Point Source Population Research Articles

Linking the Power Sources of Emission‐Line Galaxy Nuclei from the Highest to the Lowest Redshifts

This dissertation searches for common grounds for the diversity of properties exhibited by the emission-line nuclei of galaxies, from large look-back times to the local universe. I present results of (1) a program of high signal-to-noise spectroscopy for 44 quasars using the MMT and Keck obz 4 servatories; (2) a detailed analysis of the ultraviolet and optical spectral behavior of 22 narrow-line Seyfert 1 (NLS1) galaxies based on archival Hubble Space Telescope (HST) spectra; (3) an in-depth investigation of the proposed link between NLS1s and quasars by means of comparison of composite z 4 spectra and a principal component analysis; (4) a simulation of Seyferts/quasars designed to explore the role of dust in modifying their observed spectral energy distribution; and (5) a sensitive search for accretion signatures in a large sample of nearby emission-line galaxy nuclei, employing a quantitative comparison of the nebular line flux ratios in small (HST) and large (ground-based) apertures. The lowand high-redshift quasars are found to be very similar in their emission characteristics, although differences exist. In particular, the data bolster indications of supersolar metallicities in the luminous, sources, which supports z 4 scenarios that assume substantial star formation concurrent or preceding the quasar phenomena. An in-depth analysis of quasar selection effects is performed, based on comparisons of distributions of equivalent widths for quasar ensembles characterized by different detection techniques. The measurements are consistent with a common parent population, suggesting that the trends identified here are representative of high-z QSOs. The quasars are found to be more metal enriched and z 4 more spectroscopically heterogeneous than the NLS1s, indicating that a close connection between these objects remains doubtful. NLS1s reveal redder UV-blue continua than those measured in other quasars and Seyferts. The sources with UV line absorption are in general less powerful and show redder spectra, suggesting that luminosity-dependent dust absorption may be important in modifying their continua. A recedingtorus–like geometry seems to explain these trends and other observed correlations between quasar luminosity and continuum slope. This relatively simple framework also predicts a number of absorbed AGNs that is consistent with recent modeling and observations of the X-ray background light and extragalactic point source population. Finally, in most of the nearby emission-line nuclei, the expected increased AGN-like behavior at smaller scales is not seen, although the nuclear emission is resolved. This suggests that these sources are not necessarily powered by accretion onto a compact object and that the composite model proposed for the LINER/H ii transition nuclei (which assumes a central accreting-type nucleus surrounded by star-forming regions) is not generally supported. Only about 10% of the narrow-line objects reveal broad Ha features, thus changing their spectral classification from type 2 to type 1.

An α-stable approach to the study of the P(D) distribution of unresolved point sources in CMB sky maps

We present a new approach to the statistical study and modelling of number counts of faint point sources in astro- nomical images, i.e. counts of sources whose flux falls below the detection limit of a survey. The approach is based on the theory of α-stable distributions. We show that the non-Gaussian distribution of the intensity fluctuations produced by a generic point source population - whose number counts follow a simple power law - belongs to the α-stable family of distributions. Even if source counts do not follow a simple power law, we show that the α-stable model is still useful in many astrophysical scenarios. With the α-stable model it is possible to totally describe the non-Gaussian distribution with a few parameters which are closely related to the parameters describing the source counts, instead of an infinite number of moments. Using statisti- cal tools available in the signal processing literature, we show how to estimate these parameters in an easy and fast way. We demonstrate that the model proves valid when applied to realistic point source number counts at microwave frequencies. In the case of point extragalactic sources observed at CMB frecuencies, our technique is able to successfully fit the P(D) distribution of deflections and to precisely determine the main parameters which describe the number counts. In the case of the Planck mission, the relative errors on these parameters are small either at low and at high frequencies. We provide a way to deal with the presence of Gaussian noise in the data using the empirical characteristic function of the P(D). The formalism and methods here presented can be very useful also for experiments in other frequency ranges, e.g. X-ray or radio Astronomy.

