Tramp Novae Between Galaxies in the Fornax Cluster: Tracers of Intracluster Light

We report the results of a survey for novae in and between the galaxies of the Fornax cluster. Our survey provides strong evidence that intracluster novae exist and that they provide a useful, independent measure of the intracluster light in Fornax. We discovered six strong nova candidates in six distinct epochs spanning eleven years from 1993 to 2004. The data were taken with the 4m and the 1.5m telescopes at CTIO. The spatial distribution of the nova candidates is consistent with $\sim$16-41% of the total light in the cluster being in the intracluster light, based on the ratio of the number of novae we discovered in intracluster space over the total number of novae discovered plus a simple completeness correction factor. This estimate is consistent with independent measures of intracluster light in Fornax and Virgo using intracluster planetary nebulae. The accuracy of the intracluster light measurement improves with each survey epoch as more novae are discovered.Comment: 30 pages, 10 figures, accepted for publication in the Astrophysical Journal (Sep 9, 2004). Version 2: Added references. Full resolution versions of figures 1-7 and 10 can be found at http://astrowww.phys.uvic.ca/~neill/fnx

Intracluster Red Giant Stars in the Virgo Cluster

We have used the WFPC2 camera of the Hubble Space Telescope to obtain deep F814W images of a blank field in the Virgo Cluster located 41 arcmin northwest of M87. We perform star counts in that field, and in another Virgo field observed by Ferguson, Tanvir & von Hippel (1998), and show that, when compared to the Hubble Deep Field North and South, the Virgo Cluster contains an excess of objects with magnitudes I > 27. We attribute this excess to a population of intracluster red-giant branch (IC-RGB) stars. By modeling the luminosity function of these stars, we show that the tip of the Virgo RGB is at I = 27.31 +0.27/-0.17 and that the cluster contains a small, but significant, excess of stars that are up to ~1 mag brighter than this tip. If this luminous component is due entirely to stars on the asymptotic giant branch (AGB), it implies an age for the population of > 2 Gyr; if foreground RGB stars contribute to the luminous tail, then the derived age for the stars is older still. The luminosity function also suggests that most of the intracluster stars are moderately metal-rich (-0.8 < [Fe/H] <-0.2), a result consistent with that expected from stars that have been tidally stripped from intermediate luminosity galaxies. Additionally, a comparison with the planetary nebulae in our field also supports this view, although the existence of a more metal-poor population (from stripped dwarfs) cannot be ruled out. Our derived average surface brightness, mu_I = 27.9 +0.3/-0.5 mag/arcsec^2 for Virgo's diffuse component suggests that intracluster stars contribute 10% to 20% of the cluster's total I-band luminosity.Comment: 21 pages, 8 figures included, accepted for publication in the Astrophysical Journa

Cluster-Cluster Microlensing as a Probe of Intracluster Stars, MACHOs, and Remnants of the First Generation Stars

The galaxy cluster Abell 2152 is recently found to be forming a cluster-cluster system with another, more distant cluster whose core is almost perfectly aligned to that of A2152. We discuss the detectability of microlensing events where a single star in the source cluster behind A2152 is extremely magnified by an intracluster compact object in A2152. We show that a search with an 8m-class telescope with a wide field of view, such as the Subaru/Suprime-Cam, can probe intracluster compact objects with a wide mass range of m_{co} ~ 10^{-5}-10^{10} M_sun, including ranges that have not yet been constrained by any past observations. We expect that the event rate is biased for the background cluster than the foreground cluster (A2152), which would be a unique signature of microlensing, making this experiment particularly powerful. The sensitivity of this experiment for the mass fraction of compact objects would be 1-10% in the total dark matter of the cluster, which is roughly constant against m_{co}, with a reasonable telescope time for large telescopes (~10 nights). Therefore any compact objects in this mass range can be detected or rejected as the dominant component of the dark matter. About 10 events are expected if 20% of the cluster mass is in a form of compact objects with M ~ 1 M_sun, as claimed by the MACHO collaboration for the Milky Way halo. Other possibly detectable targets include intracluster stars stripped by galaxy interactions, and hypothetical very massive black holes (M >~ 100 M_sun) produced as remnants of the first generation stars, which might be responsible for the recently reported excess of the cosmic infrared background radiation that seems impossible to explain by normal galactic light.Comment: 14 pages, 6 figures, to appear in ApJ. Some minor corrections, and references adde

