Virgo Cluster Of Galaxies Research Articles

Hubble Space TelescopeImaging of Bipolar Nuclear Shells in the Disturbed Virgo Cluster Galaxy NGC 4438

We present broadband and narrowband Hubble Space Telescope (HST) images of the central region of the heavily disturbed Virgo cluster galaxy NGC 4438 (Arp 120), whose nucleus has been described as a type 1 LINER or dwarf Seyfert galaxy. Narrowband Hα and [N II] HST images reveal striking bipolar shell features, 1 kpc in projected length from end to end, that are likely the result of an outflow from the nuclear region experiencing a strong interaction with the interstellar medium (ISM). While these outflow shells share similarities with those in some other starburst or active galactic nucleus (AGN) galaxies, those in NGC 4438 are notable because NGC 4438 harbors neither a luminous circumnuclear starburst nor a luminous AGN. The shells appear to be closed at their outer ends, suggesting that the outflow in NGC 4438 is dynamically younger than those in some other galaxies. The radio continuum emission is strongly enhanced near the outer ends of the shells, suggesting working surfaces arising from collimated nuclear outflows that have impacted and shocked the surrounding ISM. The two shells are quite different, as the northwestern shell is luminous and compact, while the southeastern shell is 2.5 times longer and much fainter in both optical emission lines and the nonthermal radio continuum. The differences between the two shells may be attributed to a difference in ISM density on the two sides of the nuclear disk. Such an ISM asymmetry exists on larger scales in this heavily disturbed galaxy. At the base of the outflow is a nuclear source, which is the highest surface brightness source in the galaxy at optical wavelengths. This source is resolved with a FWHM = 03 = 25 pc and has modest luminosities, uncorrected for extinction, of 5 × 1038 ergs s-1 in Hα and MB = -13. We discuss whether the outflow is powered by a low-luminosity AGN or a compact nuclear starburst. The kinetic energy associated with the ionized gas in the shells is ~1053 ergs, which could be produced by either massive star formation or an AGN. While the northwestern shell, which contributes most of the flux in most ground-based apertures centered on the nucleus, exhibits LINER-type line ratios, the nucleus has an Hα/[N II] ratio consistent with an H II region. Although there appears to be very little massive star formation occurring in the central kpc, the nucleus may contain a partially obscured, young nuclear star cluster. On the other hand, the collimation of the southeastern shell and the strongly enhanced radio continuum emission at the outer ends of the shells are more easily explained by jets associated with a nuclear black hole than by a compact nuclear starburst, although there is no direct evidence of a jet. The 2000 km s-1 broad-line component could be due to an AGN broad-line region but might also be due to high-velocity gas in the outflow. Since NGC 4438 has a large bulge, a large nuclear black hole might be expected.

Cold Dust in Late‐Type Virgo Cluster Galaxies

We have statistically analyzed the spatially integrated far-infrared (FIR) emissions of the complete volume- and luminosity-limited sample of late-type (later than S0) Virgo cluster galaxies measured using the Infrared Space Observatory (ISO) by Tuffs et al. (2002) in bands centered on 60, 100 and 170 microns. 30 out of 38 galaxies detected at all three wavelengths contain a cold dust emission component, present within all morphological types of late-type systems ranging from early giant spirals to Blue Compact Dwarfs (BCDs), and which could not have been recognized by IRAS. The temperatures of the cold dust were found to be broadly distributed, with a median of 18 K, some 8-10 K lower than would have been predicted from IRAS. The derived total dust masses are correspondingly increased by factors of typically 6 - 13. A particularly significant result concerns the low dust temperatures (ranging down to less than 10 K) and large dust masses associated with the Im and BCD galaxies in our sample. We propose two scenarios to account for the FIR characteristics of these systems.

