Nucleus Of M81 Research Articles

Soft X-ray emission of nearby galaxy M81

The pointed observations of NGC3077 are analyzed by position sensitive proportional counter (PSPC) on board the ROSAT satellite on 1991-04-18, 1991, finding out that the X-ray emission range of M81 galaxy has, in the softer and more sensitive ROSAT PSPC band (0.2–2.4 keV), extended by 40% larger than ever observed by image proportional counter (IPC) on board the Einstein satellite, which shows that M81 galaxy has a larger X-ray corona. And by the aid of the timing analysis, it is concluded that M81 nucleus has ho variabilities on time scale of a few minutes; hence the possibility of M81 nucleus having the marginal variability over time scale of about 2 min that Fabbiano (1988) found from the Einstein IPC observation is ruled out.

The Arc and other structures in the Center of M81

view Abstract Citations (17) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Arc and other structures in the Center of M81 Kaufman, Michele ; Bash, Frank N. ; Crane, Patrick C. ; Jacoby, George H. Abstract Multi-configuration VLA λ6 cm and λ20 cm radio continuum observations of the extended emission surrounding the mini-Seyfert nucleus of M81 are presented and compared with optical emission-line images and with near-IR images. The extended radio emission is dominated by a large, highly polarized, nonthermal arc with a projected length of 0.9 kpc in the plane of the sky and centered 45" (0.8 kpc) northeast of the nucleus. The radio arc, which has no apparent optical counterpart, is connected by three bridges to the extended emission of the central source. The two outer bridges give the arc the appearance of a giant loop coming off the central source; the middle bridge has a nonthermal spectral index. In the arc, the magnetic field perpendicular to the line- of-sight is parallel to the curve of the arc. Most likely, the arc is highly inclined to the plane of the thermal gas and its shape is governed by magnetic and cosmic-ray pressures. It may be a large-scale version of the Galactic Center Lobe in the center of the Milky Way or a small-scale version of the nuclear radio lobes extending perpendicular to the disk in certain edge-on Seyfert galaxies. The middle bridge and a nonthermal "little jet" (2.5"=44 pc long) at the nucleus line up at a position angle of -30^deg^ and could be supplying cosmic ray electrons to the arc. In addition, two oval structures are detected in the radio continuum. The larger one coincides with the near-infrared mini-bar. The smaller one (possibly an inclined gas disk) has a different position angle and no near-infrared counterpart. A prominent radio continuum filament extends west from the northern major axis of the smaller oval but does not correspond to either an Hα filament or an obvious dust lane. Publication: The Astronomical Journal Pub Date: September 1996 DOI: 10.1086/118074 Bibcode: 1996AJ....112.1021K Keywords: GALAXIES: INDIVIDUAL: M81; RADIO LINES: GALAXIES; GALAXIES: NUCLEI; GALAXIES: SEYFERT full text sources ADS | data products SIMBAD (4) NED (1)

