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Old Globular Clusters in Dwarf Irregular Galaxies Argelander Institut f¨ur Astronomie der Universit¨at Bonn Old Globular Clusters in Dwarf Irregular Galaxies Dissertation zur Erlangung des Doktorgrades (Dr. rer. nat.) der Mathematisch-Naturwissenschaftlichen Fakult¨at der Rheinischen Friedrich-Wilhelms-Universit¨at Bonn vorgelegt von Iskren Y. Georgiev aus Bulgarien Bonn, July 2008 ii Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakult¨at der Rheinischen Friedrich-Wilhelms-Universit¨at Bonn 1. Referent: Prof. Dr. Klaas S. de Boer 2. Referent: Prof. Dr. Pavel Kroupa Tag der Promotion: 17.9.2008 Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonn unter http://hss.ulb.uni-bonn.de/diss online elektronisch publiziert. Erscheinungsjahr 2008 iii “We are star-stuff”, Carl Sagan iv To my caring family Summary One of the most pressing questions in astrophysics to date is resolving the history of galaxy assembly and their subsequent evolution. In the currently popular “hierar- chical merging” scenario, massive galaxies are believed to grow via numerous galaxy mergers. Present day dwarf irregular (dIrr) galaxies are widely regarded as the pris- tine analogs being the closest to resemble those pre-galactic building blocks. One way to approach this question is through studying the oldest stellar population in a given galaxy, and in particular the ancient globular star clusters whose properties unveil the conditions at the time of their early formation. To address the question of galaxy assembly the current work deals with studying the old globular clusters (GCs) in dIrr galaxies. By comparing the characteristics of GCs in massive galaxies an attempt is made to quantify the contribution of dIrr galaxies to the assembly of massive present-day galaxies and their globular cluster systems (GCSs). In particular, the Galactic GCS is known to harbor sub-populations of GCs whose properties (colors, luminosities, horizontal branch morphologies, struc- tural parameters and orbital kinematics) suggest that they might have formed in low-mass satellites and were later incorporated into the Milky Way GCS. A starting point of my PhD thesis was to see if the GC formation in dIrrs is influenced by the cluster environment. Therefore, based on deep multi-wavelength observations with the eight meter “European Southern Observatory” (ESO) “Very Large Telescope” (VLT) with the FORS1 instrument in U,B,V,I,Hα and J filters, we studied the old GCs of the Magellanic-type dIrr galaxy NGC 1427A in the central regions of Fornax galaxy cluster. We showed (Georgiev et al. 2006, and presented in Chapter 1) that this galaxy, which is as massive as the Large Magellanic Cloud (LMC), hosts 38 8 old metal- poor GCs, more than two times more than the equally massive∼ Large± Magellanic Cloud (LMC). This translates into a present-day GC specific frequency SN = 1.6 0.23, which is significantly higher than S 0.5 for Local Group dIrrs. Therefore,± N ∼ the increased SN value in galaxies in dense environments versus the low SN values for galaxies of similar mass in low-density group environments might be the effect of the dense cluster environment. This finding was recently confirmed by Peng et al. (2008) studying 100 early-type giant and dwarf galaxies in the Virgo galaxy cluster. They showed that galaxies closer to the cluster center tend to have higher GC mass fractions. Another result of our study came from the contamination analysis which indicated that the density distribution of GCs in the outskirts of the Fornax central cD galaxy NGC 1399 may not be spherically symmetric. This was proved to be the case (Firth et al. 2007) by the spectroscopically obtained radial velocity distribution of NGC 1399 and NGC 1427A GCs. Since NGC 1427A resides in a dense cluster environment a necessary comparison, v vi Summary in order to access the impact of the environment, is to study the GCSs of dIrrs in “field” environments. Taking advantage of the supreme resolution of the “Advanced Camera for Surveys” (ACS) on board “Hubble Space Telescope” (HST) to probe down to three magnitudes beyond the expected GC luminosity function (GCLF) turnover magnitude MV,TO, we have performed a search for old GCs in 70 dIrrs on archival F 606W and F 814W images (results are published in Georgiev et al. (2008a) and in Georgiev et al. (2008c), in prep., and presented in Chapter 3). The entire sample consists of 55 dIrrs, 3 dEs, 5 dSphs and 5 low-mass late-type dwarf spirals). Those galaxies reside in nearby (2 - 8 Mpc) associations containing only dwarf galaxies or in the remote halo regions of the galaxy group. All galaxies have absolute magnitudes fainter than or equal to the Small Magellanic Cloud (SMC) (M = 16.2 mag). NGC 121 is the only old ( 11 Gyr, Glatt et al. 2008) GC V − ∼ in the SMC, which is about one magnitude brighter (MV = 8.33 mag) than the typical GCLF turnover magnitude. Hence, one would expect to− find, on average, no more than one GC in each of these galaxies. However, we detect old GC candidates (GCCs) in 30 dIrrs, 2 dEs, 2 dSphs and in 4 Sms. In the studied galaxies we found in total 175 GC candidates, being old and metal-poor according to their colors and magnitudes. The total sample contains 97 metal-poor “blue” (0.7 < (V I)0 < 1.0 mag) GCs (bGCs), 63 red (1.0 < (V I)0 < 1.0 mag) GCs (rGCs) and the− rest are very likely background contaminants.