ABSTRACT We use 317,000 emission-line galaxies from the Sloan Digital Sky Survey to investigate line-ratio selection of active galactic nuclei (AGNs). In particular, we demonstrate that "star ...formation (SF) dilution" by H ii regions causes a significant bias against AGN selection in low-mass, blue, star-forming, disk-dominated galaxies. This bias is responsible for the observed preference of AGNs among high-mass, green, moderately star-forming, bulge-dominated hosts. We account for the bias and simulate the intrinsic population of emission-line AGNs using a physically motivated Eddington ratio distribution, intrinsic AGN narrow line region line ratios, a luminosity-dependent bolometric correction, and the observed relation. These simulations indicate that, in massive ( ) galaxies, AGN accretion is correlated with specific star formation rate (SFR) but is otherwise uniform with stellar mass. There is some hint of lower black hole occupation in low-mass ( ) hosts, although our modeling is limited by uncertainties in measuring and interpreting the velocity dispersions of low-mass galaxies. The presence of SF dilution means that AGNs contribute little to the observed strong optical emission lines (e.g., and ) in low-mass and star-forming hosts. However the AGN population recovered by our modeling indicates that feedback by typical (low- to moderate-accretion) low-redshift AGNs has nearly uniform efficiency at all stellar masses, SFRs, and morphologies. Taken together, our characterization of the observational bias and resultant AGN occupation function suggest that AGNs are unlikely to be the dominant source of SF quenching in galaxies, but instead are fueled by the same gas which drives SF activity.
We present the Super Eight galaxies-a set of very luminous, high-redshift (7.1 < z < 8.0) galaxy candidates found in the Brightest of Reionizing Galaxies (BoRG) Survey fields. The original sample ...includes eight galaxies that are Y-band dropout objects with H-band magnitudes of mH < 25.5. Four of these objects were originally reported in Calvi et al. Combining new Hubble Space Telescope (HST) WFC3/F814W imaging and Spitzer IRAC data with archival imaging from BoRG and other surveys, we explore the properties of these galaxies. Photometric redshift fitting places six of these galaxies in the redshift range of 7.1 < z < 8.0, resulting in three new high-redshift galaxies and confirming three of the four high-redshift galaxy candidates from Calvi et al. We calculate the half-light radii of the Super Eight galaxies using the HST F160W filter and find that the Super Eight sizes are in line with the typical evolution of size with redshift. The Super Eights have a mean mass of log (M*/M ) ∼10, which is typical for sources in this luminosity range. Finally, we place our sample on the UV z ∼ 8 luminosity function and find that the Super Eight number density is consistent with other surveys in this magnitude and redshift range.
Redshift z = 9-10 object selection is the effective limit of Hubble Space Telescope (HST) imaging capability, even when confirmed with Spitzer. If only a few photometry data points are available, it ...becomes attractive to add criteria based on their morphology in these J- and H-band images. One could do so through visual inspection, a size criterion, or alternate morphometrics. We explore a vetted sample of Brightest of Reionizing Galaxies (BoRG) z ∼ 9 and z ∼ 10 candidate galaxies and the object rejected by Morishita+ to explore the utility of a size criterion in z = 9-10 candidate selection. A stringent, point-spread function (PSF)-corrected effective radius criterion ( ) would result in the rejection of 65%-70% of the interlopers visually rejected by Morishita et al. It may also remove up to ∼20% of bona fide brightest ( ) z = 9 or 10 candidates from a BoRG selected sample based on the Mason et al. luminosity functions, assuming the Holwerda et al. size-luminosity relation. We argue that including a size constraint in lieu of a visual inspection may serve in wide-field searches for these objects in, e.g., Euclid or HST archival imaging with the understanding that some brightest ( ) candidates may be missed. The sizes of the candidates found by Morishita et al. follow the expected size distribution of z ∼ 9 for bright galaxies, consistent with the log normal in Shibuya et al. and single objects. Two candidates show high star formation surface density ( ) and all merit further investigation and follow-up observations.
