We employ UV and optical photometry, from the GALEX and SDSS surveys respectively, to study the star formation histories of 561 luminous infrared galaxies (LIRGs) in the nearby Universe. A small fraction (~4%) of these galaxies have spheroidal or near-spheroidal morphologies and could be progenitors of elliptical galaxies. The remaining galaxies are morphologically late-type or ongoing mergers. 61% of the LIRGs do not show signs of interactions, while the remaining objects are either interacting (~18%) or show post-merger morphologies (~19%). The (SSP-weighted) average age of the underlying stellar populations in these objects is typically 5-9 Gyrs, with a mean value of ~6.8 Gyrs. ~60% of the LIRG population began their recent star formation (RSF) episode within the last Gyr, while the remaining objects began their RSF episodes 1 to 3 Gyrs in the past. Up to 35% of the stellar mass in the remnant forms in these episodes - the mean value is ~15%. The (decay) timescales of the star formation are typically a few Gyrs, indicating that the star formation rate does not decline significantly during the course of the burst. 14% of the LIRG population host (Type 2) AGN. The AGN hosts exhibit UV and optical colours that are redder than those of the normal (non-AGN) population. However, there is no evidence for a systematically higher dust content in the AGN hosts. AGN typically appear ~0.5-0.7 Gyrs after the onset of star formation and the redder colours are a result of older RSF episodes, with no measurable evidence of negative feedback from the AGN on the star formation in their host galaxies. (abridged)
Hot, X-ray emitting gaseous halos around massive elliptical galaxies are a result of both stellar mass loss and inflow toward the overdensity from which giant ellipticals and their associated galaxy groups formed. The metal abundance in this gas contains important information about early star formation and past supernova activity. We find that Type II supernovae based on a Salpeter IMF, plus a small number of additional Type Ia supernovae, can explain the the density, temperature and abundance profiles currently observed in gaseous halos around massive ellipticals. Within the central, opticall…
We report preliminary results of a project to homogeneously measure star formation histories for the majority of Local Group dwarf galaxies, based on archival WFPC2 observations. For the dwarf spheroidal and elliptical galaxies, we find that the more luminous systems all show extended star formation episodes, while the less luminous systems typically formed most of their stars over ten Gyr ago. Irregular galaxies show no comparable trend. We find that a galaxy's classification as an irregular or spheroidal is entirely explained by its star formation within the past Gyr or less. Several ellipti…
We present a detailed investigation into the recent star formation histories of 5,697 Luminous Red Galaxies (LRGs) based on the Hdelta (4101A) and [OII] (3727A) lines. LRGs are luminous (L>3L*), galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study we utilise LRGs from the recently completed 2dF-SDSS LRG and QSO survey (2SLAQ). Equivalent widths of the Hdelta and [OII] lines are measured and used to define three spectral types, those with only strong Hdelta absorption (k+a), those with strong [OII] in emis…
The Burst and Transient Source Experiment (BATSE) aboard CGRO monitored the whole sky through 8 NaI(Tl) crystal scintillators sensitive in the 20 keV - 2 MeV energy band continuously from April 1991 until June 2000. Results are presented on the long term variabilty observed in the brightest Active Galactic Nuclei (AGN) present in the BATSE data archive. This was achieved through the application of the Earth Occultation technique to data flat-fielded using the BATSE Mass Model. Removal of the temporal background variations from the data should allow a more sensitive extraction of source paramet…