− The total number of GCCs in our sample dIrrs is 119, of which 64 bGCs, 42 rGCs; in dEs/dSphs GCCs sums up to 31, with 21 bGCs and 10 rGCs; the low-mass late- type spirals contain 26 GCCs in total, 13 bGCs and 11 rGCs. The color distribution of all bGCs in dIrrs peaks at (V I) = 0.96 mag and their luminosity function − turnover peaks at MV,TO = 7.6 mag, similar to that of the old LMC GCs. Gaussian fits to the histogram distributions− of bGCs return M = 7.53 0.17 mag and V − ± σ = 1.15 0.11 for dIrrs, MV = 7.12 0.10 mag and σ = 1.09 0.11 for dE/dSph, and M ±= 7.41 0.08 mag and− σ =± 0.92 0.12 for Sm galaxies.± The general V − ± ± trend of MV,TO is such that it becomes fainter from late- to early-type galaxies and σ narrower (with the cautionary note of the low statistical confidence due to small numbers). Given the completeness tests performed, the brighter MV,TO for dwarf galaxies and the lack of faint blue GCs is not due to incompleteness and might reflect relatively young GC systems in these dIrrs. Hence, a subsequent stellar evolutionary fading will make the MV,TO fainter. Also the destructive dynamical processes (mass loss due to stellar evolution, relaxation, tidal shocking, encounters with giant molecular clouds) shaping the GC mass function have to be taken into account. These destroy preferentially low-mass clusters, an effect suggested to shape the GC characteristic mass function (e.g. Gnedin & Ostriker 1997; Fall & Zhang 2001; Kroupa & Boily 2002; Lamers et al. 2005; Lamers & Gieles 2006; Parmentier et al. 2008; Baumgardt et al. 2008; McLaughlin & Fall 2008). It is interesting to note that the cluster disruption time-scale varies with the ambient density and can be shorter in starburst regions where encounters with GMCs are shown to be very destructive (Lamers et al. 2005; Lamers & Gieles 2006). We have discovered that nine dIrrs are nucleated harboring bright (MV 8 to 11 mag) central GCs, similar to nuclear clusters of dEs. These nuclear GCs∼ − have luminosity,− color, and structural parameters similar to those of the peculiar Galactic clusters ω Cen and M 54, suggesting that the latter might have had their origin in Summary vii the central regions of similar Galactic building blocks as the dIrrs in this study. A comparison between half-light radii and ellipticities of bGCs in our sample, old GCs in the LMC and Galactic “Young Halo” (YH) GCs, suspected to have originated from similar dIrrs, is performed. The cluster half-light radius (rh) versus cluster mass plane shows a very similar distribution between bGCs in dIrrs, LMC GCs and YH clusters, which indicates evolution in similar environments. Comparison with theoretical models of cluster dissolution indicates that GCs in low-mass galaxies dynamically evolve mainly as self-gravitating systems, i.e. their rh and Mcl primarily evolves due to internal processes, like two-body relaxation. GCs in dIrrs as well as in the old LMC GCs differ in their ellipticity distributions from Galactic GCs, being on average more flattened (¯ǫ = 0.1). Dynamical models of clusters evolving in isolation show that they reach this asymptotic ǫ value after few cluster relaxation times (> 5trh). This is another indication that old GCs in low-mass galaxies are dynamically evolved stellar systems which evolved in a weak tidal environments. Considering all the similarities found between old GCs in dIrrs and Galactic GCs with complex stellar populations (ω Cen, M 54, NGCs 2808, 6388, 6441 etc.) our results support the suggested accretion origin of a fraction of the Galactic GCs, later incorporated into the Galactic GCS. Quantitatively, we conclude that about 10 dwarf galaxies such as the dIrrs studied here are enough to populate the sub-population of 30 old metal-poor GCs in the halo of our Galaxy. In order for this scenario to apply to other massive and distant galaxies, where such a detailed comparison with their GCs is currently not possible, it is required that the accreted dwarfs have high specific frequencies such as to preserve the observed high ratio between metal-poor GCs and metal-poor field stars in massive galaxies.
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