اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدRadio structures of the nuclei of nearby Seyfert galaxies and the nature of the missing diffuse emission
368
0
0.0
(
0
)
تأليف
M. Orienti
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present archival high spatial resolution VLA and VLBA data of the nuclei of seven of the nearest and brightest Seyfert galaxies in the Southern Hemisphere. At VLA resolution (~0.1 arcsec), the nucleus of the Seyfert galaxies is unresolved, with the exception of MCG-5-23-16 and NGC 7469 showing a core-jet structure. Three Seyfert nuclei are surrounded by diffuse radio emission related to star-forming regions. VLBA observations with parsec-scale resolution pointed out that in MRK 1239 the nucleus is clearly resolved into two components separated by ~30 pc, while the nucleus of NGC 3783 is unresolved. Further comparison between VLA and VLBA data of these two sources shows that the flux density at parsec scales is only 20% of that measured by the VLA. This suggests that the radio emission is not concentrated in a single central component, as in elliptical radio galaxies, and an additional low-surface brightness component must be present. A comparison of Seyfert nuclei with different radio spectra points out that the ``presence of undetected flux on milli-arcsecond scale is common in steep-spectrum objects, while in flat-spectrum objects essentially all the radio emission is recovered. In the steep-spectrum objects, the nature of this ``missing flux is likely due to non-thermal AGN-related radiation, perhaps from a jet that gets disrupted in Seyfert galaxies because of the denser environment of their spiral hosts.
قيم البحث
اقرأ أيضاً
We present 18-cm radio maps of four Seyfert nuclei, Mrk 1, Mrk 3, Mrk 231 and Mrk 463E, made with the European VLBI Network (EVN). Linear radio structures are present in three out of four sources on scales of ~100 pc to ~1 kpc, and the 20-mas beam of
the EVN enables us to resolve details within the radio structures on scales of <10 pc. Mrk 3 was also imaged using MERLIN and the data combined with the EVN data to improve the sensitivity to extended emission. We find an unresolved flat-spectrum core in Mrk 3, which we identify with the hidden Seyfert 1 nucleus in this object, and we also see marked differences between the two highly-collimated radio jets emanating from the core. The western jet terminates in a bright hotspot and resembles an FRII radio structure, whilst the eastern jet has more in common with an FRI source. In Mrk 463E, we use the radio and optical structure of the source to argue that the true nucleus lies approximately 1 arcsec south of the position of the radio and optical brightness peaks, which probably represent a hotspot at the working surface of a radio jet. The EVN data also provide new evidence for a 100-pc radio jet powering the radio source in the Type 1 nucleus of Mrk 231. However, the Seyfert 2 galaxy Mrk 1 shows no evidence for radio jets down to the limits of resolution (~10 pc). We discuss the range of radio source size and morphology which can occur in the nuclei of Seyfert galaxies and the implications for Seyfert unification schemes and for radio surveys of large samples of objects.
An important aspect of solving the long-standing question as to what triggers various types of Active Galactic Nuclei involves a thorough understanding of the overall properties and formation history of their host galaxies. This is the second in a se
ries of papers that systematically study the large-scale properties of cold neutral hydrogen (HI) gas in nearby radio galaxies. The main goal is to investigate the importance of gas-rich galaxy mergers and interactions among radio-loud AGN. In this paper we present results of a complete sample of classical low-power radio galaxies. We find that extended Fanaroff & Riley type-I radio sources are generally not associated with gas-rich galaxy mergers or ongoing violent interactions, but occur in early-type galaxies without large (> 10^8 M_sun) amounts of extended neutral hydrogen gas. In contrast, enormous discs/rings of HI gas (with sizes up to 190 kpc and masses up to 2 x 10^10 M_sun) are detected around the host galaxies of a significant fraction of the compact radio sources in our sample. This segregation in HI mass with radio source size likely indicates that these compact radio sources are either confined by large amounts of gas in the central region, or that their fuelling is inefficient and different from the fuelling process of classical FR-I radio sources. To first order, the overall HI properties of our complete sample (detection rate, mass and morphology) appear similar to those of radio-quiet early-type galaxies. If confirmed by better statistics, this would imply that low-power radio-AGN activity may be a short and recurrent phase that occurs at some point during the lifetime of many early-type galaxies.
