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The presence of systematic modulations in the colour distributions in composite samples of globular clusters (GCs) associated with galaxies in the Virgo and Fornax clusters has been reported in a previous work. In this paper, we focus on the 27 brightest galaxies in Virgo, and in particular on NGC 4486, the dominant system in terms of GC population. The new analysis includes ≈7600 cluster candidates brighter than g  = 24.5 (or T 1 ≈ 23.70). The results indicate the presence of the characteristic Virgo pattern in these galaxies and that this pattern is detectable over a galactocentric range from 3 to 30 kpc in NGC 4486. This finding gives more support to the idea that the pattern has been the result of an external, still not identified phenomenon, capable of synchronizing the cluster formation in a kind of viral process, and on supragalactic scales (also having, presumably, an impact on the overall star formation history in the entire Virgo cluster). [ABSTRACT FROM AUTHOR]

New long-term Very Long Baseline Array observations of the well-known jet in the M87 radio galaxy at 43 GHz show that the jet experiences a sideways shift with an approximately 8–10 yr quasi-periodicity. Such jet wobbling can be indicative of a relativistic Lense–Thirring precession resulting from a tilted accretion disc. The wobbling period together with up-to-date kinematic data on jet rotation opens up the possibility for estimating angular momentum of the central supermassive black hole. In the case of a test-particle precession, the specific angular momentum is J / Mc = (2.7 ± 1.5) × 1014 cm, implying moderate dimensionless spin parameters a = 0.5 ± 0.3 and 0.31 ± 0.17 for controversial gas-dynamic and stellar-dynamic black hole masses. However, in the case of a solid-body-like precession, the spin parameter is much smaller for both masses, 0.15 ± 0.05. Rejecting this value on the basis of other independent spin estimations requires the existence of a magnetically arrested disc in M87. [ABSTRACT FROM AUTHOR]

New high-resolution Very Long Baseline Interferometer observations of the prominent jet in the M87 radio galaxy show a persistent triple-ridge structure of the transverse 15-GHz profile with a previously unobserved ultra-narrow central ridge. This radio structure can reflect the intrinsic structure of the jet, so that the jet as a whole consists of two embedded coaxial jets. A relativistic magnetohydrodynamic model is considered in which an inner jet is placed inside a hollow outer jet and the electromagnetic fields, pressures and other physical quantities are found. The entire jet is connected to the central engine that plays the role of a unipolar inductor generating voltage between the jets and providing opposite electric currents, and the charge neutrality and current closure together with the electromagnetic fields between the jets can contribute to the jet stabilization. The constant voltage is responsible for the similar widening laws observed for the inner and outer jets. This jet-in-jet structure can indicate simultaneous operation of two different jet-launching mechanisms, one relating to the central supermassive black hole and the other to the surrounding accretion disc. An inferred magnetic field of 80 G at the base is sufficient to provide the observed jet luminosity. [ABSTRACT FROM AUTHOR]

Due to its proximity, M87 is a prime target for next-generation high-resolution VLBI at short millimeter wavelengths, by which the jet launching region and the black hole shadow are expected to be resolved and imaged sometime soon. Along with this situation, high-quality VLBI imaging and monitoring at lower frequencies play an important role in complementing the high-frequency data. Here, we present our recent and ongoing observational studies of the M87 jet on pc-to-subpc scales based on ultra-deep VLBI imaging programs at 86GHz and 15 GHz. The high-dynamic-range images have allowed us to obtain some remarkably improved views on this jet. We also introduce the KVN and VERA Array (KaVA), a new regularly-operating VLBI network in East Asia, which is quite suitable for studying the structure and propagation of relativistic jets. Some early results from our pilot study for M87--including the detection of superluminal motions near the jet base--implying an efficient magnetic-to-kinetic conversion at these scales, are reported. [ABSTRACT FROM AUTHOR]

