Massive Galaxy Groups Research Articles

THE MOST BOUND HALO PARTICLE–GALAXY CORRESPONDENCE MODEL: COMPARISON BETWEEN MODELS WITH DIFFERENT MERGER TIMESCALES

ABSTRACT We develop a galaxy assignment scheme that populates dark matter halos with galaxies by tracing the most bound member particles (MBPs) of simulated halos. Several merger timescale models based on analytic calculations and numerical simulations are adopted as the survival times of mock satellite galaxies. We build mock galaxy samples from halo merger data of the Horizon Run 4 N-body simulation from z = 12–0. We compare group properties and two-point correlation functions (2pCFs) of mock galaxies with those of volume-limited SDSS galaxies, with r-band absolute magnitudes of and −20 at z = 0. It is found that the MBP-galaxy correspondence scheme reproduces the observed population of SDSS galaxies in massive galaxy groups ( ) and the small-scale 2pCF ( ) quite well for the majority of the merger timescale models adopted. The new scheme outperforms the previous subhalo-galaxy correspondence scheme by more than 2σ.

An extremely dense group of massive galaxies at the centre of the protocluster atz= 3.09 in the SSA22 field

We report the discovery of an extremely dense group of massive galaxies at the centre of the protocluster at $z=3.09$ in the SSA22 field from near-infrared spectroscopy conducted with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS) equipped on the Subaru Telecope. The newly discovered group comprises seven galaxies confirmed at $z_{\rm spec}\approx3.09$ within 180 kpc including five massive objects with the stellar masses larger than $10^{10.5}~M_{\odot}$ and is associated with a bright sub-mm source SSA22-AzTEC14. The dynamical mass of the group estimated from the line-of-sight velocity dispersion of the members is $M_{\rm dyn}\sim1.6\pm0.3\times10^{13}~M_{\odot}$. Such a dense group is expected to be very rare at high redshift as we found only a few comparable systems in large-volume cosmological simulations. Such rare groups in the simulations are hosted in collapsed halos with $M_{\rm vir}=10^{13.4}-10^{14.0}~M_{\odot}$ and evolve into the brightest cluster galaxies (BCGs) of the most massive clusters at present. The observed AzTEC14 group at $z=3.09$ is therefore very likely to be a proto-BCG in the multiple merger phase. The observed total stellar mass of the group is $5.8^{+5.1}_{-2.0}\times10^{11}~M_{\odot}$. It suggests that over half the stellar mass of its descendant had been formed by $z=3$. Moreover, we identified over two members for each of the four Ly$\alpha$ blobs (LABs) using our new spectroscopic data. This verifies our previous argument that many of the LABs in the SSA22 protocluster associated with multiple developed stellar components.

The masses of satellites in GAMA galaxy groups from 100 square degrees of KiDS weak lensing data

We use the first 100 sq. deg. of overlap between the Kilo-Degree Survey (KiDS) and the Galaxy And Mass Assembly (GAMA) survey to determine the galaxy halo mass of ~10,000 spectroscopically-confirmed satellite galaxies in massive ($M > 10^{13}h^{-1}{\rm M}_\odot$) galaxy groups. Separating the sample as a function of projected distance to the group centre, we jointly model the satellites and their host groups with Navarro-Frenk-White (NFW) density profiles, fully accounting for the data covariance. The probed satellite galaxies in these groups have total masses $\log M_{\rm sub} /(h^{-1}{\rm M}_\odot) \approx 11.7 - 12.2$ consistent across group-centric distance within the errorbars. Given their typical stellar masses, $\log M_{\rm \star,sat}/(h^{-2}{\rm M}_\odot) \sim 10.5$, such total masses imply stellar mass fractions of $M_{\rm \star,sat} /M_{\rm sub} \approx 0.04 h^{-1}$ . The average subhalo hosting these satellite galaxies has a mass $M_{\rm sub} \sim 0.015M_{\rm host}$ independent of host halo mass, in broad agreement with the expectations of structure formation in a $\Lambda$CDM universe.