A comparison of stellar populations in galaxy spheroids across a wide range of Hubble types

We present line-strengths and kinematics from the central regions of 32 galaxies with Hubble types ranging from E to Sbc. Spectral indices, based on the Lick system, are measured in the optical and near-infrared (NIR). The 24 indices measured, in conjunction with models of the effects of varying abundance ratios, permit the breaking of age/metallicity degeneracy, and allow estimation of enhancements in specific light elements (particularly C and Mg). The large range of Hubble types observed allows direct comparison of line-strengths in the centres of early-type galaxies (E and S0) with those in spiral bulges, free from systematic differences that have plagued comparisons of results from different studies. Our sample includes field and Virgo cluster galaxies. For early-type galaxies our data are consistent with previously reported trends of Mg2 and Mgb with velocity dispersion. In spiral bulges we find trends in all indices with velocity dispersion. We estimate luminosity-weighted ages, metallicities and heavy-element abundance ratios (enhancements) from optical indices. These show that bulges are less enhanced in light (α-capture) elements and have lower average age than early-type galaxies. Trends involving age and metallicity also differ sharply between early and late types. An anticorrelation exists between age and metallicity in early types, while, in bulges, metallicity is correlated with velocity dispersion. We consider the implications of these findings for models of the formation of these galaxies. We find that primordial collapse models of galaxy formation are ruled out by our observations, while several predictions of hierarchical clustering (merger) models are confirmed.

Temperature Map of the Virgo Cluster of Galaxies Observed withASCA

The temperature distribution of the intracluster medium (ICM) in the Virgo Cluster of galaxies has been derived from extensive mapping observations with ASCA covering an area of 19 deg2. In the spectral analysis, the inner region within a radius of ~60' from M87 is characterized by an ICM temperature of kT ~ 2.5 keV with little variation. On the other hand, the outer regions indicate significant variation of the temperature with an amplitude of about 1 keV. The temperature map was produced from the hardness ratio (HR) values with a resolution of about 5'. Besides the previously reported hot region with kT > 4 keV between M87 and M49, several hot regions with kT = 3-4 keV are detected in the cluster outskirts. The autocorrelation function for the HR variation shows that the temperature variation is correlated within a size of about 300 kpc, suggesting that gas blobs falling in the Virgo Cluster have a typical size of groups of galaxies. A correlation with the velocity dispersion of member galaxies shows that only the northwest region indicates an unusually large βspec value of 2-4. The upper limit for extended nonthermal emission in the Virgo Cluster is obtained to be LX ~ 9 × 1041 ergs s-1 in the 2-10 keV band. We discuss that these features consistently indicate that the Virgo Cluster is in a relatively early stage of the cluster evolution.

Some New Results Concerning the Study of the Virgo Cluster Structure

The creation of a new version of MERCG the catalogue enriched by the new information, which was reduced to the uniform system in RC3 and transferred together with data from other lists into our catalogue, made it possible to study the structure of the Virgo cluster based on the statistical cviterion offered by Anosova. The study of the spatial distribution of galaxies in A and B clusters, forming the main concentration of galaxies in the Virgo cluster, made it possible to consider galaxy number density, and velocity and magnitude means for early and late type galaxies in the direction from the center to the periphery. The mean value of galaxy velocities of the B cluster appeared to be larger than that for the A cluster thus placing it farther. Investigation of the distribution of galaxy colors in the areas of the A and the B clusters revealed a statistically significant distinction, expressed in the noticeable reddening of the galaxies observed in B cluster region, with can be explained possibly by the location of HI deficient galaxies in this area of the Virgo cluster.

Faint Features of Interacting Galaxies Revealed by the Digital Coaddition of 13 Tech-Pan Films of the Virgo Cluster

We have digitally coadded APM scans of 13 Kodak TechPan films of the SE region of the Virgo Cluster of galaxies. The ≈6.2°×6.2°field of the R-band films combined with the resolution of ≈2 arcsec pixel- results in data-file sizes of about 222 MBytes. The 13 scanned films have been aligned, coadded, corrected for vignetting effects and cleaned of stellar features. To illustrate the astrophysical uses of this technique, we present high-contrast images of a sample of interacting galaxies in the field. Several very faint features (but very clearly seen in the coadded array) – such as the interaction between IC 3481,IC 3481A and IC 3483, and NGC 4410A and B and IC 790 – have been presented with clarity for the first time.

