Virgo Cluster Core Research Articles

Kinematics of the diffuse intragroup and intracluster light in groups and clusters of galaxies in the local universe within 100 Mpc distance

Nearly all intragroup (IGL) and intracluster light (ICL) comes from stars that are not bound to any single galaxy but were formed in galaxies and later unbound from them. In this review we focus on the physical properties–phase space properties, metallicity and age distribution–of the ICL and IGL components of the groups and clusters in the local universe, within 100 Mpc distance. Kinematic–information on these very low surface brightness structures mostly comes from discrete tracers such as planetary nebulae and globular clusters, showing highly unrelaxed velocity distributions. Cosmological hydrodynamical simulations provide key predictions for the dynamical state of IGL and ICL and find that most IC stars are dissolved from galaxies that subsequently merge with the central galaxy. The increase of the measured velocity dispersion with radius in the outer halos of bright galaxies is a physical feature that makes it possible to identify IGL and ICL components. In the local groups and clusters, IGL and ICL are located in the dense regions of these structures. Their light fractions relative to the total luminosity of the satellite galaxies in a given group or cluster are between a few to ten percent, significantly lower than the average values in more evolved, more distant clusters. IGL and ICL in the Leo I and M49 groups, and the Virgo cluster core around M87, has been found to arise from mostly old (≥10 Gyr) metal-poor ([Fe/H] <-1.0) stars of low-mass progenitor galaxies. New imaging facilities such as LSST, Euclid, and the “big eyes’’ on the sky–ELT and JWST with their advanced instrumentation–promise to greatly increase our knowledge of the progenitors of the IGL and ICL stars, their ages, metal content, masses and evolution, there by increasing our understanding of this enigmatic component.

The Next Generation Virgo Cluster Survey. XXXIII. Stellar Population Gradients in the Virgo Cluster Core Globular Cluster System

We present a study of the stellar populations of globular clusters (GCs) in the Virgo Cluster core with a homogeneous spectroscopic catalog of 692 GCs within a major-axis distance R maj = 840 kpc from M87. We investigate radial and azimuthal variations in the mean age, total metallicity, [Fe/H], and α-element abundance of blue (metal-poor) and red (metal-rich) GCs using their co-added spectra. We find that the blue GCs have a steep radial gradient in [Z/H] within R maj = 165 kpc, with roughly equal contributions from [Fe/H] and [α/Fe], and flat gradients beyond. By contrast, the red GCs show a much shallower gradient in [Z/H], which is entirely driven by [Fe/H]. We use GC-tagged Illustris simulations to demonstrate an accretion scenario where more massive satellites (with more metal- and α-rich GCs) sink further into the central galaxy than less massive ones, and where the gradient flattening occurs because of the low GC occupation fraction of low-mass dwarfs disrupted at larger distances. The dense environment around M87 may also cause the steep [α/Fe] gradient of the blue GCs, mirroring what is seen in the dwarf galaxy population. The progenitors of red GCs have a narrower mass range than those of blue GCs, which makes their gradients shallower. We also explore spatial inhomogeneity in GC abundances, finding that the red GCs to the northwest of M87 are slightly more metal-rich. Future observations of GC stellar population gradients will be useful diagnostics of halo merger histories.

Structure and kinematics of the Virgo cluster of galaxies

Aims. This work considers the Virgo cluster of galaxies, focusing on its structure, kinematics, and morphological landscape. Our principal aim is to estimate the virial mass of the cluster. For this purpose, we present a sample of 1537 galaxies with radial velocities VLG < 2600 km s−1 situated within a region of ΔSGL = 30° and ΔSGB = 20° around M 87. About half of the galaxies have distance estimates. Methods. We selected 398 galaxies with distances in a (17 ± 5) Mpc range. Based on their 1D and 2D number-density profiles and their radial velocity dispersions, we made an estimate for the virial mass of the Virgo cluster. Results. We identify the infall of galaxies towards the Virgo cluster core along the Virgo Southern Extension filament. From a 1D profile of the cluster, we obtain the virial mass estimate of (6.3 ± 0.9)×1014 M⊙, which is in tight agreement with its mass estimate via the external infall pattern of galaxies. Conclusions. We conclude that the Virgo cluster outskirts between the virial radius and the zero-velocity radius do not contain significant amounts of dark matter beyond the virial radius.

