159 research outputs found
Local Analogs for High-redshift Galaxies: Resembling the Physical Conditions of the Interstellar Medium in High-redshift Galaxies
We present a sample of local analogs for high-redshift galaxies selected in
the Sloan Digital Sky Survey (SDSS). The physical conditions of the
interstellar medium (ISM) in these local analogs resemble those in
high-redshift galaxies. These galaxies are selected based on their positions in
the [OIII]/H versus [NII]/H nebular emission-line diagnostic
diagram. We show that these local analogs share similar physical properties
with high-redshift galaxies, including high specific star formation rates
(sSFRs), flat UV continuums and compact galaxy sizes. In particular, the
ionization parameters and electron densities in these analogs are comparable to
those in galaxies, but higher than those in normal SDSS galaxies
by 0.6~dex and 0.9~dex, respectively. The mass-metallicity
relation (MZR) in these local analogs shows ~dex offset from that in SDSS
star-forming galaxies at the low mass end, which is consistent with the MZR of
the galaxies. We compare the local analogs in this study with those
in other studies, including Lyman break analogs (LBA) and green pea (GP)
galaxies. The analogs in this study share a similar star formation surface
density with LBAs, but the ionization parameters and electron densities in our
analogs are higher than those in LBAs by factors of 1.5 and 3, respectively.
The analogs in this study have comparable ionization parameter and electron
density to the GP galaxies, but our method can select galaxies in a wider
redshift range. We find the high sSFR and SFR surface density can increase the
electron density and ionization parameters, but still cannot fully explain the
difference in ISM condition between nearby galaxies and the local
analogs/high-redshift galaxies.Comment: 13 pages, 11 figures, accepted by Ap
Constraining Sub-Parsec Binary Supermassive Black Holes in Quasars with Multi-Epoch Spectroscopy. II. The Population with Kinematically Offset Broad Balmer Emission Lines
A small fraction of quasars have long been known to show bulk velocity
offsets in the broad Balmer lines with respect to the systemic redshift of the
host galaxy. Models to explain these offsets usually invoke broad-line region
gas kinematics/asymmetry around single black holes (BHs), orbital motion of
massive (~sub-pc) binary black holes (BBHs), or recoil BHs, but single-epoch
spectra are unable to distinguish between these scenarios. The line-of-sight
(LOS) radial velocity (RV) shifts from long-term spectroscopic monitoring can
be used to test the BBH hypothesis. We have selected a sample of 399 quasars
with offset broad H-beta lines from the SDSS DR7 quasar catalog, and have
conducted second-epoch optical spectroscopy for 50 of them. Combined with the
existing SDSS spectra, the new observations enable us to constrain the LOS RV
shifts of broad H-beta lines with a rest-frame baseline of a few years to
nearly a decade. Using cross-correlation analysis, we detect significant (99%
confidence) radial accelerations in the broad H-beta lines in 24 of the 50
objects. We suggest that 9 of the 24 detections are sub-pc BBH candidates,
which show consistent velocity shifts independently measured from a second
broad line (either H-alpha or Mg II) without significant changes in the
broad-line profiles. Combining the results on the general quasar population
studied in Paper I, we find a tentative anti-correlation between the velocity
offset in the first-epoch spectrum and the average acceleration between two
epochs, which could be explained by orbital phase modulation when the time
separation between two epochs is a non-negligible fraction of the orbital
period of the motion causing the line displacement. We discuss the implications
of our results for the identification of sub-pc BBH candidates in offset-line
quasars and for the constraints on their frequency and orbital parameters.
[Abridged]Comment: 23 pages, 18 figures, ApJ in pres
A Thirty-Four Billion Solar Mass Black Hole in SMSS J2157-3602, the Most Luminous Known Quasar
From near-infrared spectroscopic measurements of the MgII emission line
doublet, we estimate the black hole (BH) mass of the quasar, SMSS
J215728.21-360215.1, as being (3.4 +/- 0.6) x 10^10 M_sun and refine the
redshift of the quasar to be z=4.692. SMSS J2157 is the most luminous known
quasar, with a 3000A luminosity of (4.7 +/- 0.5) x 10^47 erg/s and an estimated
bolometric luminosity of 1.6 x 10^48 erg/s, yet its Eddington ratio is only
~0.4. Thus, the high luminosity of this quasar is a consequence of its
extremely large BH -- one of the most massive BHs at z > 4.Comment: 7 pages, 3 figures. Accepted for publication in MNRA
The z < 1.2 optical luminosity function from a sample of ∼410,000 galaxies in Boötes
Using a sample of ~410,000 galaxies to a depth of IAB=24 over 8.26 deg2 in the Boötes field (~10 times larger than the z~1 luminosity function (LF) studies in the prior literature), we have accurately measured the evolving B-band LF of red galaxies at z<1.2 and blue galaxies at z<1.0 In addition to the large sample size, we utilize photometry that accounts for the varying angular sizes of galaxies, photometric redshifts verified with spectroscopy, and absolute magnitudes that should have very small random and systematic errors. Our results are consistent with the migration of galaxies from the blue cloud to the red sequence as they cease to form stars and with downsizing in which more massive and luminous blue galaxies cease star formation earlier than fainter less massive ones. Comparing the observed fading of red galaxies with that expected from passive evolution alone, we find that the stellar mass contained within the red galaxy population has increased by a factor of ~3.6 from z~1.1 to z~0.1 The bright end of the red galaxy LF fades with decreasing redshift, with the rate of fading increasing from ~0.2 mag per unit redshift at z = 1.0 to ~0.8 at z = 0.2. The overall decrease in luminosity implies that the stellar mass in individual highly luminous red galaxies increased by a factor of ~2.2 from z = 1.1 to z = 0.1
What Drives the Redshift Evolution of Strong Emission Line Ratios?