A SPECTROSCOPIC SURVEY OF NEARBY GALACTIC NUCLEI

ABSTRACT An optical spectroscopic survey of the nuclei of a magnitude-limited (BT 0 degrees) sample of nearby galaxies was completed to investigate a number of problems related to nuclear activity. Long-slit spectra of exceptionally high quality were obtained using the Hale 5 m telescope at Palomar Observatory for nearby 500 galaxies spanning all morphological types. The data have moderate resolution (2.5-4 A) and cover the wavelength regions 4230-5110 A and 6210-6860 A. After careful subtraction of the stellar light, emission lines with a wide range of intensities and excitation patterns were detected in ~85% of the nuclei. The relative intensity ratios of the emission lines in approximately half the cases (H II nuclei) can be explained by photoionization by young stars, while those in the other half (Seyfert nuclei and LINERs) require excitation by a source of more energetic photons, most likely originating from an active galactic nucleus (AGN). The most important results from the survey include the following. (1) LINERs are extremely common, being present in 1/3 of all galaxies, and they show a clear preference for galaxies of early Hubble types (E thorugh Sb). (2) If all LINERs are powered by nonstellar activity, then nearly half (46%) of all galaxies host active nuclei. (3) At least 10% of all nuclei were found to emit broad H-alpha emission qualitatively similar to the broad-line emission seen in classical Seyfert 1 nuclei and QSOs. This implies that the space density of AGNs in the current epoch is much larger than previously thought. (4) The presence of a bar has a negligible effect on the incidence and strength of nonstellar activity, but it appears to have a pronounced impact on nuclear star formation for some spiral galaxies. Among spirals of early Hubble types (S0-Sbc), bars enhance the probability of nuclear star formation as well as the ratio in which the massive stars form. (5) The spectroscopic properties of H II regions in the nuclear regions of galaxies change systematically with Hubble type. Nuclear H II regions also distinguish themselves from disk H II regions by emitting on average stronger low-ionization emission lines. This behavior can be understood most simply by the effects of higher dust content in galaxy nuclei. To aid in the interpretation of the data, a new set of photoionization models was calculated to predict the behavior of the optical and near-infrared emission-line spectrum under a variety of physical conditions appropriate for Seyfert nuclei and LINERs. Photoionization by a nonstellar ionizing continuum with a low ionization parameter can consistently explain the optical to near-infrared spectroscopic properties of most LINERs. Finally, a multiwavelength (radio to X-ray) analysis was performed on the LINER nucleus of M81. Spectra taken with the Hubble Space Telescope unambiguously detected a featureless, power-law, nonstellar continuum extending from optical to ultraviolet wavelengths. Numerous broad emission lines characteristic of Seyfert 1 nuclei were also found. It was possible to deduce for the first time a number of detailed properties concerning the central engine, the spectral energy distribution, and the regions emitting broad and narrow lines.

Double-Peaked Broad Emission Lines in the Nucleus of M81

We present recent optical spectroscopy of the nucleus of the spiral galaxy M81 obtained with the post-COSTAR Faint Object Spectrograph (FOS) aboard Hubble Space Telescope (HST). The spectra reveal a new double- peaked broad component in the profiles of Hα and Hβ, the FWZI of which are ~14000 and 11000 km s^-1^, respectively. Such double-peaked broad emission lines are known to exist in some active galactic nuclei (AGNs) classified as broad-line radio galaxies or radio-loud quasars, but M 81 is only the second AGN with low radio power and in a spiral host galaxy known to exhibit such lines. This detection in M81 strongly supports the trend for such double-peaked broad lines to be found preferentially in galactic nuclei with LINER-like narrow emission-line spectra. We emphasize that all AGNs known to exhibit broad, double-peaked line profiles also show clear evidence for jets, even though these jets may not be luminous radio emitters. In M81, the red peak of the double- peaked profile is stronger than the blue peak, while the blue wing extends further from systemic velocity than does the red wing. These properties are contrary to the prediction of a model in which the line originates in a circular, relativistic accretion disk. Either the disk is elliptical or the double-peaked line originates in a bipolar outflow (e.g., jets). Continued spectroscopic monitoring of the broad Hα profile in M81 is needed to constrain such models.

New Insights into the Physical Nature of LINERs from a Multiwavelength Analysis of the Nucleus of M81