Surveying the Local Supercluster Plane

We investigate the distribution and velocity field of galaxies situated in a band of 100 by 20 degrees centered on M87 and oriented along the Local supercluster plane. Our sample amounts 2158 galaxies with radial velocities less than 2000 km/s. Of them, 1119 galaxies (52%) have distance and peculiar velocity estimates. About 3/4 of early-type galaxies are concentrated within the Virgo cluster core, most of the late-type galaxies in the band locate outside the virial radius. Distribution of gas-rich dwarfs with $M_{\rm H\,I} > M_*$ looks to be insensitive to the Virgo cluster presence. Among 50 galaxy groups in the equatorial supercluster band 6 groups have peculiar velocities about 500-1000 km/s comparable with virial motions in rich clusters. The most cryptic case is a flock of nearly 30 galaxies around NGC4278 (Coma I cloud), moving to us with the mean peculiar velocity of $-$840 km/s. This cloud (or filament?) resides at a distance of 16.1 Mpc from us and approximately 5 Mpc away from the Virgo center. Galaxies around Virgo cluster exhibit Virgocentric infall with an amplitude of about 500 km/s. Assuming the spherically symmetric radial infall, we estimate the radius of the zero-velocity surface to be $R_0 = (7.0\pm0.3)$ Mpc that yields the total mass of Virgo cluster to be $(7.4\pm0.9)\times 10^{14} M_{\odot}$ in tight agreement with its virial mass estimates. We conclude that the Virgo outskirts does not contain significant amounts of dark mater beyond its virial core.

NEW CONSTRAINTS ON A COMPLEX RELATION BETWEEN GLOBULAR CLUSTER COLORS AND ENVIRONMENT

ABSTRACT We present an analysis of high-quality photometry for globular clusters (GCs) in the Virgo cluster core region, based on data from the Next Generation Virgo Cluster Survey (NGVS) pilot field, and in the Milky Way (MW), based on Very Large Telescope/X-Shooter spectrophotometry. We find significant discrepancies in color–color diagrams between sub-samples from different environments, confirming that the environment has a strong influence on the integrated colors of GCs. GC color distributions along a single color are not sufficient to capture the differences we observe in color–color space. While the average photometric colors become bluer with increasing radial distance to the cD galaxy M87, we also find a relation between the environment and the slope and intercept of the color–color relations. A denser environment seems to produce a larger dynamic range in certain color indices. We argue that these results are not due solely to differential extinction, Initial Mass Function variations, calibration uncertainties, or overall age/metallicity variations. We therefore suggest that the relation between the environment and GC colors is, at least in part, due to chemical abundance variations, which affect stellar spectra and stellar evolution tracks. Our results demonstrate that stellar population diagnostics derived from model predictions which are calibrated on one particular sample of GCs may not be appropriate for all extragalactic GCs. These results advocate a more complex model of the assembly history of GC systems in massive galaxies that goes beyond the simple bimodality found in previous decades.

Planetary Nebulae and their parent stellar populations. Tracing the mass assembly of M87 and Intracluster light in the Virgo cluster core

