The UV–Metallicity Characteristics of UCDs/IGCs in the Fornax Cluster

Poster for the American Astronomical Society (AAS) Meeting, Seattle, Jan 2009

Transcript

1. The UV–Metallicity Characteristics of UCDs/IGCs in the Fornax Cluster Arna Karick & Michael Gregg – University of California, Davis & Institute of Geophysics and Planetary Physics, LLNL email: akarick@igpp.ucllnl.org (until February 2009), arnakarick@gmail.com Abstract The remarkable variation of UV-optical ratios in elliptical galaxies suggests that the observed “UV-excess” may be uniquely sensitive to their star formation histories and chemical enrichment. While the stellar evolutionary phase responsible for creating this excess is fairly well understood (Helium-rich hot-HB stars), the UV emission of an integrated stellar population and its dependence on global char- acteristics such as metallicity, is far more complex. Since globular clusters (GCs) have relatively simple stellar populations with small dispersion in age and abundance, they are considered good calibra- tors for understanding these global characterstics. More luminous than globular clusters, the ultra-compact dwarf galaxies (UCDs) in Fornax and Virgo are unique objects for tracing the evolution of the galaxy clusters and the processes governing the formation of individual galaxies. We present a preliminary analysis of the far-UV metallicity relation for these objects using GALEX Far- UV imaging and Magellan/IMACS long-slit spectroscopy. The data reveal tantalizing hints that at least some UCDs exhibit a UV-excess comparable to the the “UV-luminous” GCs in the massive Virgo clus- ter elliptical M87. Optical Properties of Fornax UCDs: First thought to be remnant nuclei of tidally stripped dE,N galaxies, Frnax UCDs have M/L∼3-4, and internal velocity dispersions rang- ing from σv =24–37 kms−1, much larger than the brightest Milky Way globulars. Although they appear scattered across the cluster core, a projected over density near the massive elliptical galaxy NGC1399, suggests an intimate connection with its populous globular cluster system. Figure 1: Distribution of UCDs/NGC1399 GCs in Fornax. The size of the boxes are directly proportioal to their luminosity. Background: B, V, I ∼3x2 degree mosaic of the cluster taken with the CTIO Curtis Schmidt Telescope (Karick, Gregg & Drinkwater) Spectroscopic Catalogs Gregg et al. 2009: AAT/2dF 60 objects within 50′′ of NGC1399 Luminosities: 15.6 < V < 22 mag. Colors:(V – I)<1.5 Firth et al. 2007, 2008 - VLT/FLAMES & AAT/AAOmega 29 objects within 30′′ of NGC1399. Luminosities: 20.6 < V < 21 mag. Colors: 0.3<g – r<1.0, 0<r – i<0.4 (CTIO/4m-MOASIC images, Karick PhD) Mieske et al. 2004 - Las Campanas 50* objects within 20′′ of NGC1399 - *overlaps 2dF sample Luminosities: 18 < V < 21 mag. Colors: (V – I) < 1.5 Figure 2: Multi-color photometry (u,g,r,i,z) from CTIO/4m-MOSAIC imaging of the clus- ter core (Karick, PhD) Left: Luminosities of GCs from Dirsch et al. (2004), Mieske et al.(2002,2004), and Bergond et al.(2007). 2dF UCDs from Gregg et al. (2009). The solid and dashed black lines represent the GCLF and its upper and lower limits, defined by Harris et al. (1991), where MV = −7.67 ± 0.15 mag (Mg = −7.28 ± 0.49 mag) and σV =1.39 ± 0.09 (σg=1.78 ± 0.40 mag). Right: The g−z color-magnitude relation for all compact objects in our catalog. GCs associated with NGC1399 (Dirsch et al. 2004) are represented by black dots as are IGCs from Mieske et al.(2004) and Bergond et al.(2007). 2dF UCDs from Gregg et al. (2009) are distinguished from NGC1399 GCs by black circles. For comparison we also also include Dwarf-Galaxy Transition Objects (DGTOs) from Hasegan et al.(2005), Virgo Cluster UCDs from Jones et al. (2006) and the colors of Fornax Cluster dwarf nuclei. The tick marks at the top of the plot repr esent the mean colors of the blue (g−z = 1.03) ”metal-poor” and red (g−z = 1.49) ”metal-rich” populations derived for the entire sample. Magellan/IMACS Spectroscopy Medium resolution (1.3˚ A), high S/N spectroscopy of UCDs and NGC1399 GCs. Observations by Dr. Marla Geha were kindly provided for our analysis. Mean [Fe/H] values were derived from numerous absorption lines using calibrations from Puzia et al. (2002). GALEX NUV & FUV Archival Imaging We used SExtractor source detection software to measure NUV (1344–1786˚ A) and FUV (1771–2831 ˚ A) magnitudes from 34ks coadded exposures (NGS - DR4) centered to the west of the cluster, which included the bulk of the UCDs near NGC 1399. Optical – UV Properties of UCDs Optical (V&I) photometry is from our CTIO/4m photometry and HST/ACS imaging of UCDs (SNAP-10137). GALEX magnitudes were converted to HSTSTMAG . Figure 3: Left: 34ks coadded GALEX FUV image. Circles higlighting UCDs/GCs near NGC1404. Right: Figure 6. from Sohn et al. (2006): (FUV – V) vs. (V – I) color-color diagram for M87 globular clusters (black points) and Milky Way clusters (triangles). Red points are UCDs and the brightest NGC 1399 globulars. UV – Mg2 Properties of UCDs Preliminary results indicate that a significant fraction of Fornax UCDs and bright NGC1399 globulars have similar UV properties to M87 globulars. Figure 4: Left: Fig 8. from Sohn et al. (2006): Metallicity dependence of (FUV – V ) colors of M87 globular clusters (black points), Milky Way globular clusters (triangles) and early-type galaxies (squares). Fornax UCDs/NGC1399 GCs are represented by the red points. Right: Fig 12. from Sohn et al. (2006): (FUV – V) vs. [Fe/H] estimated from the colours of M87 globulars (black points). Fornax UCDs/NGC1399 globulars are represented by the red circles. [Fe/H] is derived from absorption line measurements from our IMACS spectra. Theoretical isochrones are from Lee et al. (2002), where ∆t = 0 Gyr corresponds to 12 Gyr. Discussion • Previous studies of the “UV-luminous” M87 globulars provide convincing evi- dence that their high-UV fluxes are produced by old, low mass stars on the “extreme” horizontal branch. • Preliminary analysis of our GALEX UV imaging and Magellan/IMACS spec- troscopy indicates that the brightest Fornax UCDs and NGC1399 GCs exhibit a similar “UV-excess”. • If UCDs and NGC1399 globulars have solar abundance ratios, [α/Fe]=0 then the observed “UV-excess” can be explained by invoking a small super-He rich sub-population. • This suggests that perhaps these UCDs and the brightest NGC1399 formed in situ, and their UV-metallicity characteristics may reflect NGC1399’s initial for- mation conditions. • In Progress: (1) deriving more accurate age estimates from the D4000˚ Abreak, (2) measuring accurate [α/Fe] ratios to check the validity of model comparisons. This material is based on part upon work supported by NSF grant no. 0407445 and carried out at IGPP under the auspices of the US DOE by LLNL under contract no. W-7405-Eng-48 and DE-AC52-07NA27344.