Testing Sunyaev–Zel'dovich measurements of the hot gas content of dark matter haloes using synthetic skies

The thermal Sunyaev-Zel'dovich (tSZ) effect offers a means of probing the hot gas in and around massive galaxies and galaxy groups and clusters, which is thought to constitute a large fraction of the baryon content of the Universe. The Planck collaboration recently performed a stacking analysis of a large sample of `locally brightest galaxies' (LBGs) and, surprisingly, inferred an approximately self-similar relation between the tSZ flux and halo mass. At face value, this implies that the hot gas mass fraction is independent of halo mass, a result which is in apparent conflict with resolved X-ray observations. We test the robustness of the inferred trend using synthetic tSZ maps generated from cosmological hydrodynamical simulations and using the same tools and assumptions applied in the Planck study. We show that, while the detection and the estimate of the `total' flux (within $5 r_{500}$) is reasonably robust, the inferred flux originating from within $r_{500}$ (i.e. the limiting radius to which X-ray observations typically probe) is highly sensitive to the assumed pressure distribution of the gas. Using our most realistic simulations with AGN feedback, that reproduce a wide variety of X-ray and optical properties of groups and clusters, we estimate that the derived tSZ flux within $r_{500}$ is biased high by up to to an order of magnitude for haloes with masses $M_{500} \sim 10^{13}$ M$_{\odot}$. Moreover, we show that the AGN simulations are consistent with the total tSZ flux-mass relation observed with Planck, whereas a self-similar model is ruled out.

Properties of 34 massive galaxy groups within 0.012 < z < 0.027

In order to extend the relationship between the infrared luminosity of groups and clusters of galaxies and their dynamic mass to the 1–5 × 1013 M ⊙ mass region, we selected from the study of Ramella et al. (2004) certain galaxy groups in the region of 0.012 < z < 0.027. Based on the archive data from the SDSS, NED, and 2MASX, for these 34 galaxy groups we determined the dynamic and photometric characteristics. Overall, the sample has the following median characteristics: z = 0.0226, M 200 = 0.58 × 1014 M ⊙, L K = 1.26 × 1012 L ⊙, and N(M K < −21m) = 22. Having this sample added to the earlier prepared sample of 148 groups and clusters of galaxies, we found the following relationships between the dynamic mass M 200, infrared (IR) luminosity, and the number of galaxies within the R 200 radius: L K (M K < −21m) ∝ M 0.77, N(M K < −21m) ∝ M 0.82, and M/L K ∝ L 0.22K . Practically all the members of the studied sample show a good agreement between the dynamic mass of groups M 200 and IR luminosity of galaxies, and eventually their stellar mass (only the group MKW12 deviates more than 2σ in all the dependencies).

SARCS strong-lensing galaxy groups

Our work is based on the stacked weak-lensing analysis of a sample of 80 strong-lensing galaxy groups. Our main results are the following: (i) the lensing signal does not allow us to firmly reject a simple singular isothermal sphere mass distribution compared to the expected NFW mass profile; (ii) we obtain an average concentration $c_{200}=8.6_{-1.3}^{+2.1}$ that is much higher than the expected value from numerical simulations for the corresponding average mass $M_{200}=0.73_{-0.10}^{+0.11}\times10^{14}\mathrm{M_{\odot}}$; (iii) the combination of our results with those at larger mass scales gives a mass-concentration relation $c(M)$ over nearly two decades in mass, with a slope in disagreement with predictions from numerical simulations using unbiased populations of dark matter haloes; (iv) our combined $c(M)$ relation matches results from simulations using only haloes with a large strong-lensing cross section, i.e. elongated with a major axis close to the line of sight; (v) for the simplest case of prolate haloes, we estimate with a toy model a lower limit on the minor:major axis ratio $a/c=0.5$ for the average SARCS galaxy group. Our analysis based on galaxy groups confirmed the results obtained at larger mass scales: strong lenses present apparently too large concentrations, which can be explained by triaxial haloes preferentially oriented with the line of sight. Because more massive systems already have large lensing cross sections, they do not require a large elongation along the line of sight, contrary to less massive galaxy groups. Therefore, it is natural to observe larger lensing (projected) concentrations for such systems, resulting in an overall mass-concentration relation steeper than that of non-lensing haloes.

