| 1. |
- Adamo, Angela, et al.
(författare)
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Star cluster formation and evolution in Mrk 930 : properties of a metal-poor starburst
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 415:3, s. 2388-2406
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Tidskriftsartikel (refereegranskat)abstract
- We present the analysis of the large population of star clusters in the blue compact galaxy (BCG) Mrk 930. The study has been conducted by means of a photometric analysis of multiband data obtained with the Hubble Space Telescope (HST). We have reconstructed the spectral energy distributions of the star clusters and estimated the age, mass and extinction for a representative sample. Similar to previous studies of star clusters in BCGs, we observe a very young cluster population with 70 per cent of the systems formed less than 10 Myr ago. In Mrk 930, the peak in the star cluster age distribution at 4 Myr is corroborated by the presence of Wolf-Rayet spectral features, and by the observed optical and infrared (IR) line ratios [OIII]/H beta and [Ne III]/[Ne II]. The recovered extinction in these very young clusters shows large variations, with a decrease at older ages. It is likely that our analysis is limited to the optically brightest objects (i.e. systems only partially embedded in their natal cocoons; the deeply embedded clusters being undetected). We map the extinction across the galaxy using low-resolution spectra and the H alpha-to-H beta ratio, as obtained from ground-based narrow band imaging. These results are compared with the extinction distribution recovered from the clusters. We find that the mean optical extinction derived in the starburst regions is close to the averaged value observed in the clusters [more than 80 per cent of the systems have E(B - V) <= 0.2mag], but locally, do not trace the more extinguished clusters. Previous HST studies of BCGs have revealed a population of young and extremely red super star clusters. We detect a considerable fraction of clusters affected by a red excess also in Mrk 930. The nature of the red excess, which turns up at near-IR wavelengths (I band and longwards), remains unknown. We compare the cluster formation history and the star formation history, the latter derived from the fit of spectral population synthesis models to the spectra. We find a general agreement between the two independently estimated quantities. Using the cluster properties, we perform a study of the host environmental properties. We find that the cluster formation efficiency (the fraction of star formation happening in clusters) is significantly higher, suggesting a key role of the environment for the formation of these massive objects.
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| 2. |
- Adamo, Angela, et al.
(författare)
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Super star clusters in Haro 11: properties of a very young starburst and evidence for a near-infrared flux excess
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 407:2, s. 870-890
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Tidskriftsartikel (refereegranskat)abstract
- We have used multiband imaging to investigate the nature of an extreme starburst environment in the nearby Lyman break galaxy analogue Haro 11 (ESO350-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 2.16μm 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 40Myr, 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 (<5Myr). A fraction of low-mass (below 104 Msolar), very young (1-3Myr) 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 105 Msolar, qualifying them as super star clusters. Almost half of the cluster sample is affected by flux excesses at wavelengths >8000Å 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 F814W filter, which is typically used to constrain cluster physical properties. The clusters which show the red excess are younger than 40Myr 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.
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| 3. |
- Micheva, Genoveva, et al.
(författare)
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A Super-Deep Study of the Outskirts of Haro 11
- 2010
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Ingår i: Hunting for the dark: The hidden side of galaxy formation. - New York : American Institute of Physics. - 9780735407862 ; , s. 295-296
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The low surface brightness host of Haro 11 has been observed to have anomalously red optical/NIR colors of V-K = 4.2+/-0.8 [1]. Predictions from current stellar evolutionary models for any normal stellar population over a wide range of metallicities are inconsistent with this color. We present the deepest V and K band observations to date of Haro 11 and derive a new V-K color for the host galaxy. Our new data suggest a far less extreme colour of V-K = 2.3+/-0.2, which is perfectly consistent with the expectations for an old host galaxy with the same metallicity as that derived from nebular emission lines in the star-forming center.
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| 4. |
- Micheva, Genoveva, et al.
