Black holes in dwarf/bulgeless galaxies play a crucial role in studying the co-evolution of galaxies and their central black holes. Identifying massive black holes in dwarf galaxies suggests that the growth of black holes could precede that of galaxies. However, some of the most intriguing candidate active galactic nuclei (AGN) in small galaxies have such low luminosities that the sample is vulnerable to contamination by other sources, such as supernova remnants. We re-analysed Chandra X-ray Observatory observations of candidate AGN in Henize 2-10 and NGC 4178, considering the potential signals of emission lines in the minimally-binned X-ray spectra. We find that hot plasma models, which are typical of supernova remnants, explain the observed spectra much better than simple power-law models, which are appropriate for AGN. We identify clear signals of X-ray lines in the faint X-ray source identified with the radio source in Henize 2-10 by Reines et al. 2016. Combining our work with the MUSE measurement of the ionization parameter in this region by Cresci et al. 2017 indicates that this radio and X-ray source is more likely a supernova remnant than an AGN. A similar analysis of the low-count X-ray spectrum of a candidate AGN in NGC 4178 shows that a hot plasma model is about seventeen times more probable than a simple power-law model. Our results indicate that investigation of X-ray spectra, even in a low-count regime, can be a crucial tool to identify thermally-dominated supernova remnants among AGN candidates.
A candidate accreting massive black hole (BH) with M_BH ~ 10^6 Msun has recently been identified at the center of the dwarf starburst galaxy Henize 2-10 (He 2-10). This discovery offers the first possibility of studying a growing BH in a nearby galaxy resembling those in the earlier universe, and opens up a new class of host galaxies to search for the smallest supermassive BHs. Here we present very long baseline interferometry observations of He 2-10 taken with the Long Baseline Array (LBA) at 1.4 GHz with an angular resolution of ~0.1" x 0.03". A single compact radio source is detected at the…
We report on sensitive 1.4-GHz VLBI radio observations of the unusually luminous supernova remnant SNR 4449-1 in the galaxy NGC 4449, which gave us the first well-resolved image of this object. The remnant's radio morphology consists of two approximately parallel bright ridges, suggesting similarities to the barrel shape seen for many older Galactic supernova remnants or possibly to SN 1987A. The angular extent of the remnant is 65 x 40 mas, corresponding to (3.7 x 2.3) x 10^{18} (D/3.8 Mpc) cm. We also present a new, high signal-to-noise optical spectrum. By comparing the remnant's linear siz…
NGC6946 is a high star formation rate face-on spiral galaxy that has hosted ten supernovae since 1917. Not surprisingly, a large number of supernova remnants and candidates have been identified either as optical nebulae with high [S II]:H$α$ line ratios (147) or as compact non-thermal radio sources (35). However, there are only seven overlaps between these two samples. Here, we apply [Fe II] 1.644 $μ$m emission as a new diagnostic to search for supernova remnants in an attempt to resolve this discrepancy. [Fe II] is expected to be relatively strong in the radiative shocks of supernova remnants…
We present results from a study of the Supernova Remnant (SNR) population in a sample of six nearby galaxies (NGC 2403, NGC 3077, NGC 4214, NGC 4449, NGC 4395 and NGC 5204) based on Chandra archival data. We have detected 244 discrete X-ray sources down to a limiting flux of 10^{-15} erg/s. We identify 37 X-ray selected thermal SNRs based on their X-ray colors or spectra, 30 of which are new discoveries. In many cases the X-ray classification is confirmed based on counterparts with SNRs identified in other wavelengths. Three of the galaxies in our sample (NGC 4214, NGC 4395 and NGC 5204) are s…
The presence of large ionised gaseous nebulae found around some ultraluminous X-ray sources (ULXs) provides the means to assess the mechanical and radiative feedback of the central source, and hence constrain the efficiency and impact on the surroundings of the super-Eddington regime powering most of these sources. NGC 1313 X--1 is an archetypal ULX which has been reported to be surrounded by abnormally high [O I]$λ$6300/H$α>$ 0.1 ratios and for which high-resolution spectroscopy in X-rays has hinted the presence of powerful outflows. We report observations taken with the Multi-Unit Spectrosco…
