Extragalactic H_2O masers and X-ray absorbing column densities

Having conducted a search for the 22 GHz water vapor line towards galaxies with nuclear activity, large nuclear column densities or high infrared luminosities, we present H_2O spectra for NGC2273, UGC5101 and NGC3393 with isotropic luminosities of 7, 1500, and 400 L_sun. The H_2O maser in UGC5101 is by far the most luminous yet found in an ultraluminous infrared galaxy. NGC3393 reveals the classic spectrum of a `disk maser', represented by three distinct groups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below 1000 km/s. Based on the literature and archive data, X-ray absorbing column densities are compiled for the 64 galaxies with reported maser sources beyond the Magellanic Clouds. For NGC2782 and NGC5728, we present Chandra archive data that indicate the presence of an active galactic nucleus in both galaxies. The correlation between absorbing column and H_2O emission is analyzed. There is a striking difference between kilo- and megamasers with megamasers being associated with higher column densities. All kilomasers (L_H_2O < 10 L_sun) except NGC2273 and NGC5194 are Compton-thin, i.e. their absorbing columns are < 10^24 cm^-2. Among the H_2O megamasers, 50% arise from Compton-thick and 85% from heavily obscured (> 10^23 cm^-2) active galactic nuclei. These values are not larger but consistent with those from samples of Seyfert 2 galaxies not selected on the basis of maser emission. The similarity in column densities can be explained by small deviations in position between maser spots and nuclear X-ray source and a high degree of clumpiness in the circumnuclear interstellar medium.Comment: 16 pages, 7 figures, 5 tables. Accepted for publication in A&

XMM-Newton view of galaxy pairs: activation of quiescent black holes?

We report on XMM-Newton observations of three nearby galaxy pairs, AM0707-273, AM1211-465, and AM2040-674. All six galaxies were previously classified as HII galaxies based on optical and IR spectroscopic analysis. All galaxies were detected with XMM-Newton and each member was isolated and analyzed independently. The X-ray spectra reveal strong evidence of AGN activity in the NE member of AM1211-465 pair. We measured a luminosity of 1.94(+0.11/-0.15)x10^42 erg/s in the 2-10 keV band and the presence of a neutral FeK_alpha line with a confidence level of 98.8%. The high nH value, 2.2+/-0.2x10^22 cm^-2, would explain the misclassification of the source. Marginal evidence of AGN nature was found in the X-ray spectra of AM1211-465SW and AM0707-273E. The X-ray emission of the three remaining galaxies can be explained by starburst activity.Comment: Accepted for publication in A&

The composite starburst/AGN nature of the superwind galaxy NGC 4666

We report the discovery of a Compton-thick AGN and of intense star-formation activity in the nucleus and disk, respectively, of the nearly edge-on superwind galaxy NGC 4666. Spatially unresolved emission is detected by BeppoSAX only at energies <10 keV, whereas spatially resolved emission from the whole disk is detected by XMM-Newton. A prominent (EW ~ 1-2 keV) emission line at ~6.4 keV is detected by both instruments. From the XMM-Newton data alone the line is spectrally localized at E ~ 6.42 +/- 0.03 keV, and seems to be spatially concentrated in the nuclear region of NGC 4666. This, together with the presence of a flat (Gamma ~ 1.3) continuum in the nuclear region, suggests the existence of a strongly absorbed (i.e., Compton-thick) AGN, whose intrinsic 2-10 keV luminosity is estimated to be L_{2-10} > 2 x 10^{41} erg/s. At energies <1 keV the integrated (BeppoSAX) spectrum is dominated by a ~0.25 keV thermal gas component distributed throughout the disk (resolved by XMM-Newton). At energies ~2-10 keV, the integrated spectrum is dominated by a steep (G > 2) power-law (PL) component. The latter emission is likely due to unresolved sources with luminosity L ~ 10^{38} - 10^{39} erg/s that are most likely accreting binaries (with BH masses <8 M_sun). Such binaries, which are known to dominate the X-ray point-source luminosity in nearby star-forming galaxies, have Gamma ~ 2 PL spectra in the relevant energy range. A Gamma ~ 1.8 PL contribution from Compton scattering of (the radio-emitting) relativistic electrons by the ambient FIR photons may add a truly diffuse component to the 2-10 keV emission.Comment: A&A, in press (10 pages, 14 figures.) Full gzipped psfile obtainable from http://www.bo.iasf.cnr.it/~malaguti/r_stuff.htm

