The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
Left: An intermediate-luminosity galaxy (VCC1431) in the Virgo Cluster observed with the Advanced Camera for Surveys (ACS) on HST as part of the ACS Virgo Cluster Survey, which targeted 100 early-type galaxies (Cote et al. 2004). Note the central nucleus or “luminosity excess” (see below). A similar survey of the Fornax Cluster, targeting 43 galaxies, is described in Jordan et al. (2007). Top: The ACS/WFC CCDs before the camera was assembled.
The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
The ACS Virgo and Fornax cluster surveys produced more than two dozen publications on topics ranging from the core and global structure of early-type galaxies, to globular cluster systems, new families of hot stellar systems (such as “Ultra Compact Dwarf Galaxies” and “Faint Fuzzies”) and the extragalactic distance scale. Some scientific highlights and data products from the surveys include:
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The first simultaneous characterization of the central and global structure for a large sample of early-type galaxies in the nearby universe (Virgo), made possible by the large field of view of the ACS instrument on HST (Ferrarese et al. 2006a; Cote et al. 2006; Cote et al. 2007).
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The demonstration that the Sersic family of models provides a remarkably accurate description of the brightness profiles of early-type galaxies spanning nearly three orders of magnitude in luminosity (i.e., from “giant” to “dwarf” galaxies, Ferrarese et al. 2006a). These findings build upon pioneering studies by Caon et al. (1993), Graham et al. (2003), Graham & Guzman (2003) and Jerjen & Binggeli (1997).
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The finding of a systematic transition from a central luminosity “deficit” to “excess” in the central regions of galaxies, relative to the global Sersic model fit, and a dramatic upward revision of the frequency of distinct nuclear components in the centers of low- and intermediate-luminosity galaxies (Ferrarese et al. 2006a; Cote et al. 2006; Cote et al. 2007). Once again, see the series of earlier papers by Graham and collaborators, including Graham et al. (2003), Graham & Guzman (2003) and Trujillo et al. (2004), as well as Carolla et al. (1998), Boker et al. (2002, 2004), Lotz et al. (2004) and Grant et al. (2005).
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The realization that these central excesses/nuclei probably arise, for at least some galaxies, through gas inflows and starbursts expected in mergers and accretions, as had been predicted by numerical models (Cote et al. 2006, Cote et al. 2007). See also Mihos & Hernquist (1994), who anticipated these results using pioneering numerical simulations.
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The discovery that the light “excesses” (i.e., compact stellar nuclei) in the faintest galaxies contain roughly the same percentage of the total galaxy mass as do the Supermassive Black Holes (SBHs) in the brightest galaxies, suggesting a possible link between these two components (Ferrarese et al. 2006b, Cote et al. 2006). See the contemporaneous papers by Rossa et al. (2006) and Wehner and Harris (2006), and the comprehensive subsequent study by Seth et al. (2008).
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A demonstration that the early-type galaxy populations do not show a dramatic “dichotomy” in terms of their central brightness profile slopes, as was previously believed; the ACS Virgo Cluster Survey was the first study to show that the previously reported class of “power-law galaxies” actually have a two-component structure on small scales (Ferrarese et al. 2006a; Cote et al. 2007). Once again, see also Jerjen & Binggeli (1997), Graham & Guzman (2003), as well as Rest et al. (2001) and Ravindranath et al. (2001).
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A large and homogeneous catalog of more than ≈ 10,000 globular cluster candidates in early-type galaxies (Jordan et al. 2009).
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The demonstration that the fundamental properties of globular cluster systems show unexpected continuous trends with host galaxy luminosity. Specific examples include their luminosity functions, size distributions, color/metallicity distributions, and formation efficiencies (Jordan et al.2005, 2006, 2007; Peng et al. 2006a,b, 2008; Mieske et al. 2006, 2010; Sivakoff et al. 2007; Masters et al. 2010; Villegas et al. 2010). These results build upon a number of previous studies by other researchers, including Gebhardt & Kissler-Patig (1998), Larsen et al. (2001) and Kundu et al. (2001).
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The discovery of Ultra-Compact Dwarf (UCD) galaxies in the Virgo Cluster, the first measurements for the dynamical masses of these systems, and the discovery of an apparently fundamental transition between globular clusters and UCDs at ≈ 2-3 million solar masses (Hasegan et al. 2005).
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The measurement of accurate SBF distances (i.e., typical errors of ≈ 0.5 Mpc) for a large sample of galaxies in both the Virgo and Fornax Clusters, the direct measurement of the line-of-sight depth of Virgo and a precise measurement of the relative distance of the two clusters (Mei et al. 2005a,b, 2007; Blakeslee et al. 2009).
Please see the science highlights to learn more about these and other topics. To read or download individual papers, see the publications section.
The Survey Teams
The Survey Teams
The ACS Virgo and Fornax Cluster Surveys were unprecedented surveys of early-type galaxies belonging to two benchmark clusters in the local universe, Virgo and Fornax. The surveys were based on ACS imaging from the Hubble Space Telescope (HST), and helped change the way astronomers think about galaxy formation.
