Off-nuclear Sources Research Articles

Extended X-ray emission from the z = 4.26 radio galaxy 4C 63.20

ABSTRACT We report on deep Chandra X-ray Telescope imaging observations of 4C 63.20, one of the few known radio galaxies at z > 3.5. The X-ray counterpart is resolved into a core plus two off-nuclear sources that (combined) account for close to 30 per cent of the total X-ray flux. Their morphology and orientation are consistent with a diffuse, lobe-like nature, albeit compact hotspots cannot be ruled out. The broad-band spectral energy distribution of 4C 63.20 can be reproduced with a jet model where the majority of the radio flux can be ascribed to synchrotron emission from the hotspots, whereas the (non-nuclear) X-ray emission is produced via inverse Compton (IC) off of cosmic microwave background (CMB) photons within the extended lobes. This scenario is broadly consistent with the expectation from highly magnetized lobes in a hotter CMB, and supports the view that IC/CMB may quench less extreme radio lobes at high redshifts.

XMM–Newton view of X-ray overdensities from nearby galaxy clusters: the environmental dependencies

In this work, we studied ten nearby ($z$$ \leq$0.038) galaxy clusters to understand possible interactions between hot plasma and member galaxies. A multi-band source detection was applied to detect point-like structures within the intra-cluster medium. We examined spectral properties of a total of 391 X-ray point sources within cluster's potential well. Log $N$ - Log $S$ was studied in the energy range of 2-10 keV to measure X-ray overdensities. Optical overdensities were also calculated to solve suppression/triggering phenomena for nearby galaxy clusters. Both X-ray to optical flux/luminosity properties, ($X/O$, $L_{X}$/$L_{B}$, $L_{X}$/$L_{K}$), were investigated for optically identified member galaxies. X-ray luminosity values of our point sources are found to be faint (40.08 $\leq$ log($L_{X}$) $\leq$ 42.39 erg s$^{-1}$). The luminosity range of point sources reveals possible contributions to X-ray emission from LLAGNs, X-ray Binaries and star formation. We estimated $\sim$ 2 times higher X-ray overdensities from galaxies within galaxy clusters compared to fields. Our results demonstrate that optical overdensities are much higher than X-ray overdensities at the cluster's centre, whereas X-ray overdensities increase through the outskirts of clusters. We conclude that high pressure from the cluster's centre affects the balance of galaxies and they lose a significant amount of their fuels; as a result, clustering process quenches X-ray emission of the member galaxies. We also find evidence that the existence of X-ray bright sources within cluster environment can be explained by two main phenomena: contributions from off-nuclear sources and/or AGN triggering caused by galaxy interactions rather than AGN fuelling.

A deepChandraobservation of the interacting star-forming galaxy Arp 299

We present results from a 90 ks Chandra ACIS-S observation of the X-ray luminous interacting galaxy system Arp 299 (NGC 3690/IC 694). We detect 25 discrete X-ray sources with luminosities above 4.0x10^38 erg s^-1 covering the entire Ultra Luminous X-ray source (ULX) regime. Based on the hard X-ray spectra of the non-nuclear discrete sources identified in Arp 299, and their association with young, actively star-forming region of Arp 299 we identify them as HMXBs. We find in total 20 off-nuclear sources with luminosities above the ULX limit, 14 of which are point-like sources. Furthermore we observe a marginally significant deficit in the number of ULXs, with respect to the number expected from scaling relations of X-ray binaries with the star formation rate (SFR). Although the high metalicity of the galaxy could result in lower ULX numbers, the good agreement between the observed total X-ray luminosity of ULXs, and that expected from the relevant scaling relation indicates that this deficit could be the result of confusion effects. The integrated spectrum of the galaxy shows the presence of a hot gaseous component with kT = 0.72+-0.03 keV, contributing 20% of the soft (0.1-2.0 keV) unabsorbed luminosity of the galaxy. A plume of soft X-ray emission in the west of the galaxy indicates a large scale outflow. We find that the AGN in NGC 3690 contributes only 22% of the observed broad-band X-ray luminosity of Arp 299.

THECHANDRAVIEW OF NGC 4178: THE LOWEST MASS BLACK HOLE IN A BULGELESS DISK GALAXY?