We present a general method to identify infalling substructure in discrete data sets with position and line-of-sight velocity data. We exploit the fact that galaxies falling on to a brightest cluster galaxy (BCG) in a virialized cluster, or dwarf satellites falling on to a central galaxy like the Milky Way, follow nearly radial orbits. If the orbits are exactly radial, we show how to find the probability distribution for a satellite's energy, given a tracer density for the satellite population, by solving an Abel integral equation. This is an extension of Eddington's classical formula for the isotropic distribution function. When applied to a system of galaxies, clustering in energy space can then be quantified using the Kullback-Leibler divergence, and groups of objects can be identified which, though separated in the sky, may be falling in on the same orbit. This method is tested using mock data and applied to the satellite galaxy population around M87, the BCG in Virgo, and a number of associations that are found, which may represent infalling galaxy groups. [ABSTRACT FROM AUTHOR]

The flux density of the radio galaxy Virgo A has been measured at 38 and 151.5 MHz. These measurements and published flux densities at 59 other frequencies from 5.6 MHz to 857 GHz are used to obtain a fit to the frequency dependence of the logarithm of the flux density in the form of a second-order polynomial of the logarithm of the frequency, This spectrum, which has a small positive curvature, agrees slightly better with the observed flux densities than a purely power-lawspectrumwith the spectral index 0.793±0.003 and S = 1187±23 Jy. The relative errors in using the power-law spectrum instead of this spectrum with its small, positive curvature lie in the range (−5.2−1.6)% for frequencies from 70 MHz to 857 GHz. The expected peak frequency of the spectrum of Virgo A is estimated. [ABSTRACT FROM AUTHOR]

Chandra X-ray observations of the nearby brightest cluster galaxy M87 resolve the hot gas structure across the Bondi accretion radius of the central supermassive black hole (SMBH), a measurement possible in only a handful of systems but complicated by the bright nucleus and jet emission. By stacking only short frame-time observations to limit pileup, and after subtracting the nuclear point spread function, we analysed the X-ray gas properties within the Bondi radius at 0.12-0.22 kpc (1.5-2.8 arcsec), depending on the black hole mass. Within 2 kpc radius, we detect two significant temperature components, which are consistent with constant values of 2 and 0.9 keV down to 0.15 kpc radius. No evidence was found for the expected temperature increase within ~0.25 kpc due to the influence of the SMBH. Within the Bondi radius, the density profile is consistent with -ρ ∝ r-1. The lack of a temperature increase inside the Bondi radius suggests that the hot gas structure is not dictated by the SMBH's potential and, together with the shallow density profile, shows that the classical Bondi rate may not reflect the accretion rate on to the SMBH. If this density profile extends in towards the SMBH, the mass accretion rate on to the SMBH could be at least two orders of magnitude less than the Bondi rate, which agrees with Faraday rotation measurements for M87. We discuss the evidence for outflow from the hot gas and the cold gas disc and for cold feedback, where gas cooling rapidly from the hot atmosphere could feed the cirumnuclear disc and fuel the SMBH. At 0.2 kpc radius, the cooler X-ray temperature component represents ~20 per cent of the total X-ray gas mass and, by losing angular momentum to the hot gas component, could provide a fuel source of cold clouds within the Bondi radius. [ABSTRACT FROM AUTHOR]

We performed a systematic X-ray study of eight nearby γ-ray bright radio galaxies with Suzaku in order to understand the origins of their X-ray emissions. The Suzaku spectra for five of those have been presented previously, while the remaining three (M87, PKS 0625-354, and 3C78) are presented here for the first time. Based on the Fe-K line strength, X-ray variability, and X-ray power-law photon indices, and using additional information on the [O iii] line emission, we argue for a jet origin of the observed X-ray emission in these three sources. We also analyzed five years of Fermi Large Area Telescope (LAT) GeV gamma-ray data on PKS 0625-354 and 3C 78 to understand these sources within the blazar paradigm. We found significant γ-ray variability in the former object. Overall, we note that the Suzaku spectra for both PKS 0625-354 and 3C78 are rather soft, while the LAT spectra are unusually hard when compared with other γ-ray detected low-power (FR I) radio galaxies. We demonstrate that the constructed broadband spectral energy distributions of PKS 0625-354 and 3C 78 are well described by a one-zone synchrotron/synchrotron self-Compton model. The results of the modeling indicate lower bulk Lorentz factors compared to those typically found in other BL Lacertae (BL Lac) objects, but consistent with the values inferred from modeling other LAT-detected FR I radio galaxies. Interestingly, the modeling also implies very high peak (~1016 Hz) synchrotron frequencies in the two analyzed sources, contrary to previously suggested scenarios for Fanaroff-Riley (FR) type I/BL Lac unification. We discuss the implications of our findings in the context of the FR I/BL Lac unification schemes. [ABSTRACT FROM AUTHOR]