Our current understanding of the star formation histories of early-type galaxies is reviewed, in the context of recent observational studies of their ultra-violet (UV) properties. Combination of UV and optical spectro-photometric data indicates that the bulk of the stellar mass in the early-type population forms at high redshift (z > 2), typically over short timescales (< 1 Gyr). Nevertheless, early-types of all luminosities form stars over the lifetime of the Universe, with most luminous (-23 < M(V) < -21) systems forming 10-15% of their stellar mass after z = 1 (with a scatter to higher valu…
We report on the results of 3--4 $μ$m spectroscopy of the ultra-luminous infrared galaxy (ULIRG) UGC 5101. It has a cool far-infrared color and a LINER-type optical spectrum, and so, based on a view gaining some currency, would be regarded as dominated by star formation. However, we find that it has strong 3.4 $μ$m carbonaceous dust absorption, low-equivalent-width 3.3 $μ$m polycyclic aromatic hydrocarbon (PAH) emission, and a small 3.3 $μ$m PAH to far-infrared luminosity ratio. This favors an alternative scenario, in which an energetically dominant AGN is present behind obscuring dust. The AG…
We used the mm/sub-mm receivers on the James Clerk Maxwell Telescope (JCMT) to observe the CO J=3--2, 2--1 lines in five local, optically powerful AGN and the J=4--3 line in 3C 293 (a powerful radio galaxy). Luminous CO J=3--2 emission and high CO (3--2)/(1--0) intensity ratios are found in all objects, indicating highly excited molecular gas. In 3C 293 an exceptionally bright CO J=4--3 line is found which cannot be easily explained given its quiescent star-forming environment and low AGN X-ray luminosity. In this object shocks emanating from a well-known interaction of a powerful jet with a d…
We report the detection of optical and X-ray variability in the least luminous known Seyfert galaxy, NGC4395. The featureless continuum changed by a factor of 2 in 6 months, which is typical of more luminous AGN. The largest variation was seen at shorter wavelengths, so that the spectrum becomes `harder' during higher activity states. In a one week optical broad band monitoring program, a 20% change was seen between successive nights. In a 1 month period the spectral shape changed from a power law with spectral index alpha ~0 (characteristic of quasars) to a spectral index alpha ~2 (as observe…
We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z>0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 micron sources are selected from a Spitzer/MIPS survey of the NOAO Deep Wide-Field Survey Boötes field and have corresponding 8 micron data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 micron to 8 micron flux ratio. The X-ray detected sources fall within the peak corresponding to a flat spectrum in nufnu, implying that it is populated by AGN-dominated LIRGs, whereas the peak corr…
We briefly review and extend our discussion of the ROSAT detection of the extraordinarily luminous (>10^{42} erg/s) partly extended (> 30 kpc diameter) X-ray emission from the double-nucleus ultraluminous infrared galaxy NGC 6240. The ROSAT spectrum can be well fit by emission from two components in roughly equal proportions: a thermal optically thin plasma with kT \sim 0.6 keV and a hard component that can be represented by a canonical AGN powerlaw. Source counts appear to have dropped by 30% within a year. Altogether, these findings can be well explained by a contribution of radiation from a…
To understand luminous AGNs in the z<1 universe, the ASCA AGN samples are the best at present. Combining the identified sample of AGNs from ASCA Large Sky Survey and Medium Sensitivity Survey, the sample of hard X-ray selected AGNs have been expanded up to 108 AGNs above the flux limit of 10^{-13} erg s^{-1} cm^{-2} in the 2-10 keV hard X-ray band. We discuss the fraction of absorbed AGNs in the hard X-ray selected AGN sample, and nature of absorbed luminous AGNs.
