arXiv subjects

M. J. Geller

Publications and source records attributed to M. J. Geller.

At least 37 records · Page 2Linked to original sources

arXiv2001

Infrared Mass-to-Light Profile Throughout the Infall Region of the Coma Cluster

Using a redshift survey of 1779 galaxies and photometry from the 2-Micron All-Sky Survey (2MASS) covering 200 square degrees, we calculate independent mass and light profiles for the infall region of the Coma cluster of galaxies. The redshift survey is complete to $K_s=12.2$ (622 galaxies), 1.2 magnitudes fainter than $M^*_{K_s}$ at the distance of Coma. We confirm the mass profile obtained by Geller, Diaferio, & Kurtz. The enclosed mass-to-light ratio measured in the $K_s$ band is approximately constant to a radius of $10 \Mpc$, where $M/L_{K_s}= 75\pm 23\mlsun$, in agreement with weak lensing results on similar scales. Within $2.5\Mpc$, X-ray estimates yield similar mass-to-light ratios (67$\pm32h$). The constant enclosed mass-to-light ratio with radius suggests that K-band light from bright galaxies in clusters traces the total mass on scales $\lesssim10 \Mpc$. Uncertainties in the mass profile imply that the mass-to-light ratio inside $r_{200}$ may be as much as a factor of 2.5 larger than that outside $r_{200}$. These data demonstrate that K-band light is not positively biased with respect to the mass; we cannot rule out antibias. These results imply $Ω_m = 0.17 \pm 0.05$. Estimates of possible variations in $M/L_{K_s}$ with radius suggest that the density parameter is no smaller than $Ω_m \approx 0.08$.

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arXiv2001

Redshifts for 2410 Galaxies in the Century Survey Region

The `Century Survey' strip covers 102 square degrees within the limits 8.5h \leq α_{1950} \leq 16.5h, 29.0 degrees \leq δ_{1950} \leq 30.0 degrees. The strip passes through the Corona Borealis supercluster and the outer region of the Coma cluster. Within the Century Survey region, we have measured 2410 redshifts which constitute four overlapping complete redshift surveys: (1) 1728 galaxies with Kron-Cousins R_{phot} \leq 16.13 covering the entire strip, (2) 507 galaxies with R_{phot} \leq 16.4 in the right ascension range 8h 32m \leq α_{1950} \leq 10h 45m, (3) 1251 galaxies with absorption- and K-corrected R_{CCD, corr} \leq 16.2 covering the right ascension range 8.5h \leq α_{1950} \leq 13.5h and (4) 1255 galaxies with absorption- and K-corrected V_{CCD, corr} \leq 16.7 also covering the right ascension range 8.5h \leq α_{1950} \leq 13.5h. All of these redshift samples are more than 98 % complete to the specified magnitude limit.

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arXiv2001

Star Formation in a Complete Spectroscopic Survey of Galaxies

The 15R-North galaxy redshift survey is a uniform spectroscopic survey (S/N $\sim $10) covering the range 3650---7400Åfor 3149 galaxies with median redshift 0.05. The sample is 90% complete to $R=15.4$. The median slit covering fraction is 24% of the galaxy, apparently sufficient to minimize the effects of aperture bias on the EW(H$α$). Forty-nine percent of the galaxes in the survey have one or more emission lines detected at $\geq 2 σ$. In agreement with previous surveys, the fraction of absorption-line galaxies increases steeply with galaxy luminosity. We use H$β$, O[III], H$α$, and [N\II] to discriminate between star-forming galaxies and AGNs. We use the EW(H$α$ + [N\II]) to estimate the Scalo birthrate parameter, $b$, the ratio of the current star formation rate to the time averaged star formation rate. Finally, we examine the way galaxies of different spectroscopic type trace the large-scale galaxy distribution.