view Abstract Citations (126) References (96) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS New Insights into the Physical Nature of LINERs from a Multiwavelength Analysis of the Nucleus of M81 Ho, Luis C. ; Filippenko, Alexei V. ; Sargent, Wallace L. W. Abstract We discuss the physical conditions and excitation of the nucleus of M81 by combining ground-based optical spectra, archival Hubble Space Telescope (HST) Ultraviolet (UV) and optical spectral, and published X- ray measurements. This well known low-ionization nuclear emission-line region (LINER) is representative of a general class of objects thought to be the less luminous counterparts of active galactic nuclei (AGNs) present in Seyfert galaxies and quasars. A featureless UV continuum, most likely of non-stellar origin is detected unambiguously for the first time in M81. Compared with "classical", more luminous AGNs, the UV continuum is weak relative to the X-rays, the slope between 1200 and 3000 A is significantly steeper (fnu_ is proportional to ν^-2.0+/-0.3^ rather than ν^-0.5^ to ν^-1^), and the "big blue bump" is absent. The nonstellar continuum of the nucleus has an absolute blue magnitude of - 11.6, Only a factor of 5 brighter than that of the least luminous Seyfert 1 nucleus in NGC 4395. We suggest that these characteristics of the spectral energy distribution may be a generic property of galactic nuclei having low-level activity, perhaps being a manifestation of extremely low accretion rates. The UV-optical spectrum exhibits broad emission lines with FWHM ~ 2500 km s^-1^, further reinforcing the similarity to Seyfert 1 nuclei. Detailed properties have been derived for the regions emitting narrow and broad lines. The narrow-line region is characterized by velocities of 200-1000 km s^-1^, a large, radially decreasing gradient in density ranging from a few times 10^2^ to 10^7^ cm^-3^, and a spatial extent of less than 1 to several parsecs from the central ionizing source. We deduce that the gas has roughly solar metallicity, an ionization parameter of ~10^-3.3^, and electron temperatures typical of photoionized nebulae. Lacking information on the structure of the broad-line region, we can obtain only crude estimates of its density (10^9^-10^10^ cm^-3^), ionization parameter (<~ 10^-2.8^), and radius (0.001-0.004 pc). Combining the velocities and dimensions of the broad-line region, and assuming that gravity dominates the motions of the clouds, we derive a central mass of (0.7-3) x 10^6^ M_sun_ for the nucleus of M81. The low bolometric luminosity of the nucleus implies that it is radiating only at (2-10) x 10^-4^ of the Eddington limit. Our extensive ground-based monitoring reveals that the broad component of the Hα emission line has not varied significantly during the past 15 yr, although X-ray variability on timescales of years has been reported. This variability pattern is not yet understood. The broad Hα luminosity (1.8 x 10^39^ ergs s^-1^) is 15 times larger than in NGC 4395, assuming that the reddening intrinsic to the broad-line region is negligible in both objects. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177140 Bibcode: 1996ApJ...462..183H Keywords: GALAXIES: INDIVIDUAL MESSIER NUMBER: M81; GALAXIES: NUCLEI; GALAXIES: SEYFERT; LINE: PROFILES; ULTRAVIOLET: SPECTRA; SPECTROPHOTOMETRY full text sources ADS | data products SIMBAD (15) NED (1) MAST (1) ESA (1)

Narrow-band imaging of the circumnuclear emission-line region of M81

We present new flux-calibrated narrow-band images of the circumnuclear region in M81≡ NGC 3031 in the light of [O iii] A5007, [O i.] λ6300, Hα + [N ii] λλ6548, 84 and [S ii] λλ6717 + 31 emission lines. The Hα + [N ii] and [O iii] images as well as [O i] and [S ii] ones reveal the existence of an extended emission-line structure around the nucleus of M81. The Hα + [N ii] λλ6548, 84 extends to ∼ 8 arcsec west and ∼ 13 arcsec east of the nucleus at the 5σ noise level of the image. The estimation of the filling factor, between 5 × 10−4 and 3 × 10−3, was made based on the emissivity in the [O iii] and Hα regions, respectively. An increase of the [O iii]/Hα ratio was detected, growing more prominent farther from the nucleus. This is caused by increasing [O iii] λ5007 emission rather than decreasing Hα + [N ii] emission. The [O iii]/Hβ against [O i]/Hα diagnostic diagram for the circumnuclear emission-line region (CELR) around the nucleus implies that part of the ionization, particularly in the innermost nuclear regions, could be accounted for by an AGN-like power-law ionization spectrum. Another part of the ionization, particularly in outer regions of the CELR, is caused by diffuse thermal X-ray continuum.

X-Ray Properties of the Nucleus of M81

Due to the appearance of SN 1993J, the region of M81 (NGC 3031) was observed with ASCA ten times between 1993 April and 1995 April. Data from eight of these observations have been used to study the 0.5–10 keV spectral and temporal properties of the source designated X-5, coincident with the M81 nucleus. The 2–10 keV luminosity of X-5 varied by a factor of 1.7 over a period of two years, around a mean of ∼ 2 × 1040 erg s−1 . Variations by ∼ 20% on a time scale of one day were also detected. The average spectrum of X-5 can be represented in the 2–10 keV range by a power-law continuum of the photon index, Γ = 1.85± 0.04, and a relatively low intrinsic absorption column of NH ∼ 1 × 1021 cm−2 . In the soft (&amp;lt; 2 keV) energy range, an additional thermal component with a temperature of 0.6–0.8 keV has been detected. An apparently broad or complex Fe-K emission line centered at 6.6–6.7 keV was also detected at an equivalent width of 170 ± 60 eV. These results indicate that the M81 nucleus is a low-luminosity active galactic nucleus, and suggest that similar objects are relatively numerous. A comparison is made between the nucleus of M81 and those of Seyfert galaxies.