We investigate the relative importance of starbursts and AGN in nuclear activities of ultra-luminous infrared galaxies (ULIRGs) based on chemodynamical simulations combined with spectrophotometric synthesis codes. We numerically investigate both the gas accretion rates (m_acc) onto super massive black holes (SMBHs) and the star formation rates (m_sf) in ULIRGs formed by gas-rich galaxy mergers and thereby discuss what powers ULIRGs. Our principal results, which can be tested against observations, are as follows. (1) ULIRGs powered by AGN can be formed by major merging between luminous, gas-ric…
IRAM 30m observations reveal that the deeply obscured IR-luminous galaxy NGC4418 has a rich molecular chemistry - including unusually luminous HC3N line emission. We furthermore detect: ortho-H2CO 2-1, 3-2; CN 1-0, 2-1; HCO+, 1-0. 3-2, HCN 3-2, HNC 1-0, 3-2 (and tentatively OCS 12-11). The HCN, HCO+, H2CO and CN line emission can be fitted to densities of n=5 x 10E4 - 10E5 cm-3 and gas temperatures Tk=80-150 K. Both HNC and HC3N are, however, significantly more excited than the other species which requires higher gas densities - or radiative excitation through e.g. mid-IR pumping. The HCN line…
We present mid-infrared spectroscopy of a sample of 16 optically faint infrared luminous galaxies obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. These sources were jointly selected from Spitzer and Chandra imaging surveys in the NDWFS Bootes field and were selected from their bright X-ray fluxes to host luminous AGN. None of the spectra show significant emission from polycyclic aromatic hydrocarbons (PAHs; 6.2um equivalent widths <0.2um), consistent with their infrared emission being dominated by AGN. Nine of the X-ray sources show 9.7um silicate absorption featu…
The nature and origin of the soft X-ray excess in radio quiet AGN is still an open issue. The interpretation in terms of thermal disc emission has been challanged by the discovery of the constancy of the effective temperature despite the wide range of Black Hole masses of the observed sources. Alternative models are reflection from ionized matter and absorption in a relativistically smeared wind. We analyzed XMM-Newton observations of four luminous radio quiet AGN with the aim of characterising their main properties and in particular the soft excess. Different spectral models for the soft exce…
We present a study of a sample of luminous infrared galaxies (LIGs, L_IR > 10^11 L_sun) observed in the hard (2-10 keV) X rays. The main results are: 1) most LIGs are powered both by AGN and starburst activity; 2) the AGNs in our sample are absorbed in the infrared by a lower N_H than in the X rays or, alternatively, the dust-to-gas ratio is lower than galactic; 3) the study of a subsample of sources observed in the 20-200 keV band indicates that most of the AGNs hosted by the LIGs are heavily obscured up to 100 keV and, therefore, their contribution to the X-ray background must be small.
We present sensitive 850 micron SCUBA photometry of a statistically-complete sample of six of the most luminous, z>4 radio-quiet quasars, reaching noise levels comparable with the deep blank sky surveys. One quasar (BR2237-0607; z=4.55) is detected at 850micron with a flux of 5.0mJy (4.5sigma), whilst a second (BR0019-1522; z=4.52) has a detection at the 2 sigma level. When combined with our previous millimetre measurements of z>4 quasars, we find that there is a large range (5--10) in far infrared (FIR) luminosity at fixed UV luminosity, and that the typical quasar has a L_FIR and mass of coo…
Hyper-Luminous Infrared Galaxies (HLIRGs) are the most luminous persistent objects in the Universe. They exhibit extremely high star formation rates, and most of them seem to harbour an AGN. They are unique laboratories to investigate utmost star formation, and its connection to super-massive black hole growth. X-ray studies of HLIRGs have the potential to unravel the AGN contribution to the bolometric output from these bright objects. We have selected a sample of 14 HLIRGs observed by XMM-Newton (type 1, type 2 AGN and starburst), 5 of which are candidates to be Compton-thick objects. This is…
I discuss the formation of alpha-enhanced metal-rich stellar populations in the nuclei of luminous ellipticals. Based on hierarchical clustering, different galaxy formation scenarios, which imply different star formation histories, are considered. In contrast to the fast clumpy collapse mode, the late merger of two spiral galaxies fails to reproduce significantly $α$-enhanced abundance ratios, unless the IMF is flattened. Following the star formation history predicted by semi-analytic models of hierarchical clustering for the average elliptical, solar abundance ratios are obtained with Salpete…
Morphological disturbances and gas kinematics of the SB0 galaxy NGC 7679=Arp 216 are investigated to get clues to the history of this highly composite object, where AGN and starburst signatures dominate each other in the X-ray and optical/IR regime, respectively. Perturbations of the ionized gas velocity field appear quite mild within 15'' (~5 kpc) from the center, so as it can be straightforwardly modeled as a circularly rotating disk. On the contrary, outside that radius, significant disturbances show up. In particular, the eastern distorted arm as well as the huge neutral hydrogen bridge co…
