arXiv subjects

Iskren Y. Georgiev

Publications and source records attributed to Iskren Y. Georgiev.

23 records · Page 2Linked to original sources

arXiv2010

Globular Cluster Systems in Nearby Dwarf Galaxies: III. Formation Efficiencies of Old Globular Clusters

We investigate the globular cluster (GC) system scaling parameters as a function of galaxy mass, i.e. specific frequency (S_N), specific luminosity (S_L), specific mass (S_M), and specific number (^T) of GCs. We sample the entire range in galaxy luminosity (Mv = -11 to -23 mag = 10^6 - 10^11 L_sol), environment, and morphology. Irrespective of galaxy type, we confirm the increase of the S_N-value above and below a galaxy magnitude of Mv = -20 mag. Over the full mass range, the S_L-value of early-type galaxies is, on average, twice that of late-types. To investigate the observed trends we derive theoretical predictions of GC system scaling parameters as a function of host galaxy mass based on the models of Dekel & Birnboim (2006) in which star-formation processes are regulated by stellar and supernova feedback below a stellar mass of 3x10^10 M_sol, and by virial shocks above it. We find that the analytical model describes remarkably well the shape of the GC system scaling parameter distributions with a universal specific GC formation efficiency, eta, which relates the total mass in GCs to the total galaxy halo mass. Early-type and late-type galaxies show a similar mean value of eta = 5.5e-5, with an increasing scatter towards lower galaxy masses. This can be due to the enhanced stochastic nature of the star and star-cluster formation processes for such systems. Some massive galaxies have excess eta values compared to what is expected from the mean model prediction for galaxies more luminous than Mv = -20 mag (Lv=10^10L_sol). This may be attributed to a very efficient early GC formation, less efficient production of field stars or accretion of predominantly low-mass/luminosity high-eta galaxies, or a mixture of all these effects. (Abridged)

astro-ph.CO↗

arXiv2009

Globular cluster systems in nearby dwarf galaxies - II. Nuclear star clusters and their relation to massive Galactic globular clusters

(Abridged) Using luminosities and structural parameters of globular clusters (GCs) in the nuclear regions (nGCs) of low-mass dwarf galaxies from HST/ACS imaging we derive the present-day escape velocities (v_esc) of stellar ejecta to reach the cluster tidal radius and compare them with those of Galactic GCs with extended (hot) horizontal branches (EHBs-GCs). For EHB-GCs, we find a correlation between the present-day v_esc and their metallicity as well as (V-I)_0 colour. The similar v_esc, (V-I)_0 distribution of nGCs and EHB-GCs implies that nGCs could also have complex stellar populations. The v_esc-[Fe/H] relation could reflect the known relation of increasing stellar wind velocity with metallicity, which in turn could explain why more metal-poor clusters typically show more peculiarities in their stellar population than more metal-rich clusters of the same mass do. Thus the cluster v_esc can be used as parameter to describe the degree of self-enrichment. The nGCs populate the same Mv vs. rh region as EHB-GCs, although they do not reach the sizes of the largest EHB-GCs like wCen and NGC 2419. We argue that during accretion the rh of an nGC could increase due to significant mass loss in the cluster vicinity and the resulting drop in the external potential in the core once the dwarf galaxy dissolves. Our results support the scenario in which Galactic EHB-GCs have originated in the centres of pre-Galactic building blocks or dwarf galaxies that were later accreted by the Milky Way.

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arXiv2008

Globular Cluster Systems in Nearby Dwarf Galaxies: I. HST/ACS Observations and Dynamical Properties of Globular Clusters at Low Environmental Density

