Nuclear activity and massive star formation in the low luminosity AGN NGC4303: Chandra X-ray observations

We present evidence of the co-existence of either an AGN or an ultraluminous X-ray source (ULX), together with a young super stellar cluster in the 3 central parsecs of NGC4303. The galaxy contains a low luminosity AGN and hosts a number of starburst regions in a circumnuclear spiral, as well as in the nucleus itself. A high spatial resolution Chandra image of this source reveals that the soft X-ray emission traces the ultraviolet nuclear spiral down to a core, which is unresolved both in soft and hard X-rays. The astrometry of the X-ray core coincides with the UV core within the Chandra positioning accuracy. The total X-ray luminosity of the core, 1.5*10^{39} erg/s, is similar to that from some LINERs or from the weakest Seyferts detected so far. The soft X-rays in both the core and the extended structure surrounding it can be well reproduced by evolutionary synthesis models (which include the emission expected from single stars, the hot diffuse gas, supernova remnants and binary systems), consistent with the properties of the young stellar clusters identified in the UV. The hard X-ray tail detected in the core spectrum, however, most likely requires the presence of an additional source. This additional source could either be a weak active nucleus black hole or an ultraluminous X-ray object. The implications of these results are discussed.Comment: 37 pages, 7 figures, ApJ accepte

Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/5: Tidal ULXs, Emergent Wind, and Resolved HII Regions

We present Chandra X-ray images for the interacting galaxy pair NGC 7714/5. In addition to the unresolved starburst nucleus, a variable point source with L(X) ~ 10^40 erg/s was detected 1.5" (270 pc) northwest of the nucleus, coincident with a blue, extremely optically-luminous (M(V) ~ -14.1) point source on HST images. Eleven more candidate point-like ultraluminous X-ray sources (ULXs) are seen, two >= 10^40 erg/s. Ten of these are associated with interaction-induced features but only two with star formation. We found diffuse emission with L(X) ~ 3 X 10^40 erg/s extending 11" (1.9 kpc) to the north of the nucleus. Its spectrum can be fit with a 2-temperature Mekal function (0.6/8 keV) or a 0.6 keV Mekal function plus a power law. The hard component may be due to high mass X-ray binaries (HMXBs) with contributions from inverse Compton radiation, while the soft component is likely from a superwind. We also detected extended X-ray emission from four extra-nuclear HII region complexes. This emission may be due to HMXBs or to diffuse gas heated by winds from supernovae, if the X-ray production efficiency L(X)/L(mech) is high (~5%). To estimate L(X)/L(mech), we collected published data for well-studied HII regions and superbubbles in nearby galaxies. For young HII regions (<3.5 Myrs), the median L(X)/L(mech) ~ 0.02%, while for older regions, L(X)/L(mech) ~ 0.2-7%. Thus gas heating by supernovae may be sufficient to account for the HII region emission. In galaxies much more distant than NGC 7714, for example, the Cartwheel galaxy, HII region complexes similar to those in NGC 7714 will be unresolved by Chandra and will mimick ULXs.Comment: Accepted by the Astronomical Journal. Figures also available at http://www.etsu.edu/physics/bsmith/research/n7714_chandra.htm

An X-ray halo in the "hot-spot" galaxy NGC 2903

In this paper we present ROSAT PSPC and HRI observations of the "hot-spot" galaxy NGC 2903. This isolated system strikingly reveals a soft extended X-ray feature reaching in north-west direction up to a projected distance of 5.2 kpc from the center into the halo. The residual X-ray emission in the disk reveals the same extension as the Ha disk. No eastern counterpart of the western X-ray halo emission has been detected. The luminosity of the extraplanar X-ray gas is several 10(38) erg s(-1), comparable to X-ray halos in other starburst galaxies. It has a plasma temperature of about 0.2 keV. The estimated star formation rate derived from X-rays and Halpha results in 1-2 M-circle dot yr(-1). Since galactic superwinds, giant kpc-scale galactic outflows, seem to be a common phenomenon observed in a number of edge-on galaxies, especially in the X-ray regime, and are produced by excess star-formation activity, the existence of hot halo gas as found in NGC 2903 can be attributed to events such as central starbursts. That such a starburst has taken place in NGC 2903 must be proven. The detection of hot gas above galaxy disks also with intermediate inclination, however, encounters the difficulty of discriminating between that contribution from disk and active nuclear region