Astron. Astrophys. 360, 447-456 (2000)

4. Discussion

From the quality of the spectral fits alone there is no significance for favoring a single-component model or a combination of two components. Due to the lack of any spatial information in the PSPC image there is no possibility to distinguish between different spatial and spectral components simultaneously. So only the combined information from the PSPC and HRI data allows a more detailed interpretation of the X-ray results.

Several points speak against the single PO component model, as discussed in Sect. 3.2. A more likely scenario is a composition of several different emission sources, like an active nucleus, HMXBs, and supernova remnants (SNRs). In the following we will therefore discuss a composite emission model and the comparison with the UV and optical observations of the galactic core.

4.1. The nucleus of NGC 4303

As can be discerned from the HRI image (Fig. 3 and Table 4), most of the X-ray emission of NGC 4303 (83%) comes from the central region of the galaxy. Three different pictures are imaginable for the nucleus: a central active nucleus, a central or circumnuclear region with enhanced star formation, or a combination of both phenomena. Any of these cases requires a sufficient gas density at the galactic center. This can be achieved by a barred potential which triggers radial gas flow from the outer regions toward the nucleus. On the other hand, from numerical simulations including gas dynamics bar formation has proved to be only a transient phenomenon (Combes 2000). In this picture, the galactic bar will be destroyed by the gas inflow after only a few cycles. A new bar-phase can follow this gas infall due to a subsequent gravitational instability from the accreted central mass. The problem with this picture is the contradiction of a necessary gas inflow to form and feed any nuclear activity (starburst and/or AGN) and the fact that this gas inflow destroys the bar. It seems that a sufficiently massive black hole can provide for its fuelling (Fukuda et al. 1998). Another efficient way for gas to flow further into the center is a second smaller bar embedded into the first one due to a second inner Lindblad resonance (Friedli & Martinet 1993). In some cases, a gaseous circumnuclear ring is formed at the end of the second bar.

The concentrated X-ray emission from the galactic core in NGC 4303 may originate from an AGN and/or a nuclear or circumnuclear starburst. A starburst can contribute in two different ways to the X-ray flux. First, the produced star population contains HMXBs, emitting an X-ray radiation in spectral shape similar to an AGN. HMXBs cannot be distinguished from AGN in the ROSAT data. Additionally, high-mass stars (above 8-10 ) evolve to SNe  II at an age of 10⁷ yr, depending on their initial mass. The SNe  II from one star cluster form a cumulative expanding superbubble filled with hot gas which can be described by a thermal Bremsstrahlung spectrum and additional emission lines and recombination edges of highly ionized heavy elements produced in high-mass stars and released by their SN  II explosions, e.g. O, Ne, Mg, Si, and Fe. Theoretical models for the spectral emission of such a hot diffuse gas are MEKA (Mewe et al. 1985) and the model by Raymond & Smith (1977), which we used in our spectral fit.

If we apply a two-component model to describe the X-ray spectrum of NGC 4303, the comparison of the flux ratio from the nucleus (83% in the HRI) and the disk sources (17% in the HRI) suggests that the central source can be described by the power-law component (87% in the spectral fit). The thermal RS emission exclusively originates from the disk sources, indicating ongoing star formation. To consider the possible extension of 25" of the central source, as represented by the lowest contours, it is imaginable that a small fraction of the X-ray flux is emitted by a circumnuclear starburst at a distance of 1 kpc around the core. This would add a thermal component to the non-thermal X-ray nucleus. On the other hand, a fraction of the X-ray flux from the disk sources may come from HMXBs within these star forming regions.

Fuelling an AGN on scales of a few parsec at the center of the galaxy leads to the problem of reducing the angular momentum of the central gas by several orders of magnitude, as dynamical simulations show (Barnes & Hernquist 1991). Concentration of gas in a ring-like feature around the nucleus with a radius of 1 kpc is dynamically much easier to achieve. The HRI image agrees with the picture of an extended X-ray source with a diameter of the order of 2 kpc at the galactic center of NGC 4303, explained by a circumnuclear starburst region with an additional possible compact nuclear source.The decovered massive rotating circumnuclear disk in NGC 4303 can provide by its spiral-like structure of massive star forming regions an effective mechanism to channel gas from the circumnuclear regions further down to the nucleus to feed the AGN. But one has to consider that the spiral structure in the UV has a diamater of only 225 pc, while the extension of the central X-ray source is about 2 kpc in diameter. The spiral feature detected in the UV cannot be resolved with the HRI.

From the analysis of UV and optical magnitudes and colors of the central 250 pc Colina & Wada (2000) estimated ages of 5-25 Myr for the star-forming regions. Consequently a contribution to the central X-ray flux from SNRs and cumulatively expanding hot gas has to be expected. The question remains whether we observe a pure nucleus of massive star-forming clusters or a composition of these star clusters and a low luminous AGN. If NGC 4303 contains a non-thermal active nucleus, the X-ray luminosity of 410 erg s^-1 points to only a low luminous AGN (LINER). Koratkar et al. (1995) found a correlation between and for low luminous AGNs of / 14. Pérez-Olea & Colina (1996) investigated the correlation between optical and X-ray luminosities of several AGNs with circumnuclear star-forming rings, pure AGNs, and pure starburst galaxies. The pure starbursts in their galaxy sample show / values of 0.03-0.3, 100 times smaller than for pure AGNs. If we take the H luminosity of NGC 4303 derived by Keel (1983) and assume that 10% originate from the nucleus, we get log (nucleus) = 39.2 (adopted for a distance of 16.1 Mpc). Therefore the X-ray-to-H ratio amounts to log(/) = 1.4, which agrees with the value found by Koratkar et al. (1995). Even the lower value from a single RS model (2.510 erg s^-1 for the nucleus) results in log(/) = 1.2. Typical pure starburst galaxies show H luminosities of the order of their X-ray luminosities or higher.