We study the physical mechanisms that cause the offset between low-redshift
and high-redshift galaxies on the [OIII]/H versus [NII]/H
``Baldwin, Phillips & Terlevich'' (BPT) diagram using a sample of local
analogues of high-redshift galaxies. These high-redshift analogue galaxies are
selected from the Sloan Digital Sky Survey. Located in the same region on the
BPT diagram as the ultra-violet selected galaxies at , these
high-redshift analogue galaxies provide an ideal local benchmark to study the
offset between the local and high-redshift galaxies on the BPT diagram. We
compare the nitrogen-to-oxygen ratio (N/O), the shape of the ionising radiation
field, and ionisation parameters between the high-redshift analogues and a
sample of local reference galaxies. The higher ionisation parameter in the
high-redshift analogues is the dominant physical mechanism driving the BPT
offset from low- to high-redshift, particularly at high {\nii/\ha}.
Furthermore, the N/O ratio enhancement also plays a minor role to cause the BPT
offset. However, the shape of the ionising radiation field is unlikely to cause
the BPT offset because the high-redshift analogues have a similar hard ionising
radiation field as local reference galaxies. This hard radiation field cannot
be produced by the current standard stellar synthesis models. The stellar
rotation and binarity may help solve the discrepancy.Comment: 7 pages, 5 figures. accepted for publication in MNRA
Discovery of Eight z ~ 6 Quasars in the Sloan Digital Sky Survey Overlap Regions
We present the discovery of eight quasars at z~6 identified in the Sloan
Digital Sky Survey (SDSS) overlap regions. Individual SDSS imaging runs have
some overlap with each other, leading to repeat observations over an area
spanning >4000 deg^2 (more than 1/4 of the total footprint). These overlap
regions provide a unique dataset that allows us to select high-redshift quasars
more than 0.5 mag fainter in the z band than those found with the SDSS
single-epoch data. Our quasar candidates were first selected as i-band dropout
objects in the SDSS imaging database. We then carried out a series of follow-up
observations in the optical and near-IR to improve photometry, remove
contaminants, and identify quasars. The eight quasars reported here were
discovered in a pilot study utilizing the overlap regions at high galactic
latitude (|b|>30 deg). These quasars span a redshift range of 5.86<z<6.06 and a
flux range of 19.3<z_AB<20.6 mag. Five of them are fainter than z_AB=20 mag,
the typical magnitude limit of z~6 quasars used for the SDSS single-epoch
images. In addition, we recover eight previously known quasars at z~6 that are
located in the overlap regions. These results validate our procedure for
selecting quasar candidates from the overlap regions and confirming them with
follow-up observations, and provide guidance to a future systematic survey over
all SDSS imaging regions with repeat observations.Comment: AJ in press (8 pages
The gas-phase metallicities of star-forming galaxies in aperture-matched SDSS samples follow potential rather than mass or average surface density
We present a comparative study of the relation between the aperture-based
gas-phase metallicity and three structural parameters of star-forming galaxies:
mass (), average potential () and average surface mass density (; where is the effective radius). We use a
volume-limited sample drawn from the publicly available SDSS DR7, and base our
analysis on aperture-matched sampling by selecting sets of galaxies where the
SDSS fibre probes a fixed fraction of . We find that between 0.5
and 1.5 , the gas-phase metallicity correlates more tightly with
than with either or , in that for all
aperture-matched samples, the potential-metallicity relation has (i) less
scatter, (ii) higher Spearman rank correlation coefficient and (iii) less
residual trend with than either the mass-metallicity relation
and the average surface density-metallicity relation. Our result is broadly
consistent with the current models of gas enrichment and metal loss. However, a
more natural explanation for our findings is a local relation between the
gas-phase metallicity and escape velocity.Comment: Accepted by MNRAS; 17 pages, 11 figures, 1 tabl
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