PDF hosted at the Radboud Repository of the Radboud University Nijmegen The following full text is a preprint version which may differ from the publisher's version. For additional information about this publication click this link. http://hdl.handle.net/2066/111270 Please be advised that this information was generated on 2021-10-05 and may be subject to change. The Snapshot Hubble U-Band Cluster Survey (SHUCS). I. Survey Description and First Application to the Mixed Star Cluster Population of NGC 4041.∗ I. S. Konstantopoulos1,2, L. J. Smith3, A. Adamo4, E. Silva-Villa5, J. S. Gallagher6, N. Bastian7,8, J. E. Ryon6, M. S. Westmoquette9, E. Zackrisson10, S. S. Larsen11, D. R. Weisz12, J. C. Charlton2 ABSTRACT We present the Snapshot Hubble U-band Cluster Survey (SHUCS), a project aimed at char- acterizing the star cluster populations of ten nearby galaxies (d< 23 Mpc, half within ≈ 12 Mpc) through new F336W (U band equivalent) imaging from Wide Field Camera 3, and archival BVI - equivalent data with the Hubble Space Telescope. Completing the UBVI baseline reduces the age-extinction degeneracy of optical colours, thus enabling the measurement of reliable ages and masses for the thousands of clusters covered by our survey. The sample consists chiefly of face-on spiral galaxies at low inclination, in various degrees of isolation (isolated, in group, merging), and includes two AGN hosts. This first paper outlines the survey itself, the observational datasets, the analysis methods, and presents a proof-of-concept study of the large-scale properties and star cluster population of NGC 4041, a massive SAbc galaxy at a distance of ≈ 23 Mpc, and part of a small grouping of six giant members. We resolve two structural components with distinct stellar populations, a morphology more akin to merging and interacting systems. We also find strong evidence of a truncated, Schechter-type mass function, and a similarly segmented lumi- nosity function. These results indicate that binning must erase much of the substructure present in the mass and luminosity functions, and might account for the conflicting reports on the in- trinsic shape of these functions in the literature. We also note a tidal feature in the outskirts of the galaxy in GALEX UV imaging, and follow it up with a comprehensive multi-wavelength study of NGC 4041 and its parent group. We deduce a minor merger as a likely cause of its segmented structure and the observed pattern of a radially decreasing star formation rate. We propose that combining the study of star cluster populations with broad-band metrics is not only advantageous, but often easily achievable thorough archival datasets. arXiv:1302.6598v1 [astro-ph.CO] 26 Feb 2013 Subject headings: surveys: SHUCS — galaxies: individual (NGC 4041) — galaxies: star clusters: general — galaxies: interactions — galaxies: star formation — galaxies: groups: individual: LGG 266 1Australian Astronomical Observatory, PO Box 915, Qu´ebec (CRAQ), Universit´eLaval, Qu´ebec, Canada. North Ryde NSW 1670, Australia; [email protected]. 6Department of Astronomy, University of Wisconsin- 2Department of Astronomy & Astrophysics, The Penn- Madison, 5534 Sterling, 475 North Charter Street, Madison sylvania State University, University Park, PA 16802, USA. WI 53706, USA. 7 3Space Telescope Science Institute and European Space Excellence Cluster Universe, Boltzmann-Strasse 2, Agency, 3700 San Martin Drive, Baltimore, MD 21218, 85748 Garching bei M¨unchen, Germany. USA. 8Astrophysics Research Institute, Liverpool John 4Max-Planck-Institut for Astronomy, K¨onigstuhl 17, D- Moores University, Egerton Wharf, Birkenhead, CH41 69117 Heidelberg, Germany. 1LD, UK 9 5D´epartement de Physique, de G´enie Physique et European Southern Observatory, Karl-Schwarzschild- d’Optique, and Centre de Recherche en Astrophysique du Strasse 2, 85748 Garching bei M¨unchen, Germany. 10Department of Astronomy, Stockholm University, Os- 1 1. Introduction ters that survive this period of “infant mortal- ity” can disrupt through stellar evolution, two- The launch of the Hubble Space Telescope body relaxation and the tidal field of the host (HST ) over two decades ago started a revolu- galaxy on ∼ Gyr timescales (Bastian & Gieles tion in the study of extragalactic star clusters. 2008). There has been much debate in the The discovery of large numbers of young com- literature over whether infant mortality exists, pact clusters in star-forming galaxies led to the whether the later phases are mass-dependent suggestion that they could be the present-day or not (e. g. Lamers et al. 2005; Fall et al. 2005; analogues of globular clusters (see reviews of Whitmore et al. 2007; Chandar et al. 2010; Bastian et al. Whitmore 2003; Larsen 2004a). The question 2011), and even if most stars do indeed form in of whether these young clusters can survive for a clusters (Bressert et al. 2010). By conducting a Hubble time has still not been settled but their survey of local star-forming galaxies, it will be longevity appears to be critically dependent on en- possible to obtain large samples of clusters cov- vironmental conditions within their host galaxies ering a wide range of masses and ages, and thus (de Grijs & Parmentier 2007; Bastian et al. 2011). help answer several open questions. The installation of Wide Field Camera 3 (WFC3) In addition, the local environment has recently on HST has vastly upgraded the imaging capa- been suggested as a major contributor to clus- bilities of the telescope shortward of 4000 A.