Nearby Sculptor Group Research Articles

Low-density star cluster formation: discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247

ABSTRACT The classical globular clusters found in all galaxy types have half-light radii of rh ∼ 2–4 pc, which have been tied to formation in the dense cores of giant molecular clouds. Some old star clusters have larger sizes, and it is unclear if these represent a fundamentally different mode of low-density star cluster formation. We report the discovery of a rare, young ‘faint fuzzy’ star cluster, NGC 247-SC1, on the outskirts of the low-mass spiral galaxy NGC 247 in the nearby Sculptor group, and measure its radial velocity using Keck spectroscopy. We use Hubble Space Telescope imaging to measure the cluster half-light radius of rh ≃ 12 pc and a luminosity of LV ≃ 4 × 105L⊙. We produce a colour–magnitude diagram of cluster stars and compare to theoretical isochrones, finding an age of ≃300 Myr, a metallicity of [Z/H] ∼ −0.6 and an inferred mass of M⋆ ≃ 9 × 104M⊙. The narrow width of blue-loop star magnitudes implies an age spread of ≲50 Myr, while no old red-giant branch stars are found, so SC1 is consistent with hosting a single stellar population, modulo several unexplained bright ‘red straggler’ stars. SC1 appears to be surrounded by tidal debris, at the end of an ∼2 kpc long stellar filament that also hosts two low-mass, low-density clusters of a similar age. We explore a link between the formation of these unusual clusters and an external perturbation of their host galaxy, illuminating a possible channel by which some clusters are born with large sizes.

AT 2019qyl in NGC 300: Internal Collisions in the Early Outflow from a Very Fast Nova in a Symbiotic Binary* †

Abstract Nova eruptions, thermonuclear explosions on the surfaces of white dwarfs (WDs), are now recognized to be among the most common shock-powered astrophysical transients. We present the early discovery and rapid ultraviolet (UV), optical, and infrared (IR) temporal development of AT 2019qyl, a recent nova in the nearby Sculptor Group galaxy NGC 300. The light curve shows a rapid rise lasting ≲1 day, reaching a peak absolute magnitude of M V = −9.2 mag and a very fast decline, fading by 2 mag over 3.5 days. A steep dropoff in the light curves after 71 days and the rapid decline timescale suggest a low-mass ejection from a massive WD with M WD ≳ 1.2 M ⊙. We present an unprecedented view of the early spectroscopic evolution of such an event. Three spectra prior to the peak reveal a complex, multicomponent outflow giving rise to internal collisions and shocks in the ejecta of an He/N-class nova. We identify a coincident IR-variable counterpart in the extensive preeruption coverage of the transient location and infer the presence of a symbiotic progenitor system with an O-rich asymptotic-giant-branch donor star, as well as evidence for an earlier UV-bright outburst in 2014. We suggest that AT 2019qyl is analogous to the subset of Galactic recurrent novae with red-giant companions such as RS Oph and other embedded nova systems like V407 Cyg. Our observations provide new evidence that internal shocks between multiple, distinct outflow components likely contribute to the generation of the shock-powered emission from such systems.

Abundance of dwarf galaxies around low-mass spiral galaxies in the Local Volume

The abundance of satellite dwarf galaxies has long been considered a crucial test for the current model of cosmology leading to the well-known missing satellite problem. Recent advances in simulations and observations have allowed the study of dwarf galaxies around host galaxies in more detail. Using the Dark Energy Camera we surveyed a 72 deg2 area of the nearby Sculptor group, also encompassing the two low-mass Local Volume galaxies NGC 24 and NGC 45 residing behind the group, to search for as yet undetected dwarf galaxies. Apart from the previously known dwarf galaxies we found only two new candidates down to a 3σ surface brightness detection limit of 27.4 r mag arcsec−2. Both systems are in projection close to NGC 24. However, one of these candidates could be an ultra-diffuse galaxy associated with a background galaxy. We compared the number of known dwarf galaxy candidates around NGC 24, NGC 45, and five other well-studied low-mass spiral galaxies (NGC 1156, NGC 2403, NGC 5023, M 33, and the LMC) with predictions from cosmological simulations, and found that for the stellar-to-halo mass models considered, the observed satellite numbers tend to be on the lower end of the expected range. This could mean either that there is an overprediction of luminous subhalos in ΛCDM or that we are missing some of the satellite members due to observational biases.

