We report on the discovery and observations of the extremely luminous optical transient CSS100217:102913+404220 (CSS100217 hereafter). Spectroscopic observations showed this transient was coincident with a galaxy at redshift z = 0.147, and reached an apparent magnitude of V ∼ 16.3. After correcting for foreground Galactic extinction we determine the absolute magnitude to be M V = −22.7 approximately 45 days after maximum light. Based on our unfiltered optical photometry the peak optical emission was L = 1.3 × 10 45 erg s −1 , and over a period of 287 rest-frame days had an integrated bolometric luminosity of 1.2 × 10 52 erg.Analysis of the pre-outburst SDSS spectrum of the source shows features consistent with a Narrowline Seyfert1 (NLS1) galaxy. High-resolution HST and Keck followup observations show the event occurred within 150pc of nucleus of the galaxy, suggesting a possible link to the active nuclear region. However, the rapid outburst along with photometric and spectroscopic evolution are much more consistent with a luminous supernova. Line diagnostics suggest that the host galaxy is undergoing significant star formation.We use extensive follow-up of the event along with archival CSS and SDSS data to investigate the three most likely sources of such an event; 1) an extremely luminous supernova; 2) the tidal disruption of a star by the massive nuclear black hole; 3) variability of the central AGN. We find that CSS100217 was likely an extremely luminous type IIn supernova that occurred within range of the narrow-line region of an AGN. We discuss how similar events may have been missed in past supernova surveys because of confusion with AGN activity.

We present a 9 million star color-magnitude diagram (9M CMD) of the Large Magellanic Cloud (LMC) bar. The 9M CMD reveals a complex superposition of different age and metallicity stellar populations, with important stellar evolutionary phases occurring over three orders of magnitude in number density. First, we count the non-variable red and blue supergiants, the associated Cepheid variables, and measure the stellar effective temperatures defining the Cepheid instability strip. Lifetime predictions of stellar evolution theory are tested, with implications for the origin of low-luminosity Cepheids. The highly-evolved asymptotic giant branch (AGB) stars in the 9M CMD have a bimodal distribution in brightness, which we interpret as discrete old populations ( > ∼ 1 Gyr). The faint AGB sequence may be metal-poor and very old. Comparing the mean properties of giant branch and horizontal branch (HB) stars in the 9M CMD to those of clusters, we identify NGC 411 and M3 as templates for the admixture of old stellar populations in the bar. However, there are several indications that the old and metal-poor field population has a red HB morphology: the RR Lyrae variables lie preferentially on the red edge of the instability strip, the AGB-bump is very red, and the ratio of AGB-bump stars to RR Lyraes is quite large. If the HB second parameter is age, the old and metal-poor field population in the bar likely formed after the oldest LMC clusters. Lifetime predictions of stellar evolution theory lead us to associate a significant fraction of the ∼1 million red HB clump giants in the 9M CMD with the same old and metal-poor population producing the RR Lyraes and the AGB-bump. In this case, compared to the age-dependent luminosity predictions of stellar evolution theory, the red HB clump is too bright relative to the RR Lyraes and AGB-bump. Last, we show that the surface density profile of RR Lyraes is fit by an exponential, favoring a disk-like rather than spheroidal distribution. We conclude that the age of the LMC disk is probably similar to the age of the Galactic disk. 17 The Image Reduction and Analysis Facility, v2.10.2, operated by the National Optical Astronomy Observatories.(B) The giant branch. The stars on the giant branch are old, but in a mix of different evolutionary phases. Most of these stars are on the first-ascent red giant branch (RGB); they have degenerate helium cores and hydrogen-burning shells (Schwarzschild 1958). The base of the RGB (i.e. the subgiant branch) is not visible in the 9M CMD (it is too faint). However, the termination, or "tip" of the RGB is seen at feature (E). Stars at the tip of the RGB ignite helium in their cores and evolve very rapidly to the horizontal branch, near feature (C). Some of the stars on the giant branch are also on the asymptotic giant branch (AGB). These stars are helium shell-burners, and are in an evolutionary state more advanced than the horizontal branch. For many stars in the 9M CMD, the AGB begins at (D), continues past (E) and into region (F). We do not resolve the R...

We report Giant Metrewave Radio Telescope (GMRT) continuum observations of six nearby normal galaxies at 333 MHz. The galaxies are observed with angular resolutions better than ∼20  arcsec (corresponding to a linear scale of about 0.4–1 kpc). These observations are sensitive to all angular scales of interest, since the resolution of the shortest baseline in GMRT is greater than the angular size of the galaxies. Furthermore, for five of these galaxies we show that at 333 MHz, the mean thermal fraction is less than 5 per cent. Using archival data at about 1 GHz, we estimate the mean thermal fraction to be about 10 per cent at that frequency. We also find that the non‐thermal spectral index is generally steeper in regions with low thermal fraction and/or located in the outer parts of the galaxy. In regions of high thermal fraction, the non‐thermal spectral index is flatter, and has a narrow distribution peaking at ∼−0.78 with a spread of 0.16, putting stringent constraints on the physical conditions for generation, diffusion and energy losses of cosmic ray electrons at scales of ∼1 kpc.

We present maps of total magnetic field using 'equipartition' assumptions for five nearby normal galaxies at sub-kpc spatial resolution. The mean magnetic field is found to be ∼ 11 µG. The field is strongest near the central regions where mean values are ∼ 20 − 25 µG and falls to ∼ 15 µG in disk and ∼ 10 µG in the outer parts. There is little variation in the field strength between arm and interarm regions, such that, in the interarms, the field is 20 percent weaker than in the arms. There is no indication of variation in magnetic field as one moves along arm or interarm after correcting for the radial variation of magnetic field. We also studied the energy densities in gaseous and ionized phases of the interstellar medium and compared to the energy density in the magnetic field. The energy density in the magnetic field was found to be similar to that of the gas within a factor of 2 at sub-kpc scales in the arms, and thus magnetic field plays an important role in pressure balance of the interstellar medium. Magnetic field energy density is seen to dominate over the kinetic energy density of gas in the interarm regions and outer parts of the galaxies and thereby helps in maintaining the large scale ordered fields seen in those regions.

The spectral index of synchrotron emission is an important parameter in understanding the properties of cosmic ray electrons (CREs) and the interstellar medium (ISM). We determine the synchrotron spectral index (α nt ) of four nearby star-forming galaxies, namely NGC 4736, NGC 5055, NGC 5236 and NGC 6946 at sub-kpc linear scales. The α nt was determined between 0.33 and 1.4 GHz for all the galaxies. We find the spectral index to be flatter ( −0.7) in regions with total neutral (atomic + molecular) gas surface density, Σ gas 50 M ⊙ pc −2 , typically in the arms and inner parts of the galaxies. In regions with Σ gas 50 M ⊙ pc −2 , especially in the interarm and outer regions of the galaxies, the spectral index steepens sharply to < −1.0. The flattening of α nt is unlikely to be caused due to thermal free-free absorption at 0.33 GHz. Our result is consistent with the scenario where the CREs emitting at frequencies below ∼ 0.3 GHz are dominated by bremsstrahlung and/or ionization losses. For denser medium (Σ gas 200 M ⊙ pc −2 ), having strong magnetic fields (∼ 30 µG), α nt is seen to be flatter than −0.5, perhaps caused due to ionization losses. We find that, due to the clumpy nature of the ISM, such dense regions cover only a small fraction of the galaxy ( 5 percent). Thus, the galaxy-integrated spectrum may not show indication of such loss mechanisms and remain a power-law over a wide range of radio frequencies (between ∼ 0.1 to 10 GHz).