Line Galaxies Research Articles

The Two‐sided Parsec‐Scale Structure of the Low‐Luminosity Active Galactic Nucleus in NGC 4278

We present new Very Long Baseline Interferometry observations of the LINER galaxy NGC 4278. The observations were taken with the Very Long Baseline Array (VLBA) and a single antenna of the Very Large Array (VLA) at 5 GHz and 8.4 GHz and have a linear resolution of <0.1 pc. Our radio data reveal a two sided structure, with symmetric S-shaped jets emerging from a flat spectrum core. We fit the jet brightness with gaussian components, which we identify from a previous observation taken five years before. By comparing the positions of the components in the two epochs, we measure motions between 0.45 +/- 0.14 and 3.76 +/- 0.65 mas, corresponding to apparent velocities < 0.2c, and to ages in the range 8.3 - 65.8 years. Assuming that the radio morphology is intrinsically symmetric and its appearance is governed by Doppler beaming effects, we find that NGC4278 has mildly relativistic jets (beta ~ 0.75), closely aligned to the line-of-sight (2 degrees < theta < 4 degrees). Alternatively, the source could be oriented at a larger angle and asymmetries could be related to the jet interaction with the surrounding medium. We also present new simultaneous VLA observations between 1.4 and 43 GHz, and a 5 GHz light curve between 1972 and 2003. The radio spectrum can be fit by a relatively steep power-law (alpha = 0.54). We find significant variability at 5 GHz. All these arguments indicate that the radiation from NGC 4278 is emitted via the synchrotron process by relativistic particles accelerated by a supermassive black hole. Despite a much lower power, this is the same process that takes place in ordinary radio loud AGNs.

The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties

In a recently completed survey of the stellar population properties of low-ionization nuclear emission-line regions (LINERs) and LINER/H II transition objects (TOs), we have identified a numerous class of galactic nuclei which stand out because of their conspicuous 10 8-9 yr populations, traced by high-order Balmer absorption lines and other stellar indices. These objects are called 'young-TOs', because they all have TO-like emission-line ratios. In this paper we extend this previous work, which concentrated on the nuclear properties, by investigating the radial variations of spectral properties in low-luminosity active galactic nuclei (LLAGNs). Our analysis is based on high signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500 A interval for a sample of 47 galaxies. The data probe distances of typically up to 850 pc from the nucleus with a resolution of ∼100 pc (∼1 arcsec) and S/N ∼30. Stellar population gradients are mapped by the radial profiles of absorption-line equivalent widths and continuum colours along the slit. These variations are further analysed by means of a decomposition of each spectrum in terms of template galaxies representative of very young (≤10 7 yr), intermediate age (10 8-9 yr) and old (10 10 yr) stellar populations. This study reveals that young-TOs also differ from old-TOs and old-LINERs in terms of the spatial distributions of their stellar populations and dust. Specifically, our main findings are as follows. (i) Significant stellar population gradients are found almost exclusively in young-TOs. (ii) The intermediate age population of young-TOs, although heavily concentrated in the nucleus, reaches distances of up to a few hundred pc from the nucleus. Nevertheless, the half width at half-maximum of its brightness profile is more typically 100 pc or less. (iii) Objects with predominantly old stellar populations present spatially homogeneous spectra, be they LINERs or TOs. (iv) Young-TOs have much more dust in their central regions than other LLAGNs. (v) The B-band luminosities of the central ≤1 Gyr population in young-TOs are within an order of magnitude of M B = -15, implying masses of the order of ∼10 7 -10 8 M ○. . This population was 10-100 times more luminous in its formation epoch, at which time young massive stars would have completely outshone any active nucleus, unless the AGN too was brighter in the past.

The X-ray spectrum of NGC 7213 and the Seyfert-LINER connection

We present an XMM‐Newton observation of the Seyfert‐LINER (low-ionization nuclear emission-line region) galaxy NGC 7213. The RGS soft X-ray spectrum is well fitted with ap o wer law plus soft X-ray collisionally ionized thermal plasma (kT = 0.18 +0.03 keV). We confirm the presence of Fe I ,F eXXV and Fe XXVI Kα emission in the EPIC spectrum and set tighter constraints on their equivalent widths of 82 +10 ,2 4 +9 and 24 +10 eV, respectively. We compare the observed properties together with the inferred mass accretion rate of NGC 7213 with those of other Seyfert and LINER galaxies. We find that NGC 7213 has intermediate X-ray spectral properties lying between those of the weak active galactic nucleus found in the LINER M81 and higher-luminosity Seyfert galaxies. There appears to be a continuous sequence of X-ray properties from the Galactic Centre through LINER galaxies to Seyferts, probably determined by the amount of material available for accretion in the central regions. Ke yw ords: galaxies: active ‐ galaxies: individual: NGC 7213 ‐ galaxies: Seyfert ‐ X-rays: galaxies.