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arXiv2001

X-ray Emitting Groups in the Infall Region of Abell 2199

Using a large redshift survey covering 95 square degrees, we demonstrate that the infall region of Abell 2199 contains Abell 2197, one or two X-ray emitting groups, and up to five additional groups identified in redshift surveys. Our survey shows that the X-ray emitting systems, located at projected radii of $1.^\circ4, 1.^\circ9$, and $5.^\circ1$ (2.2, 3.1, and $8.0 h^{-1}$Mpc), are connected kinematically to A2199. A2197 is itself an optically rich cluster; its weak X-ray emission suggests that it is much less massive than A2199. The absence of a sharp peak in the infall pattern at the position of A2197 supports this hypothesis. The outermost group is well outside the virial region of A2199 and it distorts the infall pattern in redshift space. The two X-ray emitting groups are roughly colinear, suggesting the existence of an extended ($8.0 h^{-1}$Mpc) filament. The identification of these infalling groups provides direct support of hierarchical structure formation; studies of these systems will provide insights into structure evolution. Groups in the infall regions of nearby clusters may offer a unique probe of the physics of the warm/hot ionized medium (WHIM) which is difficult to observe directly with current instruments.

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arXiv2000

The Infall Region of Abell 576: Independent Mass and Light Profiles

We describe observations of the nearby cluster of galaxies A576 beyond the virial radius and into the infall region. Using 1057 redshifts, we use the infall pattern in redshift space to determine the mass profile of A576 to a radius of ~4 Mpc/h. This mass estimation technique makes no assumptions about the equilibrium state of the cluster. Within 1 Mpc/h, the mass profile we derive exceeds that determined from X-ray observations by a factor of 2.5. At \~2.5 Mpc/h, however, the mass profile agrees with virial mass estimates. Our mass profile is consistent with a NFW or Hernquist profile, but it is inconsistent with an isothermal sphere. R-band images of a $3^\circ x 3^\circ$ region centered on the cluster allow an independent determination of the cluster light profile. We calculate the integrated mass-to-light ratio as a function of cluster radius; it decreases smoothly from the core to ~4 Mpc/h. The differential dM/dL_R profile decreases more steeply; we find M/L_R ~100 h at ~4 Mpc/h, in good agreement with the mass-to-light ratios of individual galaxies. This value implies $Ω_m \lesssim 0.4$ at 95% confidence. For a Hernquist model, the best-fit mass profiles differ from the observed surface number density of galaxies; the galaxies have a larger scale radius than the mass. This result is consistent with the centrally peaked $M/L_R$ profile. Similarly, the scale radius of the light profile is larger than that of the mass profile. We discuss some potential systematic effects; none can easily reconcile our results with a constant mass-to-light ratio. (abstract edited)

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arXiv2000

Kinematics and Mass Profile of AWM 7

We have measured 492 redshifts (311 new) in the direction of the poor cluster AWM~7 and have identified 179 cluster members (73 new). We use two independent methods to derive a self-consistent mass profile, under the assumptions that the absorption-line galaxies are virialized and that they trace an underlying Navarro, Frenk & White (1997) dark matter profile: (1) we fit such an NFW profile to the radial distribution of galaxy positions and to the velocity dispersion profile; (2) we apply the virial mass estimator to the cluster. With these assumptions, the two independent mass estimates agree to \sim 15% within 1.7 h^{-1} Mpc, the radial extent of our data; we find an enclosed mass \sim (3+-0.5)\times 10^{14} h^{-1} M_\odot. The largest potential source of systematic error is the inclusion of young emission-line galaxies in the mass estimate. We investigate the behavior of the surface term correction to the virial mass estimator under several assumptions about the velocity anisotropy profile, still within the context of the NFW model, and remark on the sensitivity of derived mass profiles to outliers. We find that one must have data out to a large radius in order to determine the mass robustly, and that the surface term correction is unreliable at small radii.