AbstractThe diffuse extended outer regions of galaxies are hard to study because they are faint, with typical surface brightness of 1% of the dark night sky. We can tackle this problem by using resolved star tracers which remain visible at large distances from the galaxy centers. This article describes the use of Planetary Nebulae as tracers and the calibration of their properties as indicators of the star formation history, mean age and metallicity of the parent stars in the Milky Way and Local Group galaxies. We then report on the results from a deep, extended, planetary nebulae survey in a 0.5 deg2region centered on the brightest cluster galaxy NGC 4486 (M87) in the Virgo cluster core, carried out with SuprimeCam@Subaru and FLAMES-GIRAFFE@VLT. Two planetary nebulae populations are identified out to 150 kpc distance from the center of M87. One population is associated with the M87 halo and the second one with the intracluster light in the Virgo cluster core. They have different line-of-sight velocity and spatial distributions, as well as different planetary nebulae specific frequencies and luminosity functions. The intracluster planetary nebulae in the surveyed region correspond to a luminosity of four times the luminosity of the Large Magellanic Cloud. The M87 halo planetary nebulae trace an older, more metal-rich, parent stellar population. A substructure detected in the projected phase-space of the line-of-sight velocity vs. major axis distance for the M87 halo planetary nebulae provides evidence for the recent accretion event of a satellite galaxy with luminosity twice that of M33. The satellite stars were tidally stripped about 1 Gyr ago, and reached apocenter at a major axis distance of 60–90 kpc from the center of M87. The M87 halo is still growing significantly at the distances where the substructure is detected.

THE NEXT GENERATION VIRGO CLUSTER SURVEY. VIII. THE SPATIAL DISTRIBUTION OF GLOBULAR CLUSTERS IN THE VIRGO CLUSTER

We report on a large-scale study of the distribution of globular clusters (GCs) throughout the Virgo cluster, based on photometry from the Next Generation Virgo Cluster Survey, a large imaging survey covering Virgo's primary subclusters to their virial radii. Using the g', (g'-i') color-magnitude diagram of unresolved and marginally-resolved sources, we constructed 2-D maps of the GC distribution. We present the clearest evidence to date showing the difference in concentration between red and blue GCs over the extent of the cluster, where the red (metal-rich) GCs are largely located around the massive early-type galaxies, whilst the blue (metal-poor) GCs have a more extended spatial distribution, with significant populations present beyond 83' (215 kpc) along the major axes of M49 and M87. The GC distribution around M87 and M49 shows remarkable agreement with the shape, ellipticity and boxiness of the diffuse light surrounding both galaxies. We find evidence for spatial enhancements of GCs surrounding M87 that may be indicative of recent interactions or an ongoing merger history. We compare the GC map to the locations of Virgo galaxies and the intracluster X-ray gas, and find good agreement between these baryonic structures. The Virgo cluster contains a total population of 67300$\pm$14400 GCs, of which 35% are located in M87 and M49 alone. We compute a cluster-wide specific frequency S_N,CL=$2.8\pm0.7$, including Virgo's diffuse light. The GC-to-baryonic mass fraction is e_b=$5.7\pm1.1\times10^{-4} $and the GC-to-total cluster mass formation efficiency is e_t=$2.9\pm0.5\times10^{-5}$, values slightly lower than, but consistent with, those derived for individual galactic halos. Our results show that the production of the complex structures in the unrelaxed Virgo cluster core (including the diffuse intracluster light) is an ongoing process.(abridged)

An Oxford SWIFT integral field spectroscopy study of 14 early-type galaxies in the Coma cluster

As a demonstration of the capabilities of the new Oxford SWIFT integral field spectrograph, we present first observations for a set of 14 early-type galaxies in the core of the Coma cluster. Our data consist of I- and z-band spatially resolved spectroscopy obtained with the Oxford SWIFT spectrograph, combined with r-band photometry from the SDSS archive for 14 early- type galaxies. We derive spatially resolved kinematics for all objects from observations of the calcium triplet absorption features at \sim 8500 {AA} . Using this kinematic information we classify galaxies as either Fast Rotators or Slow Rotators. We compare the fraction of fast and slow rotators in our sample, representing the densest environment in the nearby Universe, to results from the ATLAS3D survey, finding the slow rotator fraction is \sim 50 per cent larger in the core of the Coma cluster than in the Virgo cluster or field, a 1.2 {\sigma} increase given our selection criteria. Comparing our sample to the Virgo cluster core only (which is 24 times less dense than the Coma core) we find no evidence of an increase in the slow rotator fraction. Combining measurements of the effective velocity dispersion {\sigma_e} with the photometric data we determine the Fundamental Plane for our sample of galaxies. We find the use of the average velocity dispersion within 1 effective radius, {\sigma_e}, reduces the residuals by 13 per cent with respect to comparable studies using central velocity dispersions, consistent with other recent integral field Fundamental Plane determinations.