The Stellar Populations in the Central Parsecs of Galactic Bulges

We present Hubble Space Telescope blue spectra at intermediate spectral resolution for the nuclei of 23 nearby disk galaxies. These objects were selected to have nebular emission in their nuclei and span a range of emission-line classifications, as well as Hubble types. In this paper we focus on the stellar population as revealed by the continuum spectral energy distribution measured within the central 013 (~8 pc) of these galaxies. The data were modeled with linear combinations of single-age stellar population synthesis models. The large majority (~80%) of the surveyed nuclei have spectra whose features are consistent with a predominantly old (5 × 109 yr) stellar population. Approximately 25% of these nuclei show evidence of a component with age younger than 1 Gyr, with the incidence of these stars related to the nebular classification. Successful model fits imply an average reddening corresponding to AV ≈ 0.4 mag and a stellar metallicity of 1-2.5 Z☉. We discuss the implications of these results for understanding the star formation history in the environment of quiescent and active supermassive black holes. Our findings reinforce the picture wherein Seyfert nuclei and the majority of low-ionization nuclear emission-line regions are predominantly accretion-powered and suggest that much of the central star formation in H II nuclei is actually circumnuclear.

Discovery of high-amplitude X-ray variability in the Seyfert-LINER transition galaxy NGC 7589

We present the first result of a programme to search for large flux variations in the X-ray sources of the XMM Serendipitous Survey compared to previous ROSAT observations. An increase in X-ray flux by a factor >10 was discovered from the nucleus of the galaxy NGC 7589 on a time-scale of less than 5 yr. The 0.4-10 keV XMM spectrum can be approximated by a power law with photon index of 1.7-1.8, though it seems to flatten above 5 keV, suggesting a possible complex model, such as partial covering or disc reflection. A classification based on an analysis of its optical spectrum places NGC 7589 in the Seyfert region, but close to the Seyfert-LINER (low-ionization nuclear emission-line region) borderline on the active galactic nucleus (AGN) diagnostic diagrams. We classify NGC 7589 as either Seyfert 1.9 or LINER I, in the light of the detection of a broad Ha line, which makes NGC 7589 an AGN in the low-luminosity regime. We interpret the observed variability in terms of either changes in covering factor of absorbing gas in the AGN, or variability in the intrinsic X-ray luminosity. Should the latter be the case, the inferred Eddington accretion rate increased from the radiatively inefficient accretion-dominated regime to a value close to the putative critical value, at which a transition of the accretion mode is supposed to take place. This possibility presents a new prospect of studying accretion physics in the central black holes of external galaxies by direct observing changes of 'spectral state', as is common in stellar black hole binary systems.

QSOs and Active Galactic Nuclei Associated with NGC 2639

NGC 2639 is a bright, active LINER galaxy that has previously been found to have a pair of bright optical, X-ray quasars paired across it (z = 0.305 and 0.323). In addition, PSPC X-ray observations reveal a line of seven X-ray sources lying along the northeast minor axis. Four of these have now been measured spectroscopically and are reported here to be QSO/active galactic nuclei with redshifts ranging between z = 0.337 and z = 2.63. FIRST radio sources are found to be aligned along this same minor axis, some of which are coincident with the optical objects. Redshifts of all of the known QSOs in the field lie close to the well-known periodicity peaks.

Jet emission in NGC 1052 at radio, optical, and X-ray frequencies

We present a combined radio, optical, and X-ray study of the nearby LINER galaxy NGC 1052. Data from a short (2.3 ks) CHANDRA observation of NGC 1052 reveal the presence of various jet-related X-ray emitting regions, a bright compact core and unresolved knots in the jet structure as well as an extended emitting region inside the galaxy well aligned with the radio synchrotron jet-emission. The spectrum of the extended X-ray emission can best be fitted with a thermal model with keV, while the compact core exhibits a very flat spectrum, best approximated by an absorbed power-law with . We compare the radio structure to an optical “structure map” from a Hubble Space Telescope (HST) observation and find a good positional correlation between the radio jet and the optical emission cone. Bright, compact knots in the jet structure are visible in all three frequency bands whose spectrum is inconsistent with synchrotron emission.