Using high resolution Chandra data, we report the presence of a weak X-ray point source coincident with the nucleus of NGC 4178, a late-type bulgeless disk galaxy known to have high ionization mid-infrared (mid-IR) lines typically associated with active galactic nuclei (AGNs). Although the faintness of this source precludes a direct spectral analysis, we are able to infer its basic spectral properties using hardness ratios. X-ray modeling, combined with the nuclear mid-IR characteristics, suggests that NGC 4178 may host a highly absorbed AGN accreting at a high rate with a bolometric luminosity on order of 10^43 ergs/s. The black hole mass estimate, based on our Chandra data and archival VLA data using the most recent fundamental plane relations is \sim 10^4 - 10^5 M\odot, possibly the lowest mass nuclear black hole currently known. There are also three off-nuclear sources, two with a similar brightness to the nuclear source at 36" and 32" from the center. As with the nuclear source, hardness ratios are used to estimate spectra for these two sources, and both are consistent with a simple power- law model with absorption. These two sources have X-ray luminosities of the order of \sim 10^38 ergs/s, which place them at the threshold between X-ray binaries and ultra-luminous X-ray sources (ULXs). The third off-nuclear source, located 49" from the center, is the brightest source detected, with an X-ray luminosity of \sim 10^40 ergs/s. Its spectrum is well-fit with an absorbed power law model, suggesting that it is a ULX. We also fit its spectrum with the Bulk Motion Comptonization (BMC) model and suggest that this source is consistent with an intermediate-mass black hole (IMBH) of mass (6\times2)\times10^3 M\odot.

PARSEC-SCALE RADIO EMISSION FROM THE LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE DWARF STARBURST GALAXY HENIZE 2-10

A candidate accreting massive black hole (BH) with M_BH ~ 10^6 Msun has recently been identified at the center of the dwarf starburst galaxy Henize 2-10 (He 2-10). This discovery offers the first possibility of studying a growing BH in a nearby galaxy resembling those in the earlier universe, and opens up a new class of host galaxies to search for the smallest supermassive BHs. Here we present very long baseline interferometry observations of He 2-10 taken with the Long Baseline Array (LBA) at 1.4 GHz with an angular resolution of ~0.1" x 0.03". A single compact radio source is detected at the precise location of the putative low-luminosity active galactic nucleus. The physical size of the nuclear radio emission is < 3 pc x 1 pc, an order of magnitude smaller than previous constraints from the Very Large Array (VLA), and the brightness temperature of T_B > 3 x 10^5 K confirms a non-thermal origin. These LBA observations indicate that the nuclear radio emission originates from a single object and exclude the possibility of multiple supernova remnants as the origin of the nuclear radio emission previously detected with the VLA at lower resolution. A weaker, more extended, off-nuclear source is also detected with the LBA and a comparison with multi-wavelength ancillary data indicate that, unlike the nuclear source, the off-nuclear source is co-spatial with a super star cluster, lacks a detectable X-ray point-source counterpart, and is almost certainly due to a supernova remnant in the host star cluster.

Ultraluminous X-ray sources out toz~ 0.3 in the COSMOS field

<i>Context. <i/>Using Chandra observations we identified a sample of seven off-nuclear X-ray sources in the redshift range <i>z<i/> = 0.072–0.283, located within optically bright galaxies in the COSMOS Survey. All of them, if associated with their closest bright galaxy, would have <i>L<i/>[0.5–7 keV] <i>><i/> 10<sup>39<sup/> erg s<sup>-1<sup/> and therefore can be classified as ultraluminous X-ray sources (ULXs). <i>Aims. <i/>Using the multi-wavelength coverage available in the COSMOS field, we studied the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from H<i>α<i/> measurements and their stellar masses using SED fitting techniques with the aim to compute the probability to have an off-nuclear source based on the host galaxy properties. We divided the host galaxies in different morphological classes with the available ACS/HST imaging.<i>Methods. <i/>We selected off-nuclear candidates with the following criteria: 1) the distance between the X-ray and the optical centroid has to be larger than 0.9´´, larger than 1.8 times the radius of the Chandra positional error circle and smaller than the Petrosian radius of the host galaxy; 2) the optical counterpart is a bright galaxy (<i>R<i/><sub>AB<sub/><i><<i/>22); 3) the redshift of the counterpart is lower than <i>z<i/> = 0.3; 4) the source has been observed in at least one Chandra pointing at an off-axis angle smaller than 5´; 5) the X-ray positional error is smaller than 0.8´´. We verified each candidate super-imposing the X-ray contours on the optical/IR images. We expect less than one misidentified AGN due to astrometric errors and on average 1.3 serendipitous background source matches.<i>Results. <i/>We find that our ULXs candidates are located in regions of the SFR versus plane where one or more off-nuclear detectable sources are expected. From a morphological analysis of the ACS imaging and the use of rest-frame colours, we find that our ULXs are hosted both in late and early type galaxies. Finally, we find that the fraction of galaxies hosting a ULX ranges from <i>≈<i/>0.5% to <i>≈<i/>0.2% going from = 3 × 10<sup>39<sup/> erg s<sup>-1<sup/> to = 2 × 10<sup>40<sup/> erg s<sup>-1<sup/>.

