Browsing by Author "Morris, SL"

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    Dynamically close galaxy pairs and merger rate evolution in the CNOC2 redshift survey
    (2002) Patton, DR; Pritchet, CJ; Carlberg, RG; Marzke, RO; Yee, HKC; Hall, PB; Lin, H; Morris, SL; Sawicki, M; Shepherd, CW; Wirth, GD
    We investigate redshift evolution in the galaxy merger and accretion rates, using a well-defined sample of 4184 galaxies with 0.12 less than or equal toz 0.55 and R-C less than or equal to 21.5. We identify 88 galaxies in close (5 less than or equal to r(p) less than or equal to 20 h(-1) kpc) dynamical (Deltav less than or equal to 1500 km s(-1)) pairs. These galaxies are used to compute global pair statistics, after accounting for selection effects resulting from the flux limit, k-corrections, luminosity evolution, and spectroscopic incompleteness. We find that the number of companions per galaxy (for -21 less than or equal to M-B(k,e) less than or equal to -18) is N-c = 0.0321 +/- 0.0077 at z = 0.3. The luminosity in companions, per galaxy, is L-c = 0.0294 +/- 0.0084 x 10(10) h(2) L-circle dot. We assume that is proportional to the galaxy merger rate, while L-c is directly related to the mass accretion rate. After increasing the maximum pair separation to 50 h(-1) kpc and comparing with the low-redshift SSRS2 pair sample, we infer evolution in the galaxy merger and accretion rates of (1+z)(2.3+/-0.7) and (1+z)(2.3+/-0.9), respectively. These are the first such estimates to be made using only confirmed dynamical pairs. When combined with several additional assumptions, this implies that approximately 15% of present epoch galaxies with -21 less than or equal to M-B less than or equal to -18 have undergone a major merger since z = 1.
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    Environment and galaxy evolution at intermediate redshift in the CNOC2 survey
    (2001) Carlberg, RG; Yee, HKC; Morris, SL; Lin, H; Hall, PB; Patton, DR; Sawicki, M; Shepherd, CW
    The systematic variation of galaxy colors and types with clustering environment could either be the result of local conditions at formation or subsequent environmental effects as larger scale structures draw together galaxies whose stellar mass is largely in place. Below redshift 0.7 galaxy luminosities (k-corrected and evolution compensated) are relatively invariant, whereas galaxy star formation rates, as reflected in their colors, are a "transient" property that have a wide range for a given luminosity. The relations between these galaxy properties and the clustering properties are key statistics for understanding the forces driving late-time galaxy evolution. At z similar to 0.4 the comoving galaxy correlation length, r(o), measured in the CNOC2 sample is strongly color dependent, rising from 2 h(-1) Mpc to nearly 10 h(-1) Mpc as the volume-limited subsamples range from blue to red. The luminosity dependence of r(o) at z similar to 0.4 is weak below L-* in the R band, although there is an upturn at high luminosity, where its interpretation depends on separating it from the r(o)-color relation. In the B band there is a slow, smooth increase of r(o) with luminosity, at least partially related to the color dependence. Study of the evolution of galaxies within groups, which create much of the strongly nonlinear correlation signal, allows a physical investigation of the source of these relations. The dominant effect of the group environment on star formation is seen in the radial gradient of the mean galaxy colors, which on the average become redder than the field toward the group centers. The color differentiation begins around the dynamical radius of virialization of the groups. The redder-than-field trend applies to groups with a line-of-sight velocity dispersion, sigma (1) > 150 km s(-1). There is an indication, somewhat statistically insecure, that the high-luminosity galaxies in groups with sigma (1) < 125 km s(-1) become bluer toward the group center. Monte Carlo orbit integrations initiated at the measured positions and velocities show that the rate of galaxy merging in the (1) > 150 km s(-1) groups is very low, whereas for sigma (1) < 150 km s(-1) about 25% of the galaxies will merge in 0.5 Gyr. We conclude that the higher velocity dispersion groups largely act to suppress star formation relative to the less clustered field, leading to "embalmed" galaxies. On the other hand, the low velocity dispersion groups are prime sites of both strong merging and enhanced star formation that leads to the formation of some new massive galaxies at intermediate redshifts. The tidal fields within the groups appear to be a strong candidate for the physical source of the reduction of star formation in group galaxies relative to field. Tides operate effectively at all velocity dispersions to remove gas-rich companions and low-density gas in galactic halos. We find a close resemblance of the color-dependent galaxy luminosity function evolution in the field and groups, suggesting that the clustering-dependent star formation reduction mechanism is important for the evolution of field galaxies as a whole.