Testing Radiatively Inefficient Accretion Flow Theory: AnXMM‐NewtonObservation of NGC 3998

We present the results of a 10 ks XMM-Newton observation of NGC 3998, a ``type-I'' LINER galaxy. Our goal is to test the extent to which radiatively-inefficient accretion flow (RIAF) models and/or scaled-down AGN models are consistent with the observed properties of NGC 3998. The upper-limit for narrow Fe-K emission derived from a combined fit of the XMM-Newton and BeppoSAX spectra is 25 eV, which is one of the strictest limits to date for any AGN. This significantly rules out Fe-K emission as is expected to be observed in typical Seyfert 1 galaxies. The lack of any reflection features suggests that any optically-thick, geometrically-thin accretion disk must be truncated, probably at a radius of order 100-300 R_s. RIAF models fit the UV to X-ray spectral energy distribution of NGC 3998 reasonably well. In these models the mid-IR flux also constrains the emission from any outer thin disk component that might be present. The UV to X-ray SED is also consistent with a Comptonized thin disk with a very low accretion rate, in which case the lack of Fe-K emission may be due to an ionized accretion disk. Accretion models in general do not account for the observed radio flux of NGC 3998, and the radio flux may be due to a jet. Recent jet models may also be consistent with the nuclear fluxes of NGC 3998 in general, including the X-ray, optical/UV and mid-IR bands. We also derive nuclear fluxes using archival HST WFPC2 data to constrain the SED of NGC 3998. We discuss a possible OM U band and USNO-B detection of the NGC 3998 ULX.

Stellar population in LLAGN

LLAGN that include low-ionization nuclear emission-line regions (LINERs), and transition-type objects (TOs) represent the most common type of nuclear activity. Here, a study of the central stellar population of LLAGN is presented. Our goal is to search for spectroscopic signatures of young and intermediate age stars, and to investigate their relationship with the ionization mechanism in LLAGN. The method used is based on the stellar population synthesis of the UV-optical continuum of the innermost (20-100 pc) regions in these galaxies. Half of the LINERs and TOs of the Palomar catalogue are analysed. It is found that weak-[OI] ([OI]/Hα≤0.25) LLAGN have an intermediate age stellar population that dominates the optical light. But young stellar clusters dominate the UV continuum in these objects. These clusters can co-exist with a black-hole in spatial scales of a few pc. Most of the strong-[OI] LLAGN have a predominantly old stellar population. These results suggest that young and intermediate age stars do not play a significant role in the ionization of LLAGN with strong [OI].To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The stellar populations in the central parsecs of galactic bulges

We present the analysis of Hubble Space Telescope blue spectra at intermediate spectral resolution for the nuclei of 23 nearby disk galaxies. These objects were selected to have nebular emission in their nuclei, and span a range of emission-line classifications as well as Hubble types. Here we focus on the stellar population as revealed by the continuum spectral energy distribution measured within the central 0.″13 (∼8 pc) of these galaxies. The data were modeled with linear combinations of single-age stellar population synthesis models. The large majority (∼80%) of the surveyed nuclei have spectra whose features are consistent with a predominantly old (. Our findings reinforce the picture wherein Seyfert nuclei and the majority of low-ionization nuclear emission-line regions (LINERs) are predominantly accretion-powered, and suggest that much of the central star formation in HII nuclei is actually circumnuclear.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Prevalence of galaxy-galaxy interactions in AGN hosts

Studies in optical starlight have failed to reach a consensus on the importance of either galaxy interactions, bars, or nuclear spirals in triggering luminous active galactic nuclei (AGNs). Here, we present the first systematic imaging study of Seyfert (disk) galaxies in the 21-cm line of neutral atomic hydrogen (HI) gas. HI is the most sensitive and enduring tracer of galaxy interactions, and can reveal tidal features not otherwise visible in optical starlight. Our sample comprises all twenty-eight galaxies in the Véron-Cetty &amp; Véron (1998) catalog with nuclear magnitudes −19 ≥ MB &gt; −23 (including Seyfert, LINER, and HII galaxies) at 0.015 ≤ z ≤ 0.017 in the northern hemisphere, and a matched control sample of twenty-seven inactive galaxies at z≈0.008. We have detected nearly all the galaxies observed, and find a much higher incidence of tidal interactions — usually not seen in optical starlight — among the Seyfert galaxies by comparison with the matched control sample. Those Seyferts with uncertain or no clear tidal features show disturbed HI morphologies and/or kinematics, as well as HI companion galaxies, more frequently than the control sample. Our study suggests that the undisturbed optical appearence of active galaxies may be deceptive, and imply that galaxy-galaxy interactions trigger a significant fraction luminous AGNs at low redshifts. The majority of the Seyfert galaxies in our sample appear to be at a relatively early stage of an encounter rather than late in a merger.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A joint mid-infrared spectroscopic and X-ray imaging investigation of LINER galaxies