XMM-Newtonobservations of the merger-remnant galaxies NGC 3921 and 7252

Using the high sensitivity of XMM‐Newton ,w e h ave studied the X-ray emission of the two prototypical late-stage merger remnants, NGC 3921 and 7252. In the case of NGC 7252, this is complemented by archival Chandra data. We investigate the nature of the discrete X-ray point source populations and the hot diffuse gas components in these two galaxies, and compare them in the light of their different merger ages and histories. We detect three candidate ultraluminous X-ray point sources (ULXs) in NGC 3921 and at least six in NGC 7252, for which we have high spatial resolution Chandra data. These have luminosities ranging from ∼1.4 × 10 39 ‐10 40 erg s −1 (for H 0 = 75 km s −1 Mpc −1 ). We expect these ULXs to be high-mass X-ray binaries, associated with the recent star formation in these two galaxies. Extended hot gas is observed in both galaxies. We have sufficient counts in the XMM‐ Newton data to fit two-component hot plasma models to their X-ray spectra, and estimate the X-ray luminosities of the hot diffuse gas components to be 2.75 × 10 40 erg s −1 and 2.09 × 10 40 erg s −1 in NGC 3921 and 7252, respectively. These luminosities are low compared with the luminosities observed in typical mature elliptical galaxies (L X ∼ 10 41 ‐10 42 erg s −1 ), into which these merger remnants are expected to evolve. We do not see evidence that the X-ray haloes of these galaxies are currently being regenerated to the masses and luminosities seen in typical elliptical galaxies. The mass of atomic gas available to fall back into the main bodies of these galaxies and shock-heat to X-ray temperatures is insufficient for this to be the sole halo regeneration mechanism. We conclude that halo regeneration is most likely a long-term (>10 Gyr) process, occurring predominantly via mass loss from evolving stars, in a subsonic outflow stage commencing ∼2 Gyr after the merging event. Ke yw ords: galaxies: evolution ‐ galaxies: individual: NGC 3921 ‐ galaxies: individual: NGC 7252 ‐ X-rays: binaries ‐ X-rays: galaxies ‐ X-rays: ISM.

Old and Young X‐Ray Point Source Populations in Nearby Galaxies

We analyzed 1441 Chandra X-ray point sources in 32 nearby galaxies. The total point-source X-ray luminosity L_XP is well correlated with B, K, and FIR+UV luminosities of spiral host galaxies, and with the B and K luminosities for ellipticals. This suggests an intimate connection between L_XP and both the old and young stellar populations, for which K and FIR+UV luminosities are proxies for the galaxy mass M and star-formation rate SFR. We derive proportionality constants 1.3E29 erg/s/Msol and 0.7E39 erg/s/(Msol/yr), which can be used to estimate the old and young components from M and SFR, respectively. The cumulative X-ray luminosity functions for the point sources have quite different slopes for the spirals (gamma ~= 0.5-0.8) and ellipticals (gamma ~= 1.4), implying *the most luminous point sources dominate L_XP* for the spirals. Most of the point sources have X-ray colors that are consistent with either LMXBs or Ultraluminous X-ray sources (ULXs a.k.a. IXOs) and we rule out classical HMXBs (e.g. neutron-star X-ray pulsars) as contributing much to L_XP. Thus, for spirals, the ULXs dominate L_XP. We estimate that >~20% of all ULXs found in spirals originate from the older (pop II) stellar populations, indicating that many of the ULXs that have been found in spiral galaxies are in fact pop II ULXs, like those in elliptical galaxies. The linear dependence of L_XP on the SFR argues for either a steepening in the X-ray luminosity function of the young (pop I) X-ray source population at L_X >~10^(38.5-39) erg/s, or a decreasing efficiency for producing all types of young X-ray point sources as the galaxy SFR increases.