SINGLE PARAMETER GALAXY CLASSIFICATION: THE PRINCIPAL CURVE THROUGH THE MULTI-DIMENSIONAL SPACE OF GALAXY PROPERTIES

We propose to describe the variety of galaxies from SDSS by using only one affine parameter. To this aim, we build the Principal Curve (P-curve) passing through the spine of the data point cloud, considering the eigenspace derived from Principal Component Analysis of morphological, physical and photometric galaxy properties. Thus, galaxies can be labeled, ranked and classified by a single arc length value of the curve, measured at the unique closest projection of the data points on the P-curve. We find that the P-curve has a "W" letter shape with 3 turning points, defining 4 branches that represent distinct galaxy populations. This behavior is controlled mainly by 2 properties, namely u-r and SFR. We further present the variations of several galaxy properties as a function of arc length. Luminosity functions variate from steep Schechter fits at low arc length, to double power law and ending in Log-normal fits at high arc length. Galaxy clustering shows increasing autocorrelation power at large scales as arc length increases. PCA analysis allowed to find peculiar galaxy populations located apart from the main cloud of data points, such as small red galaxies dominated by a disk, of relatively high stellar mass-to-light ratio and surface mass density. The P-curve allows not only dimensionality reduction, but also provides supporting evidence for relevant physical models and scenarios in extragalactic astronomy: 1) Evidence for the hierarchical merging scenario in the formation of a selected group of red massive galaxies. These galaxies present a log-normal r-band luminosity function, which might arise from multiplicative processes involved in this scenario. 2) Connection between the onset of AGN activity and star formation quenching, which appears in green galaxies when transitioning from blue to red populations. (Full abstract in downloadable version)

Satellites around massive galaxies since z∼ 2

Accretion of minor satellites has been postulated as the most likely mechanism to explain the significant size evolution of the massive galaxies over cosmic time. Using a sample of 629 massive (Mstar~10^11 Msun) galaxies from the near-infrared Palomar/DEEP-2 survey, we explore which fraction of these objects has satellites with 0.01 Msat < Mcentral < 1 (1:100) up to z=1 and which fraction has satellites with 0.1 Msat < Mcentral < 1 (1:10) up to z=2 within a projected radial distance of 100 kpc. We find that the fraction of massive galaxies with satellites, after the background correction, remains basically constant and close to ~30% for satellites with a mass ratio down to 1:100 up to z=1, and ~15% for satellites with a 1:10 mass ratio up to z=2. The family of spheroid-like massive galaxies presents a 2-3 times larger fraction of objects with satellites than the group of disk-like massive galaxies. A crude estimation of the number of 1:3 mergers a massive spheroid-like galaxy experiences since z~2 is around 2. For a disk-like galaxy this number decreases to ~1.

A CENSUS OF BARYONS AND DARK MATTER IN AN ISOLATED, MILKY WAY SIZED ELLIPTICAL GALAXY

We present a study of the dark and luminous matter in the isolated elliptical galaxy NGC720, based on deep X-ray observations made with Chandra and Suzaku. The gas is reliably measured to ~R2500, allowing us to place good constraints on the enclosed mass and baryon fraction (fb) within this radius (M2500=1.6e12+/-0.2e12 Msun, fb(2500)=0.10+/-0.01; systematic errors are <~20%). The data indicate that the hot gas is close to hydrostatic, which is supported by good agreement with a kinematical analysis of the dwarf satellite galaxies. We confirm a dark matter (DM) halo at ~20-sigma. Assuming an NFW DM profile, our physical model for the gas distribution enables us to obtain meaningful constraints at scales larger than R2500, revealing that most of the baryons are in the hot gas. We find that fb within Rvir is consistent with the Cosmological value, confirming theoretical predictions that a ~Milky Way-mass (Mvir=3.1e12+/-0.4e12 Msun) galaxy can sustain a massive, quasi-hydrostatic gas halo. While fb is higher than the cold baryon fraction typically measured in similar-mass spiral galaxies, both the gas fraction (fg) and fb in NGC720 are consistent with an extrapolation of the trends with mass seen in massive galaxy groups and clusters. After correcting for fg, the entropy profile is close to the self-similar prediction of gravitational structure formation simulations, as observed in galaxy clusters. Finally, we find a strong heavy metal abundance gradient in the ISM similar to those observed in massive galaxy groups.