We present a comprehensive comparative high resolution mid-IR spectroscopic and X-ray imaging investigation of LINERs using archival observations from the ISO-SWS and the Chandra Advanced CCD Imaging Spectrometer. Although the sample is heterogenous and incomplete, this is the first comprehensive study of the mid-infrared fine structure line emission of LINERs. These results have been compared with similar observations of starburst galaxies and AGN. We find that LINERs very clearly fall between starbursts and AGN in their mid-IR fine structure line spectra, showing /LFIR and / ratios, both measures of the dominant nuclear energy source in dust-enshrouded galaxies, intermediate between those of AGN and starbursts. Chandra imaging observations of the LINERs reveal hard nuclear point sources morphologically consistent with AGN in most (67%) of the sample, with a clear trend with IR-brightness. Most LINERs that show a single dominant hard compact X-ray core are IR-faint (LFIR/), whereas most LINERs that show scattered X-ray sources are IR-bright. A comparative X-ray/mid-IR spectroscopic investigation of LINERs reveals some puzzling results. Objects that display strong hard nuclear X-ray cores should also display high excitation lines in the IR. However, we find two LINERs disagree with this expectation. The galaxy NGC 404 shows weak soft X-ray emission consistent with a starburst but has the most prominent highest excitation mid-IR spectrum of our entire sample. Using IR emission line diagnostics alone, this galaxy would be classified as hosting a dominant AGN. Conversely, the IR luminous LINER NGC 6240 has an extremely luminous binary AGN as revealed by the X-rays but shows weak IR emission lines. With the advent of SIRTF, and future IR missions such as Herschel and JWST, it is increasingly critical to determine the origin of these multiwavelength anomalies.

The nuclear region of low luminosity flat radio spectrum sources

We report on the spectroscopic study of 19 low luminosity Flat Radio Spectrum (LL FRS) sources selected from Marchã's et al. ([CITE]) 200 mJy sample. In the optical, these objects are mainly dominated by the host galaxy starlight. After correcting the data for this effect, we obtain a new set of spectra clearly displaying weak emission lines; such features carry valuable information concerning the excitation mechanisms at work in the nuclear regions of LL FRS sources. We have used a special routine to model the spectra and assess the intensities and velocities of the emission lines; we have analyzed the results in terms of diagnostic diagrams. Our analysis shows that 79% of the studied objects harbour a Low Ionization Nuclear Emission-line Region (or LINER) whose contribution was swamped by the host galaxy starlight. The remaining objects display a higher ionization spectrum, more typical of Seyferts; due to the poor quality of the spectra, it was not possible to identify any possible large Balmer components. The fact that we observe a LINER-type spectrum in LL FRS sources supports the idea that some of these objects could be undergoing an ADAF phase; in addition, such a low ionization emission-line spectrum is in agreement with the black hole mass values and sub-Eddington accretion rates published for some FRS sources.

NGC 5953/54:BVRIHαJKphotometry and [N II] Fabry-Perot interferometry

We present new BVRIHαJK imaging and [N II] scanning Fabry-Perot observations for the interacting galaxy pair NGC 5953/54. Morphology is reviewed using a combination of contrast-enhanced images, color-index maps, and geometric (radial μ, , PA and ) profile parameters. Our data are complemented by archived V/R/H images from HST. The Seyfert 2 NGC 5953 shows an underlying featureless disk (≥1.5 kpc) in all the observed bands and traces of faint external fan-like features in Hα and [N II]. A compact flocculent spiral pattern (≤1.5 kpc), and a ~60 pc bar-like central structure are also observed. The Liner galaxy NGC 5954 is an inclined and distorted spiral with a strong circumnuclear starburst region and star-forming regions throughout the disk. A tidal bridge or distorted arm appears to link the two galaxies, extending to the northwest as a linear feature (plume). NGC 5954 also shows blue filaments delineating bubble-like features on the north side of the nucleus where our Fabry-Perot observations reveal double emission line profiles. We derive BVRIJK magnitudes and surface brightness profiles for the galaxies. HST V/R and H-band surface brightness profiles are also presented for NGC 5953. Fabry-Perot observations yield an [N II] velocity field and rotation curves for the components. The observations coupled with results from a simulation atlas of tidal features allow us 1) to suggest a tentative geometry of the encounter in NGC 5953/54, and 2) to comment about a suspected secular transformation in NGC 5953/54 via the interaction process. A forthcoming paper is devoted to a more detailed analysis of our observations including HI synthesis observations and n-body numerical simulations.

