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Context. The baryonic Tully-Fisher relation (BTF) is a fundamental relation between baryonic mass and maximum rotation velocity. It can be used to estimate distances, as well as to constrain the properties of dark matter and its relation with the visible matter.

Aims.We present here a new method, MMF, for automatically segmenting cosmic structure into its basic components: clusters, filaments, and walls. Importantly, the segmentation is scale independent, so all structures are identified without prejudice as to their size or shape. The method is ideally suited for extracting catalogues of clusters, walls, and filaments from samples of galaxies in redshift surveys or from particles in cosmological N-body simulations: it makes no prior assumptions about the scale or shape of the structures.

Context.The properties of the gas in halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas.

We present results of a study of neutral hydrogen (H I) in a complete sample of nearby non-cluster radio galaxies. We find that radio galaxies with large amounts of extended H I ($M_{\rm H{\,\small I}} \gtrsim 10^9~M_{\odot}$) all have a compact radio source. The host galaxies of the more extended radio sources, all of Fanaroff & Riley type-I, do not contain these amounts of H I. We discuss several possible explanations for this segregation. The large-scale H I is mainly distributed in disk- and ring-like structures with sizes up to 190 kpc and masses up to $2 \times 10^{10}~M_{\odot}$. The formation of these structures could be related to past merger events, although in some cases it may also be consistent with a cold-accretion scenario.

We use neutral hydrogen (H I) and optical spectroscopic observations to compare the timescales of a merger event, starburst episode and radio-AGN activity in the radio galaxy B2 0648+27. We detect a large ring-like structure of H I in emission around the early-type host galaxy of B2 0648+27($M_{\rm H{\,\small I}} = 8.5 \times 10^{9}~M_{\odot}$, diameter = 190 kpc). We interpret this as the result of a major merger that occurred ≳1.5 Gyr ago. From modelling optical long-slit spectra we find that a young stellar population of 0.3 Gyr, indicative of a past starburst event, dominates the stellar light throughout the galaxy. The off-set in time between the merger event and the starburst activity in B2 0648+27suggests that the starburst was triggered in an advanced stage of the merger, which can be explained if the gas-rich progenitor galaxies contained a bulge. Although the exact age of the radio source remains uncertain, there appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity. We also observe an outflow of emission-line gas in this system, which is likely related to superwinds driven by the stars that formed during the starburst event. We argue that the radio galaxy B2 0648+27is a link in the evolutionary sequence between Ultra-Luminous Infrared Galaxies (ULIRGs) and genuine early-type galaxies.

We present the result of radio and optical observations of the S0 galaxy IC 4200. We observed the galaxy at the 21 cm wavelength with the Australian Telescope Compact Array, and we obtained optical spectroscopy and V- and R-band images with ESO/NTT/EMMI. Our aim is to determine the link between $\ion{H}{i}$and stellar content of IC 4200 and derive a coherent picture of its formation. We find that the galaxy hosts 8.5 $\times$109$M_{\odot}$of $\ion{H}{i}$rotating on a ~90 deg warped disk extended out to 60 kpc from the centre of the galaxy. Optical spectroscopy reveals a simple-stellar-population-equivalent age of 1.5 Gyr in the centre of the galaxy and V- and R-band images show stellar shells. Ionised gas is observed within the stellar body and is kinematically decoupled from the stars and characterised by LINER-like line ratios. We interpret these observational results as evidence for a major merger origin of IC 4200, and date the merger back to 1–3 Gyr ago.

We present Hiobservations of 68 early-type disk galaxies from the WHISP survey. They have morphological types between S0 and Sab and absolute B-band magnitudes between –14 and –22. These galaxies form the massive, high surface-brightness extreme of the disk galaxy population, few of which have been imaged in Hibefore. The Hiproperties of the galaxies in our sample span a large range; the average values of ${M}_{H{\sc i}}/{L}_B$and ${D}_{H{\sc i}}/{D}_{25}$are comparable to the ones found in later-type spirals, but the dispersions around the mean are larger. No significant differences are found between the S0/S0a and the Sa/Sab galaxies. Our early-type disk galaxies follow the same Himass-diameter relation as later-type spiral galaxies, but their effective Hisurface densities are slightly lower than those found in later-type systems. In some galaxies, distinct rings of Hiemission coincide with regions of enhanced star formation, even though the average gas densities are far below the threshold of star formation derived by Kennicutt (1989, ApJ, 344, 685). Apparently, additional mechanisms, as yet unknown, regulate star formation at low surface densities. Many of the galaxies in our sample have lopsided gas morphologies; in most cases this can be linked to recent or ongoing interactions or merger events. Asymmetries are rare in quiescent galaxies. Kinematic lopsidedness is rare, both in interacting and isolated systems. In the appendix, we present an atlas of the Hiobservations: for all galaxies we show Hisurface density maps, global profiles, velocity fields and radial surface density profiles.