SUZAKUOBSERVATIONS OF THE CIRCINUS GALAXY

We report Suzaku observations of the active, Compton-thick Circinus galaxy. Observations were obtained with both the X-ray Imaging spectrometer (XIS) and the Hard X-ray Detector (HXD). Below 10 keV, the nuclear spectrum is dominated by radiation reflected from cold dense gas of high column density, while above 13 keV the radiation is directly transmitted nuclear emission seen through a column density of $~ 4 x 10^{24} cm^-2. In the 0.2--10 keV band, the XIS spectrum is contaminated at 5% level by the brightest off-nuclear source in Circinus (CG X-1), but drops to 1% in the 5-10 keV and is negligible at higher energies. We find no significant evidence for variability in the hard (>12 keV) emission. The Circinus is marginally detected with the HXD/GSO in the 50--100 keV band at 2.5\sigma level. We model the 3-70 keV band XIS+PIN spectra with a four components: the Compton transmitted nuclear emission, the reflected nuclear emission, a soft power law (representing a combination of scattered nuclear emission, extended emission and contamination by sources in the galaxy below a few keV). The hard nuclear power-law is found to have a photon index Gamma_h ~= 1.6, very similar to the soft power-law. The high energy cut-off is E_C ~= 49 keV. These results agree with those from BeppoSax. An extrapolation of this model up to the GSO band shows good agreement with the GSO spectrum and supports our detection of the Circinus up to ~= 100keV.

Low‐Level Nuclear Activity in Nearby Spiral Galaxies

We are conducting a search for supermassive black holes (SMBHs) with masses below 10^7 M_sun by looking for signs of extremely low-level nuclear activity in nearby galaxies that are not known to be AGNs. Our survey has the following characteristics: (a) X-ray selection using the Chandra X-ray Observatory, since x-rays are a ubiquitous feature of AGNs; (b) Emphasis on late-type spiral and dwarf galaxies, as the galaxies most likely to have low-mass SMBHs; (c) Use of multiwavelength data to verify the source is an AGN; and (d) Use of the highest angular resolution available for observations in x-rays and other bands, to separate nuclear from off-nuclear sources and to minimize contamination by host galaxy light. Here we show the feasibility of this technique to find AGNs by applying it to six nearby, face-on spiral galaxies (NGC 3169, NGC 3184, NGC 4102, NGC 4647, NGC 4713, NGC 5457) for which data already exist in the Chandra archive. All six show nuclear x-ray sources. The data as they exist at present are ambiguous regarding the nature of the nuclear x-ray sources in NGC 4713 and NGC 4647. We conclude, in accord with previous studies, that NGC 3169 and NGC 4102 are almost certainly AGNs. Most interestingly, a strong argument can be made that NGC 3184 and NGC 5457, both of type Scd, host AGNs.

Spatially Resolved X‐Ray Spectra of NGC 4258

We report a spatially resolved, X-ray spectral analysis of NGC 4258 using archival {\it Chandra} and {\it XMM-Newton} observations. The {\it XMM-Newton} spectra of the nuclear region are well described by two power-law components, a soft (0.57 keV) thermal component, and an Fe K$\alpha$ line with EW = 40 $\pm$ 33 eV. The properties of the second, weaker power-law component are similar to those of an off-nuclear source $2.5\arcsec$ SW of the nucleus. The spectrum of the extended emission of the entire galaxy is well described by two thermal components (MEKAL) models with temperatures $\simeq 0.60$ and 0.22 keV. The {\it Chandra} and {\it XMM-Newton} spectra along the anomalous arms show that the absorbing column density to the SE anomalous arm is consistent with absorption by gas in our Galaxy, while the absorbing column to the NW anomalous arm is higher, indicating that the NW arm is partially on the far side of the galactic disk. The combined {\it Chandra} data clearly detect the X-ray emission from the hot spots at the end of the approximately N-S radio jets. By assuming the hot spots represent shocked thermal gas at the ends of the jets, we estimate shock powers of $\simeq 3 \times 10^{39} f^{-1/2}$ \ergps ($f $ is the filling factor), similar to the radiative power in the inner anomalous arms, consistent with the notion that the jets could be responsible for heating the gas in the anomalous arms.