The infrared--X-ray continuum correlation in active galactic nuclei

The correlation between the soft X-ray and near-infrared emission from active galactic nuclei (AGNs) is analysed using composite models. We find new evidence for differences in the ranges of parameters that characterize the narrow-line region (NLR) of Seyfert galaxies and low-ionization nuclear emission regions (LINERs). Soft X-rays show less variability, so they are better fitted for this kind of analysis. In our models, soft X-rays are emitted in the post-shock region of clouds with relatively high shock velocities Vs > 250 km s−1. Consequently, dust emission peaks in the mid-infrared. On the other hand, in the photoionized zone, dust is at lower temperature and usually does not contribute to the mid-infrared emission. The results are sensible enough to allow the same modelling method to be applied to different types of AGN. We found that shock velocities are between 300 and 1000 km s−1, with the NLR of low-luminosity active galactic nuclei (LLAGNs) and type 2 Seyfert galaxies (Sy2s) showing lower velocities than type 1 Seyfert galaxies (Sy1s). The intensity of the ionizing radiation flux at the Lyman limit from the central source is low for LINERs and low-luminosity AGNs (log Fh= 9 to 10), increasing towards Sy2s (log Fh∼ 11) and Sy1s (11 ≤Fh≤ 12). Results obtained by modelling the Einstein and the ROSAT samples of galaxies are in full agreement. Dust-to-gas ratios by number are ≥10−14 in LINERs and LLAGNs, between 10−15 and 3 × 10−13 in Sy1s and up to 5 × 10−13 in Sy2s. In order to fit the infrared and X-ray continua, an η factor is defined, which accounts for the emitting area of the cloud. If the infrared emission is due to bremsstrahlung and comes from the same cloud that produces the soft X-rays, the η values obtained from both emissions must be the same. Therefore, if (η)IR < (η)soft X, there must be a strong contribution of soft X-rays from the active centre. From the η values, we expect to identify the objects that could present strong variability.

Emission‐Line Diagnostics of the Central Engines of Weak‐Line Radio Galaxies

A handful of well-studied Weak-Line Radio Galaxies (WLRGs) have been traditionally classified as Low Ionization Nuclear Emission-line Regions (LINERs), suggesting that these two groups of AGNs might be related. In this paper, we present new optical emission-line measurements for twenty Weak-Line Radio Galaxies which we supplement with measurements for an additional four from the literature. Classifying these objects by their emission-line ratios, we find that 50% of the objects are robustly classified as LINERs while an additional 25% are likely to be LINERs. Photoionization calculations show that the Spectral Energy Distribution of the well-studied WLRG 3C 270 (NGC 4261) is able to produce the observed emission-line ratios, but only if the UV emission seen by the narrow emission-line gas is significantly higher than that observed, implying A_V = 2.5-4.2 magnitudes along our line of sight to the nucleus. From the photoionization calculations, we find that the emission-line gas must have an ionization parameter between 10^-3.5 and 10^-4.0 and a wide range in hydrogen density (10^2-10^6 cm^-3) to reproduce the measured emission-line ratios, similar to the properties inferred for the emission-line gas in LINERs. Thus, we find that properties of the emission-line gas as well as the underlying excitation mechanism are indeed similar in LINERs and WLRGs. By extension, the central engines of accretion-powered LINERs and WLRGs, which do host an accreting black hole, may be qualitatively similar.

HI observations of nearby galaxies V. Narrow (HI) line galaxies

We present HI observations of the galaxies in the first list of the Karachentsev catalog of previously unknown nearby dwarf galaxies (Karachentseva & Karachentsev 1998). This survey covers all known nearby galaxy groups within the Local Volume (i.e. within 10 Mpc) and their environment, that is about 25% of the total sky. A total of 257 galaxies have been observed with a detection rate of 60%. We searched a frequency band corresponding to heliocentric radial velocities from -470 km/s to about +4000 km/s. Non-detections are either due to limited coverage in radial velocity, confusion with Local HI (mainly in the velocity range -140 km/s to +20 km/s, or lack of sensitivity for very weak emission. 25% of the detected galaxies are located within the Local Volume. Those galaxies are dwarf galaxies judged by their optical linear diameter (1.4 +/- 0.2 kpc on the average), their mean total HI mass (4.6 E7 solar masses), and their observed linewidths (39 km/s).