Tidal effects on the spatial structure of the Local Group

The spatial distribution of galaxies in the Local Group (LG) is the footprint of its formation mechanism and the gravitational interactions among its members and the external massive galaxies or galaxy groups. Recently, Pasetto & Chiosi (2007), using a 3D-geometrical description of the spatial distribution of all the members of the LG (not only the satellites of the MW and M31) based on present-day data on positions and distances, found that all galaxies (MW, M31, their satellites, and even the most distant objects) are confined within a slab of about 200 kpc thickness. Examining how external galaxies or groups would gravitationally affect (and eventually alter) the planar structure (and its temporal evolution) of the LG, they found that the external force field acts parallel to the plane determined by geometry and studied this with the Least Action Principle. In this paper, we have thoroughly investigated the role played by the tidal forces exerted by external galaxies or galaxy groups on the LG galaxies (the most distant dwarfs in particular) in shaping their large scale distribution. The idea based on the well known effect of tidal interactions, according to which a system of mass-points can undergo not only tidal stripping but also tidal compression and thus become flatter. Excluding the dwarf galaxies tightly bound to the MW and M31, the same tidal forces can account for the planar distribution of the remaining dwarf galaxies. We analytically recover the results of Pasetto & Chiosi (2007) and prove that a planar distribution of the LG dwarf galaxies is compatible with the external force field. We also highlight the physical cause of this result.

A Compact Cluster of Massive Red Galaxies at a Redshift of 1.5

We describe a compact cluster or group of massive red galaxies at z = 1.5 discovered in one of the Gemini Deep Deep Survey (GDDS) fields. Deep H-band imaging from the Hubble Space Telescope (HST) reveals a high density of red galaxies associated with a galaxy with a spectroscopic redshift of 1.51. These galaxies have spectral energy distributions (SEDs) that peak between 3.6 and 4.5 μm, and fits to 12-band photometry reveal 12 or more galaxies with spectral shapes consistent with z = 1.5. Most are within ~170 comoving kpc of the GDDS galaxy, and the enclosed stellar mass is >6 × 10^(11) M_☉. The colors of the most massive galaxies are close to those expected from passive evolution of simple stellar populations (SSPs) formed at much higher redshifts. We suggest that several of these galaxies will merge to form a single, very massive galaxy by the present day. This system may represent an example of a short-lived dense group or cluster core typical of the progenitors of massive clusters in the present day and suggests that the red sequence was in place in overdense regions at early times.

X‐Ray Constraints on Galaxy‐Gas‐Jet Interactions in the Dumbbell Galaxies NGC 4782 and NGC 4783 in the LGG 316 Galaxy Group

We present results from a 49.3 ks Chandra X-ray observation of the strongly interacting dumbbell galaxies NGC4782(3C278) and NGC4783 that constrain the kinematics of the interaction and models for bending the radio jets associated with NGC4782. The galaxies are embedded in an approximately spherical distribution of group gas, centered on NGC4782,that is nearly isothermal with mean kT ~ 1.4+/- 0.4 keV. The X-ray morphology suggests that NGC4783 is infalling into a single, massive galaxy group (LGG316) with NGC4782 nearly at rest at the center of the group potential.NGC4783 shows a sharp X-ray surface brightness edge (cold front) to the east and a ~15 kpc ram-pressure-stripped tail to the west. Analysis of this cold front indicates NGC4783 is moving east with a total velocity ~870+270-400 km/s (Mach ~1.4+0.5-0.7) at an inclination angle ~46 deg.(>33 deg.)towards us with respect to theplane of the sky. A ~45Myr old X-ray cavity, with enthalpy of 4.4 x 10^{57}ergs, coincides with the eastern radio lobe of 3C278. X-ray knots are found on both the radio jet and counter-jet, coincident with peaks in the radio emission. Assuming a light, mildly relativistic jet in 3C278, ram pressure velocities of 100-200 km/s impacting the eastern jet and ~170 km/s acting on the western radio lobe are sufficient to produce their observed bending. These velocities may be caused by bulk motions established in the IGM by the high velocity interaction between the galaxies, by the motion of NGC4782 relative to the IGM, or both.