The Properties and Redshift Evolution of Intermediate-Luminosity Off-Nuclear X-Ray Sources in the Chandra Deep Fields

We analyze a population of intermediate-redshift (z ≈ 0.05-0.3) off-nuclear X-ray sources located within optically bright galaxies in the Great Observatories Origins Deep Survey and Galaxy Evolution from Morphology and SEDs fields. A total of 24 off-nuclear source candidates are classified using deep Chandra X-Ray Observatory exposures from the Chandra Deep Field-North, Chandra Deep Field-South, and Extended Chandra Deep Field-South; 15 of these are newly identified. These sources have average X-ray spectral shapes and optical environments similar to those observed for off-nuclear intermediate-luminosity (LX ≥ 1039 ergs s-1 in the 0.5-2.0 keV band) X-ray objects (IXOs; sometimes referred to as ultraluminous X-ray sources) in the local universe. This sample improves the available source statistics for intermediate-redshift off-nuclear sources with LX ≳ 1039.5 ergs s-1, and it places significant new constraints on the redshift evolution of the off-nuclear source frequency in field galaxies. The fraction of intermediate-redshift field galaxies containing an off-nuclear source with LX ≳ 1039 ergs s-1 is suggestively elevated (≈80% confidence level) with respect to that observed for IXOs in the local universe; we calculate this elevation to be a factor of ≈1.9. A rise in this fraction is plausibly expected as a consequence of the observed increase in global star formation density with redshift, and our results are consistent with the expected magnitude of the rise in this fraction.

A variable ultra-luminous X-ray source in the colliding galaxy NGC 7714

We studied the colliding galaxy NGC 7714 with two XMM-Newton observations, six months apart. The galaxy contains two bright X-ray sources: we show that they have different physical nature. The off-nuclear source is an accreting compact object, one of the brightest ultraluminous X-ray sources (ULXs) found to date. It showed spectral and luminosity changes between the two observations, from a low/soft to a high/hard state; in the high state, it reached erg s-1. Its lightcurve in the high state suggests variability on a ≈ h timescale. Its peculiar location, where the tidal bridge between NGC 7714 and NGC 7715 joins the outer stellar ring of NGC 7714, makes it an interesting example of the connection between gas flows in colliding galaxies and ULX formation. The nuclear source ( erg s-1) coincides with a starburst region, and is the combination of thin thermal plasma emission and a point-source contribution (with a power-law spectrum). Variability in the power-law component between the two observations hints at the presence of a single, bright point source ( erg s-1): either a hidden AGN or another ULX.

Lower Mass Black Holes in the Great Observatories Origins Deep Survey? Off-nuclear X-Ray Sources

We have identified a sample of 10 highly reliable off-nuclear (2 kpc) X-ray sources at z = 0.03-0.25 in late-type host galaxies within the two Great Observatories Origins Deep Survey (GOODS) fields (i.e., the two Chandra Deep Fields). The combination of the superb spatial resolution and great depth of the Hubble Space Telescope Advanced Camera for Surveys and Chandra ACIS coverage in the GOODS fields is critical to the identification of these sources. We extend the study of this enigmatic population up to higher redshifts and larger look-back times than has been possible before, finding that the fraction of optically luminous galaxies exhibiting luminous off-nuclear sources at z ≈ 0.1 is larger than at the current time (≈36% vs. ≈8%). The X-ray luminosities of the GOODS off-nuclear X-ray sources are comparable to or slightly greater than the integrated point-source X-ray luminosities of nearby galaxies, providing a possible constraint on the X-ray luminosity function at higher star formation rates. X-ray variability demonstrates that several of these sources are likely single accreting black holes; for the majority, however, the number of X-ray counts is too low for X-ray variability studies.

A [ITAL]CHANDRA[/ITAL][ITAL]Chandra[/ITAL] Study of the Circinus Galaxy Point-Source Population

