41 research outputs found
On the Automated and Objective Detection of Emission Lines in Faint-Object Spectroscopy
Modern spectroscopic surveys produce large spectroscopic databases, generally
with sizes well beyond the scope of manual investigation. The need arises,
therefore, for an automated line detection method with objective indicators for
detection significance. In this paper, we present an automated and objective
method for emission line detection in spectroscopic surveys and apply this
technique to 1574 spectra, obtained with the Hectospec spectrograph on the MMT
Observatory (MMTO), to detect Lyman alpha emitters near z ~ 2.7. The basic idea
is to generate on-source (signal plus noise) and off-source (noise only) mock
observations using Monte Carlo simulations, and calculate completeness and
reliability values, (C, R), for each simulated signal. By comparing the
detections from real data with the Monte Carlo results, we assign the
completeness and reliability values to each real detection. From 1574 spectra,
we obtain 881 raw detections and, by removing low reliability detections, we
finalize 649 detections from an automated pipeline. Most of high completeness
and reliability detections, (C, R) ~ (1.0, 1.0), are robust detections when
visually inspected; the low C and R detections are also marginal on visual
inspection. This method at detecting faint sources is dependent on the accuracy
of the sky subtraction.Comment: 19 pages, 16 figures, submitted to MNRA
Spatially Resolved Gas Kinematics within a Ly Nebula: Evidence for Large-scale Rotation
We use spatially extended measurements of Ly as well as less
optically thick emission lines from an 80 kpc Ly nebula at
to assess the role of resonant scattering and to disentangle
kinematic signatures from Ly radiative transfer effects. We find that
the Ly, CIV, HeII, and CIII] emission lines all tell a similar story in
this system, and that the kinematics are broadly consistent with large-scale
rotation. First, the observed surface brightness profiles are similar in extent
in all four lines, strongly favoring a picture in which the Ly photons
are produced in situ instead of being resonantly scattered from a central
source. Second, we see low kinematic offsets between Ly and the less
optically thick HeII line (100-200 km s), providing further
support for the argument that the Ly and other emission lines are all
being produced within the spatially extended gas. Finally, the full velocity
field of the system shows coherent velocity shear in all emission lines:
500 km s over the central 50 kpc of the nebula. The
kinematic profiles are broadly consistent with large-scale rotation in a gas
disk that is at least partially stable against collapse. These observations
suggest that the Ly nebula represents accreting material that is
illuminated by an offset, hidden AGN or distributed star formation, and that is
undergoing rotation in a clumpy and turbulent gas disk. With an implied mass of
M(<R=20 kpc) , this system may represent the
early formation of a large Milky Way mass galaxy or galaxy group.Comment: Accepted to ApJ; 25 pages in emulateapj format; 15 figures, 4 table
A Successful Broad-band Survey for Giant Lya Nebulae I: Survey Design and Candidate Selection
Giant Lya nebulae (or Lya "blobs") are likely sites of ongoing massive galaxy
formation, but the rarity of these powerful sources has made it difficult to
form a coherent picture of their properties, ionization mechanisms, and space
density. Systematic narrow-band Lya nebula surveys are ongoing, but the small
redshift range covered and the observational expense limit the comoving volume
that can be probed by even the largest of these surveys and pose a significant
problem when searching for such rare sources. We have developed a systematic
search technique designed to find large Lya nebulae at 2<z<3 within deep
broad-band imaging and have carried out a survey of the 9.4 square degree NOAO
Deep Wide-Field Survey (NDWFS) Bootes field. With a total survey comoving
volume of ~10^8 h^-3_70 Mpc^3, this is the largest volume survey for Lya
nebulae ever undertaken. In this first paper in the series, we present the
details of the survey design and a systematically-selected sample of 79
candidates, which includes one previously discovered Lya nebula.Comment: Accepted to ApJ after minor revision; 25 pages in emulateapj format;
18 figures, 3 table
Mapping the Polarization of the Radio-Loud Ly Nebula B3 J2330+3927
Lya nebulae, or "Lya blobs", are extended (up to ~100 kpc), bright (L[Lya] >
10^43 erg/s) clouds of Lya emitting gas that tend to lie in overdense regions
at z ~ 2--5. The origin of the Lya emission remains unknown, but recent
theoretical work suggests that measuring the polarization might discriminate
among powering mechanisms. Here we present the first narrowband, imaging
polarimetry of a radio-loud Lya nebula, B3 J2330+3927 at z=3.09, with an
embedded active galactic nucleus (AGN). The AGN lies near the blob's Lya
emission peak and its radio lobes align roughly with the blob's major axis.
With the SPOL polarimeter on the 6.5m MMT telescope, we map the total (Lya +
continuum) polarization in a grid of circular apertures of radius 0.6"
(4.4kpc), detecting a significant (>2sigma) polarization fraction P in nine
apertures and achieving strong upper-limits (as low as 2%) elsewhere. P
increases from <2% at ~5kpc from the blob center to ~17% at ~15-25kpc. The
detections are distributed asymmetrically, roughly along the nebula's major
axis. The polarization angles theta are mostly perpendicular to this axis.
