SUPERLUMINOUS SPIRAL GALAXIES Ogle, Patrick M.; Lanz, Lauranne; Nader, Cyril ...
The Astrophysical journal,
02/2016, Letnik:
817, Številka:
2
Journal Article
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ABSTRACT We report the discovery of spiral galaxies that are as optically luminous as elliptical brightest cluster galaxies, with r-band monochromatic luminosity Lr = 8-14L* (4.3-7.5 × 1044 erg s−1). ...These super spiral galaxies are also giant and massive, with diameter D = 57-134 kpc and stellar mass Mstars = 0.3-3.4 × 1011M . We find 53 super spirals out of a complete sample of 1616 SDSS galaxies with redshift z < 0.3 and Lr > 8L*. The closest example is found at z = 0.089. We use existing photometry to estimate their stellar masses and star formation rates (SFRs). The SDSS and Wide-field Infrared Survey Explorer colors are consistent with normal star-forming spirals on the blue sequence. However, the extreme masses and rapid SFRs of 5-65 M yr−1 place super spirals in a sparsely populated region of parameter space, above the star-forming main sequence of disk galaxies. Super spirals occupy a diverse range of environments, from isolation to cluster centers. We find four super spiral galaxy systems that are late-stage major mergers-a possible clue to their formation. We suggest that super spirals are a remnant population of unquenched, massive disk galaxies. They may eventually become massive lenticular galaxies after they are cut off from their gas supply and their disks fade.
The total infrared (IR) luminosity is very useful for estimating the star formation rate (SFR) of galaxies, but converting the IR luminosity into an SFR relies on assumptions that do not hold for all ...galaxies. We test the effectiveness of the IR luminosity as an SFR indicator by applying it to synthetic spectral energy distributions generated from three-dimensional hydrodynamical simulations of isolated disc galaxies and galaxy mergers. In general, the SFR inferred from the IR luminosity agrees well with the true instantaneous SFR of the simulated galaxies. However, for the major mergers in which a strong starburst is induced, the SFR inferred from the IR luminosity can overestimate the instantaneous SFR during the post-starburst phase by greater than two orders of magnitude. Even though the instantaneous SFR decreases rapidly after the starburst, the stars that were formed in the starburst can remain dust-obscured and thus produce significant IR luminosity. Consequently, use of the IR luminosity as an SFR indicator may cause one to conclude that post-starburst galaxies are still star forming, whereas in reality, star formation was recently quenched.
Abstract
How do galaxies transform from blue, star-forming spirals to red, quiescent early-type galaxies? To answer this question, we analyzed a set of 26 gas-rich, shocked post-starburst galaxies ...with Hubble Space Telescope (HST) imaging in
B
,
I
, and
H
bands and Sloan Digital Sky Survey (SDSS)
i
-band imaging of similar depth but lower resolution. We found that post-starbursts in our sample have intermediate morphologies between disk- and bulge-dominated (Sérsic
n
=
1.7
−
0.0
+
0.3
) and have red bulges, likely due to dust obscuration in the cores. A majority of galaxies in our sample are more morphologically disturbed than regular galaxies (88%, corresponding to >3
σ
significance) when observed with HST, with asymmetry and Sérsic residual flux fraction being the most successful measures of disturbance. Most disturbances are undetected at the lower resolution of SDSS imaging. Although ∼27% galaxies are clear merger remnants, we found that disturbances in another ∼30% of the sample are internal, caused by small-scale perturbations or dust substructures rather than tidal features, and require high-resolution imaging to detect. We found 2.8
σ
evidence that asymmetry features fade on timescales ∼200 Myr, and may vanish entirely after ∼750 Myr, so we do not rule out a possible merger origin of all post-starbursts given that asymmetric features may have already faded. This work highlights the importance of small-scale disturbances, detected only in high-resolution imaging, in understanding structural evolution of transitioning galaxies.