We present high spatial resolution observations of the 6 cm continuum emission of compact $\ion{H}{ii}$regions in well-known sites of massive star formation located in the Small and Large Magellanic Clouds. The observations include N81 in the SMC, and N4A, N83B, N11A, N160A and N159-5 in the LMC. Some of the compact $\ion{H}{ii}$regions are isolated, while others are embedded in more diffuse ionised regions. A description of the radio morphology of the sources, together with comparisons with other observations, is given in detail. The regions cover a wide range in size (from ∼0.1 to 7 pc), rms electron density (from ∼200 to 6500 cm-3), emission measure (from ~$3\times10^5$to $2\times10^7$pc cm-6), ionised gas mass (from ∼0.2 to 750 $M_{\odot}$) and rate of Lyman continuum photons (from ~$ 3\times 10^{47}$to $5\times10^{49}$s-1). The spectral types determined from the Lyman continuum fluxes are consistent with optical determinations. We have compared these Magellanic Cloud $\ion{H}{ii}$regions with their Galactic counterparts in terms of size, rms electron density and Lyman continuum flux. This comparison shows that their properties relate to each other in the same way as those in Galactic $\ion{H}{ii}$regions.

We present fully sampled high angular resolution two-dimensional kinematics in the Hαspectral line, optical and near-infrared imaging, as well as 21 cm atomic hydrogen data of the spiral galaxy NGC 278. This is a small non-barred galaxy, which has a bright star forming inner region of about 2 kpc in diameter, reminiscent of nuclear rings seen mainly in barred galaxies. The Hαkinematics show a disturbed velocity field, which may be partly the result of spiral density wave streaming motions. The 21 cm data trace the atomic hydrogen well outside the optical disk. The $\ion{H}{i}$is not abundant but clearly shows disturbed morphology and kinematics. We postulate that the current structure of NGC 278 is a result of a recent interaction with a small gas-rich galaxy, which is now dispersed into the outer disk of NGC 278. Non-axisymmetries set up in the disk by this minor merger may well be the cause of the intense star formation in the inner region, which can be interpreted as a rare example of a nuclear ring in a non-barred galaxy. Rather than being induced by a bar, this nuclear ring would then be the direct result of an interaction event in the recent history of the galaxy.

We present observations of the 4.8 and 8.6 GHz continuum emission towards 11 southern HIIregions made with the Australian Telescope Compact Array. The observed objects were selected from the Infrared Space Observatory (ISO) spectral catalogue of compact HIIregions (Peeters et al. 2002b). The morphologies observed for practically all the sources are consistent with them being ionized by a cluster of stars, rather than by a single star. The linear diameters of the regions range from 0.03 pc to 3 pc, the electron densities from 300 to $2.5\times 10^4~cm^{-3}$, and the Lyman continuum photon flux from 1047to 1050s-1. We confirm the existence of a relation between the density and size of HIIregions which can be fit by a power law shallower than that predicted by the classic Strömgren theory. The radio observations provide, in addition, information about the distribution of the ionized gas within the ISO apertures. As a result of the combined radio and infrared study, estimates of the extinction in the infrared and the metal content of the nebular gas were calculated. In this analysis, we also included several (ultra)compact HIIregions previously observed by the Very Large Array. Values for extinction in the K-band between ~0 and 6 mag are found. The elemental abundances of nitrogen, neon, argon and sulphur were found to decrease with Galactocentric distance. Finally, the degree of ionization of the nebulae is confirmed to be correlated with the metal content.

