Abstract
In this work, we use ∼500 low-redshift (
z
∼ 0.1) X-ray active galactic nuclei (AGNs) observed by XMM-Newton and the Sloan Digital Sky Survey (SDSS) to investigate the prevalence and nature ...of AGNs that apparently lack optical emission lines (“optically dull AGNs”). Although one quarter of spectra appear absorption-line dominated in visual assessment, line extraction with robust continuum subtraction from the MPA/JHU catalog reveals usable O
iii
measurements in 98% of the sample, allowing us to study O
iii
-underluminous AGNs together with more typical AGNs in the context of the
L
O
III
–
L
X
relation. We find that “optically dull AGNs” do not constitute a distinct population of AGNs. Instead, they are the O
iii
-underluminous tail of a single, unimodal
L
O
III
–
L
X
relation that has substantial scatter (0.6 dex). We find the degree to which an AGN is underluminous in O
iii
correlates with the specific star formation rate or
D
4000
index of the host, which are both linked to the molecular gas fraction. Thus the emerging physical picture for the large scatter seems to involve the gas content of the narrow-line region. We find no significant role for previously proposed scenarios for the presence of optically dull AGNs, such as host dilution or dust obscuration. Despite occasionally weak lines in SDSS spectra, >80% of X-ray AGNs are identified as such with the Baldwin–Phillips–Terlevich diagram. More than 90% are classified as AGNs based only on N
ii
/H
α
, providing more complete AGN samples when O
iii
or H
β
are weak. X-ray AGNs with LINER spectra obey essentially the same
L
O
III
–
L
X
relation as Seyfert 2s, suggesting their line emission is produced by AGN activity.
Abstract
The specific star formation rate (sSFR) is commonly used to describe the level of galaxy star formation (SF) and to select quenched galaxies. However, since it is a relative measure of the ...young-to-old population, an ambiguity in its interpretation may arise because a low sSFR can be due to either a substantial previous mass buildup or SF activity that is low. We show, using large samples spanning 0 <
z
< 2, that the normalization of the star formation rate (SFR) by the physical extent over which SF is taking place (i.e., the SFR surface density, Σ
SFR
) overcomes this ambiguity. Σ
SFR
has a strong physical basis, being tied to the molecular gas density and the effectiveness of stellar feedback, so we propose Σ
SFR
–
M
*
as an important galaxy evolution diagram to complement (s)SFR–
M
*
diagrams. Using the Σ
SFR
–
M
*
diagram we confirm the Schiminovich et al. result that the level of SF along the main sequence today is only weakly mass-dependent—high-mass galaxies, despite their redder colors, are as active as blue, low-mass ones. At higher redshift, the slope of the “Σ
SFR
main sequence” steepens, signaling the epoch of bulge buildup in massive galaxies. We also find that Σ
SFR
based on the optical isophotal radius more cleanly selects both starbursting and spheroid-dominated (early-type) galaxies than the sSFR. One implication of our analysis is that the assessment of the inside-out versus outside-in quenching scenarios should consider both sSFR and Σ
SFR
radial profiles, because ample SF may be present in bulges with low sSFRs (red color).
Abstract
Ionization sources other than H
ii
regions give rise to the right-hand branch in the standard (N
ii
) BPT diagram, populated by Seyfert 2s and LINERs. However, because the majority of ...Seyfert/LINER hosts are star-forming (SF), H
ii
regions contaminate the observed lines to some extent, making it unclear if the position along the branch is merely due to various degrees of mixing between pure Seyferts/LINERs and SF, or whether it reflects the intrinsic diversity of Seyfert/LINER ionizing sources. In this study, we empirically remove SF contributions in ∼100,000 Seyferts/LINERs from SDSS using the doppelganger method. We find that mixing is not the principal cause of the extended morphology of the observed branch. Rather, Seyferts/LINERs intrinsically have a wide range of line ratios. Variations in ionization parameter and metallicity can account for much of the diversity of Seyfert/LINER line ratios, but the hardness of the ionization field also varies significantly. Furthermore, our
k
-means classification on seven decontaminated emission lines reveals that LINERs are made up of two populations, which we call soft and hard LINERs. The Seyfert 2s differ from both types of LINERs primarily by higher ionization parameter, whereas the two LINER types mainly differ from each other (and from star-forming regions) in the hardness of the radiation field. We confirm that the N
ii
BPT diagram more efficiently identifies LINERs than S
ii
and O
i
diagnostics, because in the latter many LINERs, especially soft ones, occupy the same location as pure starformers, even after the SF has been removed from LINER emission.
