We investigate the process of rapid star formation quenching in a sample of 12 massive galaxies at intermediate redshift (z ∼ 0.6) that host high-velocity ionized gas outflows (v > 1000 km s−1). We ...conclude that these fast outflows are most likely driven by feedback from star formation rather than active galactic nuclei (AGNs). We use multiwavelength survey and targeted observations of the galaxies to assess their star formation, AGN activity, and morphology. Common attributes include diffuse tidal features indicative of recent mergers accompanied by bright, unresolved cores with effective radii less than a few hundred parsecs. The galaxies are extraordinarily compact for their stellar mass, even when compared with galaxies at z ∼ 2–3. For 9/12 galaxies, we rule out an AGN contribution to the nuclear light and hypothesize that the unresolved core comes from a compact central starburst triggered by the dissipative collapse of very gas-rich progenitor merging discs. We find evidence of AGN activity in half the sample but we argue that it accounts for only a small fraction (≲10 per cent) of the total bolometric luminosity. We find no correlation between AGN activity and outflow velocity and we conclude that the fast outflows in our galaxies are not powered by ongoing AGN activity, but rather by recent, extremely compact starbursts.
Origin of the Soft Excess in X-Ray Pulsars Hickox, Ryan C; Narayan, Ramesh; Kallman, Timothy R
The Astrophysical journal,
10/2004, Letnik:
614, Številka:
2
Journal Article
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The spectra of many X-ray pulsars show, in addition to a power law, a low-energy component that has often been modeled as a blackbody with kT sub(BB) 6 0.1 keV. However, the physical origin of this ...soft excess has remained a mystery. We examine a sample of well-studied bright X-ray pulsars, which have been observed using ROSAT, ASCA, Ginga, RXTE, BeppoSAX, Chandra, and XMM-Newton. In particular, we consider the Magellanic Cloud pulsars SMC X-1, LMC X-4, XTE J0111.2-7317, and RX J0059.2-7138 and the Galactic sources Her X-1, 4U 1626-67, Cen X-3, and Vela X-1. We show that the soft excess is a very common if not ubiquitous feature intrinsic to X-ray pulsars. We evaluate several possible mechanisms for the soft emission, using theoretical arguments, as well as observational clues, such as spectral shapes, eclipses, pulsations of the soft component, and superorbital modulation of the source flux. We find that reprocessing of hard X-rays from the neutron star by the inner region of the accretion disk is the only process that can explain the soft excess in all the pulsars with L sub(X) 10 super(38) ergs s super(-1). Other mechanisms, such as emission from diffuse gas in the system, are important in less luminous objects.
We explore the kinematics of ionized gas via the O iii λ5007 emission lines in active galactic nuclei (AGNs) selected on the basis of their mid-infrared (IR) emission, and split into obscured and ...unobscured populations based on their optical−IR colors. After correcting for differences in redshift distributions, we provide composite spectra of spectroscopically and photometrically defined obscured/Type 2 and unobscured/Type 1 AGNs from 3500 to 7000 . The IR-selected obscured sources contain a mixture of narrow-lined Type 2 AGNs and intermediate sources that have broad H emission and significantly narrower Hβ. Using both O iii luminosities and AGN luminosities derived from optical−IR spectral energy distribution fitting, we find evidence for enhanced large-scale obscuration in the obscured sources. In matched bins of luminosity we find that the obscured population typically has broader, more blueshifted O iii emission than in the unobscured sample, suggestive of more powerful AGN-driven outflows. This trend is not seen in spectroscopically classified samples, and is unlikely to be entirely explained by orientation effects. In addition, outflow velocities increase from small to moderate AGN E(B − V) values, before flattening out (as traced by FWHM) and even decreasing (as traced by blueshift). While difficult to fully interpret in a single physical model, due to both the averaging over populations and the spatially averaged spectra, these results agree with previous findings that simple geometric unification models are insufficient for the IR-selected AGN population, and may fit into an evolutionary model for obscured and unobscured AGNs.
The fraction of cluster galaxies that host luminous active galactic nuclei (AGNs) is an important probe of AGN fueling processes, the cold inter-stellar-medium at the centers of galaxies, and how ...tightly black holes and galaxies co-evolve. We present a new measurement of the AGN fraction in a sample of 13 clusters of galaxies at 1 < z < 1.5 selected from the Spitzer/IRAC Shallow Cluster Survey, as well as the field fraction in the immediate vicinity of these clusters, and combine these data with measurements from the literature to quantify the relative evolution of cluster and field AGN from the present to z ~ 3. We estimate that the cluster AGN fraction at 1 < z < 1.5 is fSUBA = 3.0sup +2.4sub -1.4% for AGNs with a rest-frame, hard X-ray luminosity greater than Lsub X,H > or = 10sup 44 erg ssup -1. The environment-dependent AGN evolution mimics the more rapid evolution of star-forming galaxies in clusters relative to the field.