A deep view on the Virgo cluster core

In this study we investigate the optical photometric properties of early-type galaxies in the Virgo cluster core region, by analysing their location on the colour magnitude relation (CMR) and the structural scaling relations down to faint magnitudes, and by constructing the luminosity function to compare it with theoretical expectations. We visually select potential cluster members based on morphology and angular size, excluding spiral galaxies. A photometric analysis has been carried out for 295 galaxies, using surface brightness profile shape and colour as further criteria to identify probable background contaminants. 216 galaxies are considered to be certain or probable Virgo cluster members. Our study reveals 77 galaxies not catalogued in the VCC (with 13 of them already found in previous studies) that are very likely Virgo cluster members because they follow the Virgo CMR and exhibit low Sersic indices. Those galaxies reach -8.7 mag in V band. The CMR shows a clear change in slope from dEs to dSphs, while the scatter of the CMR in the dSph regime does not increase significantly. Our sample might, however, be somewhat biased towards redder colours. The scaling relations given by the dEs appear to be continued by the dSphs indicating a similar origin. The observed change in the CMR slope may mark the point at which gas loss prevented significant metal enrichment. The almost constant scatter around the CMR possibly indicates a short formation period, resulting in similar stellar populations. The luminosity function shows a Schechter function's faint end slope of -1.50\pm0.17, implying a lack of galaxies related to the expected number of low-mass dark matter haloes from theoretical models. Our findings could be explained by suppressed star formation in low-mass dark matter halos or by tidal disruption of dwarfs in the dense core region of the cluster.

SPITZER/IRAC LOW SURFACE BRIGHTNESS OBSERVATIONS OF THE VIRGO CLUSTER

We present 3.6 and 4.5 micron Spitzer IRAC imaging over 0.77 square degrees at the Virgo cluster core for the purpose of understanding the formation mechanisms of the low surface brightness intracluster light features. Instrumental and astrophysical backgrounds that are hundreds of times higher than the signal were carefully characterized and removed. We examine both intracluster light plumes as well as the outer halo of the giant elliptical M87. For two intracluster light plumes, we use optical colors to constrain their ages to be greater than 3 & 5 Gyr, respectively. Upper limits on the IRAC fluxes constrain the upper limits to the masses, and optical detections constrain the lower limits to the masses. In this first measurement of mass of intracluster light plumes we find masses in the range of 5.5 x 10^8 - 4.5 x 10^9 and 2.1 x 10^8 - 1.5 x 10^9 solar masses for the two plumes for which we have coverage. Given their expected short lifetimes, and a constant production rate for these types of streams, integrated over Virgo's lifetime, they can account for the total ICL content of the cluster implying that we do not need to invoke ICL formation mechanisms other than gravitational mechanisms leading to bright plumes. We also examined the outer halo of the giant elliptical M87. The color profile from the inner to outer halo of M87 (160 Kpc) is consistent with either a flat or optically blue gradient, where a blue gradient could be due to younger or lower metallicity stars at larger radii. The similarity of the age predicted by both the infrared and optical colors (> few Gyr) indicates that the optical measurements are not strongly affected by dust extinction.

OPTICAL COLORS OF INTRACLUSTER LIGHT IN THE VIRGO CLUSTER CORE

We continue our deep optical imaging survey of the Virgo cluster using the CWRU Burrell Schmidt telescope by presenting B-band surface photometry of the core of the Virgo cluster in order to study the cluster's intracluster light (ICL). We find ICL features down to mu_b ~ 29 mag sq. arcsec, confirming the results of Mihos et al. (2005), who saw a vast web of low-surface brightness streams, arcs, plumes, and diffuse light in the Virgo cluster core using V-band imaging. By combining these two data sets, we are able to measure the optical colors of many of the cluster's low-surface brightness features. While much of our imaging area is contaminated by galactic cirrus, the cluster core near the cD galaxy, M87, is unobscured. We trace the color profile of M87 out to over 2000 arcsec, and find a blueing trend with radius, continuing out to the largest radii. Moreover, we have measured the colors of several ICL features which extend beyond M87's outermost reaches and find that they have similar colors to the M87's halo itself, B-V ~ 0.8. The common colors of these features suggests that the extended outer envelopes of cD galaxies, such as M87, may be formed from similar streams, created by tidal interactions within the cluster, that have since dissolved into a smooth background in the cluster potential.