The Globular Cluster System in the Outer Regions of NGC 4472

We have undertaken a study of the globular cluster (GC) systems of a large sample of elliptical and spiral galaxies in order to test specific predictions of galaxy formation models. Here we present results for the first elliptical in the study, the giant Virgo cluster galaxy NGC 4472 (M49). The galaxy was observed in three filters (BVR) using the wide-field Mosaic Imager on the Kitt Peak 4 m telescope. The Mosaic images roughly double the radial coverage of previous CCD observations of NGC 4472. We have combined the Mosaic data with published spectroscopic data and archival HST observations in order to study NGC 4472's GC system in detail, and to fully characterize the amount of contamination in our sample of GC candidates. We find that the radial profile of the GC system is fit fairly well by a de Vaucouleurs law of the form log σGC = (3.38 ± 0.08) - (1.56 ± 0.05) r1/4 out to 17' (~80 kpc), but that the observed profile falls slightly below the de Vaucouleurs law between 17' and 23', the limit of the data. The bimodal color distribution observed in previous studies is apparent in our data. We find a small metallicity gradient in the inner 8' of the total GC system due to the increasing ratio of blue to red clusters, consistent with results from past studies. The gradient vanishes, however, when the entire radial extent of the data is taken into account. We estimate a total of ~5900 GCs in NGC 4472 out to 23', yielding a specific frequency of 3.6 ± 0.6 within this radius. This specific frequency value is smaller than that found by previous studies of NGC 4472. We examine the implications of these results with regard to predictions made by four different galaxy formation models, and we find that all four models have at least one inconsistency with the data.

The Principal Axis of the Virgo Cluster

Using accurate distances to individual Virgo Cluster galaxies obtained by the method of surface brightness fluctuations, we show that Virgo's brightest elliptical galaxies have a remarkably collinear arrangement in three dimensions. This axis, which is inclined by ~10°-15° from the line of sight, can be traced to even larger scales where it appears to join a filamentary bridge of galaxies connecting Virgo to the rich cluster Abell 1367. The orientations of individual Virgo elliptical galaxies also show some tendency to be aligned with the cluster axis, as does the jet of the supergiant elliptical galaxy M87. These results suggest that the formation of the Virgo Cluster, and its brightest member galaxies, have been driven by infall of material along the Virgo-A1367 filament.

The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations

This paper analyzes single stellar population (SSP)–equivalent parameters for 50 local elliptical galaxies as a function of their structural parameters. The galaxy sample is drawn from the high-quality spectroscopic surveys of González (1993) and Kuntschner (1998). The basic data are central values of SSP-equivalent ages, t, metallicities, [Z/H], and "enhancement" ratios, [E/Fe], derived in Paper I, together with global structural parameters including velocity dispersions, radii, surface brightnesses, masses, and luminosities. The galaxies fill a two-dimensional plane in the four-dimensional space of [Z/H], log t, log σ, and [E/Fe]. SSP age, t, and velocity dispersion, σ, can be taken as the two independent parameters that specify a galaxy's location in this "hyperplane." The hyperplane can be decomposed into two subrelations: (1) a "Z-plane," in which [Z/H] is a linear function of log σ and log t and (2) a relation between [E/Fe] and σ in which [E/Fe] is larger in high-σ galaxies. Velocity dispersion is the only structural parameter that is found to modulate the stellar populations; adding other structural variables such as Ie or re does not predict [Z/H] or [E/Fe] more accurately. Cluster and field ellipticals follow the same hyperplane, but their (σ,t) distributions within it differ. Most Fornax and Virgo cluster galaxies are old, with a only a small sprinkling of galaxies to younger ages. The field ellipticals span a larger range in SSP age, with a tendency for lower σ galaxies to be younger. The present sample thus suggests that the distribution of local ellipticals in the (σ,t) plane may depend on environment. Since the (σ,t) distribution affects all two-dimensional projections involving SSP parameters, many of the familiar scaling laws attributed to ellipticals may also depend on environment. Some evidence for this is seen in the current sample. For example, only Fornax ellipticals show the classic mass-metallicity relation, whereas other subsamples do not. The tight Mg-σ relations of these ellipticals can be understood as two-dimensional projections of the metallicity hyperplane showing it edge-on. At fixed σ, young age tends to be offset by high [Z/H], preserving Mg nearly constant. The tightness of the Mg-σ relations does not necessarily imply a narrow range of ages at fixed σ.