We make a toy model for M87 jet to interpret its parabolic structure and acceleration in the apparent speeds, according to observations in milli-arcsecond to arcsecond scales upstream of HST-1. The outermost layer of jet is driven by the frame-dragging effect in the Kerr spacetime with a slowly to moderately-spinning black hole. The corresponding magnetosphere has a foot-point R in the vicinity of event-horizon, and rotating at a frequency Ω equal to that of the frame-dragging ω( R). [ABSTRACT FROM AUTHOR]

We explore energy densities of the magnetic fields and relativistic electrons in the M87 jet. Since the radio core at the jet base is identical to the optically thick surface against synchrotron self-absorption (SSA), the observing frequency is identical to the SSA turnover frequency. As a first step, we assume the radio core has a simple uniform sphere geometry. Using the observed angular size of the radio core measured by the Very Long Baseline Array at 43 GHz, we estimate the energy densities of magnetic fields (UB) and relativistic electrons (Ue) on the basis of the standard SSA formula. Imposing the condition that the Poynting power and kinetic power of relativistic electrons should be smaller than the total power of the jet, we find that (1) the allowed range of the magnetic field strength (Btot) is 1 G ⩽ Btot ⩽ 15 G and that (2) 1 × 10–5 ⩽ Ue/UB ⩽ 6 × 102 holds. The uncertainty of Ue/UB comes from the strong dependence on the angular size of the radio core and the minimum Lorentz factor of non-thermal electrons (γe, min) in the core. It is still unsettled whether resultant energetics are consistent with either the magnetohydrodynamic jet or the kinetic power dominated jet even on the ∼10 Schwarzschild radii scale. [ABSTRACT FROM AUTHOR]

We present the radial velocity curve of a single-lined spectroscopic binary in the faint Hercules dwarf spheroidal (dSph) galaxy, based on 34 individual spectra covering more than 2 yr of observations. This is the first time that orbital elements could be derived for a binary in a dSph. The system consists of a metal-poor red giant and a low-mass companion, possibly a white dwarf, with a 135 day period in a moderately eccentric (e = 0.18) orbit. Its period and eccentricity are fully consistent with metal-poor binaries in the Galactic halo, while the projected semimajor axis is small, at ap sin i = 38 R☼. In fact, a very close orbit could inhibit the production of heavier elements through s-process nucleosynthesis, leading to the very low abundances of neutron-capture elements that are found in this star. We discuss the further implications for the chemical enrichment history of the Hercules dSph, but find no compelling binary scenario that could reasonably explain the full, peculiar abundance pattern of the Hercules dSph galaxy. [ABSTRACT FROM AUTHOR]

Aims. We investigate the diffuse light in the outer regions of the nearby elliptical galaxy M87 in the Virgo cluster, in the transition region between galaxy halo and intracluster light (ICL). Methods. The diffuse light is traced using planetary nebulas (PNs). The surveyed areas are imaged with a narrow-band filter centred on the redshifted [OIII]λ5007 Å emission line at the Virgo cluster distance (the on-band image) and with a broad-band V-filter (the off-band image). All PNs are identified through the on-off band technique using automatic selection criteria based on the distribution of the detected sources in the colour-magnitude diagram and the properties of their point-spread function. Results. We present the results of an imaging survey for PNs within a total effective area of 0.43 deg2, covering the stellar halo of M87 up to a radial distance of 150 kpc. We extract a catalogue of 688 objects down to m5007 = 28.4, with an estimated residual contamination from foreground stars and background Lyα galaxies, which amounts to ∼35% of the sample. This is one of the largest extragalactic PN samples in number of candidates, magnitude depth, and radial extent, which allows us to carry out an unprecedented photometric study of the PN population in the outer regions of M87.We find that the logarithmic density profile of the PN distribution is shallower than the surface brightness profile at large radii. This behaviour is consistent with a model where the luminosity specific PN numbers for the M87 halo and ICL are different. Because of the depth of this survey we are also able to study the shape of the PN luminosity function (PNLF) in the outer regions of M87. We find a slope for the PNLF that is steeper at fainter magnitudes than the standard analytical PNLF formula and adopt a generalised model that treats the slope as a free parameter. Conclusions. The logarithmic PN number density profile is consistent with the superposition of two components associated with the halo of M87 and with the ICL, which have different α parameters. We derive α2.5,halo = (1.10+0.17-0.21) × 10-8 NPN L-1☉,bol and α2.5,ICL = (3.29+0.60-0.72) × 10-8 NPN L-1 ☉,bol for the halo and the intracluster stellar components, respectively. The fit of the generalised formula to the empirical PNLF for the M87 halo returns a value for the slope of 1.17 and a preliminary distance modulus to the M87 halo of 30.74. Comparing the PNLF of M87 and the M31 bulge, both normalised by the sampled luminosity, the M87 PNLF contains fewer bright PNs and has a steeper slope towards fainter magnitudes. [ABSTRACT FROM AUTHOR]