The edge of the M 87 halo and the kinematics of the diffuse light in the Virgo cluster core

We present high resolution FLAMES/VLT spectroscopy of intracluster planetary nebula (ICPN) candidates, targeting three new fields in the Virgo cluster core with surface brightness down to mu_B = 28.5. Based on the projected phase space information we separate the old and 12 newly-confirmed PNs into galaxy and intracluster components. The M87 PNs are confined to the extended stellar envelope of M87, within a projected radius of ~ 160 kpc, while the ICPNs are scattered across the whole surveyed region between M87 and M86. The velocity dispersions determined from the M87 PNs at projected radii of 60 kpc and 144 kpc show that the galaxy's velocity dispersion profile decreases in the outer halo, down to 78 +/- 25 km/s. A Jeans model for the M87 halo stars in the gravitational potential traced by the X-ray emission fits the observed velocity dispersion profile only if the stellar orbits are strongly radially anisotropic (beta ~= 0.4 at r ~= 10 kpc increasing to 0.8 at the outer edge), and if additionally the stellar halo is truncated at ~= 150 kpc average elliptical radius. From the spatial and velocity distribution of the ICPNs we infer that M87 and M86 are falling towards each other and that we may be observing them just before the first close pass. The inferred luminosity-specific PN numbers for the M87 halo and the ICL are in the range of values observed for old (> 10 Gyr) stellar populations (abridged).

Compact stellar systems and cluster environments

We have extended the search for luminous (M_{b_J} <= -10.5) compact stellar systems (CSSs) in the Virgo and Fornax galaxy clusters by targeting with the recently commissioned AAOmega spectrograph three cluster environments -- the cluster cores around M87 and NGC 1399, intracluster space, and a major galaxy merger site (NGC 1316). We have significantly increased the number of redshift-confirmed CSSs in the Virgo cluster core and located three Virgo intracluster globular clusters (IGCs) at a large distance from M87 (154-173 arcmin or ~750-850 kpc) -- the first isolated IGCs to be redshift-confirmed. We estimate luminous CSS populations in each cluster environment, and compare their kinematic and photometric properties. We find that (1) the estimated luminous CSS population in the Virgo cluster core is half of that in Fornax, possibly reflecting the more relaxed dynamical status of the latter; (2) in both clusters the luminous CSS velocity dispersions are less than those of the cD galaxy GC system or cluster dE galaxies, suggesting luminous CSSs have less energetic orbits; (3) Fornax has a sub-population of cluster core luminous CSSs that are redder and presumably more metal-rich than those found in Virgo; (4) no luminous CSSs were found in a 10-20 arcmin (60-130 kpc) radial arc east of the 3 Gyr old NGC 1316 galaxy merger remnant or in the adjacent intracluster region, implying that any luminous CSSs created in the galaxy merger have not been widely dispersed.

Expansion Velocities and Core Masses of Bright Planetary Nebulae in the Virgo Cluster

The line-of-sight velocities and [O III] 5007 A expansion velocities are measured for 11 planetary nebulae (PNs) in the Virgo Cluster core, at 15 Mpc distance, with the FLAMES spectrograph on the ESO VLT. These PNs are located about halfway between the two giant elliptical galaxies M87 and M86. From the [O III] 5007 A line profile widths, the average half-width at half-maximum expansion velocity for this sample of 11 PNs is HWHM = 16.5 km s−1 (rms = 2.6 km s−1). We use the PN subsample bound to M87 to remove the distance uncertainties and the resulting [O III] 5007 A luminosities to derive the central star masses. We find these masses to be at least 0.6 M☉ and obtain PN observable lifetimes tPN < 2000 yr, which imply that the bright PNs detected in the Virgo Cluster core are compact, high-density nebulae. We finally discuss several scenarios for explaining the high central star masses in these bright M87 halo PNs.