Based on a new data set of optical and infrared spectra described in Vermeij et al. ([CITE]), we analyse the gas-phase elemental abundances of a sample of HIIregions in the Large and Small Magellanic Cloud. The combined optical and infrared data set gives us access to all the ionization stages of astrophysically important elements such as sulfur and oxygen. We self-consistently determine the electron temperatures and densities for the O+, S++and O++ionization zones, and use these parameters in the derivation of the ionic fractions. We discuss the uncertainties on these ionic fractions. The different relations between the electron temperatures as proposed by Garnett ([CITE]) and Thuan et al. ([CITE]) are confronted with our results. We find our electron temperatures to be consistent with these relations, although the relation between Te[$\ion{S}{iii}$] and Te[$\ion{O}{iii}$] might be slightly steeper than predicted. We investigate the reliability of the Ionization Correction Factors (ICFs) used in the derivation of the full elemental abundances of sulfur and neon. We conclude that the prescription for the ICF used to derive the sulfur abundance as given by Stasińska ([CITE]) for $\alpha = 3$is accurate for O+/O > 0.20. No conclusions could be drawn for neon. Avoiding the use of ICFs as much as possible, we then proceed to derive the full elemental abundances. We calculate a grid of general photoionization models to compare our results with the “bright-line” abundance diagnostics for oxygen (R23) and sulfur (S23(4)). The reliability of the newly proposed S234 parameter (Oey & Shields [CITE]) which includes emission lines from S+, S++and S+3is checked. We find a very good agreement between the S234 models and our analysis results. Finally, we compare the heavy element-to-oxygen ratios of our sample objects to those of giant HIIregions in a large sample of low-metallicity blue dwarf galaxies (Izotov & Thuan [CITE]) and with the results from Kobulnicky & Skillman ([CITE], [CITE]) for the irregular galaxies NGC 1569 and NGC 4214.

We use a simple analytic model to deproject 2-d luminosity functions (LF) of galaxies in the Coma cluster measured by Beijersbergen et al. [CITE]. We demonstrate that the shapes of the LFs change after deprojection. It is therefore essential to correct LFs for projection effects. The deprojected LFs of the central area have best-fitting Schechter parameters of $M^{*}_{U}=-18.31^{+0.08}_{-0.08}$and $\alpha_{U}=-1.27^{+0.018}_{-0.018}$, $M^{*}_{B}=-19.79^{+0.14}_{-0.15}$and $\alpha_{B}=-1.44^{+0.016}_{-0.016}$and $M^{*}_{r}=-21.77^{+0.20}_{-0.28}$and $\alpha_{r}=-1.27^{+0.012}_{-0.012}$. The corrections are not significant enough to change the previously observed trend of increasing faint end slopes with increasing distance to the cluster center. The weighted U, B, and rband slopes of the deprojected LFs show a slightly weaker steepening with increasing projected cluster radius.

Neutral hydrogen observations with the Westerbork Synthesis Radio Telescope are presented for a sample of 73 late-type dwarf galaxies. These observations are part of the WHISP project (Westerbork $\ion{H}{i}$Survey of Spiral and Irregular Galaxies). Here we present $\ion{H}{i}$maps, velocity fields, global profiles and radial surface density profiles of $\ion{H}{i}$, as well as $\ion{H}{i}$masses, $\ion{H}{i}$radii and line widths. For the late-type galaxies in our sample, we find that the ratio of $\ion{H}{i}$extent to optical diameter, defined as 6.4 disk scale lengths, is on average 1.8 ±0.8, similar to that seen in spiral galaxies. Most of the dwarf galaxies in this sample are rich in $\ion{H}{i}$with a typical $M_{\ion{H}{i}}/L_B$of 1.5. The relative $\ion{H}{i}$content $M_{\rm \ion{H}{i}}/L_R$increases towards fainter absolute magnitudes and towards fainter surface brightnesses. Dwarf galaxies with lower average $\ion{H}{i}$column densities also have lower average optical surface brightnesses. We find that lopsidedness is as common among dwarf galaxies as it is in spiral galaxies. About half of the dwarf galaxies in our sample have asymmetric global profiles, a third has a lopsided $\ion{H}{i}$distribution, and about half shows signs of kinematic lopsidedness.