The structure and dynamics of the M87 jet from sub-milliarcsec to arcsecond scales are continuously examined. We analyzed the Very Long Baseline Array archival data taken at 43 and 86 GHz to measure the size of very long baseline interferometry (VLBI) cores. Millimeter/sub-millimeter VLBI cores are considered as innermost jet emissions, which has been originally suggested by Blandford & Königl. Those components fairly follow an extrapolated parabolic streamline in our previous study so that the jet has a single power-law structure with nearly 5 orders of magnitude in the distance starting from the vicinity of the supermassive black hole (SMBH), less than 10 Schwarzschild radius (rs). We further inspect the jet parabolic structure as a counterpart of the magnetohydrodynamic (MHD) nozzle in order to identify the property of a bulk acceleration. We interpret that the parabolic jet consists of Poynting-flux dominated flows, powered by large-amplitude, nonlinear torsional Alfvén waves. We examine the non-relativistic MHD nozzle equation in a parabolic shape. The nature of trans-fast magnetosonic flow is similar to the one of transonic solution of Parker's hydrodynamic solar wind; the jet becomes super-escape as well as super-fast magnetosonic at around ∼103rs, while the upstream trans-Alfvénic flow speed increases linearly as a function of the distance at ∼102-103rs. We here point out that this is the first evidence to identify these features in astrophysical jets. We propose that the M87 jet is magnetically accelerated, but thermally confined by the stratified interstellar medium inside the sphere of gravitational influence of the SMBH potential, which may be a norm in active galactic nucleus jets. [ABSTRACT FROM AUTHOR]

Information on the meeting of the Radio Astronomy Group at the National Space Centre in Leicester, England on October 27, 2012 is presented. The use of radio interferometers for the observation of M87 in Virgo was considered by doctor David Morgan in his presentation. It adds that the brightness of the sky was explained by doctor Chris North. Moreover, the increase of flare activity of the Sun since 2010 was detailed by John Cook.

The author discusses the galaxy hunting and focuses on the constellation Virgo. She states that in the Middle Ages, Virgo was known as the Virgin Mary, mother of Jesus Christ. She discusses the versatility of the NGC 4594 (Messier 104) Sombrero Galaxy which was discovered in May 1781 by the Frenchman Pierre Mechain. She mentions that Virgo is the most popular constellation for the Super Cluster of Galaxies including NGC 4486 (Messier 87), NGC 4303 (Messier 61) and the NGC 4406 (Messier 86).

The article discusses research using three-dimensional (3D) general relativistic magnetohydrodynamical simulations to investigate accreting black holes (BHs) with various spin vectors, disk thicknesses, and with magnetic flux reaching saturation. Topics include the affect of BH's spin magnitude and direction on the production of intense radiation and powerful relativistic jets, what the simulations revealed about a "magneto-spin alignment" mechanism, such as its influence on the alignment of magnetized disks and jets with the BH spin near BHs, and the implications of this mechanism for the evolution of BH mass and spin, BH feedback on host galaxies, and resolved BH images for the accreting BHs in the supermassive black hole Sagittarius A* (SgrA*) and the galaxy M87.