ABSTRACT We compare the physical and morphological properties of z ∼ 2 Ly emitting galaxies (LAEs) identified in the HETDEX Pilot Survey and narrow band studies with those of z ∼ 2 optical emission ...line selected galaxies (oELGs) identified via HST WFC3 infrared grism spectroscopy. Both sets of galaxies extend over the same range in stellar mass ( ), size (0.5 < R < 3.0 kpc), and star formation rate ( yr−1). Remarkably, a comparison of the most commonly used physical and morphological parameters-stellar mass, half-light radius, UV slope, SFR, ellipticity, nearest neighbor distance, star formation surface density, specific SFR, O iii luminosity, and O iii equivalent width-reveals no statistically significant differences between the populations. This suggests that the processes and conditions which regulate the escape of Ly from a z ∼ 2 star-forming galaxy do not depend on these quantities. In particular, the lack of dependence on the UV slope suggests that Ly emission is not being significantly modulated by diffuse dust in the interstellar medium. We develop a simple model of Ly emission that connects LAEs to all high-redshift star-forming galaxies where the escape of Ly depends on the sightline through the galaxy. Using this model, we find that mean solid angle for Ly escape is steradians; this value is consistent with those calculated from other studies.
Abstract
We investigate spatially resolved emission-line ratios in a sample of 219 galaxies (0.6 <
z
< 1.3) detected using the G102 grism on the Hubble Space Telescope Wide Field Camera 3 taken as ...part of the CANDELS Ly
α
Emission at Reionization survey to measure ionization profiles and search for low-luminosity active galactic nuclei (AGN). We analyze O
iii
and H
β
emission-line maps, enabling us to spatially resolve the O
iii
/H
β
emission-line ratio across the galaxies in the sample. We compare the O
iii
/H
β
ratio in galaxy centers and outer annular regions to measure ionization differences and investigate the potential of sources with nuclear ionization to host AGN. We investigate some of the individual galaxies that are candidates to host strong nuclear ionization and find that they often have low stellar mass and are undetected in X-rays, as expected for low-luminosity AGN in low-mass galaxies. We do not find evidence for a significant population of off-nuclear AGN or other clumps of off-nuclear ionization. We model the observed distribution of O
iii
/H
β
spatial profiles and find that most galaxies are consistent with a small or zero difference between their nuclear and off-nuclear line ratios, but 6%–16% of galaxies in the sample are likely to host nuclear O
iii
/H
β
that is ∼0.5 dex higher than in their outer regions. This study is limited by large uncertainties in most of the measured O
iii
/H
β
spatial profiles; therefore, deeper data, e.g., from deeper HST/WFC3 programs or from JWST/NIRISS, are needed to more reliably measure the spatially resolved emission-line conditions of individual high-redshift galaxies.
The James Webb Space Telescope (JWST) will observe several stars for long cumulative durations while pursuing exoplanets as primary science targets for both Guaranteed Time Observations (GTO) and ...very likely General Observer (GO) programs. Here we argue in favor of an automatic default parallel program to observe, e.g., using the F200W/F277W filters or grism of NIRCAM/NIRISS in order to find high redshift (z > 10) galaxies, cool red/brown dwarf substellar objects, solar system objects, and observations of serendipitous planetary transits. We argue here the need for automated exploratory astrophysical observations with unused JWST instruments during these long-duration exoplanet observations. Randomized fields that are observed in parallel mode reduce errors due to cosmic variance more effectively than single continuous fields of a typical wedding cake observing strategy. Hence, we argue that the proposed automated survey will explore a unique and rich discovery space in the high-redshift universe, Galactic structure, and solar system. We show that the GTO and highly probable GO target list of exoplanets covers the Galactic disk/halo and high redshift universe, mostly well out of the plane of the disk of the Milky Way. Exposure times are of the order of the CEERS GTO medium-deep survey in a single filter, comparable to CANDELS in Hubble Space Telescope's surveys and deep fields. The area covered by NIRISS and NIRCam combined could accumulate to a half square degree surveyed.
The galaxy catalogs generated from low-resolution emission-line surveys often contain both foreground and background interlopers due to line misidentification, which can bias the cosmological ...parameter estimation. In this paper, we present a method for correcting the interloper bias by using the joint analysis of auto- and cross-power spectra of the main and the interloper samples. In particular, we can measure the interloper fractions from the cross-correlation between the interlopers and survey galaxies, because the true cross-correlation must be negligibly small. The estimated interloper fractions, in turn, remove the interloper bias in the cosmological parameter estimation. For example, in the Hobby-Eberly Telescope Dark Energy Experiment low-redshift (z < 0.5) O ii λ3727 emitters contaminate high-redshift (1.9 < z < 3.5) Ly line emitters. We demonstrate that the joint-analysis method yields a high signal-to-noise ratio measurement of the interloper fractions while only marginally increasing the uncertainties in the cosmological parameters relative to the case without interlopers. We also show that the same is true for the high-latitude spectroscopic survey of the Wide-field Infrared Survey Telescope mission where contamination occurs between the Balmer- line emitters at lower redshifts (1.1 < z < 1.9) and oxygen (O iii λ5007 ) line emitters at higher redshifts (1.7 < z < 2.8).