ABSTRACT We present results from the Intermediate Redshift OSIRIS Chemo-Kinematic Survey (IROCKS) for sixteen z ∼ 1 and one z ∼ 1.4 star-forming galaxies. All galaxies were observed with OSIRIS with ...the laser guide star adaptive optics system at Keck Observatory. We use rest-frame nebular H emission lines to trace morphologies and kinematics of ionized gas in star-forming galaxies on sub-kiloparsec physical scales. We observe elevated velocity dispersions ( 50 km s−1) seen in z > 1.5 galaxies persist at z ∼ 1 in the integrated galaxies. Using an inclined disk model and the ratio of , we find that 1/3 of the z ∼ 1 sample are disk candidates while the other 2/3 of the sample are dominated by merger-like and irregular sources. We find that including extra attenuation toward H ii regions derived from stellar population synthesis modeling brings star formation rates (SFRs) using H and stellar population fit into a better agreement. We explore the properties of the compact H sub-component, or "clump," at z ∼ 1 and find that they follow a similar size-luminosity relation as local H ii regions but are scaled-up by an order of magnitude with higher luminosities and sizes. Comparing the z ∼ 1 clumps to other high-redshift clump studies, we determine that the clump SFR surface density evolves as a function of redshift. This suggests clump formation is directly related to the gas fraction in these systems and may support disk fragmentation as their formation mechanism since gas fraction scales with redshift.
We present quantitative morphologies of ~300 galaxies in the optically defined green valley at 0.4 < z < 1.2, in order to constrain the mechanism(s) responsible for quenching star formation in the ...bulk of this population. The sample is selected from galaxies in the All-Wavelength Extended Groth Strip International Survey (AEGIS). While the green valley is defined using optical U -- B colors, we find that using a green valley sample defined using NUV -- R colors does not change the results. Using Hubble Space Telescope/Advanced Camera for Surveys imaging, we study several quantitative morphological parameters including CAS, B/T from GIM2D, and Gini/M 20. We find that the green galaxy population is intermediate between the red and blue galaxy populations in terms of concentration, asymmetry, and morphological type and merger fraction estimated using Gini/M 20. We find that most green galaxies are not classified as mergers; in fact, the merger fraction in the green valley is lower than in the blue cloud. We show that at a given stellar mass, green galaxies have higher concentration values than blue galaxies and lower concentration values than red galaxies. Additionally, we find that 12% of green galaxies have B/T = 0 and 21% have B/T <= 0.05. Our results show that green galaxies are generally massive (M * ~ 1010.5 M ) disk galaxies with high concentrations. We conclude that major mergers are likely not the sole mechanism responsible for quenching star formation in this population and that either other external processes or internal secular processes play an important role both in driving gas toward the center of these galaxies and in quenching star formation.
UV Star Formation Rates in the Local Universe Salim, Samir; Rich, R. Michael; Charlot, Stéphane ...
The Astrophysical journal. Supplement series,
12/2007, Letnik:
173, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We measure star formation rates (SFRs) of approximately 50,000 optically selected galaxies in the local universe (z approximately 0.1)--from gas-rich dwarfs to massive ellipticals. We obtain ...dust-corrected SFRs by fitting the GALEX (ultraviolet) and SDSS photometry to a library of dust-attenuated population synthesis models. For star-forming galaxies, our UV-based SFRs compare remarkably well with those from SDSS-measured emission lines (H alpha ). Deviations from perfect agreement are shown to be due to differences in the dust attenuation estimates. In contrast to H alpha measurements, UV provides reliable SFRs for galaxies with weak H alpha , and where H alpha is contaminated with AGN emission (1/2 of the sample). Using full-SED SFRs, we calibrate a simple prescription that uses GALEX far- and near-UV magnitudes to produce dust-corrected SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGNs, (2) those hosting an AGN, and (3) galaxies without Ha emission. We find that the three have distinct star formation histories, with AGNs lying Intermediate between the star-forming and the quiescent galaxies. Star-forming galaxies without an AGN lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. On the other hand, weak AGNs, while also massive, have lower SFRs, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent galaxies via strong and weak AGNs. We confirm that some galaxies with no H alpha show signs of star formation in the UV. We derive a cosmic star formation density at z = 0.1 with significantly smaller total error than previous measurements.
