Calibrating Galaxy Redshifts Using Absorption by the Surrounding Intergalactic Medium

Rest-frame UV spectral lines of star-forming galaxies are systematically offset from the galaxies' systemic redshifts, probably because of large-scale outflows. We calibrate galaxy redshifts measured from rest-frame UV lines by utilizing the fact that the mean HI Ly-alpha absorption profiles around the galaxies, as seen in spectra of background objects, must be symmetric with respect to the true galaxy redshifts if the galaxies are oriented randomly with respect to the lines of sight to the background objects. We use 15 QSOs at z~2.5-3 and more than 600 foreground galaxies with spectroscopic redshifts at z~1.9-2.5. All galaxies are within 2 Mpc proper from the lines of sight to the background QSOs. We find that LyA emission and ISM absorption redshifts require systematic shifts of v_LyA=-295(+35)(-35) km/s and v_ISM=145(+70)(-35) km/s. Assuming a Gaussian distribution, we put 1-sigma upper limits on possible random redshift offsets of <220 km/s for LyA and <420 km/s for ISM redshifts. For the small subset (<10%) of galaxies for which near-IR spectra have been obtained, we can compare our results to direct measurements based on nebular emission lines which we confirm to mark the systemic redshifts. While our v_ISM agrees with the direct measurements, our v_LyA is significantly smaller. However, when we apply our method to the near-IR subsample which is characterized by slightly different selection effects, the best-fit velocity offset comes into agreement with the direct measurement. This confirms the validity of our approach, and implies that no single number appropriately describes the whole population of galaxies, in line with the observation that the line offset depends on galaxy spectral morphology. This method provides accurate redshift calibrations and will enable studies of circumgalactic matter around galaxies for which rest-frame optical observations are not available.Comment: 7 pages, 3 figures, accepted for publication in MNRA

The nature of Lyman-break galaxies

Using semi-analytic models of galaxy formation, we investigate the properties of $z\sim3$ galaxies and compare them with the observed population of Lyman-break galaxies (LBGs). In addition to the usual quiescent mode of star formation, we introduce a physical model for starbursts triggered by galaxy-galaxy interactions. We find that with the merger rate that arises naturally in the CDM-based merging hierarchy, a significant fraction of bright galaxies identified at high redshift (z \ga 2) are likely to be low-mass, bursting satellite galaxies. The abundance of LBGs as a function of redshift and the luminosity function of LBGs both appear to be in better agreement with the data when the starburst mode is included, especially when effects of dust extinction are considered. The objects that we identify as LBGs have observable properties including low velocity dispersions that are in good agreement with the available data. In this ``Bursting Satellite'' scenario, quiescent star formation at z\ga2 is relatively inefficient and most of the observed LBGs are starbursts triggered by satellite mergers within massive halos. In high-resolution N-body simulations, we find that the most massive dark matter halos cluster at redshift $z\sim 3$ much as the LBGs are observed to do. This is true for both the $\Omega=1$ CHDM model and low-$\Omega$ \LCDM and OCDM models, all of which have fluctuation power spectra $P(k)$ consistent with the distribution of low-redshift galaxies. The Bursting Satellite scenario can resolve the apparent paradox of LBGs that cluster like massive dark matter halos but have narrow linewidths and small stellar masses.Comment: 10 pages, latex, including 2 figures, to appear in the Proceedings of the 3rd International Symposium on Sources and Detection of Dark Matter in the Universe (DM98), Feb. 1998, ed. D. Clin

Diffuse Lyman Alpha Haloes around Lyman Alpha Emitters at z=3: Do Dark Matter Distributions Determine the Lyman Alpha Spatial Extents?