(författare)
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Deep multiband surface photometry on a sample of 24 blue compact galaxies - I
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 431:1, s. 102-144
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Tidskriftsartikel (refereegranskat)abstract
- We present deep optical and near-infrared (NIR) UBVRIHKs imaging data for 24 blue compact galaxies (BCGs). The individual exposure times are on average similar to 40 min in the optical (B) and similar to 90 min in the NIR, but on occasion up to similar to 5 h for a single target and filter, observed with 2.5, 3.5, 8.2-m telescopes. The sample contains luminous dwarf and intermediate-mass BCGs which are predominantly metal poor, although a few have near-solar metallicities. We have analysed isophotal and elliptical integration surface brightness and colour profiles, extremely deep (mu(B) less than or similar to 29 mag arcsec(-2)) contour maps and RGB images for each galaxy in the sample, and provide a morphological classification where such is missing. Separating the burst from the underlying host we find that regardless of the total luminosity the host galaxy has the properties of a low surface brightness (LSB) dwarf with M-B greater than or similar to -18. For a number of galaxies we discover a distinct LSB component dominant around and beyond the Holmberg radius. For the specific case of ESO 400-43A&B we detect an optical bridge between the two companion galaxies at the mu(V) similar to 28th mag arcsec(-2) isophotal level. Synthetic disc tests are performed to verify that we can trace such faint components with negligible errors down to mu(B) = 28 and mu(K) = 23 mag arcsec(-2). By examining the structural parameters (central surface brightness mu(0) and scalelength h(r)) derived from two radial ranges typically assumed to be dominated by the underlying host galaxy, we demonstrate the importance of sampling the host well away from the effects of the burst. We find that mu(0) and h(r) of the BCGs host deviate from those of dwarf ellipticals (dEs) and dwarf irregulars (dI) solely due to a strong burst contribution to the surface brightness profile almost down to the Holmberg radius. Structural parameters obtained from a fainter region, mu(B) = 26-28 mag arcsec(-2), are consistent with those of true LSB galaxies for the starbursting BCGs in our sample, and with dEs and dIs for the BCGs with less vigorous star formation.
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| 5. |
- Micheva, Genoveva, et al.
(författare)
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Deep multiband surface photometry on a sample of 24 blue compact galaxies - I
- 2013
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 431:1, s. 102-144
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Tidskriftsartikel (refereegranskat)abstract
- We present deep optical and near-infrared (NIR) UBVRIHKs imaging data for 24 blue compact galaxies (BCGs). The individual exposure times are on average similar to 40 min in the optical (B) and similar to 90 min in the NIR, but on occasion up to similar to 5 h for a single target and filter, observed with 2.5, 3.5, 8.2-m telescopes. The sample contains luminous dwarf and intermediate-mass BCGs which are predominantly metal poor, although a few have near-solar metallicities. We have analysed isophotal and elliptical integration surface brightness and colour profiles, extremely deep (mu(B) less than or similar to 29 mag arcsec(-2)) contour maps and RGB images for each galaxy in the sample, and provide a morphological classification where such is missing. Separating the burst from the underlying host we find that regardless of the total luminosity the host galaxy has the properties of a low surface brightness (LSB) dwarf with M-B greater than or similar to -18. For a number of galaxies we discover a distinct LSB component dominant around and beyond the Holmberg radius. For the specific case of ESO 400-43A&B we detect an optical bridge between the two companion galaxies at the mu(V) similar to 28th mag arcsec(-2) isophotal level. Synthetic disc tests are performed to verify that we can trace such faint components with negligible errors down to mu(B) = 28 and mu(K) = 23 mag arcsec(-2). By examining the structural parameters (central surface brightness mu(0) and scalelength h(r)) derived from two radial ranges typically assumed to be dominated by the underlying host galaxy, we demonstrate the importance of sampling the host well away from the effects of the burst. We find that mu(0) and h(r) of the BCGs host deviate from those of dwarf ellipticals (dEs) and dwarf irregulars (dI) solely due to a strong burst contribution to the surface brightness profile almost down to the Holmberg radius. Structural parameters obtained from a fainter region, mu(B) = 26-28 mag arcsec(-2), are consistent with those of true LSB galaxies for the starbursting BCGs in our sample, and with dEs and dIs for the BCGs with less vigorous star formation.
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| 6. |
- Micheva, Genoveva, et al.
(författare)
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Deep multiband surface photometry on a sample of 24 blue compact galaxies II. A volume-limited sample of 21 emission line galaxies
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556, s. A10-
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Tidskriftsartikel (refereegranskat)abstract
- 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 A(180) 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.
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| 7. |
- Micheva, Genoveva, et al.
(författare)
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The faint outskirts of the blue compact galaxy Haro 11 : is there a red excess?