Abstract
We present new estimates on the fraction of heavily X-ray-obscured, Compton-thick (CT) active galactic nuclei (AGNs) out to a redshift of
z
≤ 0.8. From a sample of 540 AGNs selected by ...mid-infrared (MIR) properties in observed X-ray survey fields, we forward model the observed-to-intrinsic X-ray luminosity ratio (
R
L
X
) with a Markov Chain Monte Carlo simulation to estimate the total fraction of CT AGNs (
f
CT
), many of which are missed in typical X-ray observations. We create model
N
H
distributions and convert these to
R
L
X
using a set of X-ray spectral models. We probe the posterior distribution of our models to infer the population of X-ray-nondetected sources. From our simulation we estimate a CT fraction of
f
CT
=
0.555
−
0.032
+
0.037
. We perform an X-ray stacking analysis for sources in Chandra X-ray Observatory fields and find that the expected soft (0.5–2 keV) and hard (2–7 keV) observed fluxes drawn from our model to be within 0.48 and 0.12 dex of our stacked fluxes, respectively. Our results suggests at least 50% of all MIR-selected AGNs, possibly more, are CT (
N
H
≳ 10
24
cm
−2
), which is in excellent agreement with other recent work using independent methods. This work indicates that the total number of AGNs is higher than can be identified using X-ray observations alone, highlighting the importance of a multiwavelength approach. A high
f
CT
also has implications for black hole (BH) accretion physics and supports models of BH and galaxy coevolution that include periods of heavy obscuration.
Abstract
We present a large sample of infrared-luminous candidate active galactic nuclei (AGNs) that lack X-ray detections in Chandra, XMM-Newton, and NuSTAR fields. We selected all optically ...detected SDSS sources with redshift measurements, combined additional broadband photometry from WISE, UKIDSS, 2MASS, and GALEX, and modeled the spectral energy distributions (SEDs) of our sample sources. We parameterize nuclear obscuration in our SEDs with
and uncover thousands of powerful obscured AGNs that lack X-ray counterparts, many of which are identified as AGN candidates based on straightforward WISE photometric criteria. Using the observed luminosity correlation between rest-frame 2–10 keV (
) and rest-frame AGN
(
), we estimate the intrinsic X-ray luminosities of our sample sources and combine these data with flux limits from X-ray catalogs to determine lower limits on nuclear obscuration. Using the ratio of intrinsic-to-observed X-ray luminosity (
), we find a significant fraction of sources with column densities approaching
cm
–2
, suggesting that multiwavelength observations are necessary to account for the population of heavily obscured AGNs. We simulate the underlying
distribution for the X-ray non-detected sources in our sample through survival analysis, and confirm the presence of AGN activity via X-ray stacking. Our results point to a considerable population of extremely obscured AGNs undetected by current X-ray observatories.
The morphology-density relationship states that dense cosmic environments such as galaxy clusters have an overabundance of quiescent elliptical galaxies, but it is unclear at which redshift this ...relationship is first established. We study the morphology of four clusters with 1.2 < z < 1.8 using Hubble Space Telescope imaging and the morphology computation code statmorph. By comparing the median morphology of cluster galaxies to CANDELS field galaxies using Monte Carlo analysis, we find that two out of four clusters (at z = 1.19 and 1.75) have an established morphology-density relationship with more than 3 significance. Approximately 50% of the galaxies in these clusters are bulge-dominated, compared to ∼30% in the field, and they are significantly more compact. This result is more significant for low-mass galaxies with , showing that low-mass galaxies are affected the most in clusters. We also find an intriguing system of two z 1.45 clusters at a unusually small 2D separation of 3′ and 3D separation of 73 Mpc that exhibit no morphology-density relationship but have enhanced merger signatures. We conclude that the environmental mechanism responsible for the morphology-density relationship is (1) already active as early as z = 1.75; (2) forms compact, bulge-dominated galaxies; and (3) affects primarily low-mass galaxies. However, there is a significant degree of intracluster variance that may depend on the larger cosmological environment in which the cluster is embedded.
Super spirals are the most massive star-forming disk galaxies in the universe. We measured rotation curves for 23 massive spirals with the Southern African Large Telescope (SALT) and found a wide ...range of fast rotation speeds (240-570 km s−1), indicating enclosed dynamical masses of (0.6−4) × 1012M . Super spirals with mass in stars log M stars M > 11.5 break from the baryonic Tully-Fisher relation (BTFR) established for lower-mass galaxies. The BTFR power-law index breaks from 3.75 0.11 to 0.25 0.41 above a rotation speed of ∼340 km s−1. Super spirals also have very high specific angular momenta that break from the Fall relation. These results indicate that super spirals are undermassive for their dark matter halos, limited to a mass in stars of log M stars M < 11.8 . Most giant elliptical galaxies also obey this fundamental limit, which corresponds to a critical dark halo mass of log M halo M 12.7 . Once a halo reaches this mass, its gas can no longer cool and collapse in a dynamical time. Super spirals survive today in halos as massive as log M halo M 13.6 , continuing to form stars from the cold baryons they captured before their halos reached critical mass. The observed high-mass break in the BTFR is inconsistent with the Modified Newtonian Dynamics theory.