Using stacks of Ly-a images of 2128 Ly-a emitters (LAEs) and 24 protocluster UV-selected galaxies (LBGs) at z=3.1, we examine the surface brightness profiles of Ly-a haloes around high-z galaxies as a function of environment and UV luminosity. We find that the slopes of the Ly-a radial profiles become flatter as the Mpc-scale LAE surface densities increase, but they are almost independent of the central UV luminosities. The characteristic exponential scale lengths of the Ly-a haloes appear to be proportional to the square of the LAE surface densities (r(Lya) \propto Sigma(LAE)^2). Including the diffuse, extended Ly-a haloes, the rest-frame Ly-a equivalent width of the LAEs in the densest regions approaches EW_0(Lya) ~ 200 A, the maximum value expected for young (< 10^7 yr) galaxies. This suggests that Ly-a photons formed via shock compression by gas outflows or cooling radiation by gravitational gas inflows may partly contribute to illuminate the Ly-a haloes; however, most of their Ly-a luminosity can be explained by photo-ionisation by ionising photons or scattering of Ly-a photons produced in HII regions in and around the central galaxies. Regardless of the source of Ly-a photons, if the Ly-a haloes trace the overall gaseous structure following the dark matter distributions, it is not surprising that the Ly-a spatial extents depend more strongly on the surrounding Mpc-scale environment than on the activities of the central galaxies.Comment: 7 pages, 4 figures, accepted for publication in MNRA

Parallelization of Modular Algorithms

In this paper we investigate the parallelization of two modular algorithms. In fact, we consider the modular computation of Gr\"obner bases (resp. standard bases) and the modular computation of the associated primes of a zero-dimensional ideal and describe their parallel implementation in SINGULAR. Our modular algorithms to solve problems over Q mainly consist of three parts, solving the problem modulo p for several primes p, lifting the result to Q by applying Chinese remainder resp. rational reconstruction, and a part of verification. Arnold proved using the Hilbert function that the verification part in the modular algorithm to compute Gr\"obner bases can be simplified for homogeneous ideals (cf. \cite{A03}). The idea of the proof could easily be adapted to the local case, i.e. for local orderings and not necessarily homogeneous ideals, using the Hilbert-Samuel function (cf. \cite{Pf07}). In this paper we prove the corresponding theorem for non-homogeneous ideals in case of a global ordering.Comment: 16 page

Medium-resolution spectroscopy of galaxies with redshifts 2.3 < z < 3.5

Using FORS2 at the ESO VLT we obtained medium resolution (R ~ 2000) spectra of 12 galaxies with 2.37 < z < 3.40 in the FORS Deep Field. Two individual spectra with good S/N and a composite of all 12 spectra were used to derive properties of the stellar and interstellar absorption lines of galaxies in this redshift range. Systematic differences between the individual spectra were found for the strength and profiles of the intrinsic interstellar lines. For eight spectra with sufficient S/N we measured the `1370' and `1425' metallicity indices. From these indices we find for our sample that galaxies at z > 3 have lower mean metallicity than galaxies at 2.5 < z < 3. However there remain uncertainties concerning the absolute calibration of the metallicity tracers in use for high-redshift galaxies. Additional modeling will be needed to resolve these uncertainties.Comment: 10 pages, 4 figures. Accepted by A&

An algorithm for primary decomposition in polynomial rings over the integers

We present an algorithm to compute a primary decomposition of an ideal in a polynomial ring over the integers. For this purpose we use algorithms for primary decomposition in polynomial rings over the rationals resp. over finite fields, and the idea of Shimoyama-Yokoyama resp. Eisenbud-Hunecke-Vasconcelos to extract primary ideals from pseudo-primary ideals. A parallelized version of the algorithm is implemented in SINGULAR. Examples and timings are given at the end of the article.Comment: 8 page

Diffuse Lyman Alpha Emitting Halos: A Generic Property of High Redshift Star Forming Galaxies