We discuss the dense molecular gas in central regions of nearby Seyfert galaxies, and report new arcsec resolution observations of HCN(1-0) and HCO$^+$(1-0) for 3 objects. In NGC 3079 the lines show complex profiles as a result of self-absorption and
saturated continuum absorption. H$^{13}$CN reveals the continuum absorption profile, with a peak close to the galaxys systemic velocity that traces disk rotation, and a second feature with a blue wing extending to $-350$km s$^{-1}$ that most likely traces a nuclear outflow. The morphological and spectral properties of the emission lines allow us to constrain the dense gas dynamics. We combine our kinematic analysis for these 3 objects, as well as another with archival data, with a previous comparable analysis of 4 other objects, to create a sample of 8 Seyferts. In 7 of these, the emission line kinematics imply thick disk structures on radial scales of $sim$100pc, suggesting such structures are a common occurrence. We find a relation between the circumnuclear LHCN and Mdyn that can be explained by a gas fraction of 10% and a conversion factor {alpha}HCN $sim$ 10 between gas mass and HCN luminosity. Finally, adopting a different perspective to probe the physical properties of the gas around AGN, we report on an analysis of molecular line ratios which indicates that the clouds in this region are not self-gravitating.
In this work, we investigate the contribution of dust scattering to the diffuse H-alpha emission observed in nearby galaxies. As initial conditions for the spatial distribution of HII regions, gas, and dust, we take three Milky Way-like galaxies from
state-of-the-art cosmological hydrodynamical simulations that implement different prescriptions for star formation, feedback, and chemical enrichment. Radiative transfer has been solved a posteriori, using the publicly-available Monte Carlo code Sunrise to take into account dust absorption and scattering of the H-alpha photons, originating exclusively from the HII regions. No contribution from recombinations in the diffuse ionized gas (DIG) component is explicitly or implicitly included in our model. Our main result is that the flux arising from scattered light is of the order of 1-2 per cent of the H-alpha flux coming directly from the HII regions. Building upon previous studies, we conclude that the DIG contributes lass than 50 per cent of the total H-alpha emission.
We present detailed image analysis of rest-frame optical images of 235 low-redshift ($z leq$ 0.35) type 1 active galactic nuclei (AGNs) observed with the Hubble Space Telescope. The high-resolution images enable us to perform rigorous two-dimensional
image modeling to decouple the luminous central point source from the host galaxy, which, when warranted, is further decomposed into its principal structural components (bulge, bar, and disk). In many cases, care must be taken to account for structural complexities such as spiral arms, tidal features, and overlapping or interacting companion galaxies. We employ Fourier modes to characterize the degree of asymmetry of the light distribution of the stars, as a quantitative measure of morphological distortion due to interactions or mergers. We examine the dependence of the physical parameters of the host galaxies on the properties of the AGNs, namely radio-loudness and the width of the broad emission lines. In accordance with previous studies, narrow-line (H$beta$ FWHM $leq 2000$ km~s$^{-1}$) type 1 AGNs, in contrast to their broad-line (H$beta$ FWHM $> 2000$ km~s$^{-1}$) counterparts, are preferentially hosted in later type, lower luminosity galaxies, which have a higher incidence of pseudo-bulges, are more frequently barred, and are less morphologically disturbed. This suggests narrow-line type 1 AGNs experienced a more quiescent evolutionary history driven primarily by internal secular evolution instead of external dynamical perturbations. The fraction of AGN hosts showing merger signatures is larger for more luminous sources. Radio-loud AGNs generally preferentially live in earlier type (bulge-dominated), more massive hosts, although a minority of them appears to contain a significant disk component. We do not find convincing evidence for enhanced merger signatures in the radio-loud population.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
