2,574 research outputs found
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 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 much as the LBGs are observed to do. This
is true for both the CHDM model and low- \LCDM and OCDM
models, all of which have fluctuation power spectra 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
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