Using a sample of 92 UV continuum-selected, spectroscopically identified galaxies with = 2.65, all of which have been imaged in the Ly-a line with extremely deep narrow-band imaging, we examine galaxy Ly-a emission profiles to very faint surface brightness limits. The galaxies are representative of spectroscopic samples of LBGs at similar redshifts in terms of apparent magnitude, UV luminosity, inferred extinction, and star formation rate, and were selected without regard to Ly-a emission properties. We use extremely deep stacks of UV continuum and Ly-a emission line images to show that all sub-samples exhibit diffuse Ly-a emission to radii of at least 10" (80 physical kpc), including galaxies whose spectra exhibit Ly-a in net absorption. The intensity scaling, but not the surface brightness distribution, is strongly correlated with the emission observed in the central ~1". The characteristic scale length for Ly-a line emission exceeds that of the UV continuum light for the same galaxies by factors of at least 5-10, regardless of the spectral morphology of Ly-a. Including the extended Ly-a halos increases the total Ly-a flux [and rest equivalent width W_0(Lya)] by an average factor of 5. We argue that most, if not all, of the observed Ly-a emission in the diffuse halos originates in the galaxy H II regions and is scattered in our direction by H I gas in the galaxy's circum-galactic medium (CGM). We show that whether or not a galaxy is classified as a giant "Lyman-a Blob" (LAB) depends sensitively on the Ly-a surface brightness threshold reached by an observation. Accounting for diffuse Ly-a halos, all LBGs would be LABs if surveys were routinely sensitive to 10 times lower surface brightness thresholds; also, essentially all LBGs would qualify as LAEs (W_0(Lya) > 20 A).Comment: Updated to match final version to appear in ApJ; 20 pages, 14 figure

The halo masses and galaxy environments of hyperluminous QSOs at z~2.7 in the Keck Baryonic Structure Survey

We present an analysis of the galaxy distribution surrounding 15 of the most luminous (>10^{14} L_sun; M_1450 ~ -30) QSOs in the sky with z~2.7. Our data are drawn from the Keck Baryonic Structure Survey (KBSS). In this work, we use the positions and spectroscopic redshifts of 1558 galaxies that lie within ~3', (4.2 h^{-1} comoving Mpc; cMpc) of the hyperluminous QSO (HLQSO) sightline in one of 15 independent survey fields, together with new measurements of the HLQSO systemic redshifts. We measure the galaxy-HLQSO cross-correlation function, the galaxy-galaxy autocorrelation function, and the characteristic scale of galaxy overdensities surrounding the sites of exceedingly rare, extremely rapid, black hole accretion. On average, the HLQSOs lie within significant galaxy overdensities, characterized by a velocity dispersion sigma_v ~ 200 km s^{-1} and a transverse angular scale of ~25", (~200 physical kpc). We argue that such scales are expected for small groups with log(M_h/M_sun)~13. The galaxy-HLQSO cross-correlation function has a best-fit correlation length r_0_GQ = (7.3 \pm 1.3) h^{-1} cMpc, while the galaxy autocorrelation measured from the spectroscopic galaxy sample in the same fields has r_0_GG = (6.0 \pm 0.5) h^{-1} cMpc. Based on a comparison with simulations evaluated at z ~ 2.6, these values imply that a typical galaxy lives in a host halo with log(M_h/M_sun) = 11.9\pm0.1, while HLQSOs inhabit host halos of log(M_h/M_sun) = 12.3\pm0.5. In spite of the extremely large black hole masses implied by their observed luminosities [log(M_BH/M_sun) > 9.7], it appears that HLQSOs do not require environments very different from their much less luminous QSO counterparts. Evidently, the exceedingly low space density of HLQSOs (< 10^{-9} cMpc^{-3}) results from a one-in-a-million event on scales << 1 Mpc, and not from being hosted by rare dark matter halos.Comment: 15 pages, 6 figures. Accepted for publication in Ap

Detecting the Transverse Proximity Effect: Radiative Feedback from QSOs

We present results of a search for absorbers close to QSOs near the line of sight of background quasars based on sensitive Keck/HIRES observations of the QSO triplet KP76, KP77, and KP78 (z=2.466, 2.535, and 2.615), with separations of 2-3 arcmin on the plane plane of the sky. Through the use of a high resolution spectra, together with accurate systemic redshifts of the QSOs using near-IR spectroscopy, we quantify the state of the IGM gas at the redshifts of the foreground QSOs. The inferred ionizing flux at the proper transverse distances probed is up to 100 times greater than the UV background. We show that the state of the gas has been significantly affected by the UV radiation from the foreground QSOs