MAGNITUDES AND COLORS OF SN 1993J COMPARISON STARS

ABSTRACT A catalog and find chart, with cross-identifications to other charts and catalogs, are provided for 36 comparison stars of SN 1993J within 16' from the center of M81. Revised V band magnitudes and--where available--U-B, B-V, V-Rc and V-Ic colors are listed for 35 stars with 9.5 </= V </= 15.5, and a fainter one ("a") at V=17.8. For stars more than 6' from the nucleus of M81, the GSC photovisual magnitudes, mpv, can be converted to the V system, through V*=mpv+0.40+0.15 (B-V), with a mean error of 0.03 mag when the color index is known and, less precisely, V*=mpv+0.50 ± 0.1 mag, when it is not.

The Broad-Band X-Ray Spectrum of the Nucleus of M81

We report on the results of a 2000 s observation of the nearby spiral galaxy M81 using the Broad-Band X-Ray Telescope (BBXRT). The observed X-ray emission is dominated by the flux from the nucleus, classified optically as a low-ionization nuclear emission-line region (LINER) (Filippenko & Sargent 1988). The 0.5-10 keV spectrum is well fit by a simple power law of photon index 2.20 (+0.3, -0.15), distinct from the flatter index observed in Seyfert galaxies. An upper limit of 90 eV has been placed on the equivalent width of any Fe K fluorescent feature. Taken together with the nondetection in the optical and UV of any nonthermal continuum emission, this argues strongly for the absence of an accretion disk around the M81 nucleus

Discovery of an extremely low luminosity Seyfert 1 nucleus in the dwarf galaxy NGC 4395

view Abstract Citations (185) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Discovery of an Extremely Low Luminosity Seyfert 1 Nucleus in the Dwarf Galaxy NGC 4395 Filippenko, Alexei V. ; Sargent, Wallace L. W. Abstract We have discovered a Seyfert 1 nucleus in the center of the nearby (d ~ 2.6 Mpc), late-type (Sd III-IV), dwarf galaxy NGC 4395. Such nuclei have never been seen in galaxies of this kind. The optical spectrum reveals very strong, narrow emission lines covering a wide range of ionization from [O I] to [Fe X]. Weak wings having FWZI ~ 4000-8000 km s^-1^ are visible in the permitted-line profiles, including He II λ4686. H II regions in the rest of the galaxy have normal spectra. The luminosity of the broad Hα emission is ~ 1.2 x 10^38^ ergs s^-1^, a factor of 10 fainter than in the nucleus of M81; thus, NGC 4395 contains the least luminous known Seyfert 1 nucleus. The equivalent width of broad Hα (270 A) is similar to that in normal, luminous Seyfert 1 galaxies. The optical continuum is featureless, and has M_B_ ~ 9.8 mag--less luminous than the brightest known supergiant stars. It is possible that the nuclear properties of NGC 4395 can be explained in terms of stellar phenomena, rather than by accretion onto a black hole. Publication: The Astrophysical Journal Pub Date: July 1989 DOI: 10.1086/185472 Bibcode: 1989ApJ...342L..11F Keywords: Dwarf Galaxies; Galactic Nuclei; Light (Visible Radiation); Seyfert Galaxies; Stellar Luminosity; Emission Spectra; H Alpha Line; H Ii Regions; Line Spectra; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 4395; GALAXIES: NUCLEI; GALAXIES: SEYFERT; LINE PROFILES; SPECTROPHOTOMETRY full text sources ADS | data products SIMBAD (2) NED (2)