ChandraandXMM-Newtonobservations of NGC 4214: the hot interstellar medium and the luminosity function of dwarf starbursts

We present results from Chandra and XMM-Newton X-ray observations of NGC4214, a nearby dwarf starburst galaxy containing several young regions of very active star-formation. Starburst regions are known to be associated with diffuse X-ray emission, and in this case the X-ray emission from the galaxy shows an interesting morphological structure within the galaxy, clearly associated with the central regions of active star-formation. Of the two main regions of star formation in this galaxy, X-ray emission associated with the older is identified whereas little is detected from the younger, providing an insight into the evolutionary process of the formation of superbubbles around young stellar clusters. The spectra of the diffuse emission from the galaxy can be fitted with a two temperature component thermal model with kT = 0.14keV and 0.52keV, and analysis of this emission suggests that NGC4214 will suffer a blow-out in the future. The point source population of the galaxy has an X-ray luminosity function with a slope of 0.76. This result, together with those for other dwarf starburst galaxies (NGC4449 and NGC5253), was added to a sample of luminosity functions for spiral and starburst galaxies. The slope of the luminosity function of dwarf starbursts is seen to be similar to that of their larger counterparts and clearly flatter than those seen in spirals. Further comparisons between the luminosity functions of starbursts and spiral galaxies are also made.

X‐Ray Point Sources in the Sombrero Galaxy: Very Soft Sources, the Globular Cluster/Low‐Mass X‐Ray Binary Connection, and an Overview

We report on the population of point sources discovered during an 18.5 ks Chandra ACIS-S observation of the Sombrero galaxy. We present the luminosity function and the spectra of the six brightest sources, consider correlations with globular clusters (GCs) and with planetary nebulae, and study the galaxy's population of very soft sources. We detected 122 sources. Twenty-two sources are identified as very soft; of these, five appear to be classical luminous supersoft X-ray sources (SSSs), while 17 may belong to the slightly harder class referred to as quasi-soft (QSSs). There is an overdensity of very soft sources within 2 kpc of the nucleus, which is itself the brightest X-ray source. Very soft sources are also found in the disk and halo, with one QSS in a globular cluster (GC). This source is somewhat harder than most SSSs; the energy distribution of its photons is consistent with what is expected from an accreting intermediate-mass black hole. Several sources in the Sombrero's halo are good candidates for SSS models in which the accretor is a nuclear-burning white dwarf. In total, 32 X-ray sources are associated with GCs. The majority of sources with luminosity greater than 1038 ergs s-1 are in GCs. These results for M104, an Sa galaxy, are similar to what has been found for elliptical galaxies and for the late-type spiral M31. We find that those optically bright GCs with X-ray sources house only the brightest X-ray sources. We find that, in common with other galaxies, there appears to be a positive connection between young (metal-rich) GCs and X-ray sources but that the brightest X-ray sources are equally likely to be in metal-poor GCs. The luminosity function of X-ray sources in GCs has a cut-off near the Eddington luminosity for a 1.4 M☉ object. We propose a model that can explain the trends seen in the data sets from the Sombrero and other galaxies. Thermal timescale mass transfer can occur in some of the younger clusters in which the turnoff mass is slightly greater than 0.8 M☉; multiplicity may play a role in some of the most massive clusters; accretion from giant stars may be the dominant mechanism in some older, less massive and less centrally concentrated clusters. Key elements of the model can be tested.