XMM-Newton unveils the complex iron K alpha region of Mrk 279

We present the results of a ~160 ks-long XMM-Newton observation of the Seyfert 1 galaxy Mrk 279. The spectrum shows evidence of both broad and narrow emission features. The Fe K alpha line may be equally well explained by a single broad Gaussian (FWHM~10,000 km/s) or by two components: an unresolved core plus a very broad profile (FWHM~14,000 km/s). For the first time we quantified, via the "locally optimally emitting cloud" model, the contribution of the broad line region (BLR) to the absolute luminosity of the broad component of the Fe K alpha at 6.4 keV. We find that the contribution of the BLR is only ~3%. In the two-line component scenario, we also evaluated the contribution of the highly ionized gas component, which produces the FeXXVI line in the iron K region. This contribution to the narrow core of the Fe K alpha line is marginal <0.1%. Most of the luminosity of the unresolved, component of Fe K alpha may come from the obscuring torus, while the very-broad associated component may come from the accretion disk. However, models of reflection by cold gas are difficult to test because of the limited energy band. The FeXXVI line at 6.9 keV is consistent to be produced in a high column density (N_H~10^23 cm^{-2}), extremely ionized (log\xi~5.5-7) gas. This gas may be a highly ionized outer layer of the torus.Comment: 10 pages, 9 figures, accepted for publication in Astronomy and Astrophysic

On the nature of unabsorbed Seyfert 2 galaxies

We present an analysis of six 12 um selected Seyfert 2 galaxies that have been reported to be unabsorbed in the X-ray. By comparing the luminosities of these galaxies in the mid-IR (12um), optical ([O III]) and hard X-ray (2-10 keV), we show that they are all under-luminous in the 2-10 keV X-ray band. Four of the objects exhibit X-ray spectra indicative of a hard excess, consistent with a heavily obscured X-ray component and hence a hidden nucleus. In these objects the softer X-rays may be dominated by a strong soft scattered continuum or contamination from the host galaxy, which is responsible for the unabsorbed X-ray spectra observed, and accounts for the anomalously low 2-10 keV X-ray luminosity. We confirm this assertion in NGC4501 with a Chandra observation, which shows hard X-ray emission coincident with the nucleus, consistent with heavy absorption, and a number of contaminating softer sources which account for the bulk of the softer emission. We point out that such 'Compton thick' sources need not necessarily present iron Ka emission of high EW. An example in our sample is IRASF01475-0740, which we know must host an obscured AGN as it hosts a HBLR seen in scattered light (Tran 2003). The X-ray spectrum is nonetheless relatively unobscured and the iron Ka line only moderate in strength. These observations can be reconciled if the hidden nuclear emission is dominated by transmitted, rather than reflected X-rays, which can then be weak compared to the soft scattered light or galactic emission. Despite these considerations, we conclude that NGC3147 & 3660 may intrinsically lack a BLR, confirming the recent results of Bianchi et al. (2008) for NGC3147. Neither X-ray spectrum shows signs of hidden hard emission and both sources exhibit X-ray variability leading us to believe we are viewing the nucleus directly.Comment: 10 pages, 5 figures, accepted for publication at MNRA