Observations of infrared fine-structure lines provide direct information on the metallicity and ionization structure of HIIregions and indirectly on the hardness of the radiation field ionizing these nebulae. We have analyzed a sample of Galactic and Magellanic Cloud HIIregions observed by the Infrared Space Observatory (ISO) to examine the interplay between stellar content, metallicity and the ionization structure of HIIregions. The observed [$\ion{S}{iv}$] 10.5/[$\ion{S}{iii}$] 18.7 ${\rm \, \mu m}$and [$\ion{Ne}{iii}$] 15.5/[$\ion{Ne}{ii}$] 12.8 ${\rm \, \mu m}$line ratios are shown to be highly correlated over more than two orders of magnitude. We have compared the observed line ratios to the results of photoionization models using different stellar energy distributions. The derived characteristics of the ionizing star depend critically on the adopted stellar model as well as the (stellar) metallicity. We have compared the stellar effective temperatures derived from these model studies for a few well-studied HIIregions with published direct spectroscopic determinations of the spectral type of the ionizing stars. This comparison supports our interpretation that stellar and nebular metallicity influences the observed infrared ionic line ratios. We can explain the observed increase in degree of ionization, as traced by the [$\ion{S}{iv}$]/[$\ion{S}{iii}$] and [$\ion{Ne}{iii}$]/[$\ion{Ne}{ii}$] line ratios, by the hardening of the radiation field due to the decrease of metallicity. The implications of our results for the determination of the ages of starbursts in starburst galaxies are assessed.

A set of ISOPHOT spectra from a sample of HIIregions in the Large Magellanic Cloud (LMC) is presented. In all the spectra, emission bands arising from Polycyclic Aromatic Hydrocarbons (PAHs) are clearly present. These features are observed to vary considerably in relative strength to each other from source to source and even within 30 Doradus. The LMC spectra have been compared with ISO-SWS spectra from Galactic HIIregions and with the ISOCAM observation towards a quiescent molecular cloud in the SMC (Reach et al. [CITE]). A correlation is found between the I7.7/I11.2versus I6.2/I11.2and the I8.6/I11.2versus I6.2/I11.2ratios. A segregation between the sources in the different types of environment (Milky Way – LMC – SMC) is present. Furthermore, within the LMC observations, a clear distinction between 30 Doradus and non-30 Doradus pointings is found. We discuss the variations in the relative strength of the PAH features in view of the different physical environments and highlight the relation with the PAH/dust ratio and the extinction curve. We conclude that 1) the same conditions responsible for the observed trends in the relative PAH-feature strengths also affect the carrier of the 2175 Å bump leading to the differences in strength of the latter, and 2) the molecular structure is the major cause of the observed variations in the relative strength of the PAH features. In the SMC and 30 Doradus compact PAH species dominate, while PAHs with an open, uneven structure are the dominant ones in Galactic HIIregions and the non-30 Dor LMC sources.

We have used MERLIN, at 1.4 and 5 GHz, to search for radio supernovae (RSNe) and supernova remnants (SNRs) in the unobscured irregular dwarf galaxy NGC 1569, and in particular in the region of its super star clusters (SSCs) A and B. Throughout NGC 1569 we find some 5 RSNe and SNRs but the SSCs and their immediate surroundings are largely devoid of non-thermal radio sources. Even though many massive stars in the SSCs are expected to have exploded already, when compared with M 82 and its many SSCs the absence of RSNe and SNRs in and near A and B may seem plausible on statistical arguments. The absence of RSNe and SNRs in and near A and B may, however, also be due to a violent and turbulent outflow of stellar winds and supernova ejected material, which does not provide a quiescent environment for the development of SNRs within and near the SSCs.

We present and interpret observations of atomic and molecular gas toward the southern elliptical galaxy NGC 1316 (Fornax A), a strong double-lobe radio source with a disturbed optical morphology that includes numerous shells and loops. The 12CO(1-0), 12CO(2-1), and Hiobservations were made with SEST and the VLA. CO emission corresponding to a total molecular hydrogen mass of ~$5 \times 10^8$$M_\odot$was detected towards the central position as well as northwest and southeast of the nucleus in the regions of the dust patches. The origin of that gas is likely external and due to accretion of one or several small gas-rich galaxies. Hiwas not detected in the central region of NGC 1316, but ~$2 \times 10^7$$M_\odot$of atomic gas was found towards the giant Hiiregion discovered by Schweizer (1980) located $6{\farcm}7$(or 36.2 kpc) from the nucleus. Hiwas also found at three other locations in the outer part of NGC 1316. The Hidistributions and kinematics of the two nearby spiral companions of NGC 1316, NGC 1317 (a barred galaxy to the north) and NGC 1310 (to the west) could be studied. Both galaxies have unusually small Hidisks that may have been affected by ram-pressure stripping.