We present a multiwavelength examination of the supernova remnant (SNR) S26 in the nearby galaxy NGC 300 using data from Chandra X-ray Observatory, XMM-Newton X-ray Observatory, Hubble Space Telescope (HST), the Very Large Array, and the Australia Telescope Compact Array. We simultaneously fit all of the available X-ray data with a thermal plasma model and find a temperature of $0.77 \pm 0.13$ keV with a hydrogen column density of ($9.7^{+6.4}_{-4.8}$)$\times 10^{20}$ cm$^{-2}$. HST imaging allows us to measure a semimajor axis of $0.78 \pm 0.10$ arcsec ($7.5 \pm 1.0$ pc) and a semiminor axis …
We constrained the progenitor masses for 169 supernova remnants, 8 historically observed supernovae, and the black hole formation candidate in NGC 6946, finding that they are consistent with originating from a standard initial mass function. Additionally, there were 16 remnants that showed no sign of nearby star formation consistent with a core-collapse supernova, making them good Type Ia candidates. Using $Hubble$ $Space$ $Telescope$ broadband imaging, we measured stellar photometry of ACS/WFC fields in F435W, F555W, F606W, and F814W filters as well as WFC3/UVIS fields in F438W, F606W, and F8…
We present results from a study of optically emitting Supernova Remnants (SNRs) in six nearby galaxies (NGC 2403, NGC 3077, NGC 4214, NGC 4395, NGC 4449 and NGC 5204) based on deep narrow band Hα and [SII] images as well as spectroscopic observations. The SNR classification was based on the detected sources that fulfill the well-established emission line flux criterion of [SII]/Hα > 0.4. This study revealed ~400 photometric SNRs down to a limiting Hα flux of 10^(-15) erg sec^(-1) cm^(-2). Spectroscopic observations confirmed the shock-excited nature of 56 out of the 96 sources with ([SII]/Hα)$…
The relatively nearby spiral galaxy NGC~6946 is one of the most actively star forming galaxies in the local Universe. Ten supernovae (SNe) have been observed since 1917, and hence NGC6946 surely contains a large number of supernova remnants (SNRs). Here we report a new optical search for these SNRs using narrow-band images obtained with the WIYN telescope. We identify 147 emission nebulae as likely SNRs, based on elevated [SII]:Halpha ratios compared to HII regions. We have obtained spectra of 102 of these nebulae with Gemini North-GMOS; of these, 89 have [SII]:Halpha ratios greater than 0.4, …
Supernova remnants (SNRs) represent a powerful laboratory to study the Cosmic-Ray acceleration processes at the shocks, and their relation to the properties of the circumstellar medium. With the aim of studying the high-frequency radio emission and investigating the energy distribution of accelerated electrons and the magnetic field conditions, we performed single-dish observations of the large and complex Cygnus Loop SNR from 7.0 to 24.8 GHz with the Medicina and the Sardinia Radio Telescope, focusing on the northern filament (NGC 6992) and the southern shell. Both regions show a spectrum wel…
Jets from active galactic nuclei (AGN) are known to recurrently enrich their surrounding medium with mildly-relativistic particles and magnetic fields. Here, we present a detailed multi-frequency analysis of the nearby (z=0.01646) galaxy group NGC 507. In particular, we present new high-sensitivity and high spatial resolution radio images in the frequency range 144-675 MHz obtained using LOFAR and uGMRT observations. These reveal the presence of previously undetected diffuse radio emission with complex, filamentary morphology, likely related to a previous outburst of the central galaxy. Based …
We present comprehensive X-ray point source catalogs of NGC~55, NGC~2403, and NGC~4214 as part of the Chandra Local Volume Survey. The combined archival observations have effective exposure times of 56.5 ks, 190 ks, and 79 ks for NGC~55, NGC~2403, and NGC~4214, respectively. When combined with our published catalogs for NGC 300 and NGC 404, our survey contains 629 X-ray sources total down to a limiting unabsorbed luminosity of $\sim5\times10^{35}$ erg s$^{-1}$ in the 0.35-8 keV band in each of the five galaxies. We present X-ray hardness ratios, spectral analysis, radial source distributions, …