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arXiv1999

The RASSCALS: An X-ray and Optical Study of 260 Galaxy Groups

We describe the ROSAT All-Sky Survey-Center for Astrophysics Loose Systems (RASSCALS), the largest X-ray and optical survey of low mass galaxy groups to date. We draw 260 groups from the combined Center for Astrophysics and Southern Sky Redshift Surveys, covering one quarter of the sky to a limiting Zwicky magnitude of 15.5. We detect 61 groups (23%) as extended X-ray sources. The statistical completeness of the sample allows us to make the first measurement of the X-ray selection function of groups, along with a clean determination of their fundamental scaling laws. We find robust evidence of similarity breaking in the relationship between the X-ray luminosity and velocity dispersion. Groups with sigma < 340 km/s are overluminous by several orders of magnitude compared to the familiar LX ~ sigma^4 law for higher velocity dispersion systems. An understanding of this break depends on the detailed structure of groups with small velocity dispersions sigma < 150 km/s.

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arXiv1999

An Imaging and Spectroscopic Survey of Galaxies within Prominent Nearby Voids. II. Morphologies, Star Formation and Faint Companions

We assess the morphology-density relation, H-alpha linewidth-density relation, and the effects of nearby companions for 300 galaxies selected from low-density environments of the Center for Astrophysics Redshift Survey. Both the morphological mix and the EW(H-alpha) distribution of galaxies at modest underdensites are indistinguishable from our control sample at modest overdensities (early-type and absorption-line fractions both ~35%). At the lowest densities: the absorption-line fraction drops to 15%; more galaxies have active star formation; and there are more irregular and peculiar morphologies. Our magnitude-limited redshift survey of 250 fainter projected companions reveals that the void galaxies are no less likely to have a neighbor in redshift space. However, the typical velocity separation of close pairs drops sharply at densities below half the mean, from >200 km/s to ~100 km/s. The EW(H-alpha) distribution for galaxies lacking nearby companions varies little with density, while galaxies with nearby companions are much more likely to have strong current star formation (H-alpha linewidths >20 Å) in the voids. These results and the luminosity- and color-density relations of this sample (astro-ph/9910073) suggest that the formation and evolution of field galaxies are only sensitive to large-scale underdensity below a threshold of roughly half the mean density. The differences in galaxy properties at the lower densities may be explained by: 1) a relative scarcity of the small-scale primordial density enhancements needed to form massive early-type/absorption- line galaxies; 2) more effective galaxy encounters due to the lower velocity dispersion on small scales; and 3) more gas and dust available for triggered star formation, presuming that luminous void galaxies formed more recently.

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arXiv1999

An Imaging and Spectroscopic Survey of Galaxies Within Prominent Nearby Voids. I. The Sample and Luminosity Distribution

We study the optical properties of a large sample of galaxies in low-density regions of the nearby universe. We make a 5/h Mpc-smoothed map of the galaxy density throughout the Center for Astrophysics Redshift Survey (CfA2) to identify galaxies within three prominent nearby ``voids'' with diameter >30/h Mpc. We augment the CfA2 void galaxy sample with fainter galaxies found in the same regions from the deeper Century and 15R Redshift Surveys. We obtain B and R CCD images and high-S/N longslit spectra for the resulting sample of 149 void galaxies, and also for 131 galaxies on the periphery of these voids. Here we describe the photometry for the sample, including B isophotal magnitudes and B-R colors. For the 149 galaxies which lie in regions below the mean survey density, the luminosity functions in B and R are well-fit by Schechter functions with respective parameters (alpha_B = -0.5 +/- 0.3, B_* = -18.9 +/- 0.2) and (alpha_R = -0.9 +/- 0.3, R_* = -20.4 +/- 0.3). The B luminosity function (LF) is consistent with typical survey LFs (e.g., the Southern Sky Redshift Survey), and the R LF is consistent with the Century Survey. The B and R LFs of 131 galaxies in the ``void periphery'', regions between the mean density and twice the mean, have similar Schechter parameters. The CfA2 LF is inconsistent with both samples at 3.5-sigma. When we narrow our analysis to the 46 galaxies in regions below half the mean density, the LF is steeper: alpha = -1.4 +/- 0.5. The typical survey LFs are inconsistent with this subsample at 2-sigma. The B-R color distribution of galaxies in the lowest-density regions is also shifted significantly (3-sigma) blue-ward of the higher density samples. The most luminous red galaxies (R < -21) are absent from the lowest density regions at 2.5-sigma.