˚ It is ter disruption (e. g. Elmegreen & Hunter 2010; now much easier to measure the age and mass dis- Kruijssen et al. 2011). The relationship between tributions of large populations of star clusters in the formation of clusters and the properties of the galaxies, and address fundamental questions such host galaxy is, in fact, far from clear on both global as their long term survival chances. To obtain and local scales, and a survey would help to inves- ages and extinctions for clusters younger than tigate these relationships. So far, detailed studies ∼ 2 Gyr, it is essential to obtain photometry of large numbers of clusters in small samples of across the UBVI baseline (Anders et al. 2004a). galaxies (e. g. Meurer et al. 1995; Goddard et al. Prior to WFC3, HST U band imaging of suf- 2010; Adamo et al. 2011; Silva-Villa & Larsen ficient depth and spatial coverage was feasible 2011) or small numbers of clusters in large sam- for only a few regions of nearby galaxies (e. g. ples of galaxies (Larsen 2004b; Bastian 2008; Smith et al. 2007; Anders et al. 2004b), and only Mullan et al. 2011) have suggested that environ- distant systems in their entirety (e. g. Ostlin¨ et al. ments with a higher star-forming density form a 2003; Adamo et al. 2010a). higher fraction of their stars in clusters. This con- All local, late-type giant galaxies host popu- nection is also relevant to the link between galaxy lations of young and intermediate-age star clus- interactions and increased star formation. Since ters, often with masses and densities that ri- star clusters can trace bursts of star formation, val globular clusters. It has been proposed that they can then provide a viable chronometer for the vast majority of stars are formed in clusters the interaction history of a galaxy. but that most of these clusters (∼ 90%) rapidly Naturally, understanding the physics that gov- dissolve (Lada & Lada 2003) due to the expul- ern these clusters is essential to utilizing them sion of residual gas from star formation. Clus- as tracers of star formation. After two decades of HST -driven research, several cluster parame- car Klein Centre, AlbaNova, Stockholm SE-106 91, Sweden. ters are considered as standard, such as their dis- 11Department of Astrophysics/IMAPP, Radboud Uni- versity Nijmegen, P.O. Box 9010, 6500 GL Nijmegen, The tributions of luminosity and size. Perhaps most Netherlands. notably, many studies have investigated the star 12Department of Astronomy, Box 351580, University of cluster mass distribution, with a common finding Washington, Seattle, WA 98195, USA. that it can be represented by a power law of index ∗ Based on observations made with the NASA/ESA near −2 (e. g. Zhang & Fall 1999; de Grijs et al. Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of 2003; Bik et al. 2003). More recent studies have, Universities for Research in Astronomy, Inc., under NASA however, suggested that the true form is that of a contract NAS 5-26555. These observations are associated Schechter function, a power-law distribution with with program #SNAP 12229. an exponential truncation at high masses. This re- 2 Fig. 1.— Colour composite imaging of NGC 4041, using all available HST imaging (UBVI and Hα). The imprints of various detectors are visible here, arising from uneven spatial coverage across the optical baseline. The new F336W imaging envelops the archival WFPC2 pointings, while the ACS image leaves a trace of its chip gap. The face of NGC 4041 exhibits a composite structure. Pink Hα bubbles trace the distribution of young clusters along spiral arms that wind tighter in the inner regions than the outer galaxy. A bright central component seems to define this two-step structure, while the central peak is offset from the geometrical center, the center of the outermost isophote, by ≈ 1′′. flects the mass function of Giant Molecular Clouds Another parameter of interest is the star cluster (e.g. Solomon et al. 1987), out of which star clus- size distribution and whether this is related to en- ters form. The slope also appears to be a func- vironment and/or age. Observations indicate that tion of brightness, that is to say, the brighter the effective radii are constrained to a range of 0.5 – subsample, the steeper the extracted slope (Gieles 10 pc despite the large dynamic range in cluster 2010).