Browsing by Author "Sawicki, M"
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- ItemDetermination of a protein structure by iodination(1999) Ghosh, D; Erman, M; Sawicki, M; Lala, P; Weeks, DR; Li, NY; Pangborn, W; Thiel, DJ; Jörnvall, H; Gutierrez, R; Eyzaguirre, JEnzymatic and non-enzymatic iodination of the amino acid tyrosine is a well known phenomenon. The iodination technique has been widely used for labeling proteins, Using high-resolution X-ray crystallographic techniques, the chemical and three-dimensional structures of iodotyrosines formed by non-enzymatic incorporation of I atoms into tyrosine residues of a crystalline protein are described. Acetylxylan esterase (AXE II; 207 amino-acid residues) from Penicillium purpurogenum has substrate specificities towards acetate esters of D-xylopyranose residues in xylan and belongs to a new class of alpha/beta hydrolases. The crystals of the enzyme are highly ordered, tightly packed and diffract to better than sub-angstrom resolution at 85 K. The iodination technique has been utilized to prepare an isomorphous derivative of the AXE II crystal. The structure of the enzyme determined at 1.10 Angstrom resolution exclusively by normal and anomalous scattering from I atoms, along with the structure of the iodinated complex at 1.80 Angstrom resolution, demonstrate the formation of covalent bonds between I atoms and C atoms at ortho positions to the hydroxyl groups of two tyrosyl moieties, yielding iodotyrosines.
- ItemDynamically 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, GDWe 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.
- ItemEnvironment 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, CWThe 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.
- ItemGalaxy groups at intermediate redshift(2001) Carlberg, RG; Yee, HKC; Morris, SL; Lin, H; Hall, PB; Patton, DR; Sawicki, M; Shepherd, CWGalaxy 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.
- ItemMultiple conformations of catalytic serine and histidine in acetylxylan esterase at 0.90 Å(2001) Ghosh, D; Sawicki, M; Lala, P; Erman, M; Pangborn, W; Eyzaguirre, J; Gutiérrez, R; Jörnvall, H; Thiel, DJAcetylxylan esterase (AXEII; 207 amino acids) from Penicillium purpurogenum has substrate specificities toward acetate esters of D-xylopyranose residues in xylan and belongs to a new class of alpha/beta hydrolases. The crystal structure of AXEII has been determined by single isomorphous replacement and anomalous scattering, and refined at 0.90- and 1.10-Angstrom resolutions with data collected at 85 K and 295 K, respectively. The tertiary structure consists of a doubly wound alpha/beta sandwich, having a central six-stranded parallel beta -sheet flanked by two parallel ol-helices on each side. The catalytic residues Ser(90), His(187), and Ap(175) are located at the C-terminal end of the sheet, an exposed region of the molecule. The serine and histidine side chains in the 295 K structure show the frequently observed conformations in which Ser(90) is trans and the hydroxyl group is in the plane of the imidazole ring of His(187), However, the structure at 85 K displays an additional conformation in which Ser(90) side-chain hydroxyl is away from the plane of the imidazole ring of His(187). The His(187) side chain forms a hydrogen bond with a sulfate ion and adopts an altered conformation. The only other known hydrolase that has a similar tertiary structure is Fusarium solani cutinase, The exposed nature of the catalytic triad suggests that AXEII is a pure esterase, i.e. an alpha/beta hydrolase with specificity for nonlipidic polar substrates.
- ItemRedshifts in the Hubble Deep Field South(2003) Sawicki, M; Mallén-Ornelas, GWe present a catalog of 97 spectroscopic redshifts of z < 1 galaxies in the Hubble Deep Field South (HDF-S) and its flanking fields (FFs). In the HDF-S proper we observed approximately half the galaxies brighter than I-814(AB) = 24 and obtained redshifts for 76% of them. Targets in our HDF-S sample were pre-selected to be at z < 1 based on photometric redshifts, while in the FFs a simple magnitude cut was used. The photometric redshift preselection in the HDF-S resulted in a spectroscopic success rate that is significantly higher than in the FFs, where no preselection was applied. The rms precision of our redshift measurements, determined from repeat observations, is deltaz = 0.0003. We present the photometry and redshifts for the 97 objects for which we secured spectroscopic redshifts and describe the basic properties of this sample.
- ItemThe absorption and emission kinematics in the z=0.7450 MgII absorber toward Q1331+17(2003) Ellison, SL; Mallén-Ornelas, G; Sawicki, MWe present a comparative analysis of the galaxy emission and quasar (QSO) absorption kinematics of a z(abs) similar to 0.7450 Mg II system and its candidate absorbing galaxy (G5) located 3."86 (28.3 h(70)(-1) kpc) from the QSO. We have obtained a spectrum of the galaxy candidate, previously identified as a luminous edge-on disk, and detect the [O II] lambda3727 doublet at a systemic redshift of z(sys) = 0.7450. From slit spectroscopy of this galaxy, we find v(rot) greater than or similar to 210 km s(-1), possibly as large as 350 km s(-1). Plotted on the same velocity scale, the systemic redshift of the galaxy coincides with the center of the absorption system, although the absorption components span more than 100 km s(-1) in either direction. However, once the sense of the rotation is taken into account, there is no absorbing gas at the projected velocity of the disk rotation curve. This implies incompatibility with a simple disk scenario. Moreover, a reanalysis of archival Hubble Space Telescope (HST) data reveals that the galaxy is only 0.3L(*), considerably less luminous than previously reported in the literature. This is incompatible with the established Tully-Fisher relation at this redshift, unless approximately 2 mag of total extinction is invoked. Careful inspection of the archival HST data reveals that G5 may well be composed of two galaxies, although the quality of the data does not permit a detailed investigation of this. This possibility is further supported by the identification of a second faint emission line at lambda(obs) = 5674 Angstrom, whose distinct spatial and velocity profiles indicate that it arises in a different galaxy at a different redshift. Analysis of the absorption lines shows evidence for superbubbles in the interstellar medium of the absorbing galaxy, based on the striking symmetry between components and large Mg I/Mg II and Mg I/Fe II ratios, indicative of large densities. The large velocity separations between line pairings, Deltav similar to 150 km s(-1), indicate that these bubbles may be powered by OB associations comparable to the largest observed at z = 0 and that the gas is probably enriched to at least 1/10 solar metallicity. This is consistent with observations at low redshift that extended Mg II halos are often seen in galaxies that contain disturbed gas. Superbubbles may also explain why the absorber has a relatively large Mg II equivalent width relative to the luminosity of the associated galaxy ( or galaxies).
- ItemThe 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, DRWe 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.