Comparing the Lya flux to that of the continuum, and conservatively assuming
that the continuum is highly polarized (20-100%) and aligned with the total
polarization, we place lower limits on the polarization of the Lya emission
P(Lya) ranging from no significant polarization at ~5 kpc from the blob center
to ~ 3--17% at 10--25kpc. Like the total polarization, the Lya polarization
detections occur more often along the blob's major axis.Comment: 9 pages, 7 figures, accepted for publication in Ap
The Diversity of Diffuse Ly Nebulae around Star-Forming Galaxies at High Redshift
We report the detection of diffuse Ly emission, or Ly halos
(LAHs), around star-forming galaxies at and in the NOAO
Deep Wide-Field Survey Bo\"otes field. Our samples consist of a total of
1400 galaxies, within two separate regions containing spectroscopically
confirmed galaxy overdensities. They provide a unique opportunity to
investigate how the LAH characteristics vary with host galaxy large-scale
environment and physical properties. We stack Ly images of different
samples defined by these properties and measure their median LAH sizes by
decomposing the stacked Ly radial profile into a compact galaxy-like
and an extended halo-like component. We find that the exponential scale-length
of LAHs depends on UV continuum and Ly luminosities, but not on
Ly equivalent widths or galaxy overdensity parameters. The full
samples, which are dominated by low UV-continuum luminosity Ly emitters
(), exhibit LAH sizes of 5kpc. However, the
most UV- or Ly-luminous galaxies have more extended halos with
scale-lengths of 7kpc. The stacked Ly radial profiles decline
more steeply than recent theoretical predictions that include the contributions
from gravitational cooling of infalling gas and from low-level star formation
in satellites. On the other hand, the LAH extent matches what one would expect
for photons produced in the galaxy and then resonantly scattered by gas in an
outflowing envelope. The observed trends of LAH sizes with host galaxy
properties suggest that the physical conditions of the circumgalactic medium
(covering fraction, HI column density, and outflow velocity) change with halo
mass and/or star-formation rates.Comment: published in ApJ, minor proof corrections applie
Pinpointing the Molecular Gas within a Lyman Alpha Blob at z ~ 2.7
We present IRAM PdBI observations of the CO(3-2) and CO(5-4) line transitions
from a Ly-alpha blob at z~2.7 in order to investigate the gas kinematics,
determine the location of the dominant energy source, and study the physical
conditions of the molecular gas. CO line and dust continuum emission are
detected at the location of a strong MIPS source that is offset by ~1.5" from
the Ly-alpha peak. Neither of these emission components is resolved with the
1.7" beam, showing that the gas and dust are confined to within ~7kpc from this
galaxy. No millimeter source is found at the location of the Ly-alpha peak,
ruling out a central compact source of star formation as the power source for
the Ly-alpha emission. Combined with a spatially-resolved spectrum of Ly-alpha
and HeII, we constrain the kinematics of the extended gas using the CO emission
as a tracer of the systemic redshift. Near the MIPS source, the Ly-alpha
profile is symmetric and its line center agrees with that of CO line, implying
that there are no significant bulk flows and that the photo-ionization from the
MIPS source might be the dominant source of the Ly-alpha emission. In the
region near the Ly-alpha peak, the gas is slowly receding (~100km/s) with
respect to the MIPS source, thus making the hyper-/superwind hypothesis
unlikely. We find a sub-thermal line ratio between two CO transitions,
I_CO(5-4)/I_CO(3-2)=0.97+/-0.21. This line ratio is lower than the average
values found in high-z SMGs and QSOs, but consistent with the value found in
the Galactic center, suggesting that there is a large reservoir of low-density
molecular gas that is spread over the MIPS source and its vicinity.Comment: 12 pages, 9 figures, accepted for publication in Ap
Lyman-Alpha Escape from Low-Mass, Compact, High-Redshift Galaxies
We investigate the effects of stellar populations and sizes on Ly
escape in 27 spectroscopically confirmed and 35 photometric Lyman-Alpha
Emitters (LAEs) at z 2.65 in seven fields of the Bo\"otes region of
the NOAO Deep Wide-Field Survey. We use deep /WFC3 imaging to supplement
ground-based observations and infer key galaxy properties. Compared to typical
star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive
(), younger (ages 1
Gyr), smaller ( 1 kpc), less dust-attenuated (E(BV) 0.26
mag), but have comparable star-formation-rates (SFRs ). Some of the LAEs in the sample may be very young galaxies
having low nebular metallicities ()
and/or high ionization parameters (). Motivated by
previous studies, we examine the effects of the concentration of star formation
and gravitational potential on Ly escape, by computing
star-formation-rate surface density, and specific
star-formation-rate surface density, . For a given
, the Ly escape fraction is higher for LAEs with
lower stellar masses. LAEs have higher on average compared
to SFGs. Our results suggest that compact star formation in a low gravitational
potential yields conditions amenable to the escape of Ly photons. These
results have important implications for the physics of Ly radiative
transfer and for the type of galaxies that may contribute significantly to
cosmic reionization.Comment: 36 pages, 15 figures; Accepted for publication in The Astronomical
Journa