The X-Ray Properties of the Nearby Star-forming Galaxy IC 342: TheXMM-NewtonView

We present results on the X-ray properties of IC 342 using a 10 ks XMM-Newton observation. Thirty-five sources are detected coincident with the disk of IC 342 (more than tripling the number known), of which ?31 are likely to be intrinsic to IC 342. This point-source population exhibits a diverse range of spectral properties and has an X-ray luminosity function slope and infrared luminosity comparable to that of starburst galaxies such as M82 and the Antennae, while its relative lack of extended X-ray emission is similar to the properties of quiescent spirals. Although we find no evidence of short-term variability in any of the X-ray sources, we do detect long-term variability between this observation and the 1991 ROSAT and 1993/2000 ASCA observations for five sources. Notably, the second most luminous source in IC 342 (X-2) is found to have an absorption-corrected 0.5?10 keV luminosity of 5.75 ? 1039 ergs s-1 and a spectrum best fitted by an absorbed multicolor accretion disk model with Tin = 2.17 keV and NH = 15.3 ? 1021 cm-2. This is the lowest luminosity state observed for X-2 to date, although the slope of the spectrum is intermediate between the previously observed low/hard and high/soft states. IC 342 X-1, on the other hand, is found to be in an identical state to that observed in 2000 with ASCA. Assuming X-1 is in an anomalous very high (VH) state, then either (1) X-1 has remained in this state between 2000 and 2002 and is therefore the longest duration VH-state binary ever observed, or (2) it was simply caught in a VH state by chance in both the 2000 ASCA and 2002 XMM-Newton observations. We have also confirmed the ROSAT HRI result that the nucleus of IC 342 is made up of both pointlike and extended emission, with the extended emission contributing ?55% and ?35% in the 0.3?2.0 keV and 2.0?10.0 keV bands, respectively. The spectrum of the nucleus is best fitted by an absorbed two-component model consisting of a thermal plasma with a temperature of kT ? 0.3 and a power law with a photon index of ? ? 2.53. The relative fluxes of the two spectral components suggest that the nucleus is complex, with a soft extended component contributing approximately half of the total luminosity.

CHANDRA X-RAY OBSERVATIONS OF EARLY TYPE GALAXIES

We review recent observational results on early type galaxies obtained with high spatial resolution Chandra data. With its unprecedented high spatial resolution, Chandra reveals many intriguing features in early type galaxies which were not identified with the previous X-ray missions. In particular, various fine structures of the hot ISM in early type galaxies are detected, for example, X-ray cavities which are spatially coincident with radio jets/lobes, indicating the interaction between the hot ISM and radio jets. Also point sources (mostly LMXBs) are individually resolved down to Lx = a few x <TEX>$10^{37}\;erg\;sec^{-1}$</TEX> and it is for the first time possible to unequivocally investigate their properties and the X-ray luminosity function. After correcting for incompleteness, the XLF of LMXBs is well reproduced by a single power law with a slope of -1.0 - -1.5, which is in contrast to the previous report on the existence of the XLF break at Lx, Eddington = 2 x <TEX>$10^{38}\;erg\;sec^{-1}$</TEX> (i.e., Eddington luminosity of a neutron star binary). Carefully considering both detected and undetected, hidden populations of point sources we further discuss the XLF of LMXBs and the metal abundance of the hot ISM and their impact on the properties of early type galaxies.

10 MK Gas in M17 and the Rosette Nebula: X‐Ray Flows in Galactic HiiRegions

We present the first high spatial resolution X-ray images of two high-mass star forming regions, the Omega Nebula (M17) and the Rosette Nebula (NGC 2237-2246), obtained with the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer instrument. The massive clusters powering these H II regions are resolved at the arcsecond level into more than 900 (M17) and 300 (Rosette) stellar sources similar to those seen in closer young stellar clusters. However, we also detect soft diffuse X-ray emission on parsec scales that is spatially and spectrally distinct from the point-source population. The diffuse emission has luminosity LX 3.4 × 1033 ergs s-1 in M17 with plasma energy components at kT 0.13 and 0.6 keV (1.5 and 7 MK), while in Rosette it has LX 6 × 1032 ergs s-1 with plasma energy components at kT 0.06 and 0.8 keV (0.7 and 9 MK). This extended emission most likely arises from the fast O star winds thermalized either by wind-wind collisions or by a termination shock against the surrounding media. We establish that only a small portion of the wind energy and mass appears in the observed diffuse X-ray plasma; in these blister H II regions, we suspect that most of it flows without cooling into the low-density interstellar medium. These data provide compelling observational evidence that strong wind shocks are present in H II regions.