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arXiv1999

The Mass Profile of the Coma Galaxy Cluster

We use a new redshift survey complete to m_R=15.4 within 4.25 deg from the center of the Coma cluster to measure the mass profile of the cluster to r=5.5 Mpc/h. We extend the profile to r=10 Mpc/h with a further sample complete to m_R=15.4 in 42% of the area within a 10 deg radius and to m_{Zw}=15.5 in the remaining area. Galaxies within this region are falling onto the cluster on moderately radial orbits and thus do not satisfy virial equilibrium. Nonetheless, identification of the caustics in redshift space provides an estimate of the gravitational potential at radius r and hence of the system mass, M(<10 Mpc/h)=(1.65+-0.41)10^{15} M_\odot/h (1-sigma error). Previous mass estimates derived from optical and X-ray observations are limited to r<2.5 Mpc/h. Our mass profile is consistent with these estimates but extends to distances four times as large. Over the entire range, the mass increases with r at the rate expected for a Navarro, Frenk & White (1997) density profile.

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arXiv1999

The Dynamics of Poor Systems of Galaxies

We assemble and observe a sample of poor galaxy systems that is suitable for testing N-body simulations of hierarchical clustering (Navarro, Frenk, & White 1997; NFW) and other dynamical halo models (e.g., Hernquist 1990). We (1) determine the parameters of the density profile rho(r) and the velocity dispersion profile sigma(R), (2) separate emission-line galaxies from absorption-line galaxies, examining the model parameters and as a function of spectroscopic type, and (3) for the best-behaved subsample, constrain the velocity anisotropy parameter, beta, which determines the shapes of the galaxy orbits. The NFW universal profile and the Hernquist (1990) model both provide good descriptions of the spatial data. In most cases an isothermal sphere is ruled out. Systems with declining sigma(R) are well-matched by theoretical profiles in which the star-forming galaxies have predominantly radial orbits (beta > 0); many of these galaxies are probably falling in for the first time. There is significant evidence for spatial segregation of the spectroscopic classes regardless of sigma(R).

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arXiv1998

Environments of Redshift Survey Compact Groups of Galaxies

Redshift Survey Compact Groups (RSCGs) are tight knots of N >= 3 galaxies selected from the CfA2+SSRS2 redshift survey. The selection is based on physical extent and association in redshift space alone. We measured 300 new redshifts of fainter galaxies within 1 h^{-1} Mpc of 14 RSCGs to explore the relationship between RSCGs and their environments. 13 of 14 RSCGs are embedded in overdense regions of redshift space. The systems range from a loose group of 5 members to an Abell cluster. The remaining group, RSCG 64, appears isolated. RSCGs are isolated and distinct from their surroundings to varying degrees, as are the Hickson Compact Groups. Among the 13 embedded RSCGs, 3 are distinct from their general environments (RSCG 9, RSCG 11 and RSCG 85).