Program Galaxies
Left: An intermediate-luminosity galaxy (VCC1431) in the Virgo Cluster observed with the Advanced Camera for Surveys (ACS) on HST as part of the ACS Virgo Cluster Survey, which targeted 100 early-type galaxies (Cote et al. 2004). Note the central nucleus or “luminosity excess” (see below). A similar survey of the Fornax Cluster, targeting 43 galaxies, is described in Jordan et al. (2007). Top: The ACS/WFC CCDs before the camera was assembled.
The Virgo Cluster is the rich cluster nearest to the Milky Way, and the dominant mass concentration in the local universe. It also represents the nearest large collection of early-type (red sequence) galaxies within ~50 Mpc. At a distance of ≈16.5 Mpc, it has historically played a central role in furthering our understanding of galaxy evolution, supermassive black holes, the extragalactic distance scale, the intracluster medium, extragalactic star clusters, and countless other topics in modern astrophysics.
The Virgo Cluster is the rich cluster nearest to the Milky Way, and the dominant mass concentration in the local universe. It also represents the nearest large collection of early-type (red sequence) galaxies within ~50 Mpc. At a distance of ≈16.5 Mpc, it has historically played a central role in furthering our understanding of galaxy evolution, supermassive black holes, the extragalactic distance scale, the intracluster medium, extragalactic star clusters, and countless other topics in modern astrophysics.
The Fornax Cluster is smaller and more compact than Virgo. At a slightly larger distance of ≈20.0 Mpc, it offers an unique opportunity to study the fossil record of galaxy formation in rather different environment than the Virgo Cluster.
The Next Generation Virgo Cluster Survey (NGVS)
Survey Design and Implementation
The Next Generation Virgo Cluster Survey (NGVS) is a Large Program on the Canada French Hawaii Telescope (CFHT) that was awarded nearly 800 hours (approximately 160 nights) to carry out panoramic, multi-band imaging of the Virgo Cluster. Virgo is the dominant mass concentration in the local universe and the largest collection of galaxies within ≈30 Mpc. Prior to the NGVS, approximately 1850 confirmed or probable member galaxies were catalogued by previous observing programs (Figure 1).
The science goals of the NGVS are diverse. Key topics related to Virgo itself include:
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The faint-end shape of the galaxy luminosity function.
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The characterization of galaxy scaling relations over a range of ~ one million in stellar mass.
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The study of stellar nuclei and their connection to supermassive black holes and their host galaxies.
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The connections between the cluster, galaxies and the intracluster medium.
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The properties of structurally extreme galaxies, including ultra diffuse and ultra compact systems.
The depth and areal coverage of the NGVS also makes it possible to address a number of foreground (Kuiper Belt Objects, Milky Way halo stars) and background (high-redshift clusters, cosmic shear measurements, strong lensing events) science topics.
Data acquistion for the NGVS was completed in 2014, but the scientific exploitation of the survey is ongoing. To date, the survey team has published nearly 50 refereed papers based on NGVS data.
Figure 1: Spatial distribution of Virgo cluster galaxies from Binggeli et al. (1985). The red crosses indicate the location of M87 and M49, which mark the respective centres of the A and B sub-clusters; the large dotted red circles indicate their virial radii. The area surveyed by the NGVS is the region interior to the solid red curves, a total area of 104 sq. deg.
Figure 2: The forty CCDs (each measuring 2048 x 4096 pixels) that make up the MegaCam detector system (see Boulade et al. 2003). Each MegaCam image covers an area of 0.96 deg × 0.94 deg at a scale of 0.187" per pixel.
The panchromatic nature of the NGVS is critical for achieving the survey’s science goals. NGVS uses the MegaCam instrument on CFHT (Figure 2) to survey Virgo it its entirety, from the core to virial radius, in five filters (u,g,r,i,z), to unprecedented depths. A total of 117 distinct MegaCam fields are contained within the survey footprint. The wide wavelength baseline of the survey (Figure 3) makes it possible to distinguish background galaxies from cluster members, through the use of colour-colour diagrams and photometric redshifts.
The principal challenge involved in processing NGVS data is the characterization of the scattered light component affecting each MegaCam image: traditional data reduction techniques (such as used for the CFHT Legacy Survey) proved ineffective in removing such scattered light component without compromising the extended sources present in the field. The NGVS collaboration therefore developed a new reduction pipeline, Elixir-LSB, that provides a real-time characterization of the scattered light component in MegaCam images (Figure 4). In order to be processed by Elixir-LSB, data must be acquired using a specific acquisition pattern, as was adopted for the NGVS.
Figure 3: Total transmission curves for the CFHT/MegaPrime u*griz filter set used in the NGVS. These curves show the combined transmission for mirrors, optics, and detectors. For reference, the spectra plotted in grey show SSP models from Bruzual & Charlot (2003) having solar metallicity and ages of t = 0.025, 0.10, 0.29, 0.64, 1.4, 2.5, 5, and 11 Gyr.
Figure 4: Comparison of raw, pre-processed, and stacked g-band images for the NGVS field containing M49 (the large galaxy in the lower left quadrant). The top row shows, from left to right, a raw, Elixir and Elixir-LSB processed single frame. The bottom row shows, from left to right, MegaPipe "local background," "global background," and Elixir-LSB stacks obtained by combining all five g-band dithered frames for this field. All panels use a similar grey scale.