A deep Parkes H i survey of the Sculptor group and filament: H i mass function and environment

We present the results of a deep survey of the nearby Sculptor group and the associated Sculptor filament taken with the Parkes 64-m radio telescope in the 21-cm emission line of neutral hydrogen. We detect 31 HI sources in the Sculptor group/filament, eight of which are new HI detections. We derive a slope of the HI mass function along the Sculptor filament of $\alpha = -1.10^{+0.20}_{-0.11}$, which is significantly flatter than the global mass function and consistent with the flat slopes previously found in other low-density group environments. Some physical process, such as star formation, photoionisation or ram-pressure stripping, must therefore be responsible for removing neutral gas predominantly from low-mass galaxies. All of our HI detections have a confirmed or tentative optical counterpart and are likely associated with luminous rather than 'dark' galaxies. Despite a column density sensitivity of about $4 \times 10^{17}~\mathrm{cm}^{-2}$, we do not find any traces of extragalactic gas or tidal streams, suggesting that the Sculptor filament is, at the current time, a relatively quiescent environment that has not seen any recent major interactions or mergers.

NGC 55: a disc galaxy with flat abundance gradients

We present new spectroscopic observations obtained with GMOS@Gemini-S of a sample of 25 hii regions located in NGC 55, a late-type galaxy in the nearby Sculptor group. We derive physical conditions and chemical composition through the te-method for 18 hii regions, and strong-line abundances for 22 hii regions. We provide abundances of He, O, N, Ne, S, Ar, finding a substantially homogenous composition in the ionised gas of the disc of NGC 55, with no trace of radial gradients. The oxygen abundances, both derived with \te- and strong-line methods, have similar mean values and similarly small dispersion: 12+$\log$(O/H)=8.13$\pm$0.18~dex with the former and 12+$\log$(O/H)=8.17$\pm$0.13~dex with the latter. The average metallicities and the flat gradients agree with previous studies of smaller samples of \hii regions and there is a qualitative agreement with the blue supergiant radial gradient as well. We investigate the origin of such flat gradients comparing NGC 55 with NGC 300, its companion galaxy, which is also twin of NGC 55 in terms of mass and luminosity. We suggest that the differences in the metal distributions in the two galaxies might be related to the differences in their K-band surface density profile. The flatter profile of NGC 55 probably causes in this galaxy higher infall/outflow rates than in similar galaxies. This likely provokes a strong mixing of gas and a re-distribution of metals.

Radio-continuum study of the nearby sculptor group galaxies. Part 3: NGC 7793 at λ=12.2, 6 and 3 cm

We re-examine a series of archived centimetre radio-continuum observations (lambda=16, 6 and 3 cm) focusing on NGC7793 using the Australia Telescope Compact Array. These new images are both very sensitive (rms=0.011 mJy/beam) and feature reasonably high angular resolution (down to < 3"). Using these images, a total of 76 discrete radio sources are identified, of which 57 have been classified. We also studied the radio component of the micro-quasar NGC7793-S26 which shows two distinct regions of somewhat steep spectral index between -0.3 and -0.7.

Radio-continuum study of the nearby Sculptor group Galaxies. Part 2: NGC 55 at λ=20, 13, 6 and 3 cm

A series of new radio-continuum (λ=20, 13, 6 and 3 cm) mosaic images focused on the NGC 55 galactic system were produced using archived observational data from the Australia Telescope Compact Array. These new images are both very sensitive (down to rms=33 μJy) and feature high angular resolution (down to <4″). Using these newly created images, 66 previously unidentified discrete sources are identified. Of these sources, 46 were classified as background sources, 11 as H ii regions and 6 as supernova remnant candidates. This relatively low number of SNR candidates detected coupled with the low number of large H ii regions is consistent with the estimated low star formation rate of the galaxy at 0.06 M ⊙ year−1. Our spectral index map shows that the core of galaxy appears to have a shallow spectral index between α=−0.2 and −0.4. This indicates that the core of the galaxy is a region of high thermal radiation output.