The Structure of the Virgo Cluster from Surface Brightness Fluctuations inHubble Space TelescopeImages

Surface brightness fluctuations (SBF) in Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images of elliptical galaxies provide the precision necessary to examine the internal structure of the Virgo galaxy cluster. In a sample of 15 elliptical galaxies with observations from the HST archive, we find that most galaxies are within 2 Mpc of NGC 4486, but several galaxies (including NGC 4406) are significantly in the foreground or background. The measured redshifts and distances suggest that all sample galaxies are gravitationally bound to the galaxy cluster. The SBF distances presented here show good agreement with other SBF distance measurements and with distance measurements made using other methods. We include a brief overview of the SBF method, as well as a more detailed description of the SBF calibration used with the data included in this study, which was taken in the HST F814W filter.

Identifying Gamma‐Ray Burst Remnants in Nearby Galaxies

We study the spectral signatures arising from cooling and recombination of an interstellar medium whose equilibrium state has been altered over D100 pc by the radiation of a gamma-ray burst (GRB) and its afterglow. We identify signatures in the line diagnostics that are indicative of a photoionized GRB remnant which is yr old. We estimate that at least a few such remnants should be (5 ) 104 detectable in the Virgo Cluster of galaxies. If the gamma-ray emission from GRBs is beamed to a frac- tion of their sky, then the expected number of Virgo remnants is larger by a factor of Virgo f b f b. remnants can be resolved with arcsecond imaging and are likely to be center-—lled using narrowband —lters of high-ionization lines (such as (O III) j5007 or He II j4686) and limb-brightened for low- ionization lines (such as (S II) j6717). The nonrelativistic blast wave might be visible separately, since it does not reach the outer edge of these young photoionized remnants. The remnants should show evi- dence for ionization cones if the prompt or afterglow UV emission from GRBs is beamed. Subject headings: galaxies: clusters: individual (Virgo) ¨ gamma rays: burstsISM: general

Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with ASCA.

Based on mapping observations with ASCA, an unusual hot region with a spatial extent of 1 deg2 was discovered between M87 and M49 at a center coordinate of R.A.=12h27m36s and decl.=9&j0;18' (J2000). The X-ray emission from the region has a 2-10 keV flux of 1x10-11 ergs s-1 cm-2 and a temperature of kT greater, similar4 keV, which is significantly higher than that in the surrounding medium of approximately 2 keV. The internal thermal energy in the hot region is estimated to be VnkT approximately 1060 ergs with a gas density of approximately 10-4 cm-3. A power-law spectrum with a photon index of 1.7-2.3 is also allowed by the data. The hot region suggests there is an energy input due to a shock that is probably caused by the motion of the gas associated with M49, infalling toward the M87 cluster with a velocity greater, similar1000 km s-1.

The Surface Brightness Fluctuations and Globular Cluster Populations of Virgo Elliptical and Lenticular Galaxies

The Virgo Cluster of galaxies is a complex system composed of several subclusters or ““ clouds.II Our understanding of this structure is severely limited by the small number of member galaxies for which high-precision distance measurements are available. In this dissertation, I use Hubble Space Telescope archive images to measure precise distances to 15 elliptical and lenticular galaxies in the Virgo Cluster using surface brightness Nuctuations (SBFs) (J. Tonry & D. P. Schneider, 1988, AJ, 96, 807). Furthermore, I identify the globular clusters associated with these galaxies to a limiting magnitude signiÐcantly fainter than the peak of the globular cluster luminosity function (GCLF). Because the GCLF can also be used as a standard candle (see B. C. Whitmore, 1996, in The Extragalactic Distance Scale : Proceedings of the STScI May Symposium, ed. M. Livio, M. Donahue, & N. Panagia [Cambridge : Cambridge Univ. Press], 251, for a review), these images provide independent distance measurements from the same data. In addition, the color distributions of the globular clusters provide an indication of the globular cluster formation history. This study produced the following results :

ASCA observation of the Virgo cluster of galaxies

ASCA observation of the Virgo cluster of galaxies

Subclusters in the Virgo Area

By using the data of the CfA redshift catalogue(1995.4), the structure, substructures and some astrophysical properties of the Virgo galaxy cluster are studied in depth (Shao 1996).