We have performed 23 GHz VLA observations of 7 compact, luminous infrared galaxies, selected to have evidence of starburst activity. New and published multi-frequency data are combined to obtain the spectral energy distributions of all 7 galaxies from the near-infrared to the radio (at 1.4 GHz). These SEDs are compared with new models, for dust enshrouded galaxies, which account for both starburst and AGN components. In all 7 galaxies the starburst provides the dominant contribution to the infrared luminosity; in 4 sources no contribution from an AGN is required. Although AGN may contribute up…
Infrared observations of complete samples of active galactic nuclei (AGN) have shown that a substantial fraction of their bolometric luminosity is emitted at wavelengths ~8-1000microns. In radio-loud and Blazar-like objects much of this emission appears to be direct non-thermal synchrotron radiation. However, in the much larger numbers of radio-quiet AGN it is now clear that thermal dust emission is responsible for the bulk of radiation from the near-infrared through submillimeter wavelengths. Luminous infrared-selected AGN are often surrounded by powerful nuclear starbursts, both of which app…
Radio imaging of ULIR galaxies is ideal to explore the connection between the starburst and the AGN phenomenon since it is unaffected by dust obscuration, and provides the required high angular resolution to distinguish between an AGN and starburst emission. We have made combined 18cm radio continuum, EVN and MERLIN observations of 13 ULIRGs that have the parsec and deci-parsec scale resolution necessary to distinguish between an AGN and supernovae remnants at the centres of these galaxies, and assess the contribution of each to the total energy distribution. Images of four galaxies are presen…
NGC 7679 is a nearby luminous infrared Sy2 galaxy in which starburst and AGN activities co-exist. The ionization structure is maintained by both the AGN power-law continuum and starburst. The galaxy is a bright X-ray source possessing a low X-ray column density N_H < 4 x 10^20 cm^{-2}. The Compton-thin nature of such unabsorbed objects infers that the simple formulation of the Unified model for SyGs is not applicable in their case. The main goal of this article is to investigate both gas distribution and ionization structure in the circumnuclear region of NGC 7679 in search for the presence of…
Using broadband optical imaging and Chandra X-ray data for a sample of 46 cluster central dominant galaxies (CDGs), we investigate the connection between star formation, the intracluster medium (ICM), and the central active galactic nucleus (AGN). We report the discovery of a remarkably sharp threshold for the onset of star formation that occurs when the central cooling time of the hot atmosphere falls below ~ 5x10^8 yr, or equivalently when the central entropy falls below ~ 30 keV cm^2. In addition to this criterion, star formation in cooling flows also appears to require that the X-ray and g…
We present Spitzer IRS low resolution, mid-IR spectra of a sample of 25 high luminosity QSOs at 2<z<3.5. When combined with archival IRS observations of local, low luminosity type-I AGNs, the sample spans five orders of magnitude in luminosity. We find that the continuum dust thermal emission at lambda(rest)=6.7um is correlated with the optical luminosity, following the non-linear relation L(6.7um) propto L(5100A)^0.82. We also find an anti correlation between the ratio L(6.7um)/L(5100A) and the [OIII]5007A line luminosity. These effects are interpreted as a decreasing covering factor of the c…
We present Infrared Space Observatory mid- and far-infrared spectroscopy of the merging galaxy NGC 6240, an object presenting many aspects of importance for the role of star formation and AGN activity in [ultra]luminous infrared galaxies. The mid-infrared spectrum shows starburst indicators in the form of low excitation fine-structure line emission and aromatic `PAH' features. A strong high excitation [OIV] line is observed which most likely originates in the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely powerful emission in the pure rotational lines of molecular hy…
In this paper, results of optical identification of ASCA surveys are summarized. To understand luminous AGNs in the z<1 universe, the ASCA AGN sample is still better than samples of AGNs from deep Chandra or XMM-Newton surveys. Combining the identified sample of AGNs from ASCA Large Sky Survey and Medium Sensitivity Survey, the sample of hard X-ray selected AGNs have been expanded up to about 80 AGNs above the flux limit of 10^{-13} erg s^{-1} cm^{-2} in the 2-10keV band. One fifth of the ASCA AGNs are absorbed narrow-line AGNs. The luminosity distribution of the absorbed AGNs is limited below…
We review the unique cosmological implications of the XMM-LSS survey in association with its multi-WL follow-up: (1) Large Scale Structures traced by X-ray clusters and AGNs, optical galaxies, weak lensing as well as Sunyaev-Zel'dovich effect; (2) location of IR star forming galaxies and IR (obscured) AGNs within the cosmic web. The XMM/MegaCam/VIRMOS/SIRTF data base will provide the first comprehensive study of structure formation - from hundreds of Mpc to galaxy scale - in close connection with environmental processes.