ABSTRACT The Type IIb supernova (SN) 2011dh exploded in the nearby galaxy M51 (the Whirlpool galaxy) and provides us with one of the best laboratories to study early high-energy emission from SNe. We give here a comprehensive view of the X-ray properties of SN 2011dh from the analyses of two pointed XMM-Newton early observations as well as the full Swift X-ray Telescope (XRT) data set (163 ks). Due to the high XMM-Newton throughput, we were able to satisfactorily fit the X-ray spectrum with two hot diffuse gas components including an additional absorption component to our Galaxy. A power-law model provided a worse description of the data. In addition, the early Swift XRT light curve hints of a flux excess at early times (≲3 d), consistent with the adiabatic cooling of stellar's photosphere a few days after the shock breakout. [ABSTRACT FROM AUTHOR]

Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole. [ABSTRACT FROM AUTHOR]

Context. We present the results of a long M 87 monitoring campaign in very high energy γ-rays with the MAGIC-I Cherenkov telescope. Aims. We aim to model the persistent non-thermal jet emission by monitoring and characterizing the very high energy γ-ray emission of M 87 during a low state. Methods. A total of 150 h of data were taken between 2005 and 2007 with the single MAGIC-I telescope, out of which 128.6?h survived the data quality selection. We also collected data in the X-ray and Fermi-LAT bands from the literature (partially contemporaneous). Results. No flaring activity was found during the campaign. The source was found to be in a persistent low-emission state, which was at a confidence level of 7σ. We present the spectrum between 100?GeV and 2?TeV, which is consistent with a simple power law with a photon index Γ = 2.21 ± 0.21 and a flux normalization at 300?GeV of (7.7 ± 1.3) × 10-8 TeV-1 s-1 m-2. The extrapolation of the MAGIC spectrum into the GeV energy range matches the previously published Fermi-LAT spectrum well, covering a combined energy range of four orders of magnitude with the same spectral index. We model the broad band energy spectrum with a spine layer model, which can satisfactorily describe our data. [ABSTRACT FROM AUTHOR]

ABSTRACT The supermassive black hole candidate at the centre of M87 drives an ultra-relativistic jet visible on kiloparsec scales, and its large mass and relative proximity allow for event horizon scale imaging with very long baseline interferometry at millimetre wavelengths (mm-VLBI). Recently, relativistic magnetohydrodynamic simulations of black hole accretion flows have proven capable of launching magnetically dominated jets. We construct time-dependent disc/jet models of the innermost portion of the M87 nucleus by performing relativistic radiative transfer calculations from one such simulation. We identify two types of models, jet-dominated or disc/jet, that can explain the spectral properties of M87, and use them to make predictions for current and future mm-VLBI observations. The Gaussian source size for the favoured sky orientation and inclination from observations of the large-scale jet is as ( ≃4-6 Schwarzschild radii) on current mm-VLBI telescopes, very similar to existing observations of Sgr A*. The black hole shadow, direct evidence for an event horizon, should be visible in future measurements using baselines between Hawaii and Mexico. Both models exhibit variability at millimetre wavelengths with factor of ≃2 amplitudes on year time-scales. For the low inclination of M87, the counter-jet dominates the event horizon scale millimetre wavelength emission from the jet-forming region. [ABSTRACT FROM AUTHOR]

ABSTRACT As shown in previous works, globular clusters (GCs) can be used to trace the overall structure of the diffuse stellar populations in early-type galaxies if the number of clusters per unit stellar mass depends on metallicity. In this paper we further test this assumption in the galaxy NGC 4486 (M87) by combining several data sources. The results show that GCs allow the mapping of the galaxy in terms of the surface brightness profile, integrated colour gradient, chemical abundance and mass-to-luminosity ratios up to 1000 arcsec (or 80.4 kpc) from its centre (i.e. some 10 effective radii). The analysis indicates the presence of a dominant high-metallicity bulge associated with the red globulars, whose ellipticity increases outwards, and of a more flattened low-metallicity halo connected with the blue globulars. The chemical abundance gradient of the composite stellar population is remarkably similar to that inferred from X-ray observations of hot gas. The mass-metallicity spectrum of the stellar population can, in principle, be understood in terms of inhomogeneous enrichment models. In turn, the distribution of the bluest GCs, and lowest metallicity halo stars, has an intriguing similarity with that of dark matter, a feature shared with NGC 1399. Also, in these two galaxies, the number of blue GCs per dark mass unit is identical within the errors, ≈1.0(±0.3) × 10−9. The total stellar mass derived for NGC 4486 is 6.8(±1.1) × 1011 M⊙ with a baryonic mass fraction fb= 0.08(±0.01). [ABSTRACT FROM AUTHOR]