Radio-continuum study of the Nearby sculptor group galaxies. Part 1: NGC 300 at λ=20 cm

A series of new radio-continuum (λ=20 cm) mosaic images focused on the NGC 300 galactic system were produced using archived observational data from the VLA and/or ATCA. These new images are both very sensitive (rms =60 μJy) and feature high angular resolution (<10 ″). The most prominent new feature is the galaxy’s extended radio-continuum emission, which does not match its optical appearance. Using these newly created images a number of previously unidentified discrete sources have been discovered. Furthermore, we demonstrate that a joint deconvolution approach to imaging this complete data-set is inferior when compared to an immerge approach.

ACHANDRAOBSERVATION OF THE NEARBY SCULPTOR GROUP Sd GALAXY NGC 7793

(Abridged) We conducted a Chandra ACIS observation of the nearby Sculptor Group Sd galaxy NGC 7793. At the assumed distance to NGC 7793 of 3.91 Mpc, the limiting unabsorbed luminosity of the detected discrete X-ray sources (0.2-10.0 keV) is approximately 3x10^36 ergs s^-1. A total of 22 discrete sources were detected at the 3-sigma level or greater including one ultra-luminous X-ray source (ULX). Based on multiwavelength comparisons, we identify X-ray sources coincident with one SNR, the candidate microquasar N7793-S26, one HII region and two foreground Galactic stars. We also find that the X-ray counterpart to the candidate radio SNR R3 is time-variable in its X-ray emission: we therefore rule out the possibility that this source is a single SNR. A marked asymmetry is seen in the distribution of the discrete sources with the majority lying in the eastern half of this galaxy. All of the sources were analyzed using quantiles to estimate spectral properties and spectra of the four brightest sources (including the ULX) were extracted and analyzed. We searched for time-variability in the X-ray emission of the detected discrete sources using our measured fluxes along with fluxes measured from prior Einstein and ROSAT observations. From this study, three discrete X-ray sources are established to be significantly variable. A spectral analysis of the galaxy's diffuse emission is characterized by a temperature of kT = 0.19-0.25 keV. The luminosity function of the discrete sources shows a slope with an absolute value of Gamma = -0.65+/-0.11 if we exclude the ULX. If the ULX is included, the luminosity function has a long tail to high L_X with a poor-fitting slope of Gamma = -0.62+/-0.2. The ULX-less slope is comparable to the slopes measured for the distributions of NGC 6946 and NGC 2403 but much shallower than the slopes measured for the distributions of IC 5332 and M83.

Optical spectra of supernova remnant candidates in the Sculptor Group galaxy NGC 300

We present moderate-resolution (<5A) long-slit optical spectra of 51 nebular objects in the nearby Sculptor Group galaxy NGC 300 obtained with the 2.3 meter Advanced Technology Telescope at Siding Spring Observatory, Australia. Adopting the criterion of [SII]/Ha>=0.4 to confirm supernova remnants (SNRs) from optical spectra, we find that of 28 objects previously proposed as SNRs from optical observations, 22 meet this criterion with six showing [SII]/Ha of less than 0.4. Of 27 objects suggested as SNRs from radio data, four are associated with the 28 previously proposed SNRs. Of these four, three (included in the 22 above) meet the criterion. In all, 22 of the 51 nebular objects meet the [SII]/Ha criterion as SNRs while the nature of the remaining 29 objects remains undetermined by these observations.