We report the results of our investigation on the possible existence of supernova remnants (SNRs) in the nearby galaxy, NGC 1569, using the CCD imaging and spectroscopic observations from the RTT150 telescope of TUG-TUBITAK in Antalya, Turkey for two different observing periods. Using [S\,{\sc ii}]/H$α$ $\geq$ 0.4 standard criteria, the identification of 13 new SNR candidates for this galaxy is proposed for two different epochs. We found the [S\,{\sc ii}]/H$α$ ratios ranging from 0.46--0.84 and H$α$ intensities ranging from (2.2--32) $\times$ 10$^{-15}$ erg cm$^{-2}$ s$^{-1}$. [S\,{\sc ii}]$λλ…
We present results from a deep (200 ks) Chandra observation of the early-type galaxy NGC 4552 (M89) which is falling into the Virgo cluster. Previous shallower X-ray observations of this galaxy showed a remnant gas core, a tail to the South of the galaxy, and twin `horns' attached to the northern edge of the gas core [machacek05a]. In our deeper data, we detect a diffuse, low surface brightness extension to the previously known tail, and measure the temperature structure within the tail. We combine the deep Chandra data with archival XMM-Newton observations to put a strong upper limit on the d…
4U 1820-30 is a low-mass X-ray binary near the center of the globular cluster NGC 6624 consisting of, at least, one neutron star and one helium white dwarf. Analyzing 16 years of data from the Rossi X-ray Timing Explorer (RXTE) allows us to measure its orbital period and its time derivative with unprecedented accuracy to be P = 685.01197 +- 0.00003 s and dP/dt /P = -5.3 +- 0.3x10^-8 yr^-1. Hence, we confirm that the period derivative is significantly negative at the >17 sigma level, contrary to theoretical expectations for an isolated X-ray binary. We discuss possible scenarios that could expl…
The galaxies M82, NGC 253, NGC 1068, and NGC 4945 have been detected in gamma-rays by Fermi. Previously, we developed and tested a model for cosmic ray interactions in the starburst galaxy M82. Now, we aim to explore the differences between starburst and active galactic nuclei (AGN) environments by applying our self-consistent model to the starburst galaxy NGC 253 and the Seyfert galaxy NGC 1068. Assuming constant cosmic-ray acceleration efficiency by supernova remnants with Milky-Way parameters, we calculate the cosmic-ray proton and primary and secondary electron/positron populations, predic…
Some ultraluminous X-ray sources (ULXs) are surrounded by collisionally ionized bubbles, larger and more energetic than supernova remnants: they are evidence of the powerful outflows associated with super-Eddington X-ray sources. We illustrate the most recent addition to this class: a huge (350 pc x 220 pc in diameter) bubble around a ULX in NGC 5585. We modelled the X-ray properties of the ULX (a broadened-disc source with L_X ~ 2-4 x 10^{39} erg/s) from Chandra and XMM-Newton, and identified its likely optical counterpart in Hubble Space Telescope images. We used the Large Binocular Telescop…
We present results from a study of the non-nuclear discrete sources in a sample of three nearby spiral galaxies (NGC 4395, NGC 4736, and NGC 4258) based on XMM-Newton archival data supplemented with Chandra data for spectral and timing analyses. A total of 75 X-ray sources has been detected within the D25 regions of the target galaxies. The large collecting area of XMM-Newton makes the statistics sufficient to obtain spectral fitting for 16 (about 20%) of these sources. Compiling the extensive archival exposures available, we were able to obtain the detailed spectral shapes of diverse classes …
We present very long baseline interferometry (VLBI) observations of the milliarcsecond-scale radio structure of the supernova remnant SNR 4449$-$1 in the galaxy NGC 4449. This young and superluminous remnant was observed at 1.6 GHz ($λ= 18$\,cm) with the European VLBI Network. The observations confirm earlier identifications of this object with a supernova remnant (SNR) while revealing a somewhat different morphology compared with the structure reported by Bietenholz et al. from VLBI observations at 1.4 GHz. This difference is discussed here in the context of structural sensitivity of both obs…
N103B is a Type Ia supernova remnant (SNR) projected in the outskirt of the superbubble around the rich cluster NGC 1850 in the Large Magellanic Cloud (LMC). We have obtained H$α$ and continuum images of N103B with the $\textit{Hubble Space Telescope}$ ($\textit{HST}$) and high-dispersion spectra with 4m and 1.5m telescopes at Cerro Tololo Inter-American Observatory. The $\textit{HST}$ H$α$ image exhibits a complex system of nebular knots inside an incomplete filamentary elliptical shell that opens to the east where X-ray and radio emission extends further out. Electron densities of the nebula…
