comes from behind. We argue that since this part of the sky flux can be subjected to extinction by dust present in the galaxy/group/cluster studied, standard reduction procedures may lead to a systematic oversubtraction of the EBL. Even very small amounts of extinction can lead to spurious features in radial surface brightness profiles and colour maps of extended objects. We assess the likely impact of this effect on a number of topics in extragalactic astronomy where very deep surface photometry is currently attempted, including studies of stellar haloes, starburst host galaxies, disc truncations and diffuse intragroup/intracluster light. We argue that EBL extinction may provide at least a partial explanation for the anomalously red colours reported for the haloes of disc galaxies and for the hosts of local starburst galaxies. EBL extinction effects also mimic truncations in discs with unusually high dust opacities, but are unlikely to be the cause of such features in general. Failure to account for EBL extinction can also give rise to a non-negligible underestimate of intragroup and intracluster light at the faintest surface brightness levels currently probed. Finally, we discuss how EBL extinction effects may be exploited to provide an independent constraint on the surface brightness of the EBL, using a combination of surface photometry and direct star counts. [ABSTRACT FROM AUTHOR]

Despite a general consensus in the astronomical community that all quasars are located at the distances implied by their redshifts, a number of observations still challenge this interpretation, possibly indicating that some subpopulation of quasars may harbour significant redshift components not related to the expansion of the Universe. It has been suggested that these objects may have been ejected from local galaxies and are likely to evolve into new galaxies themselves. Here, a test of such exotic scenarios is proposed, based on the spectral energy distribution of the galaxies hosting quasars with suspected ejection origin. Provided that the time-scales over which the ejected objects manifest themselves as quasars is short, one would in the framework of the ejection scenarios expect to find either no quasar host galaxy, a pseudo-host consisting of gas ionized by the quasar, or a host galaxy consisting of young stars only. It is argued that the spectral energy distributions corresponding to the last two options should differ significantly from that of most quasar host galaxies detected at low redshift so far, thus providing a potential test of the claimed existence of ejected quasars. A minimal implementation of this test, involving optical and near-infrared broad-band photometry, is suggested. [ABSTRACT FROM AUTHOR]