25 research outputs found
Identification of Outflows and Candidate Dual Active Galactic Nuclei in SDSS Quasars at z=0.8-1.6
We present a sample of 131 quasars from the Sloan Digital Sky Survey at
redshifts 0.8<z<1.6 with double peaks in either of the high-ionization narrow
emission lines [NeV]3426 or [NeIII]3869. These sources were selected with the
intention of identifying high-redshift analogs of the z<0.8 active galactic
nuclei (AGN) with double-peaked [OIII]5007 lines, which might represent AGN
outflows or dual AGN. Lines of high-ionization potential are believed to
originate in the inner, highly photoionized portion of the narrow line region
(NLR), and we exploit this assumption to investigate the possible kinematic
origins of the double-peaked lines. For comparison, we measure the [NeV]3426
and [NeIII]3869 double peaks in low-redshift (z<0.8) [OIII]-selected sources.
We find that [NeV]3426 and [NeIII]3869 show a correlation between
line-splitting and line-width similar to that of [OIII]5007 in other studies;
and the velocity-splittings are correlated with the quasar Eddington ratio.
These results suggest an outflow origin for at least a subset of the
double-peaks, allowing us to study the high-ionization gas kinematics around
quasars. However, we find that a non-neligible fraction of our sample show no
evidence for an ionization stratification. For these sources, the outflow
scenario is less compelling, leaving the dual AGN scenario as a viable
possibility. Finally, we find that our sample shows an anti-correlation between
the velocity-offset ratio and luminosity ratio of the components, which is a
potential dynamical argument for the presence of dual AGN. Therefore, this
study serves as a first attempt at extending the selection of candidate dual
AGN to higher redshifts.Comment: 19 pages, 12 figures, accepted for publication in The Astrophysical
Journa
Counts-in-Cylinders in the Sloan Digital Sky Survey with Comparisons to N-body Simulations
Environmental statistics provide a necessary means of comparing the
properties of galaxies in different environments and a vital test of models of
galaxy formation within the prevailing, hierarchical cosmological model. We
explore counts-in-cylinders, a common statistic defined as the number of
companions of a particular galaxy found within a given projected radius and
redshift interval. Galaxy distributions with the same two-point correlation
functions do not necessarily have the same companion count distributions. We
use this statistic to examine the environments of galaxies in the Sloan Digital
Sky Survey, Data Release 4. We also make preliminary comparisons to four models
for the spatial distributions of galaxies, based on N-body simulations, and
data from SDSS DR4 to study the utility of the counts-in-cylinders statistic.
There is a very large scatter between the number of companions a galaxy has and
the mass of its parent dark matter halo and the halo occupation, limiting the
utility of this statistic for certain kinds of environmental studies. We also
show that prevalent, empirical models of galaxy clustering that match observed
two- and three-point clustering statistics well fail to reproduce some aspects
of the observed distribution of counts-in-cylinders on 1, 3 and 6-Mpc/h scales.
All models that we explore underpredict the fraction of galaxies with few or no
companions in 3 and 6-Mpc/h cylinders. Roughly 7% of galaxies in the real
universe are significantly more isolated within a 6 Mpc/h cylinder than the
galaxies in any of the models we use. Simple, phenomenological models that map
galaxies to dark matter halos fail to reproduce high-order clustering
statistics in low-density environments.Comment: 17 pages, 10 figures. Accepted, Ap
Close Galaxy Counts as a Probe of Hierarchical Structure Formation
Standard LCDM predicts that the major merger rate of galaxy-size dark matter
halos rises rapidly with redshift. The average number of close companions per
galaxy, Nc, is often used to infer the galaxy merger rate, however, recent
observational studies suggest that Nc evolves very little with redshift. Here
we use a "hybrid" N- body simulation plus analytic substructure model to
predict Nc directly. We identify dark matter subhalos with galaxies and show
that the observed lack of close pair count evolution arises because the high
merger rate per halo at early times is counteracted by a decrease in the number
of halos massive enough to host a galaxy pair. We compare our results to data
compiled from the DEEP2, SSRS2, and the UZC redshift surveys. Observed pair
counts match our predictions if we assume a monotonic mapping between galaxy
luminosity and the maximum circular velocity that each subhalo had when it was
first accreted onto its host halo. This suggests that satellite galaxies are
significantly more resilient to destruction than are dissipationless dark
matter subhalos. We argue that while Nc does not provide a direct measure of
the halo merger rate, it offers a powerful means to constrain the Halo
Occupation Distribution and the spatial distribution of galaxies within halos.
Interpreted in this way, close pair counts provide a useful test of galaxy
formation processes on < 100 kpc scales.Comment: 16 pages, 16 figures, minor change to figure 10, figure captions
updated, typos corrected, Figure 4 corrected, version accepted for
publication by Ap
Close Galaxy Pairs at z = 3: A Challenge to UV Luminosity Abundance Matching
We use a sample of z~3 Lyman Break Galaxies (LBGs) to examine close pair
clustering statistics in comparison to LCDM-based models of structure
formation. Samples are selected by matching the LBG number density and by
matching the observed LBG 3-D correlation function of LBGs over the two-halo
term region. We show that UV-luminosity abundance matching cannot reproduce the
observed data, but if subhalos are chosen to reproduce the observed clustering
of LBGs we are able to reproduce the observed LBG pair fraction, (Nc), defined
as the average number of companions per galaxy. This model suggests an over
abundance of LBGs by a factor of ~5 over those observed, suggesting that only 1
in 5 halos above a fixed mass hosts a galaxy with LBG-like UV luminosity
detectable via LBG selection techniques. We find a total observable close pair
fraction of 23 \pm 0.6% (17.7 \pm 0.5%) using a prototypical cylinder radius in
our overdense fiducial model and 8.3 \pm 0.5% (5.6 \pm 0.2%) in an abundance
matched model (impurity corrected). For the matched spectroscopic slit
analysis, we find Ncs = 5.1\pm0.2% (1.68\pm0.02%), the average number of
companions observed serendipitously in our for fiducial slits (abundance
matched), whereas the observed fraction of serendipitous spectroscopic close
pairs is 4.7\pm1.5 per cent using the full LBG sample and 7.1\pm2.3% for a
subsample with higher signal-to-noise ratio. We show that the standard method
of halo assignment fails to reproduce the break in the LBG close pair behavior
at small scale. To reconcile these discrepancies we suggest that a plausible
fraction of LBGs in close pairs with lower mass than our sample experience
interaction-induced enhanced star formation that boosts their luminosity
sufficiently to be detected in observational sample but are not included in the
abundance matched simulation sample.Comment: 18 pages, 12 figures, 1 table, published in MNRA