SHAKEN, NOT STIRRED: THE DISRUPTED DISK OF THE STARBURST GALAXY NGC 253

Near-infrared images obtained with the CFHT WIRCam are used to investigate the recent history of the nearby Sculptor Group spiral NGC 253. The distribution of stars in the disk is lop-sided, in the sense that the projected density of AGB stars in the north east portion of the disk between 10 and 20 kpc from the galaxy center is ~ 0.5 dex higher than on the opposite side of the galaxy. With the exception of the central 2 kpc, the north east portion of the disk appears to have been the site of the highest levels of star-forming activity in the galaxy during the past ~ 0.1 Gyr. Diffuse stellar structures are found in the periphery of the disk, and the most prominent of these is to the south and east of the galaxy. Bright AGB stars are detected out to 15 kpc above the disk plane, and these are part of a diffusely distributed, flattened extraplanar component. Comparisons between observed and model luminosity functions suggest that the extraplanar regions contain stars that formed throughout much of the age of the Universe. It is suggested that the disk of NGC 253 was disrupted by a tidal encounter with a now defunct companion. The ages of the youngest extraplanar stars suggests that the event that produced the extraplanar population, and presumably induced the starburst, occured within the past ~ 0.2 Gyr.

Stellar vs. Hii region chemical abundances in nearby galaxies

AbstractWe have obtained new spectrophotometric data for 28 Hii regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of several auroral lines has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines. The gradient corresponds to −0.077±0.006 dex kpc−1, which agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy. The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data. This result provides increased confidence on the direct method to determine extragalactic nebular abundances.

EXTRAGALACTIC CHEMICAL ABUNDANCES: DO H II REGIONS AND YOUNG STARS TELL THE SAME STORY? THE CASE OF THE SPIRAL GALAXY NGC 300

We have obtained new spectrophotometric data for 28 H II regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of several auroral lines, including [O III] λ4363, [S III] λ6312, and [N II] λ5755, has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines, and obtain the following least-square solution: 12 + log(O/H) = 8.57(±0.02) – 0.41(±0.03)R/R 25, where the galactocentric distance is expressed in terms of the isophotal radius R 25. The characteristic oxygen abundance, measured at 0.4 × R 25, is 12 + log(O/H) = 8.41. The gradient corresponds to –0.077 ± 0.006 dex kpc–1, and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy, –0.081 ± 0.011 dex kpc–1. The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data, which is 8.59(±0.05). This allows little room for depletion of nebular oxygen onto dust grains, although in this kind of comparison we are somewhat limited by systematic uncertainties, such as those related to the atomic parameters used to derive the chemical compositions. We discuss the implications of our result with regard to strong-line abundance indicators commonly used to estimate the chemical compositions of star-forming galaxies, such as R 23. By applying a few popular calibrations of these indices based on grids of photoionization models on the NGC 300 H II region fluxes, we find metallicities that are higher by 0.3 dex (a factor of 2) or more relative to our nebular (Te based) and stellar ones. We detect Wolf-Rayet stellar emission features in ~1/3 of our H II region spectra, and find that in one of the nebulae hosting these hot stars the ionizing field has a particularly hard spectrum, as gauged by the softness parameter η = (O+/O++)/(S+/S++). We suggest that this is related to the presence of an early WN star. By considering a larger sample of extragalactic H II regions we confirm, using direct abundance measurements, previous findings of a metallicity dependence of η, in the sense that softer stellar continua are found at high metallicity.

Detection of Neutral Hydrogen in Early-Type Dwarf Galaxies of the Sculptor Group

We present results of deep 21 cm neutral hydrogen (H I) line observations of five early- and mixed-type dwarf galaxies in the nearby Sculptor group using the Australia Telescope National Facility 64 m Parkes Radio Telescope. Four of these objects, ESO 294-G010, 410-G005, 540-G030, and 540-G032, were detected in H I with neutral hydrogen masses in the range × 105 M⊙ (M/LB = 0.08, 0.13, 0.16, and 0.18 M⊙ L, respectively). These H I masses are consistent with the gas mass expected from stellar outflows over a large period of time. Higher spatial resolution H I data from the Australia Telescope Compact Array interferometer were further analyzed to measure more accurate positions and the distribution of the H I gas. In the cases of the dwarfs ESO 294-G010 and ESO 540-G030, we find significant offsets of 290 and 460 pc, respectively, between the position of the H I peak flux and the center of the stellar component. These offsets are likely to have internal causes such as the winds from star-forming regions. The fifth object, the spatially isolated dwarf elliptical galaxy Scl-dE1, remains undetected at our 3 σ limit of 22.5 mJy km s-1 and thus must contain less than 105 M⊙ of neutral hydrogen. This leaves Scl-dE1 as the only Sculptor group galaxy known in which no interstellar medium has been found to date. The object joins a list of similar systems, including the Local Group dwarfs Tucana and Cetus, that do not fit into the global picture of the morphology-density relation in which gas-rich dwarf irregular galaxies are in relative isolation and gas-deficient dwarf elliptical galaxies are satellites of more luminous galaxies.