Aims. We aim to constrain the structural variations within the HST-1 region downstream of the radio jet of M87, in general as well as in connection to the episodes of activity at very high energy (VHE). Methods. We analyzed and compared 26 VLBI observations of the M87 jet, obtained between 2006 and 2011 with the Very Long Baseline Array (VLBA) at 1.7 GHz and the European VLBI Network (EVN) at 5 GHz. Results. HST-1 is detected at all epochs; we model-fitted its complex structure with two or more components, the two outermost of which display a significant proper motion with a superluminal velocity around ∼4 c. The motion of a third feature that is detected upstream is more difficult to characterize. The overall position angle of HST-1 has changed during the time of our observations from -65° to -90°, while the structure has moved by over 80 mas downstream. Our results on the component evolution suggest that structural changes at the upstream edge of HST-1 can be related to the VHE events. [ABSTRACT FROM AUTHOR]

In this article we present a model of formation of a galaxy with a black hole in the center. It is based on the Lemaître-Tolman solution and is a refinement of an earlier model. The most important improvement is the choice of the interior geometry of the black hole allowing for the formation of Gyrs old black holes. Other refinements are the use of an arbitrary Friedmann model as the background (unperturbed) initial state and the adaptation of the model to an arbitrary density profile of the galaxy. Our main interest was the M87 galaxy (NGC 4486), which hosts a supermassive black hole of mass 3.2 × 10 M⊙. It is shown that for this particular galaxy, within the framework of our model and for the initial state being a perturbation of the ΛCDM model, the age of the black hole can be up to 12.7 Gyrs. The dependence of the model on the chosen parameters at the time of last scattering was also studied. The maximal age of the black hole as a function of the Ω and Ω parameters for the M87 galaxy can be 3.717 or 12.708 Gyr. [ABSTRACT FROM AUTHOR]

We describe a Chandra X-ray target-of-opportunity project designed to isolate the site of TeV flaring in the radio galaxy M87. To date, we have triggered the Chandra observations only once (2010 April) and by the time of the first of our nine observations, the TeV flare had ended. However, we found that the X-ray intensity of the unresolved nucleus was at an elevated level for our first observation. Of the more than 60 Chandra observations we have made of the M87 jet covering nine years, the nucleus was measured at a comparably high level only three tithes. Two of these occasions can be associated with TeV flaring, and at the time of the third event, there were no TeV monitoring activities. From the rapidity of the intensity drop of the nucleus, we infer that the size of the emitting region is of order a few light days x the unknown beaming factor; comparable to the same sort of estimate for the TeV emitting region. We also find evidence of spectral evolution in the X-ray band which seems consistent with radiative losses affecting the non-thermal population of the emitting electrons within the unresolved nucleus. [ABSTRACT FROM AUTHOR]