We investigate the old globular cluster (GC) population of 68 faint (Mv>-16 mag) dwarf galaxies located in the halo regions of nearby (<12 Mpc) loose galaxy groups and in the field environment based on archival HST/ACS images in F606W and F814W filters. The combined color distribution of 175 GC candidates peaks at (V-I)=0.96 +/- 0.07 mag and the GC luminosity function turnover for the entire sample is found at Mv,to = -7.6 +/- 0.11 mag, similar to the old metal-poor LMC GC population. Our data reveal a tentative trend of Mv,to becoming fainter from late-type to early-type galaxies. The luminosity and color distributions of GCs in dIrrs shows a lack of faint blue GCs. Our analysis reveals that this might reflect a relatively younger GC system than typically found in luminous early-type galaxies. If verified by spectroscopy this would suggest a later formation epoch of the first metal-poor star clusters in dwarf galaxies. We find several bright (massive) GCs which reside in the nuclear regions of their host galaxies. These nuclear clusters have similar luminosities and structural parameters as the peculiar Galactic clusters suspected of being the remnant nuclei of accreted dwarf galaxies, such as M54 and wCen. Except for these nuclear clusters, the distribution of GCs in dIrrs in the half-light radius vs. cluster mass plane is very similar to that of Galactic young halo clusters, which suggests comparable formation and dynamical evolution histories. A comparison with theoretical models of cluster disruption indicates that GCs in low-mass galaxies evolve dynamically as self-gravitating systems in a benign tidal environment.

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arXiv2008

Old globular clusters in magellanic-type dwarf irregular galaxies

We present a study of the old globular clusters (GC) using archival F606W and F814W HST/ACS images of 19 Magellanic-type dwarf Irregular (dIrr) galaxies found in nearby (2 - 8 Mpc) associations of only dwarf galaxies. All dIrrs have absolute magnitudes fainter than or equal to the SMC (Mv = -16.2 mag). We detect 50 GC candidates in 13 dIrrs, of which 37 have (V-I) colors consistent with "blue" (old, metal-poor) GCs (bGC). The luminosity function (LF) of the bGCs in our sample peaks at Mv = -7.41 +/- 0.22 mag, consistent with other galaxy types. The width of the LF is sigma = 1.79 +/- 0.31 which is typical for dIrrs, but broader than the typical width in massive galaxies. The half-light radii and ellipticities of the GCs in our sample (rh ~ 3.3 pc, e ~ 0.1) are similar to those of old GCs in the Magellanic Clouds and to those of "Old Halo" (OH) GCs in our Galaxy, but not as extended and spherical as the Galactic "Young Halo" (YH) GCs (rh ~ 7.7 pc, e ~ 0.06). The e distribution shows a turnover rather than a power law as observed for the Galactic GCs. This might suggest that GCs in dIrrs are kinematically young and not fully relaxed yet. The present-day specific frequencies (SN) span a broad range: 0.3 < SN < 11. Assuming a dissipationless age fading of the galaxy light, the SN values would increase by a factor of ~ 2.5 to 16, comparable with values for early-type dwarfs (dE/dSphs). A bright central GC candidate, similar to nuclear clusters of dEs, is observed in one of our dIrrs: NGC 1959. This nuclear GC has luminosity, color, and structural parameters similar to that of wCen and M54, suggesting that the latter might have their origin in the central regions of similar Galactic building blocks. A comparison between properties of bGCs and Galactic YH GCs, suspected to have originated from similar dIrrs, is performed.

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arXiv2006

The old globular cluster system of the dIrr galaxy NGC1427A in the Fornax cluster

We present a study of the old globular cluster (GC) population of the dwarf irregular galaxy NGC 1427A using multi-wavelength VLT observations in U, B, V, I, H_alpha, J, H, and Ks bands under excellent observing conditions. We applied color and size selection criteria to select old GC candidates and made use of archival ACS images taken with the Hubble Space Telescope to reject contaminating background sources and blended objects from the GC candidates' list. The H_alpha observations were used to check for contamination due to compact, highly reddened young star clusters whose colors and sizes could mimic those of old GCs. After accounting for contamination we obtain a total number of 38+/-8 GC candidates with colors consistent with an old (~10 Gyr) and metal-poor (Z < 0.4xZ_solar) population as judged by simple stellar population models. Our contamination analysis indicates that the density distribution of GCs in the outskirts of the Fornax central cD galaxy NGC1399 may not be spherically symmetric. We derive a present-day specific frequency S_N of 1.6+/-0.23 for NGC 1427A, a value significantly larger than what is observed in the Local Group dwarf irregular galaxies and comparable with the values found for the same galaxy types in the Virgo and Fornax clusters. Assuming a universal globular cluster luminosity function turnover magnitude, we derive a distance modulus to NGC 1427A of 31.01+/-0.21 mag which places it 3.2+/-2.5(statistic)+/-1.6(systematic) Mpc in front of the Fornax central cD galaxy NGC 1399. The implications of this result for the relationship between NGC 1427A and the cluster environment are briefly discussed.

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