The [ITAL]XMM-Newton[/ITAL] View of the Nucleus of NGC 4261

We present the first results from an XMM-Newton observation of the Fanaroff-Riley type I galaxy NGC 4261, which harbors a supermassive black hole and a low-ionization nuclear emission-line region. Here we focus on the X-ray properties of the nucleus, using the European Photon Imaging Camera pn data. The 0.6-10 keV continuum in best fitted by a thermal component with kT ~ 0.7 keV, plus a power law with photon index Γ ~ 1.4, absorbed by a column density NH ~ 4 × 1022 cm-2. An unresolved Fe K emission line with EW ~ 280 eV is detected at ~7 keV. We also detect, for the first time, short-term flux variability from the nucleus, on a timescale of 3-5 ks. The short-term variations rule out an advection-dominated accretion flow as the only production mechanism of the X-ray continuum. Instead, we argue that the inner jet contributes to the emission in the X-ray band.

X-ray emission line gas in the LINER galaxy M 81

We present the soft X-ray spectrum of the LINER galaxy M 81 derived from a long observation with the XMM- Newton RGS. The spectrum is dominated by continuum emission from the active nucleus, but also contains emission lines from Fe L, and H-like and He-like N, O, and Ne. The emission lines are significantly broader than the RGS point-source spectral resolution; in the cross dispersion direction the emission lines are detected adjacent to, as well as coincident with, the active nucleus. This implies that they originate in a region of a few arcminutes spatial extent (1 arcmin ∼ 1 kpc in M 81). The flux ratios of the O VII triplet suggest that collisional processes are responsible for the line emission. A good fit to the whole RGS spectrum is obtained using a model consisting of an absorbed power law from the active nucleus and a 3 temperature optically thin thermal plasma. Two of the thermal plasma components have temperatures of 0.18 ± 0.04 keV and 0.64 ± 0.04 keV, characteristic of the hot interstellar medium produced by supernovae; the combined luminosity of the plasma at these two temperatures accounts for all the unresolved bulge X-ray emission seen in the Chandra observation by Tennant et al. (2001). The third component has a higher temperature (1.7 +2.1 −0.5 keV), and we argue that this, along with some of the 0.64 keV emission, comes from X-ray binaries in the bulge of M 81.

ChandraSnapshot Observations of Low‐Luminosity Active Galactic Nuclei with a Compact Radio Source

The results of Chandra snapshot observations of 11 LINERs (Low-Ionization Nuclear Emission-line Regions), three low-luminosity Seyfert galaxies, and one HII-LINER transition object are presented. Our sample consists of all the objects with a flat or inverted spectrum compact radio core in the VLA survey of 48 low-luminosity AGNs (LLAGNs) by Nagar et al. (2000). An X-ray nucleus is detected in all galaxies except one and their X-ray luminosities are in the range 5x10^38 to 8x10^41 erg/s. The X-ray spectra are generally steeper than expected from thermal bremsstrahlung emission from an advection-dominated accretion flow (ADAF). The X-ray to Halpha luminosity ratios for 11 out of 14 objects are in good agreement with the value characteristic of LLAGNs and more luminous AGNs, and indicate that their optical emission lines are predominantly powered by a LLAGN. For three objects, this ratio is less than expected. Comparing with properties in other wavelengths, we find that these three galaxies are most likely to be heavily obscured AGN. We use the ratio RX = \nu L\nu (5 GHz)/LX, where LX is the luminosity in the 2-10 keV band, as a measure of radio loudness. In contrast to the usual definition of radio loudness (RO = L\nu(5 GHz)/L\nu(B)), RX can be used for heavily obscured (NH >~ 10^23 cm^-2, AV>50 mag) nuclei. Further, with the high spatial resolution of Chandra, the nuclear X-ray emission of LLAGNs is often easier to measure than the nuclear optical emission. We investigate the values of RX for LLAGNs, luminous Seyfert galaxies, quasars and radio galaxies and confirm the suggestion that a large fraction of LLAGNs are radio loud.