One of the conundrums in extragalactic astronomy is the discrepancy in observed metallicity distribution functions (MDFs) between the two prime stellar components of early-type galaxies--globular clusters (GCs) and halo field stars. This is generally taken as evidence of highly decoupled evolutionary histories between GC systems and their parent galaxies. Here we show, however, that new developments in linking the observed GC colors to their intrinsic metallicities suggest nonlinear color-to-metallicity conversions, which translate observed color distributions into strongly peaked, unimodal MDFs with broad metal-poor tails. Remarkably, the inferred GC MDFs are similar to the MDFs of resolved field stars in nearby elliptical galaxies and those produced by chemical evolution models of galaxies. The GC MDF shape, characterized by a sharp peak with a metal-poor tail, indicates a virtually continuous chemical enrichment with a relatively short timescale. The characteristic shape emerges across three orders of magnitude in the host galaxy mass, suggesting a universal process of chemical enrichment among various GC systems. Given that GCs are bluer than field stars within the same galaxy, it is plausible that the chemical enrichment processes of GCs ceased somewhat earlier than that of the field stellar population, and if so, GCs preferentially trace the major, vigorous mode of star formation events in galactic formation. We further suggest a possible systematic age difference among GC systems, in that the GC systems in more luminous galaxies are older. This is consistent with the downsizing paradigm whereby stars of brighter galaxies, on average, formed earlier than those of dinamer galaxies: this additionally supports the similar nature shared by GCs and field stars. Although the sample used in this study (the Hubble Space Telescope Advanced Camera for Surveys/Wide Field Channel, WFPC2, and WFC3 photometry for the GC systems in the Virgo galaxy cluster) confines our discussion to R ≲ Re for giant ellipticals and ≲10 Re for normal ellipticals, our findings suggest that GC systems and their parent galaxies have shared a more common origin than previously thought, and hence greatly simplify theories of galaxy formation. [ABSTRACT FROM AUTHOR]

The optical color distributions of globular clusters (GCs) in most large elliptical galaxies are bimodal. Based on the assumed linear relationship between GC colors and their metallicities, the bimodality has been taken as evidence of two GC subsystems with different metallicities in each galaxy and has led to a number of theories in the context of galaxy formation. More recent observations and modeling of GCs, however, suggests that the color-metallicity relations (CMRs) are inflected, and thus colors likely trace metallicities in a nonlinear manner. The nonlinearity could produce bimodal color distributions from a broad underlying metallicity spread, even if it is unimodal. Despite the far-reaching implications, whether CMRs are nonlinear and whether the nonlinearity indeed causes the color bimodality are still open questions. Given that the spectroscopic refinement of CMRs is still very challenging, we here propose a new photometric technique to probe the possible nonlinear nature of CMRs. In essence, a color distribution of GCs is a "projected" distribution of their metallicities. Since the form of CMRs hinges on which color is used, the shape of color distributions varies depending significantly on the colors. Among other optical colors, the u-band related colors (e.g., u - g and u - z) are theoretically predicted to exhibit significantly less inflected CMRs than other preferred CMRs (e.g., for g - z). As a case study, we performed the Huhble Space Telescope (HST)/WFPC2 archival u-band photometry for the M87 (NGC 4486) GC system with confirmed color bimodality, We show that the u-band color distributions are significantly different from that of g - z and consistent with our model predictions. With more u-band measurements, this method will support or rule out the nonlinear CMR scenario for the origin of GC color bimodality with high confidence. The HST/WFC3 observations in F336W for nearby large elliptical galaxies are highly anticipated in this regard. [ABSTRACT FROM AUTHOR]

During the last decade, M87's jet has been the site of an extraordinary variability event, with one knot (HST-1) increasing by over a factor 100 in brightness. Variability has also been seen on timescales of months in the nuclear flux. Here we discuss the optical-UV polarization and spectral variability of these components, which show vastly different behavior. HST-1 shows a highly significant correlation between flux and polarization, with P increasing from ~20% at minimum to >40% at maximum, while the orientation of its electric vector stayed constant. HST-1's optical-UV spectrum is very hard (αUV-O ~ 0.5, Fv ∝ v-αL and displays "hard lags" during epochs 2004.9-2005.5, including the peak of the flare, with soft lags at later epochs. We interpret the behavior of HST-1 as enhanced particle acceleration in a shock, with cooling from both particle aging and the relaxation of the compression. We set 2σ upper limits of 0.5δ pc and 1.02c on the size and advance speed of the flaring region. The slight deviation of the electric vector orientation from the jet position angle (P.A.) makes it likely that on smaller scales the flaring region has either a double or twisted structure. By contrast, the nucleus displays much more rapid variability, with a highly variable electric vector orientation and "looping" in the (I, P) plane. The nucleus has a much steeper spectrum (αUV-O ~ 1.5) but does not show UV-optical spectral variability. Its behavior can be interpreted as either a helical distortion to a steady jet or a shock propagating through a helical jet. [ABSTRACT FROM AUTHOR]