We present the results on millimeter interferometric observations of four luminous infrared galaxies (LIRGs), Arp 220, Mrk 231, IRAS 08572+3915, and VV 114, and one Wolf-Rayet galaxy, He 2-10, using the Nobeyama Millimeter Array (NMA). Both the HCN(1-0) and HCO+(1-0) molecular lines were observed simultaneously and their brightness-temperature ratios were derived. High-quality infrared L-band (2.8-4.1 micron) spectra were also obtained for the four LIRGs to better constrain their energy sources deeply buried in dust and molecular gas. When combined with other LIRGs we have previously observed …
We investigate the luminous X-ray sources in the Lockman Hole (LH) and the Extended Groth Strip (EGS) detected at 24microns using MIPS and also with IRAC on board Spitzer. We assemble optical/infrared spectral energy distributions (SEDs) for 45 X-ray/24micron sources in the EGS and LH. Only about 1/4 of the hard X-ray/24micron sources show pure type 1 AGN SEDs. More than half of the X-ray/24micron sources have stellar-emission-dominated or obscured SEDs, similar to those of local type 2 AGN and spiral/starburst galaxies. One-third of the sources detected in hard X-rays do not have a 24micron c…
We predict that the UV bump spectrum of quasars extends into the soft X-ray range with a steep power law. For radio-louds, the spectral index is alpha_s ~ 1.7 (S_nu = S_o nu^-alpha), and for radio-quiets it should be somewhat steeper alpha_s >~ 1.7. The prediction is based on a simple scenario in which all quasars, radio-quiets and radio-louds, are assumed to have jets close to their accretion disks. Soft X-rays are produced via unsaturated Comptonization of UV photons from the disk in the (marginally) optically thick jet base. Nonthermal hard X-rays are produced further downstream. The scenar…
We calculate infrared spectral energy distributions (SEDs) from simulations of major galaxy mergers and study the effect of AGN and starburst driven feedback on the evolution of the SED as a function of time. We use a self-consistent three-dimensional radiative equilibrium code to calculate the emergent SEDs and to make images. To facilitate a simple description of our findings, we describe our results in reference to an approximate analytic solution for the far-IR SED. We focus mainly on the luminous infrared galaxy (LIRG) and ultraluminous infrared galaxy (ULIRG) phases of evolution. We cont…
In the decreasing intrinsic redshift (DIR) model galaxies are assumed to be born as compact objects that have been ejected with large intrinsic redshift components, z_(i), out of the nuclei of mature AGN galaxies. As young AGN (quasars) they are initially several magnitudes sub-luminous to mature galaxies but their luminosity gradually increases over 10^8 yrs, as z_(i) decreases and they evolve into mature AGN (Seyferts and radio galaxies). Evidence presented here that low- and intermediate-redshift AGN are unquestionably sub-luminous to radio galaxies is then strong support for this model and…
We present X-ray results on the ultraluminous infrared galaxy Arp220 obtained with BeppoSAX. The X-ray emission up to 10 keV is detected. No significant signal is detected with the PDS detector in the higher energy band. The 2-10 keV emission has a hard spectrum (photon-index of 1.7), similar to M82, and a luminosity of 1e41 erg/s. A population of X-ray binaries may be a major source of this X-ray emission. The upper limit of an iron K line equivalent width at 6.4 keV is ~600 eV. This observation imposes so far the tightest constraint on an active nucleus if present in Arp220. We find that a c…