Abundance segregation in Virgo spiral galaxies

Aims. To derive O/H, N/O, and S/O abundances of Hll regions located in nine Virgo spiral galaxies, as well as to verify what is the cause of the abundance segregation found. Methods. We employed photoionization models to reproduce observed emission-line intensities of H II regions located in the galaxies considered. The abundance gradients obtained were interpreted using a grid of chemical evolution models. Results. Our models indicate that galaxies near to the core of the Virgo cluster are overabundant in O/H, N/O, and S/O by about 0.25 dex in comparison to the ones at the periphery. With one exception, models with upper stellar mass limit of M u = 30-40 M Θ and age of the ionizing star cluster ranging from 1.5 to 2.5 Myr were able to reproduce the observational data. Chemical models indicate the collapse time-scale for inner regions of galaxies near to the Virgo core is larger than the one in galaxies located at the intermediate and peripheral positions in the cluster, what can be due to the dense environment existing in Virgo Cluster core.

Intracluster Stars in the Virgo Cluster Core

We have investigated the properties of the diffuse light in the Virgo Cluster core region, based on the detection of intracluster planetary nebulae (PNs) in four fields. We eliminate the bias from misclassified faint continuum objects, using improved Monte Carlo simulations, and the contaminations by high-redshift Lyα galaxies, using the Lyα luminosity function in blank fields. Recent spectroscopic observations confirm that our photometric PN samples are well understood. We find that the diffuse stellar population in the Virgo core region is inhomogeneous on scales of 30'–90': there exist significant field-to-field variations in the number density of PNs and the inferred amount of intracluster light, with some fields empty, some fields dominated by extended Virgo galaxy halos, and some fields dominated by the true intracluster component. There is no clear trend with distance from M87. The mean surface luminosity density, its rms variation, and the mean surface brightness of diffuse light in our four fields are ΣB = 2.7 × 106 LB⊙ arcmin-2, rms = 2.1 × 106 LB⊙ arcmin-2, and B = 29.0 mag arcsec-2, respectively. Our results indicate that the Virgo Cluster is a dynamically young environment and that the intracluster component is associated partially with local physical processes like galaxy interactions or harassment. We also argue, based on kinematic evidence, that the so-called over-luminous PNs in the halo of M84 are dynamically associated with this galaxy and must thus be brighter than, and part of a different stellar population from, the normal PN population in elliptical galaxies.

Ram Pressure Stripping in the Low-Luminosity Virgo Cluster Elliptical Galaxy NGC 4476

We present a deep VLA search for H I emission from the low-luminosity Virgo Cluster elliptical galaxy NGC 4476, which contains 1.1 × 108 M⊙ of molecular gas in an undisturbed disk in regular rotation. No H I was detected. The rms noise in the final image corresponds to a 3 σ column density sensitivity of 1.2 × 1020 cm-2 at the position of NGC 4476, averaged over the 4 kpc beam. The total H I mass is less than 1.5 × 107 M⊙. If we compare our H I upper limit with the H2 content, we find that NGC 4476 is extremely deficient in H I compared with other galaxies detected in these two species. The H2/H mass ratio for NGC 4476 is greater than 7, whereas typical H2/H ratios for elliptical galaxies detected in both H I and H2 are less than 2. On the basis of this extreme H I deficiency and the intracluster medium density at the projected distance from M87, we argue that either NGC 4476 has undergone ram pressure stripping while traveling through the Virgo Cluster core or its average molecular gas density is larger and its interstellar UV field is smaller than in typical spiral galaxies. NGC 4476 is located 12' in projection from M87, which causes extreme continuum confusion problems. We also discuss in detail the techniques used for continuum subtraction. The spectral dynamic range of our final image is 50,000 to 1.

