32 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
Galactic outflows and the kinematics of damped Lyman alpha absorbers
The kinematics of damped Lyman alpha absorbers (DLAs) are difficult to
reproduce in hierarchical galaxy formation models, particularly the
preponderance of wide systems. We investigate DLA kinematics at z=3 using
high-resolution cosmological hydrodynamical simulations that include a
heuristic model for galactic outflows. Without outflows, our simulations fail
to yield enough wide DLAs, as in previous studies. With outflows, predicted DLA
kinematics are in much better agreement with observations. Comparing two
outflow models, we find that a model based on momentum-driven wind scalings
provides the best match to the observed DLA kinematic statistics of Prochaska &
Wolfe. In this model, DLAs typically arise a few kpc away from galaxies that
would be identified in emission. Narrow DLAs can arise from any halo and galaxy
mass, but wide ones only arise in halos with mass >10^11 Mo, from either large
central or small satellite galaxies. This implies that the success of this
outflow model originates from being most efficient at pushing gas out from
small satellite galaxies living in larger halos. This increases the
cross-section for large halos relative to smaller ones, thereby yielding wider
kinematics. Our simulations do not include radiative transfer effects or
detailed metal tracking, and outflows are modeled heuristically, but they
strongly suggest that galactic outflows are central to understanding DLA
kinematics. An interesting consequence is that DLA kinematics may place
constraints on the nature and efficiency of gas ejection from high-z galaxies.Comment: submitted to MNRA
Constraining Stellar Feedback: Shock-ionized Gas in Nearby Starburst Galaxies
(abridged) We investigate the properties of feedback-driven shocks in 8
nearby starburst galaxies using narrow-band imaging data from the Hubble Space
Telescope (HST). We identify the shock--ionized component via the line
diagnostic diagram \oiii/\hb vs. \sii (or \nii)/\ha, applied to resolved
regions 3--15 pc in size. We divide our sample into three sub-samples:
sub-solar (Holmberg II, NGC 1569, NGC 4214, NGC 4449, and NGC 5253), solar (He
2-10, NGC 3077) and super-solar (NGC 5236) for consistent shock measurements.
For the sub-solar sub-sample, we derive three scaling relations: (1) , (2) , and
(3) , where
is the \ha luminosity from shock--ionized gas, the SFR per
unit half-light area, the total \ha luminosity, and
the absolute H-band luminosity from 2MASS normalized to solar luminosity. The
other two sub--samples do not have enough number statistics, but appear to
follow the first scaling relation. The energy recovered indicates that the
shocks from stellar feedback in our sample galaxies are fully radiative. If the
scaling relations are applicable in general to stellar feedback, our results
are similar to those by Hopkins et al. (2012) for galactic super winds. This
similarity should, however, be taken with caution at this point, as the
underlying physics that enables the transition from radiative shocks to gas
outflows in galaxies is still poorly understood.Comment: 29 pages, 14 figures, accepted for publication in the 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
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