ABSTRACT
We present the Arizona CDFS Environment Survey (ACES), a recently completed spectroscopic redshift survey of the Chandra Deep Field-South (CDFS) conducted using the Inamori-Magellan Areal ...Camera and Spectrograph on the Magellan-Baade telescope. In total, the survey targeted 7277 unique sources down to a limiting magnitude of R
AB = 24.1, yielding 5080 secure redshifts across the ∼30 arcmin × 30 arcmin extended CDFS region. The ACES data set delivers a significant increase to both the spatial coverage and the sampling density of the spectroscopic observations in the field. Combined with previously published spectroscopic redshifts, ACES now creates a highly complete survey of the galaxy population at R < 23, enabling the local galaxy density (or environment) on relatively small scales (∼1 Mpc) to be measured at z < 1 in one of the most heavily studied and data-rich fields in the sky. Here, we describe the motivation, design and implementation of the survey and present a preliminary redshift and environment catalogue. In addition, we utilize the ACES spectroscopic redshift catalogue to assess the quality of photometric redshifts from both the COMBO-17 and Multiwavelength Survey by Yale-Chile imaging surveys of the CDFS.
Abstract
Galaxy morphology and its evolution over the cosmic epoch hold important clues for understanding the regulation of star formation (SF). However, studying the relationship between morphology ...and SF has been hindered by the availability of consistent data at different redshifts. Our sample, combining CANDELS (0.8 <
z
< 2.5) and the GALEX-SDSS-WISE Legacy Catalog (GSWLC;
z
∼ 0), has physical parameters derived using consistent SED fitting with flexible dust attenuation laws. We adopt visual classifications from Kartaltepe et al. and expand them to
z
∼ 0 using SDSS images matching the physical resolution of CANDELS rest-frame optical images and deep FUV GALEX images matching the physical resolution of the CANDELS rest-frame FUV images. Our main finding is that disks with SF clumps at
z
∼ 0 make a similar fraction (∼15%) of star-forming galaxies as at
z
∼ 2. The clumpy disk contribution to the SF budget peaks at
z
∼ 1, rather than
z
∼ 2, suggesting that the principal epoch of disk assembly continues to lower redshifts. Star-forming spheroids (“blue nuggets”), though less centrally concentrated than quenched spheroids, contribute significantly (∼15%) to the SF budget at
z
∼ 1–2, suggesting that compaction precedes quenching. Among green valley and quiescent galaxies, the pure spheroid fraction drops after
z
∼ 1, whereas spheroids with disks (S0-like) become dominant. Mergers at or nearing coalescence are enhanced in SFR relative to the main sequence at all redshifts by a factor of ∼2, but contribute ≲5% to the SF budget, with their contribution remaining small above the main sequence.
Context. One of the most important properties of a galaxy is the total stellar mass, or equivalently the stellar mass-to-light ratio (M/L). It is not directly observable, but can be estimated from ...stellar population synthesis. Currently, a galaxy’s M/L is typically estimated from global fluxes. For example, a single global g − i colour correlates well with the stellar M/L. Spectral energy distribution (SED) fitting can make use of all available fluxes and their errors to make a Bayesian estimate of the M/L. Aims. We want to investigate the possibility of using morphology information to assist predictions of M/L. Our first goal is to develop and train a method that only requires a g-band image and redshift as input. This will allows us to study the correlation between M/L and morphology. Next, we can also include the i-band flux, and determine if morphology provides additional constraints compared to a method that only uses g- and i-band fluxes. Methods. We used a machine learning pipeline that can be split in two steps. First, we detected morphology features with a convolutional neural network. These are then combined with redshift, pixel size and g-band luminosity features in a gradient boosting machine. Our training target was the M/L acquired from the GALEX-SDSS-WISE Legacy Catalog, which uses global SED fitting and contains galaxies with z ∼ 0.1. Results. Morphology is a useful attribute when no colour information is available, but can not outperform colour methods on its own. When we combine the morphology features with global g- and i-band luminosities, we find an improved estimate compared to a model which does not make use of morphology. Conclusions. While our method was trained to reproduce global SED fitted M/L, galaxy morphology gives us an important additional constraint when using one or two bands. Our framework can be extended to other problems to make use of morphological information.
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