We have used the Chandra X-Ray Observatory to resolve spatially and spectrally the X-ray emission from the Circinus galaxy. We report here on the nature of the X-ray emission from the off-nuclear point sources associated with the disk of Circinus, which make up ≈34% of the total 0.5–10 keV emission. We find that many of the serendipitous X-ray sources are concentrated along the optical disk of the galaxy, although few have optical counterparts within 1'' of their X-ray positions down to limiting magnitudes of mV = 23–25. At the distance of Circinus (≈3.8 Mpc), their intrinsic 0.5–10 keV luminosities range from ≈2 × 1037 ergs s-1 to ≈4 × 1039 ergs s-1. One-fourth of the sources are variable over the duration of the 67 ks observation, and spectral fitting of these off-nuclear sources shows a diverse range of spectral properties. The overall characteristics of the point sources suggest that most are X-ray binaries and/or ultraluminous supernova remnants within Circinus. We are able to analyze the two strongest off-nuclear sources in greater detail and find both to have remarkable properties. The average X-ray luminosities of the two sources are 3.7 × 1039 ergs s-1 and 3.4 × 1039 ergs s-1. The former displays large and periodic flux variations every 7.5 hr and is well fitted by a multicolor blackbody accretion disk model with Tin = 1.35 keV, properties consistent with an eclipsing ≳50 M⊙ black hole binary. The latter appears to be a young supernova remnant, as it coincides with a nonthermal radio counterpart and an Hα-detected H II region. This source exhibits both long-term (≈4 yr) X-ray variability and a 6.67–6.97 keV iron emission-line blend with a 1.6 keV equivalent width. These two objects further support the notion that super-Eddington X-ray sources in nearby galaxies can be explained by a mixture of intermediate-mass black holes in X-ray binaries and young supernova remnants.

New X-Ray Constraints on Starburst and Seyfert Activity in the Barred Spiral Galaxy NGC 1672

The nearby barred spiral galaxy NGC 1672 shows dramatic starburst activity and may also host a Seyfert 2 nucleus. We present new X-ray observations that set constraints on starburst and Seyfert activity in NGC 1672. Two ROSAT HRI exposures, taken in 1992 and 1997, are used to investigate long-term variability of the known X-ray sources and to search for new sources of X-ray emission. We find large-amplitude (≈69%) variability from X-3, one of the off-nuclear sources located near an end of the galactic bar. X-3 has a peak observed 0.2–2.0 keV luminosity of ≈2.5 × 1039 ergs s-1, and it is probably a luminous X-ray binary or young supernova remnant. We do not observe variability of the nuclear source X-1 or the strong off-nuclear source X-2. Our analyses also reveal two new off-nuclear sources, one of which is associated with a bright region along a spiral arm, and we find evidence for large-scale diffuse X-ray emission throughout part of the disk of NGC 1672. Furthermore, we use ASCA data taken in 1995 to constrain the hard X-ray properties of NGC 1672. While the nuclear source X-1 is the dominant soft X-ray source in NGC 1672, we find that the bulk of the 2–10 keV and 5–10 keV emission is spatially coincident with the off-nuclear source X-3, giving it an apparent 0.2–8 keV luminosity of 6 × 1039 ergs s-1. A power-law–plus–Raymond-Smith model provides an acceptable fit to the full-band ASCA spectra. We do not find any evidence for a luminous but absorbed nuclear X-ray source. If there is a luminous Seyfert 2 nucleus in NGC 1672, it must be obscured by a Compton-thick torus with a column density of 2 × 1024 cm-2.

Ionization cones and radio ejecta in active galaxies

We report radio mapping at three frequencies of the Seyfert 2 galaxy NGC 5252, which is known to exhibit a spectacular pair of 'ionization cones' in optical emission-line images. The radio structure of the galaxy comprises an unresolved (less than 50 pc) source coincident with the optical nucleus, weak, narrow features extending approximately equal to 900 pc to north and south from the nucleus, and an unresolved radio source some 10 kpc from the nucleus. The inner parts of the extended radio structure and the off-nuclear source align well with the axis of the ionization cones. There are currently 11 Seyfert galaxies known to possess an ionization cone or a bi-cone; 8 of these galaxies also contain a linear (double, triple, or jet-like) nuclear radio structure. For this limited, incomplete sample, there is a tight alignment between cone and radi axes: the formal mean difference between the measured projections of these axes on the sky is only 6 deg, and the alignment may well be better than this at the location(s) closer to the nucleus where the collimation occurs. Although the degree of collimation is much worse for the ionizing photons than for the radio plasma, it is clear that they are collimated by the same, or coplanar, nulcear disks or tori. In particular, if the ionization cones result from absorption by dusty tori on the pc scale and the radio ejecta from accretion disks around the central black hole, the absence of differential precession indicates that either the gravitating mass distribution is close to spherical or the dusty torus has settled into a preferred plane. The cones currently known in late-type (but not early-type) spirals show a trend to align with the axis of the galaxy stellar disk. We argue that this alignment is either an observational selection effect or indicates that the gas accreted to power the nuclear activity has an internal origin in late-type spirals, but may have an external origin (e.g., a galaxy merger) in early-types. .