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    Galaxy groups at intermediate redshift
    (2001) Carlberg, RG; Yee, HKC; Morris, SL; Lin, H; Hall, PB; Patton, DR; Sawicki, M; Shepherd, CW
    Galaxy groups likely to be virialized are identified within the CNOC2 intermediate-redshift galaxy survey. The resulting groups have a median velocity dispersion, sigma (1) similar or equal to 200 km s(-1). The virial mass-to-light ratios, using k-corrected and evolution-compensated luminosities, have medians in the range of 150-250 h M./L., depending on group definition details. The number-velocity dispersion relation at sigma (1) greater than or similar to 200 km s(-1) is in agreement with the low-mass extrapolation of the cluster-normalized Press-Schechter model. Lower velocity dispersion groups are deficient relative to the Press-Schechter model, The two-point group-group autocorrelation function has r(0) = 6.8 +/- 0.3 h(-1) Mpc, which is much larger than the correlations of individual galaxies, but about as expected from biased clustering. The mean number density of galaxies around group centers falls nearly as a power law with r(-2.5) and has no well-defined core. The projected velocity dispersion of galaxies around group centers is either hat or slowly rising outward. The combination of a steeper than isothermal density profile and the outward rising velocity dispersion implies that the mass-to-light ratio of groups rises with radius if the velocity ellipsoid is isotropic but could be nearly constant if the galaxy orbits are nearly circular. Such strong tangential anisotropy is not supported by other evidence. Although the implication of a rising M/L must be viewed with caution, it could naturally arise through dynamical friction acting on the galaxies in a background of "classical" collisionless dark matter.
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    The galaxy correlation function in the CNOC2 redshift survey: Dependence on color, luminosity, and redshift
    (2001) Shepherd, CW; Carlberg, RG; Yee, HKC; Morris, SL; Lin, H; Sawicki, M; Hall, PB; Patton, DR
    We examine how the spatial correlation function of galaxies from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) depends on galaxy color, luminosity, and redshift. The projected correlation w(p) function is determined for volume-limited samples of objects with 0.12 less than or equal to z < 0.51 and evolution-compensated R-C-band absolute magnitudes M-R(O) < -20, over the co-moving projected separation range 0.04 h(-1) Mpc < r(p) <10 h(-1) Mpc. Our sample consists of 2937 galaxies that are classified as being either early- or late-type objects according to their spectral energy distribution (SED), as determined from UBVRCIC photometry. For the sake of simplicity, galaxy SEDs are classified independently of redshift : Our classification scheme therefore does not take into account the color evolution of galaxies. Objects with SEDs corresponding to early-type galaxies are found to be more strongly clustered by a factor of 3 and to have a steeper correlation function than those with late-type SEDs. Modeling the spatial correlation function, as a function of comoving separation r, as xi (r) = (r/r(o))(-gamma), we find r(o) = 5.45 +/- 0.28 h(-1) Mpc and gamma = 1.91 +/- 0.06 for early-type objects, and r(o) = 3.95 +/- 0.12 h(-1) Mpc and gamma = 1.59 +/- 0.08 for late-type objects (for Omega (M) = 0.2 Omega (Lambda) = 0). While changing the cutoff between early- and late-type SEDs does affect the correlation amplitudes of the two samples, the ratio of the amplitudes remains constant to within 10%. The redshift dependence of the correlation function also depends on SED type. Modeling the redshift dependence of the comoving correlation amplitude r(o)(gamma) r(o)(gamma)(z) proportional to (1 + z)(gamma -3-epsilon), we find that early-type objects have epsilon = -3.9 +/- 1.0, and late-type objects have epsilon = -7.7 +/- 1.3. Both classes of objects therefore have clustering amplitudes, measured in comoving coordinates, which appear to decrease rapidly with cosmic time. The excess clustering of galaxies with early-type SEDs, relative to late-type objects, is present at all redshifts in our sample. In contrast to the early- and late-type SED samples, the combined sample undergoes little apparent evolution, with epsilon = -2.1 +/- 1.3, which is consistent with earlier results. The apparent increase with redshift of the clustering amplitude in the early- and late-type samples is almost certainly caused by evolution of the galaxies themselves rather than by evolution of the correlation function. If galaxy SEDs have evolved significantly since z similar to 0.5, then our method of classifying SEDs may cause us to overestimate the true evolution of the clustering amplitude for the unevolved counterparts to our early- and late-type samples. However, if color evolution is to explain the apparent clustering evolution, the color evolution experienced by a galaxy must be correlated with the galaxy correlation function. We also investigate the luminosity dependence of the correlation function for volume-limited samples with 0.12 less than or equal to z < 0.40 and M-R(o) < -19.25.