Abstract
Strong gravitational lenses are a rare and instructive type of astronomical object. Identification has long relied on serendipity, but different strategies—such as mixed spectroscopy of ...multiple galaxies along the line of sight, machine-learning algorithms, and citizen science—have been employed to identify these objects as new imaging surveys become available. We report on the comparison between spectroscopic, machine-learning, and citizen-science identification of galaxy–galaxy lens candidates from independently constructed lens catalogs in the common survey area of the equatorial fields of the Galaxy and Mass Assembly survey. In these, we have the opportunity to compare high completeness spectroscopic identifications against high-fidelity imaging from the Kilo Degree Survey used for both machine-learning and citizen-science lens searches. We find that the three methods—spectroscopy, machine learning, and citizen science—identify 47, 47, and 13 candidates, respectively, in the 180 square degrees surveyed. These identifications barely overlap, with only two identified by both citizen science and machine learning. We have traced this discrepancy to inherent differences in the selection functions of each of the three methods, either within their parent samples (i.e., citizen science focuses on low redshift) or inherent to the method (i.e., machine learning is limited by its training sample and prefers well-separated features, while spectroscopy requires sufficient flux from lensed features to lie within the fiber). These differences manifest as separate samples in estimated Einstein radius, lens stellar mass, and lens redshift. The combined sample implies a lens candidate sky density of ∼0.59 deg
−2
and can inform the construction of a training set spanning a wider mass–redshift space. A combined approach and refinement of automated searches would result in a more complete sample of galaxy–galaxy lens candidates for future surveys.
We present a Bayesian approach to the redshift classification of emission-line galaxies when only a single emission line is detected spectroscopically. We consider the case of surveys for ...high-redshift Ly -emitting galaxies (LAEs), which have traditionally been classified via an inferred rest-frame equivalent width (EW; ) greater than 20 Å. Our Bayesian method relies on known prior probabilities in measured emission-line luminosity functions and EW distributions for the galaxy populations, and returns the probability that an object in question is an LAE given the characteristics observed. This approach will be directly relevant for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), which seeks to classify ∼106 emission-line galaxies into LAEs and low-redshift emitters. For a simulated HETDEX catalog with realistic measurement noise, our Bayesian method recovers 86% of LAEs missed by the traditional > 20 Å cutoff over 2 < z < 3, outperforming the EW cut in both contamination and incompleteness. This is due to the method's ability to trade off between the two types of binary classification error by adjusting the stringency of the probability requirement for classifying an observed object as an LAE. In our simulations of HETDEX, this method reduces the uncertainty in cosmological distance measurements by 14% with respect to the EW cut, equivalent to recovering 29% more cosmological information. Rather than using binary object labels, this method enables the use of classification probabilities in large-scale structure analyses. It can be applied to narrowband emission-line surveys as well as upcoming large spectroscopic surveys including Euclid and WFIRST.
We assemble an unbiased sample of 29 galaxies with O ii λ3727 and/or O iii λ5007 detections at z < 0.15 from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) Pilot Survey (HPS). The HPS ...finds galaxies without preselection based on their detected emission lines via integral field spectroscopy. Sixteen of these objects were followed up with the second-generation, low-resolution spectrograph on the upgraded HET. Oxygen abundances were then derived via strong emission lines using a Bayesian approach. We find that most of the galaxies fall along the mass-metallicity relation derived from photometrically selected star-forming galaxies in the Sloan Digital Sky Survey (SDSS). However, two of these galaxies have low metallicity (similar to the very rare green pea galaxies in mass-metallicity space). The star formation rates (SFRs) of this sample fall in an intermediate space between the SDSS star-forming main sequence and the extreme green pea galaxies. We conclude that spectroscopic selection fills a part of the mass-metallicity-SFR phase space that is missed in photometric surveys with preselection like SDSS; i.e., we find galaxies that are actively forming stars but are faint in continuum. We use the results of this pilot investigation to make predictions for the upcoming unbiased, large spectroscopic sample of local line emitters from HETDEX. With the larger HETDEX survey, we will determine if galaxies selected spectroscopically without continuum brightness preselection have metallicities that fall on a continuum that bridges typical star-forming and rarer, more extreme systems like green peas.
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