Recent measurements of the dark matter halo masses of infrared-selected obscured quasars are in tension -- some indicate that obscured quasars have a higher halo mass compared to their unobscured ...counterparts, while others find no difference. The former result is inconsistent with the simplest models of quasar unification which rely solely on the viewing angle, while the latter may support such models. Here, using empirical relationships between dark matter halo and supermassive black hole (BH) masses, we provide a simple evolutionary picture which naturally explains these findings and is motivated by more sophisticated merger-driven quasar-fuelling models. The model tracks the growth rate of haloes, with the BH growing in spurts of quasar activity in order to 'catch up' with the M sub( bh)-M sub( stellar)-M sub( halo) relationship. The first part of the quasar phase is obscured and is followed by an unobscured phase. Depending on the luminosity limit of the sample, driven by observational selection effects, a difference in halo masses may or may not be significant. For high-luminosity samples, the difference can be large (a few to 10 times higher masses in obscured quasars), while for lower luminosity samples, the halo mass difference is very small, much smaller than current observational constraints. Such a simple model provides a qualitative explanation for the higher mass haloes of obscured quasars, as well as a rough quantitative agreement with seemingly disparate results.
ABSTRACT Hot dust-obscured galaxies (Hot DOGs) are a population of hyper-luminous infrared galaxies identified by the Wide-field Infrared Survey Explorer (WISE) mission from their very red mid-IR ...colors, and characterized by hot dust temperatures (T > 60 K). Several studies have shown clear evidence that the IR emission in these objects is powered by a highly dust-obscured active galactic nucleus (AGN) that shows close to Compton-thick absorption at X-ray wavelengths. Thanks to the high AGN obscuration, the host galaxy is easily observable, and has UV/optical colors usually consistent with those of a normal galaxy. Here we discuss a sub-population of eight Hot DOGs that show enhanced rest-frame UV/optical emission. We discuss three scenarios that might explain the excess UV emission: (i) unobscured light leaked from the AGN by reflection over the dust or by partial coverage of the accretion disk; (ii) a second unobscured AGN in the system; or (iii) a luminous young starburst. X-ray observations can help discriminate between these scenarios. We study in detail the blue excess Hot DOG WISE J020446.13-050640.8, which was serendipitously observed by Chandra/ACIS-I for 174.5 ks. The X-ray spectrum is consistent with a single, hyper-luminous, highly absorbed AGN, and is strongly inconsistent with the presence of a secondary unobscured AGN. Based on this, we argue that the excess blue emission in this object is most likely either due to reflection or a co-eval starburst. We favor the reflection scenario as the unobscured star formation rate needed to power the UV/optical emission would be 1000 M yr−1. Deep polarimetry observations could confirm the reflection hypothesis.
ABSTRACT To provide the census of the sources contributing to the X-ray background peak above 10 keV, Nuclear Spectroscopic Telescope Array (NuSTAR) is performing extragalactic surveys using a ...three-tier "wedding cake" approach. We present the NuSTAR survey of the COSMOS field, the medium sensitivity, and medium area tier, covering 1.7 deg2 and overlapping with both Chandra and XMM-Newton data. This survey consists of 121 observations for a total exposure of ∼3 Ms. To fully exploit these data, we developed a new detection strategy, carefully tested through extensive simulations. The survey sensitivity at 20% completeness is 5.9, 2.9, and 6.4 × 10−14 in the 3-24, 3-8, and 8-24 keV bands, respectively. By combining detections in 3 bands, we have a sample of 91 NuSTAR sources with 1042-1045.5 luminosities and redshift z = 0.04-2.5. Thirty-two sources are detected in the 8-24 keV band with fluxes ∼100 times fainter than sources detected by Swift-BAT. Of the 91 detections, all but 4 are associated with a Chandra and/or XMM-Newton point-like counterpart. One source is associated with an extended lower energy X-ray source. We present the X-ray (hardness ratio and luminosity) and optical-to-X-ray properties. The observed fraction of candidate Compton-thick active galactic nuclei measured from the hardness ratio is between 13%-20%. We discuss the spectral properties of NuSTAR J100259+0220.6 (ID 330) at z = 0.044, with the highest hardness ratio in the entire sample. The measured column density exceeds 1024 cm−2, implying the source is Compton-thick. This source was not previously recognized as such without the >10 keV data.
Galaxy-scale bars are expected to provide an effective means for driving material toward the central region in spiral galaxies, and possibly feeding supermassive black holes (BHs). Here we present a ...statistically complete study of the effect of bars on average BH accretion. From a well-selected sample of 50,794 spiral galaxies (with ) extracted from the Sloan Digital Sky Survey Galaxy Zoo 2 project, we separate those sources considered to contain galaxy-scale bars from those that do not. Using archival data taken by the Chandra X-ray Observatory, we identify X-ray luminous ( ) active galactic nuclei and perform an X-ray stacking analysis on the remaining X-ray undetected sources. Through X-ray stacking, we derive a time-averaged look at accretion for galaxies at fixed stellar mass and star-formation rate, finding that the average nuclear accretion rates of galaxies with bar structures are fully consistent with those lacking bars ( yr−1). Hence, we robustly conclude that large-scale bars have little or no effect on the average growth of BHs in nearby ( ) galaxies over gigayear timescales.
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