4.2. The galactic disk

At first glance the optical disk of NGC 4303 seems to have the quite symmetrical morphology of a late-type spiral. A closer look reveals that the eastern spiral arm of the galaxy has a much more prominent form with a boomerang-like shape and a lot more bright emission regions than the western counterpart. The northern disk shows a complex structure with many separate features. This asymmetry is more discernible in the H image. The H  II regions are mainly located in the northern part of the disk at the junction of the bar with the eastern spiral arm and along that arm. A close encounter of one or both of the nearby galaxies NGC 4303 A and NGC 4292 may have caused these features. The interaction within the Virgo Cluster is another possible source. Infall into the intracluster medium could cause ram pressure effects. Nevertheless, NGC 4303 is located at the outer edge of the cluster, which may produce only a moderate disturbance. This agrees with the H  I distribution over the whole optical disk. Galaxies lying nearer to the cluster center show H  I deficiencies and concentration of the neutral hydrogen in the central regions, indicating past interactions with the H  I gas been stripped off from the outer disk regions.

As a striking indication for accumulation of gas in these regions, the X-ray sources A-C and F within the galactic disk of NGC 4303 are located at the ends of the bar. Gas dynamical simulations of barred galaxies have shown this accumulation due to mass flows along the bar to the center and to the ends of the bar, respectively (Noguchi 1988; Englmaier & Gerhard 1997). The increased densities lead to enhanced star formation.

From the low inclination of NGC 4303 no direct information can be obtained whether the disk is warped or not. But it is striking that source D lies exactly at the bend of the eastern boomerang-shaped arm. This may indicate that this X-ray source is caused by the tidal force leading to a local gas concentration. Another indirect hint for a past interaction comes from the spectra of the QSO 1219+047 (source no. 7 in Fig. 1), a QSO whose line-of-sight penetrates the outer H  I disk of NGC 4303. Bowen et al. (1996) detected complex Mg II absorption, spanning a velocity range of 300 km s^-1, despite the low inclined galactic disk. This high velocity is not fully understood. One possible explanation could be the result of interactions between NGC 4303 and the nearby companions.

4.3. Star formation in the disk

Table 4 lists the observed count rates and derived 0.1-2.4 keV luminosities for the single X-ray sources in the disk of NGC 4303. These include the assumptions of a Raymond-Smith plasma with solar abundances at a temperature of 0.3 keV. A power-law model would increase the values by a factor of 2.5. A rough estimation of the SFR in the disk from the X-ray luminosities is done by calculating the SN  II rate using a SNR model by Cioffi (1990), and assuming a Salpeter IMF within a mass interval from 0.1 to 100 and with all stars with masses above 8 evolving to SNe  II . According to Cioffi, a SNR expanding into an ISM with a density of 1 cm^-3 radiates a total energy of 4.710 erg in the soft X-ray regime above 0.1 keV for a time of 10⁴ yr. Norman & Ikeuchi (1989) investigated the cumulative effect of a number of SNRs. The total SFR in the disk of NGC 4303 from the X-ray luminosity amounts to 0.5  yr^-1. This however is just a very simple estimation, containing several simplifications, as e.g. the sum of the single SN model from Cioffi (1990) for several cumulatively expanding SNRs in an evolving OB assoziation, or the derivation of the total disk SFR from single X-ray sources. A more detailed determiation of the SFR, for example using an analytic suberbubble model by Suchkov et a. (1994), would need information about the extensions and expansion time of the superbubbles, in order to determine the mechanical energy release by the SNe and, by this, the SN rate. This can be compared with the observed X-ray luminosity.

Besides the mentioned restrictions the difference between the SFR estimated from the X-ray flux and the SFR derived by the H flux by Kennicutt (1983) (14  yr^-1) may be due to several reasons. Kennicutt used a Miller-Scalo IMF which increases the SFR by a factor of about 1.5. The total H luminosity underlies a distance determination with a Hubble constant of 50 km s^-1 Mpc^-1. Taking a radial velocity of 1569 km s^-1 (de Vaucouleurs et al. 1991) leads to a 3.8 times higher luminosity than taking the distance of 16.1 Mpc which we used. Additionally, the fact that not all H  II regions may emit an adequate X-ray flux or are not strong enough to be detected lowers the estimated SFR from the X-ray luminosity which implies the existence of high-mass stars having been evolved to SNe  II . Possible X-ray emission from diffuse hot gas within the disk may lie below the detection limit of 1.110 cts s^-1 arcsec^-2 (5 above background level). A very faint component located at the spiral arms can be seen in outlines in Fig. 3, but is not detected at a 3 level. This limit corresponds to an X-ray flux of 4.610 erg s^-1 cm^-2 arcsec^-2 (ECF for a RS model as in Table 4). Kennicutt (1983)also admitted to treat the derived H flux and resulting SFR with extreme caution because of only moderate accuracy due to possibly strong extinction effects. The H flux derived by Keel (1983) is by a factor of 30 lower than the one derived by Kennicutt, after adopting the same distance.

Strikingly, the sources B and F both coincide with some of the most H luminous H  II regions (Sources no.s 27 and 69 with log =-12.12 and log =-11.99, respectively, in MR92). Depending on the fraction of the central X-ray flux steming from SNRs and superbubbles or from an AGN component, the SFR for the core is of the order of 1  yr^-1.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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