The active nucleus of M81 and its history

Optical spectra obtained in subarcsecond seeing have been used to examine the emission-line properties and stellar dynamics of the immediate nuclear region (r &lt; B pc) in M81, the closest luminous galaxy with an optically accessible active nucleus. Stellar velocity dispersions marginally imply a central mass of order 10^7^ M_sun_ (depending on details of the stellar kinematics in this region), with a firm upper limit of 2 X 10^7^ M_sun_. Comparison with such systems as M31 and M32 then confirms that the magnitude of a central compact mass is not itself a controlling parameter in the observed luminosity of most active nuclei. The efficiency L/L_Edd_ at the observed epoch must be greater than 3 X 10^-5^. If objects like M81 are the descendants of luminous quasars seen at large redshifts, as has been suggested based on the QSO luminosity function at redshifts near 2.5, the duty cycle for such luminous nuclear activity can be no more than a few percent to avoid leaving a larger remnant black hole. In addition to the broad Balmer lines, emission from (Fe VII] is found at the nucleus itself, probably from an intermediate-density region. Emission from [N I] λ 5200 is very strong and narrow, peaking sharply at the nucleus; complex and perhaps stratified ionization structure is present even on these relatively small scales. Other forbidden lines such as [0 III] λ 5007 show complex, perhaps multicomponent, profiles at the nucleus, which may complicate physical interpretation of linewidth-critical-density relations. No blue continuum, either nonthermal or due to a small number of hot stars, was detected at the nucleus, but the stellar population is increasingly strong lined as the nucleus is approached, and the color change associated with this sets a detection threshold on any featureless continuum too high for a direct test of a power law picture for photoionization of the surrounding gas.

Low-Activity Nuclei in Spiral Galaxies

Two types of active nuclei have been observed in normal spiral galaxies in X-rays with the Einstein Observatory: low-activity AGN, and starburst regions. The prototype of the first kind is the nucleus of M81 (Elvis and Van Speybroeck 1982; Fabbiano 1988a), but similar nuclei might also be those of M33 and NGC 1313. Soft X-ray spectra of these nuclei suggest relatively steep soft X-ray components (Trinchieri, Fabbiano and Peres 1988; Fabbiano and Trinchieri 1987; Fabbiano 1988a), reminiscent of those observed in QSOs by Bechtold et al (1987) and Wilkes and Elvis (1987). In M81, in particular, this soft component might supply enough photons to explain the optical line spectrum. If this soft X-ray component originates from an accretion disk surrounding a central black hole, the mass of the latter is likely to be smaller than 104–5 solar masses.

High-Resolution Observations of the M81 Nucleus

The nucleus of the nearby Sb galaxy M81 has been shown to exhibit many of the features prominent in luminous active galactic nuclei: a variable, compact VLBI radio source (Kellermann et al. 1976, Jones, Sramek and Terzian 1981), a central variable X-ray source (Elvis and van Speybroeck 1982, Fabbiano 1988), and broad wings of Balmer emission similar in extent to those typical to Seyfert 1 nuclei (Peimbert and Torres-Peimbert 1981, Shuder and Osterbrock 1981). The nucleus also shows a narrow-line spectrum in many ways typical of LINERs (Heckman 1980), now known to be representative of early-type spiral nuclei in general.

A detailed study of the emission lines in the Seyfert 1 nucleus of M81

Optical spectra of M81 having moderate resolution and exceptionally high SNR ratios are reported. The observations and reductions are presented, the subtraction of the underlying stellar continuum is described, the procedures used to measure the emission lines are outlined, and reddening is discussed. A detailed analysis of the narrow-line region is given. It is shown that a wide range of densities is present, making the observed relative intensities consistent with photoionization by dilute, nonstellar radiation. The mass of the central object in M81 is described under the assumption that the widths of the broad permitted lines are induced by gravity. There appear to have been no changes in the strength of the broad H-alpha line during the past few years, even though the X-ray flux of M81 has been observed to vary substantially. A possible cause of very rapid X-ray variability is discussed. 112 references.

The Structure of the Radio Nucleus of M81

New observations of the radio nucleus of the nearby bright spiral galaxy M81 (NGC 3031) show that the structure of the nucleus is considerably more complex than previously thought. The radio nucleus has a slightly inverted, variable spectrum (de Bruyn et al. 1976; Crane, Guffrida, and Carlson 1976). The variability and VLBI observations (Kellermann et al. 1976; Jones, Sramek, and Terzian 1981) both indicate a linear dimension of ~1500 AU. The recent VLBI observations of Bartel et al. (1982) determined that nearly 100% of the emission originates in an elongated region with linear dimensions of 1000–4000 AU. Peimbert and Torres-Peimbert (1981) have classified the optical nucleus of M81 as Seyfert type 1.5 (the weakest known) with the narrow-emission-line region extending over ~5″ (Münch 1959).