- 2010
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 405:2, s. 1203-1211
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies of the low surface brightness host of the blue compact galaxy (BCG) Haro 11 have suggested an abnormally red colour of V - K = 4.2 +/- 0.8 for the host galaxy. This colour is inconsistent with any normal stellar population over a wide range of stellar metallicities (Z = 0.001-0.02). Similar though less extreme host colours have been measured for other BCGs and may be reconciled with population synthesis models, provided that the stellar metallicity of the host is higher than that of the ionized gas in the central starburst. We present the deepest V- and K-band observations to date of Haro 11 and derive a new V - K colour for the host galaxy. Our new data suggest a far less extreme colour of V - K = 2.3 +/- 0.2, which is perfectly consistent with the expectations for an old host galaxy with the same metallicty as that derived from nebular emission lines in the star-forming centre.
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| 8. |
- Micheva, Genoveva, 1977-
(författare)
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Unveiling the nature of blue compact galaxies
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Blue compact galaxies (BCGs) are gas-rich star-forming low redshift galaxies with low metallicities. In some cases the relative strength of the starburst can be so high that it completely dominates the light output of the galaxy, an obstacle which has been countered by deeper optical imaging data and observations in the near infra-red (NIR) regime. This has revealed an older population referred to as the "host". In an effort to study the hosts of BCGs we have analyzed new and extremely deep UBVRIHKs imaging data for 46 high and low luminosity BCGs. For several BCGs the data reveal previously undetected extended low surface brightness components beyond the μB~26 mag arcsec-2 isophote. These are predominantly the luminous BCGs in the sample, and they show tails, plumes, optical bridges between companion galaxies, and other signs of merging or strong tidal interactions. The low luminosity BCGs, on the other hand, are well represented by an exponential disk profile down to the reliability limit of the data at a surface brightness level of μB~28 mag arcsec-2.The burst and host populations are examined separately. The integrated colors of both are compared to predictions from spectral evolutionary models, giving an indication of their respective ages and metallicities. Our analysis suggests that for the luminous BCGs a strong contribution by nebular emission is present almost down to the Holmberg radius, invalidating the host structural parameters obtained from brighter isophotes. Possible evolutionary links to quiescent galaxies like dEs, dIs, and LSBGs are explored by examining the structural parameters derived from two radial ranges typically assumed to be dominated by the underlying host galaxy. In this parameter space the luminous BCGs in our sample deviate from their low luminosity counterparts and from BCG data in the literature. They are instead consistent with the structural properties of giant low surface brightness galaxies with central surface brightnesses μB≥23 mag arcsec-2. We further examine the asymmetry and concentration parameters for the sample and study the correlation between the minimum asymmetry distribution in the optical and NIR vs morphological class, concentration and integrated colors to identify mergers/tidally interacting galaxies. A shift in the asymmetry distribution occurs for low luminosity BCGs from the optical to the NIR. In contrast, we find that the flocculent asymmetry component (due to star formation) completely dominates the composite asymmetry of high luminosity BCGs. We introduce an alternative asymmetry measure which successfully traces the dynamical asymmetry component (due to merging/tidal interaction) of the host.
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| 9. |
- Zackrisson, E.rik, et al.
(författare)
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Red halos and extragalactic background light
- 2010
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Ingår i: Cosmic Radiation Fields: Sources in the early Universe. ; , s. 14-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Deep surface photometry of disk galaxies at optical and near-IR wavelengths have revealed faint halos with colours much too red to be reconciled with the resolved stellar populations detected in the halos of the Milky Way or M31. Alternative scenarios involving high metallicities, nebular emission or large amounts of dust in these halos are also disfavoured. Here, we argue that extinction of the optical extragalactic background light in the halos of these galaxies may possibly be responsible for the reported colour anomalies. We also discuss how an independent measurement of the optical extragalactic background light might be accomplished by combining direct star counts with surface photometry for a single nearby galaxy.
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| 10. |
- Zackrisson, Erik, et al.
(författare)
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Red Halos Revealed by Surface Photometry
- 2010
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Ingår i: Hunting for the dark: The hidden side of galaxy formation. - New York : American Institute of Physics. ; , s. 305-306
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- There are two complementary techniques for studying the luminous halos of galaxies: direct star counts and surface photometry. Star counts can trace the structure of stellar halos out to much larger projected distances, but surface photometry gives a more direct measurement of surface brightness. Curiously, a number of attempts to study the halos of galaxies through optical/near-infrared surface photometry have revealed integrated colours that are too red to be reconciled with the halo properties inferred from the study of resolved stars. We argue that these anomalously red colours can be explained either by a stellar halo population with a bottom-heavy initial mass function, or by dust extinction of extragalactic background light.
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