Ongoing Gas Stripping in the Virgo Cluster Spiral Galaxy NGC 4522

The Virgo Cluster galaxy NGC 4522 is one of the best spiral candidates for intracluster medium–interstellar medium (ICM-ISM) stripping in action. Optical broadband and Hα images from the WIYN telescope of the highly inclined galaxy reveal a relatively undisturbed stellar disk and a peculiar distribution of Hα emission. Ten percent of the Hα emission arises from extraplanar H II regions which appear to lie within filamentary structures ≥3 kpc long above one side of the disk. The filaments emerge from the outer edge of a disk of bright Hα emission which is abruptly truncated beyond 0.35R25. Together the truncated Hα disk and extraplanar Hα filaments are reminiscent of a bow shock morphology, which strongly suggests that ISM of NGC 4522 is being stripped by the gas pressure of the ICM. The galaxy has a line-of-sight velocity of 1300 km s-1 with respect to the mean Virgo Cluster velocity and thus is expected to experience a strong interaction with the intracluster gas. The existence of H II regions apparently located above the disk plane suggests that star formation is occurring in the stripped gas, and that newly formed stars will enter the galaxy halo and/or intracluster space. The absence of H II regions in the disk beyond 0.35R25 and the existence of H II regions in the stripped gas suggest that even molecular gas has been effectively removed from the disk of the galaxy.

Galactic‐Scale Outflow and Supersonic Ram‐Pressure Stripping in the Virgo Cluster Galaxy NGC 4388

Galactic‐Scale Outflow and Supersonic Ram‐Pressure Stripping in the Virgo Cluster Galaxy NGC 4388

Results of the digital co-addition of thirteen Schmidt films of the Virgo cluster of galaxies

We have digitally co-added APM scans of 13 Kodak TechPan lms of the SE region of the Virgo clus- ter of galaxies. The6:2 6:2 eld of the R-band lms combined with the resolution of2 arcsec pixel 1 results in data le sizes of about 222 MBytes. The 13 scanned lms have been aligned, co-added, corrected for vignetting eects and cleaned of stellar features. To illustrate the as- trophysical uses of this technique, we present high con- trast images of a sample of Virgo cluster and background galaxies. Several very faint, but very clearly seen features, such as the interaction between IC 3481, IC 3481A and IC 3483 and laments outside of the common envelope of NGC 4438 & NGC 4435, can be seen for the rst time. We present an image of the halo of M 87 showing its great extent, and an image of the central regions of the cluster which shows no real evidence for interactions between the galaxies in this region. We also present high-quality im- ages of the previously-identied \jet and shell features around M 89. We also present an image of the whole eld which appears to show large variations in brightness of the intra-cluster medium across the region, with the brightest regions in the northern part in the central regions of the cluster, though we caution against this interpretation un- til we have investigated the large variation in emulsion sensitivity across individual lms more thoroughly.

How to find MACHOs in the Virgo cluster

We discuss the feasibility of finding extra-galactic MACHOs by monitoring quasars behind the Virgo cluster of galaxies. We show that with only a modest observing programme one could detect several MACHOs in the mass range 1 × 10 −5 −2 × 10 −2 M ⊙ if they make a significant contribution to the mass of Virgo. The contamination by events from cosmologically distributed MACHOs is estimated and is negligible if either the MACHO mass is ≳ 10 −4 M ⊙ or the quasar radius is ≳ 3 × 10 15 cm.

Constraints on the Hubble constant from observations of the brightest red-giant stars in a Virgo-cluster galaxy

The nearest large groups of elliptical galaxies, in the Virgo and Fornax clusters, play a central role in determinations of the Hubble constant, H0, and hence the cosmological rate of expansion. Because the relative distances between these two clusters and more remote clusters are well known, absolute distance determinations to Virgo and Fornax should establish the Hubble constant for the local Universe. In addition, elliptical galaxies reside predominantly in the cores of clusters, so distance calibrations for ellipticals should minimize the uncertainties due to the possibly large extent of the clusters along the line of sight. A powerful and direct way of establishing such distances is to use thebrightest red-giant stars, which have nearly uniform luminosities1,2. Here we report the direct observation of old red-giant stars in a dwarf elliptical galaxy in the Virgo cluster. We determine a distance to this galaxy, and thus to the core of the Virgo cluster, of 15.7 ± 1.5 megaparsecs, from which we estimate a Hubble constant of H0 = 77 ± 8 km s−1 Mpc−1. Under the assumption of a low-density Universe with the simplest cosmology, the age of the Universe is no more than 12–13 billion years.