The H I content of the recently discovered field dwarf galaxy APPLES 1

We present observations in a search for neutral hydrogen associated with the recently detected field dwarf galaxy APPLES 1, performed with the Parkes radiotelescope. The observed radio spectrum shows no evident (> 3 sigma rms) line emission indicating an upper limit for the HI content of the galaxy of similar to 10(6) M-circle dot and providing an upper value for the M-HI/L-B ratio equal to 2.4 M-circle dot/L-circle dot. The low value of the HI content suggested by the observations, together with the galaxy optical morphology, might indicate that APPLES 1 is a dwarf spheroidal. This indication is in contrast with the evidence of recent star formation, which is typical for dwarf irregular galaxies. This may suggest that APPLES 1 belongs to the class of mixed dwarf irregular/spheroidal transition-type galaxies. We also conclude that the relatively low neutral gas mass in APPLES 1 can be explained by an extended and inefficient star formation process, without the need for a dramatic event such as enhanced star formation or a past encounter with a massive galaxy or galaxy group.

ESO 3060170: A Massive Fossil Galaxy Group with a Heated Gas Core?

We present a detailed study of the ESO 3060170 galaxy group, combining Chandra, XMM-Newton, and optical observations. The system is found to be a fossil galaxy group. The group X-ray emission is composed of a central, dense, cool core (10 kpc in radius) and an isothermal medium beyond the central 10 kpc. The region between 10 and 50 kpc (the cooling radius) has the same temperature as the gas from 50 to 400 kpc, although the gas cooling time between 10 and 50 kpc (2-6 Gyr) is shorter than the Hubble time. Thus, the ESO 3060170 group does not have a group-sized cooling core. We suggest that the group cooling core may have been heated by a central active galactic nucleus (AGN) outburst in the past and that the small, dense, cool core is the truncated relic of a previous cooling core. The Chandra observations also reveal a variety of X-ray features in the central region, including a finger, an edgelike feature, and a small tail, all aligned along a north-south axis, as are the galaxy light and group galaxy distribution. The proposed AGN outburst may cause gas to slosh around the center and produce these asymmetric features. The observed flat temperature profile to rvir is not consistent with the predicted temperature profile in recent numerical simulations. We compare the entropy profile of the ESO 3060170 group with those of three other groups and find a flatter relation than that predicted by simulations involving only shock heating, S ∝ r~0.85. This is direct evidence of the importance of nongravitational processes in group centers. We derive the mass profiles within rvir and find that the ESO 3060170 group is the most massive fossil group known [ × 1014 M☉]. The M/L ratio of the system, ~150 at 0.3rvir, is normal.

Warm Gas and Ionizing Photons in the Local Group

Several lines of argument suggest that a large fraction of the baryons in the universe may be in the form of warm (T ~ 105-107 K) gas. In particular, loose groups of galaxies may contain substantial reservoirs of such gas. Observations of the cosmic microwave background by COBE place only weak constraints on such an intragroup medium within the Local Group. The idea of a Local Group corona dates back at least 40 years (Kahn & Woltjer). Here we show that gas at T ~ (2-3) × 106 K (the approximate virial temperature of the Local Group)—extremely difficult to observe directly—can in principle radiate a large enough flux of ionizing photons to produce detectable Hα emission from embedded neutral clouds. However, additional constraints on the corona—the most stringent being pulsar dispersion measures toward the Magellanic Clouds, and the timing mass—rule out an intragroup medium whose ionizing flux dominates over the cosmic background or the major Local Group galaxies. A cosmologically significant coronal gas mass could remain invisible to Hα observations. More massive galaxy groups could contain extensive coronae which are important for the baryon mass and produce a strong, local ionizing flux.