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arXiv1998

The Galaxy Luminosity Function at z < 0.05: Dependence on Morphology

We investigate the dependence of the local galaxy luminosity function on morphology using 5,404 galaxies from the recently enlarged Second Southern Sky Redshift Survey (SSRS2). Over the range -22 \le M_B \le -14 ($H_0 = 100 km/s/Mpc), the luminosity function of early-type galaxies is well fitted by a Schechter function with parameters M_*=-19.37^{+0.10}_{-0.11}, alpha=-1.00^{+0.09}_{-0.09} and phi_*=0.0044 +/- 0.0008 Mpc^{-3}. The spiral luminosity function is very similar and is well fitted by the parameters M_*=-19.43^{+0.08}_{-0.08}, alpha=-1.11^{+0.07}_{-0.06} and phi_*=0.0080 +/- 0.0014 Mpc^{-3} over the same range in absolute magnitude. The flat faint end of the early-type luminosity function is consistent with earlier measurements from the CfA Redshift Survey (Marzke et al. 1994) but is significantly steeper than the slope measured in the Stromlo-APM survey (Loveday et al. 1992). Combined with the increased normalization of the overall LF measured from intermediate redshift surveys, the flat faint-end slope of the E/S0 LF produces no-evolution models which reproduce the deep HST counts of early-type galaxies remarkably well. However, the observed normalization of the SSRS2 LF is consistent with the low value measured in other local redshift surveys. The cause of this low-redshift anomaly remains unknown. The luminosity function of irregular and peculiar galaxies in the SSRS2 is very steep: M_*=-19.78^{+0.40}_{-0.50}, alpha=-1.81^{+0.24}_{-0.24} and phi_*=0.0002 +/- 0.00008 Mpc^{-3}. The steep slope is consistent with the LFs measured for Sm-Im galaxies in the CfA survey, UV-selected galaxies (Treyer et al. 1997), star-forming field galaxies (Bromley et al. 1997) and the bluest galaxies in the SSRS2 (Marzke & da Costa 1997).

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arXiv1998

The Southern Sky Redshift Survey

We report redshifts, magnitudes and morphological classifications for 5369 galaxies with $m_B \leq 15.5$ and 57 galaxies fainter than this limit, in two regions covering a total of 1.70 steradians in the southern celestial hemisphere. The galaxy catalog is drawn primarily from the list of non-stellar objects identified in the Guide Star Catalog (Lasker et al. 1990, AJ 99, 2019; hereafter GSC). The galaxies have positions accurate to about 1 arc sec and magnitudes with an rms scatter of about 0.3 mag. We compute magnitudes ($m_{SSRS2}$) from the relation between instrumental GSC magnitudes and the photometry by Lauberts & Valentijn (1989). From a comparison with CCD photometry, we find that our system is homogeneous across the sky and corresponds to magnitudes measured at the isophotal level $\sim$ 26 mag arcsec$^{-2}$. The precision of the radial velocities is of \sim 40 km/s and the redshift survey is more than 99% complete to the $m_{SSRS2}$ = 15.5 magnitude limit. This sample is in the direction opposite to the CfA2; in combination the two surveys provide an important database for studies of the properties of galaxies and their large-scale distribution in the nearby Universe.

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arXiv1996

A Dynamical Analysis of the Poor Galaxy Clusters A2626 and A2440

We use 189 new radial velocities, an $R$ band CCD mosiac, archival X-ray images, and gas temperatures to study substructure, masses, luminosity functions, and mass-to-light ratios in A2626 and A2440. The symmetric X-ray emission from A2626 suggests a relaxed, single-component system; however, 159 redshifts reveals a complex, three component cluster at cz~17,500km/s. One components is a typical X-ray bright cluster, a second has a cluster-like galaxy population with lower central galaxy and gas density, and the third is a background structure with field galaxy composition. A comparison of the magnitude distributions within the two subclusters suggests that A2626 is a merger in progress. Virial masses and a composite luminosity function for a region with projected radius r=1.5h^{-1}Mpc yield a mass to light ratio M/L_R~610h. Analysis of the X-ray emission from the primary component yields a gas mass fraction of 2.2h^{-3/2}% and a baryon fraction of 3.4%. A radial infall model implies that the virial mass may be an underestimate. In A2440, deep CCD $R$ band photometry indicates a striking correspondence between the galaxy and gas distributions. The galaxy distribution has three main components, each associated with a giant elliptical galaxy. The two larger peaks in the galaxy distribution coincide with the primary peaks in the X-ray emission, and the third is associated with a significant X-ray surface brightness enhancement. We use this galaxy-gas correspondence and 48 redshifts to argue that the subclusters are bound within a single system, and that the two primary components are beginning to merge. The composite luminosity function and estimates of the subcluster virial masses indicate a mass-to-light ratio from 660h to 880h.