We study properties of the host galaxies of 15 hard X-ray selected type-2 active galactic nuclei (AGNs) at intermediate redshifts (0.05$<z<$0.6) detected in $ASCA$ surveys. The absorption corrected hard X-ray luminosities $L_{\rm 2-10 keV}$ range from 10$^{42}$ erg s$^{-1}$ to $10^{45}$ erg s$^{-1}$. We took the $R$-band image of these AGNs with the University of Hawaii 2.2 m telescope. Thanks to the intrinsic obscuration of nuclear light, we can decompose the galaxies with a spheroid component and a disk component. The resulting spheroid luminosities correlate with $L_{\rm 2-10 keV}$; higher …
We study the size of BLRs of low luminosity Active Galactic Nuclei, also called 'dwarf AGN', defined as having ($L_{Hα}\le10^{41}{\rm erg\cdot s^{-1}}$)). We more than double the sample size analyzed previously (Wang & Zhang 2003, hereafter Paper I). In this study we first confirm our previous result that the sizes of BLRs of low luminosity AGN are larger than the ones expected from the empirical relation $R_{BLRs} - L_{Hα}$ valid for 'normal' AGN: Seyfert 1s and quasars, except for the objects with accretion rate $\dot{m_{Hα}}>10^{-5.5}$. Second, we find a positive correlation between the lin…
Spectroscopic observations of a sample of 73 very luminous IRAS galaxies (log(LIR/Lsun)>=11.5 for H0=50km s^{-1} Mpc^{-1},q0=0.5) from the 2Jy red- shift survey catalogue were carried out using the 2.16m telescope at the Beijing Astronomical Observatory. The observational data, including the optical images (extracted from Digital Sky Survey) and spectra for these galaxies, are presented in paper I (Wu et al., 1998). In this paper, we give the spectral and morphological classifications for these very luminous IRAS galaxies (VLIRGs). We show that about 60% of VLIRGs exhibit AGN-like spectra (Sey…
The dust-to-gas ratios in three different samples of luminous, ultraluminous, and hyperluminous infrared galaxies are calculated by modelling their radio to soft X-ray spectral energy distributions using composite models which account for the photoionizing radiation from HII regions, starbursts, or AGNs, and for shocks. The models are limited to a set which broadly reproduces the mid-IR fine structure line ratios of local, IR bright, starburst galaxies. The results show that two types of clouds contribute to the IR emission. Those characterized by low shock velocities and low preshock densitie…
We constrain the past star formation histories of a sample of 25 distant (z~0.7) luminous infrared galaxies (LIRGs) detected with the mid infrared cameras ISOCAM and MIPS onboard the ISO and Spitzer satellites. We use high resolution VLT-FORS2 spectroscopy in addition to a comprehensive library of 200,000 model optical spectra to derive Bayesian likelihood estimates of the star formation histories of these galaxies based on the analysis of Balmer absorption lines and the 4000 A break. The locus of distant LIRGs in the diagram defined by Hdelta_A and D4000 is roughly comparable to that of local…
We present the results of infrared 2.8-4.1 micron (L-band) spectroscopy of nearby infrared luminous galaxies with possible signatures of dust-obscured active galactic nuclei (AGNs) in data at other wavelengths. The samples are chosen to include sources with a radio excess relative to far-infrared emission, strong absorption features in mid-infrared 5-11.5 micron spectra, unusually weak [CII] 158 micron emission relative to the far-infrared continuum, and radio galaxies classified optically as narrow-line objects. Our aim is to investigate whether the signatures of possible obscured AGNs can be…
Context: A long-standing challenge of observational AGN research is to find type 2 quasars, the luminous analogues of Seyfert-2 galaxies. Aims: We search for luminous narrow-line type 2 AGN, characterise their properties, and compare them with broad-line type 1 AGN. Methods: Combining the ISOCAM parallel survey at 6.7 mu with 2MASS, we have selected AGN via near-mid-infrared colours caused by the hot nuclear dust emission. We performed spectroscopy in the optical and, for a subset of the sample, also in the mid-infrared with Spitzer. Results: We find nine type 2 AGN at redshift 0.1<z<0.5, thre…