We present new subarcsecond-resolution Karl G. Jansky Very Large Array (VLA) imaging at 10 GHz of 155 ultraluminous (Lbol ∼ 1011.7-1014.2 L ) and heavily obscured quasars with redshifts z ∼ 0.4-3. ...The sample was selected to have extremely red mid-infrared-optical color ratios based on data from the Wide-Field Infrared Survey Explorer (WISE) along with a detection of bright, unresolved radio emission from the NRAO VLA Sky Survey (NVSS) or Faint Images of the Radio Sky at Twenty cm Survey. Our high-resolution VLA observations have revealed that the majority of the sources in our sample (93 out of 155) are compact on angular scales <0 2 (≤1.7 kpc at z ∼ 2). The radio luminosities, linear extents, and lobe pressures of our sources are similar to young radio active galactic nuclei (e.g., gigahertz-peaked spectrum GPS and compact steep-spectrum CSS sources), but their space density is considerably lower. Application of a simple adiabatic lobe expansion model suggests relatively young dynamical ages (∼104-7 yr), relatively high ambient ISM densities (∼1-104 cm−3), and modest lobe expansion speeds (∼30-10,000 km s−1). Thus, we find our sources to be consistent with a population of newly triggered, young jets caught in a unique evolutionary stage in which they still reside within the dense gas reservoirs of their hosts. Based on their radio luminosity function and dynamical ages, we estimate that only ∼20% of classical large-scale FR I/II radio galaxies could have evolved directly from these objects. We speculate that the WISE-NVSS sources might first become GPS or CSS sources, of which some might ultimately evolve into larger radio galaxies.
Abstract Spitzer spectral maps reveal a disk of highly luminous, warm (>150 K) H 2 in the center of the massive spiral galaxy Messier 58, which hosts a radio-loud active galactic nucleus (AGN). The ...inner 2.6 kpc of the galaxy appears to be overrun by shocks from the radio jet cocoon. Gemini NIRI imaging of the H 2 1–0 S(1) emission line, Atacama Large Millimeter/submillimeter Array CO 2–1, and Hubble Space Telescope multiband imagery indicate that much of the molecular gas is shocked in situ, corresponding to lanes of dusty molecular gas that spiral toward the galactic nucleus. The CO 2–1 and ionized gas kinematics are highly disturbed, with velocity dispersion up to 300 km s −1 . Dissipation of the associated kinetic energy and turbulence, likely injected into the interstellar medium by radio-jet-driven outflows, may power the observed molecular and ionized gas emission from the inner disk. The polycyclic aromatic hydrocarbon (PAH) fraction and composition in the inner disk appear to be normal, in spite of the jet and AGN activity. The PAH ratios are consistent with excitation by the interstellar radiation field from old stars in the bulge, with no contribution from star formation. The phenomenon of jet-shocked H 2 may substantially reduce star formation and help to regulate the stellar mass of the inner disk and supermassive black hole in this otherwise normal spiral galaxy. Similarly strong H 2 emission is found at the centers of several nearby spiral and lenticular galaxies with massive bulges and radio-loud AGNs.
A primary aim of the (NuSTAR) mission is to find and characterize heavily obscured Active Galactic Nuclei (AGNs). Based on mid-infrared photometry from the Wide-Field Infrared Survey Explorer (WISE) ...and optical photometry from the Sloan Digital Sky Surveys, we have selected a large population of luminous obscured AGNs (i.e., "obscured quasars"). Here we report NuSTAR observations of four WISE-selected heavily obscured quasars for which we have optical spectroscopy from the Southern African Large Telescope and W. M. Keck Observatory. Optical diagnostics confirm that all four targets are AGNs. With NuSTAR hard X-ray observations, three of the four objects are undetected, while the fourth has a marginal detection. We confirm that these objects have observed hard X-ray (10-40 keV) luminosities at or below ∼1043 erg s−1. We compare X-ray and IR luminosities to obtain estimates of the hydrogen column densities (NH) based on the suppression of the hard X-ray emission. We estimate NH of these quasars to be at or larger than 1025 cm−2, confirming that WISE and optical selection can identify very heavily obscured quasars that may be missed in X-ray surveys, and they do not contribute significantly to the cosmic X-ray background. From the optical Balmer decrements, we found that our three extreme obscured targets lie in highly reddened host environments. This galactic extinction cannot adequately explain the more obscured AGNs, but it may imply a different scale of obscuration in the galaxy.
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