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arXiv1995

PAIRWISE VELOCITIES OF GALAXIES IN THE CFA AND SSRS2 REDSHIFT SURVEYS

(compressed version) We combine the CfA Redshift Survey (CfA2) and the Southern Sky Redshift Survey (SSRS2) to estimate the pairwise velocity dispersion of galaxies $\sig12$ on a scale of $\sim 1 \hmpc$. Both surveys are complete to an apparent magnitude limit $B(0)=15.5$. Our sample includes 12,812 galaxies distributed in a volume $1.8 \times 10^6 \hmpc3$. We conclude: 1) The pairwise velocity dispersion of galaxies in the combined CfA2+SSRS2 redshift survey is $\sig12=540 \kms \pm 180 \kms$. Both the estimate and the variance of $\sig12$ significantly exceed the canonical values $\sig12=340 \pm40$ measured by Davis \& Peebles (1983) using CfA1. 2) We derive the uncertainty in $\sig12$ from the variation among subsamples with volumes on the order of $7 \times 10^5$ \hmpc3. This variation is nearly an order of magnitude larger than the formal error, 36 $\kms$, derived using least-squares fits to the CfA2+SSRS2 correlation function. This variation among samples is consistent with the conclusions of Mo \etal (1993) for a number of smaller surveys and with the analysis of CfA1 by Zurek \etal (1994). 3) When we remove Abell clusters with $R\ge1$ from our sample, the pairwise velocity dispersion of the remaining galaxies drops to $295 \pm 99 \kms$. Thus the dominant source of variance in $\sig12$ is the shot noise contributed by dense virialized systems. 4) The distribution of pairwise velocities is consistent with an isotropic exponential with velocity dispersion independent of scale.

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arXiv1995

COSMOLOGICAL CONSTRAINTS FROM CLUSTER X-RAY MORPHOLOGIES

We use a representative sample of 65 galaxy clusters observed with the Einstein IPC to constrain the range of cluster X-ray morphologies. We develop and apply quantitative and reproducible measures to constrain the intrinsic distributions of emission weighted centroid variation, axial ratio, orientation, and the radial fall--off. We then use the range of cluster X--ray morphologies to constrain three generic cosmological models (Omega=1, Omega_0=0.2, and Omega_0=0.2 & lambda_0=0.8). For each of these models, we evolve eight sets of Gaussian random initial conditions consistent with an effective power spectrum $P(k)\propto k^{-1}$ on cluster scales. Using this sample of 24 numerical cluster simulations, we compare the X--ray morphologies of the observed and simulated clusters. The comparisons indicate that: (i) cluster centroid variations, axial ratios, and radial fall-offs are sensitive to the underlying cosmological model, and (ii) galaxy clusters with the observed range of X--ray morphologies are very unlikely in low Omega_0 cosmologies. The analysis favors the Omega=1 model, though some discrepancies remain. We discuss the effects of changing the initial conditions and including additional physics in the simulations.

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arXiv1994

The Power Spectrum of Galaxies in the Nearby Universe

We compute the power spectrum of galaxy density fluctuations in a recently completed redshift survey of optically-selected galaxies in the southern hemisphere (SSRS2). The amplitude and shape of the SSRS2 power spectrum are consistent with results of the Center for Astrophysics redshift survey of the northern hemisphere (CfA2), including the abrupt change of slope on a scale of 30-50Mpc/h; these results are reproducible for independent volumes of space and variations are consistent with the errors estimated from mock surveys. Taken together, the SSRS2 and CfA2 form a complete sample of 14,383 galaxies which covers one-third of the sky. The power spectrum of this larger sample continues to rise on scales up to ~ 200Mpc/h, with weak evidence for flattening on the largest scales. The SSRS2+CfA2 power spectrum and the power spectrum constraints implied by COBE are well-matched by an Omega*h ~ 0.2, Omega+lambda_0=1 CDM model with minimal biasing of optically-selected galaxies.

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