A Chandra Observation of the Nearby Sculptor Group Sd Galaxy NGC 7793

We present the results of a Chandra observation made of the nearby spiral galaxy NGC 7793: the effective exposure time of this observation was 49094 seconds. Twenty-two discrete sources were identified at a minimum of a 3 $\sigma$ level to an estimated limiting luminosity of ∼ $2\times10^{36}$ ergs sec $^{-1}$ . We have performed a spectral analysis of the known ultraluminous X-ray source (ULX) in this galaxy: statistically-acceptable fits to the spectrum can be obtained with either a power law model, a bremsstrahlung model or a DISKBB model. We have also searched for counterparts at multiple wavelengths to these sources: based on this search, we have classified two supernova remnants, one HII region and two foreground stars.

Multi-frequency study of extragalactic supernova remnants and HII regions

We present a multi-frequency study of supernova remnants (SNRs) and $\ion{H}{ii }$ regions in the nearby Sculptor Group Sd galaxy NGC 300, based on new ATCA observations at the wavelengths of 13 and 20 cm, XMM-Newton observations, newly-processed ROSAT (PSPC/HRI; Read & Pietsch 2001) and VLA (20/6 cm) images of this galaxy. We have investigated the physical properties at the X-ray and radio wavelengths of the 28 optical SNRs found by Blair & Long (1997) and have expanded on the multi-wavelength work by Pannuti et al. (2000) on this same galaxy. From a total of 54 radio sources and 11 X-ray sources, we report 18 SNRs and three (3) SNR candidates (classified by spectral index alone) in NGC 300. Five of these 18 SNRs are associated with reported optical SNRs and three have X-ray counterparts. An additional 12 radio SNRs are seen in the Blair & Long (1997) [S II] images. We also investigate the luminosity function of our SNRs. Three background radio sources are confirmed and 12 other sources could represent additional background objects. Twenty two radio correlations with OB associations within NGC 300 correspond to either $\ion{H}{ii }$ regions or SNRs making them a good tracer of SNRs near star-forming regions. Additionally, two radio sources coincide with potential globular clusters of NGC 300 reported by Kim et al. (2002).

An X‐Ray, Optical, and Radio Search for Supernova Remnants in the Nearby Sculptor Group Sd Galaxy NGC 7793

We present a multi-wavelength study of the properties of supernova remnants (SNRs) in the nearby Sculptor Group Sd galaxy NGC 7793. Using our own Very Large Array radio observations at 6 and 20 cm, as well as archived ROSAT X-ray data, previously published optical results and our own H-alpha image, we have searched for X-ray and radio counterparts to previously known optically-identified SNRs and for new previously unidentified SNRs at these two wavelength regimes. Only two of the 28 optically-identified SNRs are detected at another wavelength. The most noteworthy source in our study is N7793-S26, which is the only SNR that is detected at all three wavelengths. It features a long (approximately 450 pc) filamentary morphology that is clearly seen in both the optical and radio images. N7793-S26's radio luminosity exceeds that of the Galactic SNR Cas A, and based on equipartition calculations we determine that an energy of at least 10^52 ergs is required to maintain this source. A second optically identified SNR, N7793-S11, has detectable radio emission but is not detected in the X-ray. Complementary X-ray and radio searches for SNRs have yielded five new candidate radio SNRs, to be added to the 28 SNRs in this galaxy that have already been detected by optical methods. We find that the density of the ambient interstellar medium (ISM) surrounding these SNRs significantly impacts the spectral characteristics of SNRs in this galaxy, consistent with surveys of the SNR populations in other galaxies.