Diffuse Optical Light in Galaxy Clusters I: Abell 3888

We are undertaking a program to measure the characteristics of the intracluster light (total flux, profile, color, and substructure) in a sample of 10 galaxy clusters with a range of cluster mass, morphology, and redshift. We present here the methods and results for the first cluster in that sample, A3888. We have identified an intracluster light (ICL) component in A3888 in V and r that contains 13\pm5% of the total cluster light and extends to 700h_{70}^{-1}kpc (~0.3 r_{200}) from the center of the cluster. The ICL color in our smallest radial bin is V-r = 0.3 \pm 0.1, similar to the central cluster ellipticals. The ICL is redder than the galaxies at 400 < r < 700h_{70}^{-1}kpc although the uncertainty in any one radial bin is high. Based on a comparison of V-r color with simple stellar models, the ICL contains a component which formed more than 7 Gyr ago (at z > 1), coupled with a high metallicity (1.0Z_{\odot} < Z_{ICL} \la 2.5Z_{\odot}), and a more centralized component which contains stars formed within the past 5 Gyr (at z ~ 1). The profile of the ICL can be roughly fit by a shallow exponential in the outer regions and a steeper exponential in the central region. We also find a concentration of diffuse light around a small group of galaxies 1.4h_{70}^{-1}Mpc from the center of the cluster. In addition, we find 3 low surface brightness features near the cluster center which are blue (V-r = 0.0) and contain a total flux of 0.1M*. Based on these observations and X-ray and galaxy morphology, we suggest that this cluster is entering a phase of significant merging of galaxy groups in the core, whereupon we expect the ICL fraction to grow significantly with the formation of a cD galaxy as well as the in-fall of groups.Comment: 16 pages, 13 figures, 3 tables, AJ accepte

Intracluster Planetary Nebulae in Virgo: Photometric selection, spectroscopic validation and cluster depth

We have imaged an empty area of 34'x34' one and a half degree north of the Virgo cluster core to survey for intracluster planetary nebula candidates. We have implemented and tested a fully automatic procedure for the selection of emission line objects in wide-field images, based on the on-off technique from Ciardullo and Jacoby. Freeman et al. have spectroscopically confirmed a sample of intracluster planetary nebulae in one Virgo field. We use the photometric and morphological properties of this sample to test our selection procedure. In our newly surveyed Virgo field, 75 objects were identified as best candidates for intracluster PNe. The luminosity function of the spectroscopically confirmed PNe shows a brighter cut-off than the planetary nebula luminosity function for the inner regions of M87. Such a brighter cut-off is also observed in the newly surveyed field and indicates a smaller distance modulus, implying that the front end of the Virgo cluster is closer to us by a significant amount: 14% closer (2.1 Mpc) than M87 for the spectroscopic field, using the PN luminosity function distance of 14.9 Mpc to M87, and 19% closer (2.8 Mpc) than M87 for the newly surveyed field. Independent distance indicators (Tully-Fisher relation for Virgo spirals and surface brightness fluctuations for Virgo ellipticals) agree with these findings. From these two Virgo cluster fields there is no evidence that the surface luminosity density for the diffuse stellar component in the cluster decreases with radius. The luminosity surface density of the diffuse stellar population is comparable to that of the galaxies.Comment: 30 pages, 8 figures, accepted for pubblication in A

Intracluster Planetary Nebulae in the Virgo Cluster I. Initial Results

We report the initial results of a survey for intracluster planetary nebulae in the Virgo Cluster. In two 16' x 16' fields, we identify 69 and 16 intracluster planetary nebula candidates, respectively. In a third 16' x 16' field near the central elliptical galaxy M87, we detect 75 planetary nebula candidates, of which a substantial fraction are intracluster in nature. By examining the number of the planetaries detected in each field and the shape of the planetary nebula luminosity function, we show that 1) the intracluster starlight of Virgo is distributed non-uniformly, and varies between subclumps A and B, 2) the Virgo Cluster core extends ~3 Mpc in front of M87, and thus is elongated along the line-of-sight, and 3) a minimum of 22% of Virgo's stellar luminosity resides between the galaxies in our fields, and that the true number may be considerably larger. We also use our planetary nebula data to argue that the intracluster stars in Virgo are likely derived from a population that is of moderate age and metallicity.Comment: 20 pages, 3 figures, accepted for publication in the Astrophysical Journal. Paper is also available at http://www.astro.psu.edu/users/johnf/Text/research.htm