We present rotation curves derived from H?I?observations for a sample of 62?galaxies that have been observed as part of the Westerbork H?I?Survey of Spiral and Irregular Galaxies (WHISP) project. These rotation curves have been derived by interactively fitting model data cubes to the observed cubes. This procedure takes the rotation curve shape, the H?I?distribution, the inclination, and the size of the beam into account, and makes it possible to correct for the effects of beam smearing. A comparison with higher spatial resolution H??rotation curves available in the literature shows that there is general agreement between the two. The late-type dwarf galaxies in our sample have rotation-curve shapes that are similar to those of late-type spiral galaxies, in the sense that their rotation curves, when expressed in units of disk scale lengths, rise as steeply in the inner parts and start to flatten at two disk scale lengths. None of the galaxies in our sample have solid-body rotation curves that extend beyond three disk scale lengths. The logarithmic slopes between two disk scale lengths and the last measured point on the rotation curve is similar between late-type dwarf and spiral galaxies. Thus, whether the flat part of the rotation curve is reached or not seems to depend more on the extent of the rotation curve than on its amplitude. We also find that the outer rotation curve shape does not strongly depend on luminosity, at least for galaxies fainter than MR~ -19. We find that in spiral galaxies and even in the central regions of late-type dwarf galaxies, the shape of the central distribution of light and the inner rise of the rotation curve are related. This implies that galaxies with stronger central concentrations of light also have higher central mass densities, and it suggests that the luminous mass dominates the gravitational potential in the central regions, even in low surface brightness dwarf galaxies.

We present a study of the distribution and kinematics of the neutral gas in the low-inclination Scd galaxy NGC?6946. The galaxy has been observed for 192?h at 21-cm with the Westerbork Synthesis Radio Telescope. These are among the deepest observations ever obtained for a nearby galaxy. We detect widespread high-velocity H?I?(up to about 100?km?s-1) and find 121 H?I?holes, most of which are located in the inner regions where the gas density and the star formation rate are higher. Much of the high-velocity gas appears to be related to star formation and to be, in some cases, associated with H?I?holes. The overall kinematics of the high-velocity gas is characterized by a slower rotation as compared with the regular disk rotation. ?We conclude that the high-velocity gas in NGC?6946 is extra-planar and has the same properties as the gaseous halos observed in other spiral galaxies including the Milky Way. Stellar feedback (galactic fountain) is probably at the origin of most of the high-velocity gas and of the H?I?holes. There are also indications, especially in the outer regions, ??an extended H?I?plume, velocity anomalies, sharp edges, and large-scale asymmmetries?? pointing to tidal encounters and recent gas accretion.

We present results of an HI study of a complete sample of nearby radio galaxies. Our goal is to investigate whether merger or interaction events could be at the origin of the radio-AGN activity. Around five of our radio galaxies, hosted mainly by early-type galaxies, we detect extended HI in emission. In most cases this HI is distributed in large (up to 190 kpc) and massive (up to MHI ∼ 1010 M⊙) disk- or ring-like structures, that show fairly regular rotation around the host galaxy. This suggests that in these systems a major merger likely occurred, but at least several Gyr ago. For the HI-rich radio galaxy B2 0648+27 we confirm such a merger origin through the detection of a post-starburst stellar population that dominates the visible light throughout this system. The timescale of the current episode of radio-AGN activity in our HI-rich radio galaxies is several orders of magnitude smaller than the merger timescales. Therefore the radio-AGN activity either started late in the lifetime of the merger event, or is not directly related to the merger event at all. Another intriguing result is that the HI-rich (>109 M⊙) radio galaxies in our sample all have compact radio sources, while none of the extended radio sources contain these amounts of extended H I. This strongly suggests that there is a relation between the size of the radio jet and the presence of large amounts of neutral gas associated with the host galaxy. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Observations of the nearby starburst galaxy NGC 253 in the 21-cm line reveal the presence of neutral hydrogen in the halo, up to 12 kpc from the galactic plane. This extra-planar H i is found in only one half of the galaxy and is concentrated in a half-ring structure and plumes which are lagging in rotation with respect to the disk. The H i plumes are seen bordering the bright H$\alpha$ and X-ray halo emission. It is likely that, as proposed earlier for the H$\alpha$ and the X-rays, the origin of the extra-planar H i is also related to the central starburst and to the active star formation in the disk. A minor merger and gas accretion are also discussed as possible explanations. The H i disk is less extended than the stellar disk. This may be the result of ionization of its outer parts or, alternatively, of tidal or ram pressure stripping.