The nucleus of M81 - Simultaneous 2.3 and 8.3 GHz Mark III VLBI observations

Mark III very long baseline interferometry (VLBI) observations have been made of the nucleus in the normal galaxy M81 (NGC 3031) simultaneously at 2.3 and 8.3 GHz, and it was found that nearly 100% of the flux density of the nuclear region originates in an elongated radio core with linear dimensions 1000-4000 AU, dependent on frequency. This galactic nucleus is the most compact observed in any extragalactic source. The position coincides with that of the nucleus at optical and X-ray wavelengths within the larger uncertainties of the latter two. The position angle of the major axis of the M81 nucleus is within 3 deg of 75 deg at 2.3 GHz and within 6 deg of 50 deg at 8.3 GHz and is frequency dependent at the 4 sigma level. These values straddle the 62 deg position angle of the projection on the sky of the rotation axis of the galaxy. The spectrum of the core is slightly inverted, and the length of its major axis decreases with frequency. These results are consistent with the emission's being incoherent electron synchrotron radiation from an inhomogeneous region with an optical depth varying along the rotation axis of the galaxy.

The UV continuum spectrum of M81

view Abstract Citations (23) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The UV continuum spectrum of M81 Bruzual A., G. ; Peimbert, M. ; Torres-Peimbert, S. Abstract From IUE data the absolute continuum spectrum for the nucleus of M81 in the 1200-3200 A range is presented. The spectrum can be fitted by an old stellar population with a small contribution from horizontal-branch stars. It is found that the number of stars with m less than 0.3 solar mass is negligible. An upper limit for the nonthermal contribution to the UV spectrum is obtained, which together with the radio and X-ray fluxes yields a power law spectrum with alpha(ux) ranging from 0.7 to 1.3. The power-law radiation field produces from 5 percent to 50 percent of the photons needed to maintain the nuclear regions of M81 ionized. Publication: The Astrophysical Journal Pub Date: September 1982 DOI: 10.1086/160271 Bibcode: 1982ApJ...260..495B Keywords: Continuous Spectra; Galactic Nuclei; Seyfert Galaxies; Ultraviolet Spectra; Horizontal Branch Stars; Iue; Star Distribution; Stellar Mass; Ultraviolet Astronomy; Astrophysics full text sources ADS | data products NED (2) MAST (1) INES (1)

Mark III VLBI Observations of the Nucleus of M81 at 2.3 and 8.3 GHz

The normal spiral galaxy M81, which has some characteristics of a Seyfert (Peimbert, Torres-Peimbert, 1981), has a flat spectrum in the radio range (de Bruyn et al., 1976), variable on the time scale of days (Crane et al., 1976), and detectable radiation at infrared (Rieke, Lebofsky, 1978) and X-ray wavelengths (Elvis, van Speybroeck, 1981). At a distance of ∼3.3 Mpc, M81 is the nearest extragalactic object with a nucleus detectable with VLBI (Kellermann et al., 1976). We report here on simultaneous VLBI observations made with the Mark III system at 2.3 and 8.3 GHz. Observations on 14 and 16 March 1981 utilized the 100 m diameter telescope in Effelsberg, W. Germany (MPIR); the 43 m telescope at Green Bank, WV (NRAO); and the 40 m telescope near Big Pine, CA (OVRO).

Physical conditions in the nucleus of M81

Physical conditions in the nucleus of M81

Observations of a radio source in the nucleus of M81 with dimensions less than 1300 astronomical units

view Abstract Citations (46) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of a radio source in the nucleus of M81 with dimensions less than 1300 astronomical units Kellermann, K. I. ; Shaffer, D. B. ; Pauliny-Toth, I. I. K. ; Preuss, E. ; Witzel, A. Abstract Very-long-baseline interferometer observations of the nucleus of M81 show an unresolved radio source with linear dimensions not exceeding about 1300 AU. This is the smallest extragalactic radio source whose size has been directly measured. Its luminosity is intermediate between that of the compact radio source at the galactic center and the luminosity of compact sources found in the centers of extended radio galaxies and quasars. Publication: The Astrophysical Journal Pub Date: December 1976 DOI: 10.1086/182318 Bibcode: 1976ApJ...210L.121K Keywords: Extragalactic Radio Sources; Galactic Nuclei; Interferometry; Radiant Flux Density; Spiral Galaxies; Brightness Temperature; Galactic Structure; Spectrum Analysis; Astronomy full text sources ADS | data products SIMBAD (1) NED (1)