The line–of–sight distribution of the gas in the inner 60 pc of the Galaxy

2MASS K S band data of the inner 60 pc of the Galaxy are used to reconstruct the line-of-sight distances of the giant molecular clouds located in this region. Using the 2MASS H hand image of the same region, two differentpopulations of point sources are identified according to their flux ratio in the two bands. The population of blue point sources forms a homogeneous foreground that has to he subtracted before analyzing the K S band image. The reconstruction is made using two basic assumptions: (i) an axis-symmetric stellar distribution in the region of interest and (ii) optically thick clouds with an area filling factor of ∼1 that block all light of stars located behind them. Due to the reconstruction method, the relative distance between the different cloud complexes is a robust result, whereas it is not excluded that the absolute distance with respect to Sgr A* of structures located more than 10 pc in front of Sgr At are understimated by up to a factor of 2. It is shown that all structures observed in the 1.2 mm continuum and in the CS(2-1) line are present in absorption. We place the 50 km s - 1 cloud complex close to, hut in front of, Sgr A*. The 20 kin s - 1 cloud complex is located in front of the 50 km s - 1 cloud complex and has a large LOS distance gradient along the direction of the galactic longitude. The bulk of the Circumnuclear Disk is not seen in absorption. This leads to an upper limit of the cloud sizes within the Circumnuclear Disk of ∼0.06 pc.

Chandra observations of the dwarf starburst and Wolf-Rayet galaxies NGC 4449 and NGC 5253

We present an analysis of the Chandra observations of two dwarf starburst, Wolf-Rayet galaxies (NGC 4449 and NGC 5253). We have identified at least three different classes of objects within the X-ray point source populations, and we have found the diffuse emission, resulting from the stellar winds and supernova explosions of massive stars, to have a complex morphology and to consist of several components. Comparison with the Hα emission suggests the presence of ~ kpc-scale wind-blown bubbles and ruptured superbubbles.

ChandraObservation of Low‐Mass X‐Ray Binaries in the Elliptical Galaxy M84

We present characteristics of the X-ray point-source population in the M84 galaxy observed by the Chandra Advanced CCD Imaging Spectrometer S-Array. We find an excess in the number of sources centered on M84 with a spatial distribution closely corresponding to the M84 stellar light. Given an absence of recent star formation, accreting binaries are the only candidates for the M84 X-ray sources. The majority of M84 sources (with luminosities exceeding 1038 ergs s-1) exhibit hardness ratios expected from multitemperature blackbody disk emission. The most luminous sources, which we attribute to accreting black holes, exhibit X-ray colors typical of a blackbody spectrum. We also identify the sources whose X-ray colors match the expectations for constituents of the cosmic X-ray background. The number of such sources agrees with that expected to be background sources. After correcting for incompleteness in the source detection, we find a log N- log S for M84 similar to that of the elliptical galaxy NGC 4697, i.e., having a break at a luminosity of Lb = 2.4 × 1038 ergs s-1, approximately the Eddington limit on the isotropic luminosity for accretion onto a neutron star. The slope of the luminosity function above the break provides evidence for a mass distribution in the M84 accreting black holes.

DeepROSATHRI Observations of the NGC 1399/NGC 1404 Region: Morphology and Structure of the X‐Ray Halo

We present the analysis of a deep (167 ks) ROSAT HRI observation of the cD galaxy NGC 1399 in the Fornax Cluster, comparing it with previous work on this galaxy and with recent Chandra data. We find, in agreement with previous observations, an extended and asymmetric gaseous halo with a luminosity (in the 0.1-2.4 keV energy band) of LX = (5.50 ± 0.04) × 1041 ergs s-1 within 46 kpc (assuming a distance of D = 19 Mpc). Using both HRI and, at larger radii, archival PSPC data, we find that the radial behavior of the X-ray surface brightness profile is not consistent with a simple β-model and suggests instead three distinct components. We use a multicomponent bidimensional model to study in detail these three components, which we identify respectively with the cooling flow region, the galactic halo, and the cluster halo. From these data we derive a binding mass distribution in agreement with that suggested by optical dynamical indicators, with an inner core dominated by luminous matter and an extended dark halo differently distributed on galactic and cluster scales. The HRI data and a preliminary analysis of Chandra public data allow us to detect significant density fluctuations in the halo. We discuss possible nonequilibrium scenarios to explain the hot halo structure, including tidal interactions with neighboring galaxies, ram stripping from the intracluster medium, and merging events. In the innermost region of NGC 1399, the comparison between the X-ray and radio emission suggests that the radio-emitting plasma is displacing and producing shocks in the hot X-ray-emitting gas. We do not detect the nuclear source in X-rays, and we pose an upper limit of ~4 × 1039 ergs s-1 (0.1-2.4 keV) to its X-ray luminosity. We found that the NGC 1404 halo is well represented by a single symmetric β-model and follows the stellar light profile within the inner 8 kpc. The mass distribution is similar to the "central" component of the NGC 1399 halo. At larger radii, ram pressure stripping from the intracluster gas produces strong asymmetries in the galactic halo. Finally, we discuss the properties of the point-source population, finding evidence of correlation between the source excess and NGC 1399.