Deep far-infrared (FIR) imaging data obtained with ISOPHOT at $\mathrm{90\,\mu m}$, $\mathrm{150\,\mu m}$, and $\mathrm{200\,\mu m}$ detected the thermal emission from cold dust in the northern shell region of NGC 5128 (Centaurus A), where previously neutral hydrogen and molecular gas has been found. A somewhat extended FIR emission region is present in both the $\mathrm{150\,\mu m}$ and $\mathrm{200\,\mu m}$ map, while only an upper flux limit could be derived from the $\mathrm{90\,\mu m}$ data. The FIR spectral energy distribution can be reconciled with a modified blackbody spectrum with very cold dust color temperatures and emissivity indices in the range $\mathrm{13\,K} < T_{\rm Dust} < 15.5$ K and $\mathrm{2 > {\beta} > 1}$, respectively, where the data favor the low temperature end. A representative value for the associated dust mass is ${M_{\rm Dust} \approx 7\times10^4\,M_{\sun}}$, which together with the HI gas mass gives a gas-to-dust ratio of $\approx$300, close the average values of normal inactive spiral galaxies. This value, in conjunction with the atomic to molecular gas mass ratio typical for a spiral galaxy, indicates that the interstellar medium (ISM) from the inner part of a captured disk galaxy is likely the origin of the outlying gas and dust. These observations are in agreement with recent theoretical considerations that in galaxy interactions leading to stellar shell structures the less dissipative clumpy component of the ISM from the captured galaxy can lead to gaseous shells. Alternatively, the outlying gas and dust could be a rotating ring structure resulting from an interaction or even late infall of tidal material of a merger in the distant past. With all three components (atomic gas, molecular gas, dust) of the ISM present in the northern shell region, local star formation may account for the chains of young blue stars surrounding the region to the east and north. The dust cloud may also be involved in the disruption of the large scale radio jet before entering the brighter region of the northern radio lobe.

A sample of 100 Sbc galaxies has been observed at different wavelengths and resolutions. The ultimate aim of the project is to probe the origin of their nuclear activity, e.g., blackhole, starbust, etc.

The electronic structure of the lowest excited triplet state of the vanadate ion is investigated by determining the polarization of the emission from the individual spin sublevels with optically detected magnetic resonance techniques in zero field at 1·3 K. In addition, the polarization of the total emission is determined as a function of temperature between 7 and 530 K. An interpretation of the results based on an analysis of spin-orbit coupling points towards a localized excitation in which an electron is excited from a 2pπ orbital of one of the oxygen ligands to a molecular orbital with a major contribution from the vanadium 3d∼x∼y orbital.

Neutral hydrogen observations of the nearby, edge-on spiral galaxy NGC 891 reveal the presence of an H Ihalo extending up to at least 5 kpc from the plane. This halo gas appears to rotate 25 to 100 km s-1more slowly than the gas in the plane. If this velocity difference is due to the gradient in the gravitational potential, then it may serve to discriminate between disk and spheroidal mass models. The classic picture of a large outer flare in the H Ilayer of NGC 891 may no longer be valid.

It has become clear in the past few years that young supernovae can become rather powerful radio sources at an early stage of their evolution. Three young supernovae have been detected so far: SN1970g in M101 (ref. 1), SN1979c in M100 (ref. 2) and SN1980k in NGC6946 (refs 3,4). The data on these objects and on older supernova remnants suggest that the radio emission turns on between 1 week and 1 yr after the supernova event, and that the supernova continues to be a radio emitter for at least a few years. About 50–100yr later, when the supernova shell has swept up enough interstellar gas, the remnant may become visible again in the radio domain5. Previous searches for radio emission from young supernovae have been based on optically discovered supernovae. However, with the many detailed maps of the radio continuum emission from galaxies now available, it becomes feasible to find supernovae by comparing radio data alone. We report here the appearance of a radio source in the northern, inner spiral arm of NGC4258. Inspection of existing optical data (ref. 6, and W. L. W. Sargent and C. Kowal, personal communication) confirms that this source is a young supernova, making it the first radio detected supernova event.