The Line-of-Sight Velocity Distributions of Intracluster Planetary Nebulae in the Virgo Cluster Core

Radial velocities of 40 intracluster planetary nebulae (ICPNs) in the Virgo Cluster were obtained with the new multifiber FLAMES spectrograph on UT2 at the Very Large Telescope. For the first time, the λ4959 line of the [O III] doublet is seen in a large fraction (50%) of ICPNs spectra, and a large fraction of the photometric candidates with m(5007) ≲ 27.2 is spectroscopically confirmed. ICPNs with the velocity dispersion of the Virgo Cluster are found in our CORE field 12 from M87. These may have originated from tidal mass loss of smaller galaxies in the M87 subcluster halo. In a field 025 from M87, we see an extended stellar halo of M87 in approximate dynamical equilibrium but with few ICPNs. Finally, in a field near M84/M86, the ICPN velocities are highly correlated with the galaxy velocities, showing that any well-mixed intracluster population is yet to form. Overall, the measured velocity distributions confirm the nonuniform dynamical structure and ongoing assembly of the Virgo Cluster.

The dispersion in the Cepheid period-luminosity relation and the consequences for the extragalactic distance scale

Using published Hubble Space Telescope (HST) Cepheid data from 25 galaxies, we have found a correlation between the dispersion in the Cepheid period-luminosity (P-L) relation and host galaxy metallicity, which is significant at the 3σ level in the V band. In the I band the correlation is less significant, although the tighter intrinsic dispersion of the P-L relation in I may make it harder to detect such a correlation in the HST sample. One possibility is that low metallicity galaxies have smaller metallicity gradients than high metallicity galaxies; if the Cepheid P-L relation has a significant dependence on metallicity then this might explain the higher P-L dispersion in the higher metallicity galaxies. A second possibility is that the increased P-L dispersion is driven by metallicity dispersion but now due to a relation between metallicity and Cepheid colour rather than luminosity. A third possibility is that the increased P-L dispersion is caused by an increase in the width of the instability strip with metallicity. Whatever the explanation, the high observed dispersions in the HST Cepheid P-L relations have the important consequence that the bias due to incompleteness in the P-L relation at faint magnitudes is more significant than previously thought. Using a maximum likelihood technique which takes into account the effect on the P-L relations of truncation by consistently defined magnitude completeness limits, we rederive the Cepheid distances to the 25 galaxies. In the case of the galaxies with the highest P-L dispersion at the largest distances, we find that the published distance modulus underestimates the true distance modulus by up to 0.5 mag. When both metallicity and magnitude incompleteness corrections are made, a scale error in the published Cepheid distances is seen in the sense that the published distance moduli are increasingly underestimated at larger distances. This results in the average distance modulus to the four galaxies in the Virgo cluster core increasing from (m - M) 0 = 31.2 ± 0.19 to (m - M) 0 = 31.4 ± 0.19 if the γ VI = -0.24 mag dex -1 metallicity correction of Kenni-cutt et al. is assumed. For the 18 HST galaxies with good Tully-Fisher (TF) distances and (m - M) 0 > 29.5 the Cepheid-TF distance modulus average residual increases from 0.44 ± 0.09 to 0.63 ± 0.1 mag with γ VI = -0.24. This indicates a significant scale error in TF distances, which reduces the previous Pierce & Tully TF estimate of H 0 = 85 ± 10 km s -1 Mpc -1 to H 0 = 63 ± 7 km s -1 Mpc -1 , assuming y VI = -0.24 and a still uncertain Virgo infall model. Finally, for the eight HST galaxies with Type Ia supernovae (SNIa), the metallicity and incompleteness corrected Cepheid distances marginally suggest there may be a metallicity dependence of SNIa peak luminosity in the sense that metal-poor hosts have lower luminosity SNIa. Thus, SNIa Hubble diagram estimates of both H 0 and q 0 may therefore also require significant corrections for metallicity, once the exact sizes of the Cepheid metallicity corrections become better established.

Atomic gas far away from the Virgo cluster core galaxy NGC 4388

Atomic gas far away from the Virgo cluster core galaxy NGC 4388