Diffuse Stellar Light at 100 kpc Scales in M87

In a new survey of nearby galaxies from stacked photographic images, M87 shows a diffuse fan of stellar material which extends along the projected SE (major) axis out to about 100 kpc at a limiting (B) surface brightness of $28 \hbox{mag arcsec}^{-2}$. We suggest that disruption of a small spheroidal galaxy within a larger potential is the most likely explanation for the diffuse structure. Our simulations include a rigid primary potential with characteristics similar to those derived for M87 and a populated intruder. The orbit is required to pass close to the center of the potential to produce a fan which nearly aligns with the major axis and has a large opening angle, as seen in M87. The structure of the dark matter at large galactic radii is investigated by representing M87 with different potentials. The morphology and luminosity of the fan and the kinematics of debris in the center of the primary potential are analyzed and compared with substructure in M87. The short lifetimes (t_fan < 5 10^8 years) of the simulated diffuse fans indicate that several accretion events could be hidden in galaxies.Comment: 24 pages, Latex (aaspp4.sty), with 15 figures. Accepted for publication in Ap

Mapping the ionized gas of the metal-poor HII galaxy PHL 293B with MEGARA

Here we report the first spatially resolved spectroscopic study for the galaxy PHL293B using the high-resolution GTC/MEGARA IFU. PHL293B is a local, extremely metal-poor, high ionization galaxy. This makes PHL 293B an excellent analogue for galaxies in the early Universe. The MEGARA aperture (~12.5''x 11.3'') covers the entire PHL 293B main body and its far-reaching ionized gas. We created and discussed maps of all relevant emission lines, line ratios and physical-chemical properties of the ionized ISM. The narrow emission gas appears to be ionized mainly by massive stars according to the observed diganostic line ratios, regardless of the position across the MEGARA aperture. We detected low intensity broad emission components and blueshifted absorptions in the Balmer lines (H$\alpha$,H$\beta$) which are located in the brightest zone of the galaxy ISM. A chemically homogeneity, across hundreds of parsecs, is observed in O/H. We take the oxygen abundance 12+log(O/H)=7.64 $\pm$ 0.06 derived from the PHL293B integrated spectrum as the representative metallicity for the galaxy. Our IFU data reveal for the first time that the nebular HeII4686 emission from PHL 293B is spatially extended and coincident with the ionizing stellar cluster, and allow us to compute its absolute HeII ionizing photon flux. Wolf-Rayet bumps are not detected excluding therefore Wolf-Rayet stars as the main HeII excitation source. The origin of the nebular HeII4686 is discussed.Comment: 14 pages, 9 Figures, 3 Tables; Accepted for publication in MNRA

Detection of intergalactic red-giant-branch stars in the Virgo cluster

It has been suspected for nearly 50 years that clusters of galaxies contain a population of intergalactic stars, ripped from galaxies during cluster formation or when the galaxies' orbits take them through the cluster center. Support for the existence of such a population of free-floating stars comes from measurements of the diffuse light in clusters, and from recent detections of planetary nebulae with positions and/or velocities far removed from any observed cluster galaxy. But estimates for the mass of the diffuse population and its distribution relative to the galaxies are still highly uncertain. Here we report the direct detection of intergalactic stars in deep images of a blank field in the Virgo Cluster. The data suggest that approximately 10% of the stellar mass of the cluster is in intergalactic stars. We observe a relatively homogeneous distribution of stars, with evidence of a slight gradient toward M87.Comment: Accepted for publication in Nature. 10 pages, 2 postscript figures included. Uses nature.sty and astrobib.sty. (Astrobib is available from http://www.stsci.edu/software/TeX.html.