BeppoSAX observations of LINER-2 galaxies

We present BeppoSAX observations of 6 ``type-2'' LINER and ``transition'' galaxies (NGC3379, NGC3627, NGC4125, NGC4374, NGC5195 and NGC5879)from the Ho et al. (1997) spectroscopic sample of nearby galaxies. All objects are detected in the 2-10 keV band, having luminosities in the range L(2-10 keV) ~ 1x10^{39}- 1x10^{40} erg s-1. The PDS upper limits above 10 keV place constraints on the presence of a heavily obscured AGN in the case of NGC3379 and NGC4125. No significant variability is detected in any of the objects. The spectra are described in most cases by a simple power-law model with a spectral slope of 1.7-2.5 while there is evidence neither for a significant absorption above the Galactic nor for an FeK emission line. Therefore, based on the spectral properties alone, it is difficult to differentiate between a low-luminosity AGN or a star-forming galaxy scenario. However, imaging observations of NGC3627 and NGC5195 with Chandra ACIS-S reveal very weak nuclear sources while most of the X-ray flux originates either in off-nuclear point sources or in diffuse emission. The above clearly argue in favour of a star-forming origin for the bulk of the X-ray emission, at least in the above two sources.Comment: 10 pages, to appear in A&

The X-ray emission of local luminous infrared galaxies

We study the X-ray emission of a representative sample of 27 local luminous infrared galaxies (LIRGs). The median IR luminosity of our sample is log L_IR/L_sun = 11.2, thus the low-luminosity end of the LIRG class is well represented. We used new XMM-Newton data as well as Chandra and XMM-Newton archive data. The soft X-ray (0.5-2 keV) emission of most of the galaxies (>80%), including LIRGs hosting a Seyfert 2 nucleus, is dominated by star-formation related processes. These LIRGs follow the star-formation rate (SFR) versus soft X-ray luminosity correlation observed in local starbursts. We find that ~15% of the non-Seyfert LIRGs (3 out of 20) have an excess hard X-ray emission relative to that expected from star-formation that might indicate the presence of an obscured AGN. The rest of the non-Seyfert LIRGs follow the SFR versus hard X-ray (2-10 keV) luminosity correlation of local starbursts. The non-detection of the 6.4 keV Fe K alpha emission line in the non-Seyfert LIRGs allows us to put an upper limit to the bolometric luminosity of an obscured AGN, L_bol <1043 erg s-1 . That is, in these galaxies, if they hosted a low luminosity AGN, its contribution to total luminosity would be less than 10%. Finally we estimate that the AGN contribution to the total luminosity for our sample of local LIRGs is between 7% and 10%.Comment: Accepted for Publication in A&A, 22 pages, 9 figure

Megamaser detection and nuclear obscuration in Seyfert galaxies

We revisit the relation between H2O maser detection rate and nuclear obscuration for a sample of 114 Seyfert galaxies, drawn from the CfA, 12um and IRAS F25/F60 catalogs. These sources have mid-infrared spectra from the Spitzer Space Telescope and they are searched for X-ray and [O III], 5007Angstrom fluxes from the literature. We use the strength of the [O IV], 25.9um emission line as tracer for the intrinsic AGN strength. After normalization by [O IV] the observed X-ray flux provides information about X-ray absorption. The distribution of X-ray / [O IV] flux ratios is significantly different for masers and non-masers: The maser detected Seyfert-2s (Sy 1.8-2.0) populate a distinct X-ray / [O IV] range which is, on average, about a factor four lower than the range of Seyfert-2 non-masers and about a factor of ten lower than the range of Seyfert-1s (Sy 1.0-1.5). Non-masers are almost equally distributed over the entire X-ray / [O IV] range. This provides evidence that high nuclear obscuration plays a crucial role for the probability of maser detection. Furthermore, after normalization with [O IV], we find a similar but weaker trend for the distribution of the maser detection rate with the absorption of the 7um dust continuum. This suggests that the obscuration of the 7 um continuum occurs on larger spatial scales than that of the X-rays. Hence, in the AGN unified model, at moderate deviation from edge-on, the 7um dust absorption may occur without proportionate X-ray absorption. The absorption of [O III] appears unrelated to maser detections. The failure to detect masers in obscured AGN is most likely due to insufficient observational sensitivity