Tip of the red giant branch distance for the Sculptor group dwarf ESO 540-032

We present the rst VICCD photometry for the Sculptor group galaxy ESO 540-032 obtained at the Very Large Telescope UT1+FORS1. The (I; V I) colour-magnitude diagram indicates that this intermediate- type dwarf galaxy is dominated by old, metal-poor ((Fe=H) 1:7 dex) stars, with a small population of slightly more metal-rich ((Fe=H) 1:3 dex), young (age 150 500 Myr) stars. A discontinuity in the I-band luminosity function is detected at I0 =2 3:44 0:09 mag. Interpreting this feature as the tip of the red giant branch and adopting MI = 4:20 0:10 mag for its absolute magnitude, we have determined a Population II distance modulus of (m M)0 =2 7:64 0:14 mag (3:4 0:2 Mpc). This distance conrms ESO 540-032 as a member of the nearby Sculptor group but is signicantly larger than a previously reported value based on the Surface Brightness Fluctuation (SBF) method. The results from stellar population synthesis models suggest that the application of the SBF technique on dwarf galaxies with mixed morphology requires a detailed knowledge of the underlying stellar composition and thus oers no advantage over a direct distance measurement using the tip of the red giant branch as distance indicator. We produce the surface brightness proles for ESO 540-032 and derive the photometric and structural parameters. The global properties follow closely the relations between metallicity and both absolute magnitude and central surface brightness dened by dwarf elliptical galaxies in the Local Group. Finally, we identify and discuss a non-stellar object near the galaxy center which may resemble a globular cluster.

An X‐Ray, Optical, and Radio Search for Supernova Remnants in the Nearby Sculptor Group Sd Galaxy NGC 300

An X‐Ray, Optical, and Radio Search for Supernova Remnants in the Nearby Sculptor Group Sd Galaxy NGC 300

Testing the Surface Brightness Fluctuations Method for Dwarf Elliptical Galaxies in the Centaurus A Group

We have obtained deep B and R-band CCD photometry for five dwarf elliptical galaxies that were previously identified on Schmidt films covering the region of the Centaurus A (Cen A) group. From a Fourier analysis of the R-band CCD images we determined the surface brightness fluctuation (SBF) magnitude R for each stellar system. All magnitudes are similar, and, given the small color spread, they suggest that these low surface brightness galaxies lie approximately at the same distance, regardless of the assumed SBF zero point. Long-slit spectra have been acquired to derive redshifts for two of the dwarfs, ESO 269-066 and ESO 384-016. The velocities, v? = 784 km s-1 and v? = 561 km s-1, respectively, identify them unambiguously as Cen A group members. An age (H?)-metallicity (C2 ?4668) analysis of the spectra reveals an underlying old and metal-poor stellar population in both cases. Combining photometric and spectroscopic results we find strong evidence that indeed all dwarf galaxies are Cen A group members. Based on Cepheid, TRGB, and PNLF distances published for the two main Cen A group galaxies, NGC 5128 and NGC 5253, we adopted a mean group distance of 3.96 Mpc to calibrate the apparent fluctuation magnitudes. The resulting absolute SBF magnitudes R of the dEs correlate with the dereddened colors (B-R)0 as predicted by Worthey's stellar synthesis models, using the theoretical isochrones of Bertelli and collaborators. This good agreement allows a calibration of the SBF method for dwarf ellipticals in the color range 0.8 1.2, the R-color relation is steep, and accurate colors are needed to achieve SBF distances with an uncertainty of only 10%. One of the dwarfs, ESO 219-010, is located slightly behind the core of the Cen A group at about 4.8 Mpc, while the remaining four recover the mean group distance of 3.96 Mpc that was put into the calibration. The depth of the group is only 0.5 Mpc, which identifies the Cen A group as a spatially well-isolated galaxy aggregate, in contrast to the nearby Sculptor group.