ChandraObservations of the X‐Ray Point Source Population in Centaurus A

We present results from two Chandra X-ray Observatory observations of the X-ray point source population in the nearby radio galaxy Centaurus A (NGC 5128). Using a wavelet decomposition detection algorithm, we detect 246 individual point sources above a limiting luminosity of ~2 × 1036 ergs s-1, 82 of which are detected in both data sets where the fields of view overlap. Thirty-eight sources were detected in only one observation but were within the field of view of both pointings, implying considerable variability. We identify eight foreground stars in our observations. We also identify nine of the sources with known globular clusters in Centaurus A. All previously observed ROSAT sources within our field of view are detected. The number of luminous (Lx > 1037 ergs s-1) X-ray binaries per unit optical luminosity in Cen A is roughly consistent with that observed for the M31 bulge. There are approximately 50% more X-ray binaries per unit optical luminosity in Centaurus A than in the whole of M31, however. We find considerably fewer luminous X-ray binaries per unit optical luminosity in Cen A than in two other elliptical galaxies, M84 and NGC 4697, recently observed by Chandra. This result directly confirms the variance in the X-ray binary population with host galaxy optical luminosity previously inferred from spectral analysis of ROSAT and ASCA observations of elliptical galaxies.

Predicted Planck extragalactic point-source catalogue

An estimation of the number and amplitude (in flux) of the extragalactic point sources that will be observed by the Planck Mission is presented in this paper. The study is based on the Mexican Hat wavelet formalism introduced by Cayón et al. Simulations at Planck observing frequencies are analysed, taking into account all the possible cosmological, Galactic and extragalactic emissions together with noise. With the technique used in this work, the Planck Mission will produce a catalogue of extragalactic point sources above the following flux values: 1.03 Jy (857 GHz), 0.53 Jy (545 GHz), 0.28 Jy (353 GHz), 0.24 Jy (217 GHz), 0.32 Jy (143 GHz), 0.41 Jy (100 GHz, high-frequency instrument), 0.34 Jy (100 GHz, low-frequency instrument), 0.57 Jy (70 GHz), 0.54 Jy (44 GHz) and 0.54 Jy (30 GHz), which are only slightly model dependent (see text). Amplitudes of these sources are estimated with errors below ∼15 per cent. Moreover, we also provide a complete catalogue (for the point-source simulations analysed) with errors in the estimation of the amplitude below ∼10 per cent. In addition we discuss the possibility of identifying different point-source populations in the Planck catalogue by estimating their spectral indices.

Spatially resolved spectroscopy of the cluster of galaxies A 2199

The X-ray spectrum of the bright cD cluster of galaxies A 2199 is studied using all instruments of BeppoSAX. Spatially resolved spectroscopy with the MECS instrument shows the presence of spectral hardening in the outer parts of the cluster. The presence of a hard tail is also confirmed by the simultaneous PDS observations. Our analysis shows that the hard component cannot be due to a population of point sources like AGN but corresponds to a diffuse non-thermal component. We also discuss the presence of resonance scattering in this cluster.