Ring Of NGC Research Articles

Hidden Gems on a Ring: Infant Massive Clusters and Their Formation Timeline Unveiled by ALMA, HST, and JWST in NGC 3351

We use 0.1″ observations from the Atacama Large Millimeter Array (ALMA), Hubble Space Telescope (HST), and JWST to study young massive clusters (YMCs) in their embedded “infant” phase across the central starburst ring in NGC 3351. Our new ALMA data reveal 18 bright and compact (sub-)millimeter continuum sources, of which 8 have counterparts in JWST images and only 6 have counterparts in HST images. Based on the ALMA continuum and molecular line data, as well as ancillary measurements for the HST and JWST counterparts, we identify 14 sources as infant star clusters with high stellar and/or gas masses (∼105 M ⊙), small radii (≲ 5 pc), large escape velocities (6–10 km s−1), and short freefall times (0.5–1 Myr). Their multiwavelength properties motivate us to divide them into four categories, likely corresponding to four evolutionary stages from starless clumps to exposed H ii region–cluster complexes. Leveraging age estimates for HST-identified clusters in the same region, we infer an evolutionary timeline, ranging from ∼1–2 Myr before cluster formation as starless clumps, to ∼4–6 Myr after as exposed H ii region–cluster complexes. Finally, we show that the YMCs make up a substantial fraction of recent star formation across the ring, exhibit a nonuniform azimuthal distribution without a very coherent evolutionary trend along the ring, and are capable of driving large-scale gas outflows.

The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: The Case of the Circumnuclear Starburst Ring in NGC 7552

Luminous infrared galaxies (LIRGs), the most extreme star-forming galaxies in the nearby (D < 30 Mpc) universe, show a notable X-ray emission deficiency (up to a factor of ∼10) compared with predictions from scaling relations of galaxy-wide high-mass X-ray binary (HMXB) luminosity with star formation rate. In the nearby (≈20 Mpc) LIRG NGC 7552, the majority of the IR emission originates in a circumnuclear starburst ring, which has been resolved into several discrete knots of star formation. We present results from recent Chandra observations of NGC 7552, which reveal significant deficits in the 2–7 keV X-ray luminosities from two of the most powerful star-forming knots. We hypothesize that the expected luminous HMXB populations in these knots are either (1) obscured by very large column densities or (2) suppressed due to the knots having relatively high metallicity and/or very young ages (≲5 Myr). We distinguish between these possibilities using data from recent NuSTAR observations, whose sensitivity above 10 keV is capable of uncovering heavily obscured HMXB populations, since emission at these energies is more immune to absorption effects. We find no evidence of a heavily obscured HMXB population in the central region of NGC 7552, suggesting suppressed HMXB formation. We further show that metallicity-dependent scaling relations cannot fully account for the observed deficit from the most powerful star-forming knots or the central region as a whole. Thus, we suggest that recent bursts in local star formation activity likely drive the high L IR within these regions on timescales ≲5 Myr, shorter than the timescale required for the formation of HMXBs.

GOALS-JWST: NIRCam and MIRI Imaging of the Circumnuclear Starburst Ring in NGC 7469

We present James Webb Space Telescope (JWST) imaging of NGC 7469 with the Near-Infrared Camera and the Mid-InfraRed Instrument. NGC 7469 is a nearby, z = 0.01627, luminous infrared galaxy that hosts both a Seyfert Type-1.5 nucleus and a circumnuclear starburst ring with a radius of ∼0.5 kpc. The new near-infrared (NIR) JWST imaging reveals 66 star-forming regions, 37 of which were not detected by Hubble Space Telescope (HST) observations. Twenty-eight of the 37 sources have very red NIR colors that indicate obscurations up to A v ∼ 7 and a contribution of at least 25% from hot dust emission to the 4.4 μm band. Their NIR colors are also consistent with young (<5 Myr) stellar populations and more than half of them are coincident with the mid-infrared (MIR) emission peaks. These younger, dusty star-forming regions account for ∼6% and ∼17% of the total 1.5 and 4.4 μm luminosity of the starburst ring, respectively. Thanks to JWST, we find a significant number of young dusty sources that were previously unseen due to dust extinction. The newly identified 28 young sources are a significant increase compared to the number of HST-detected young sources (4–5). This makes the total percentage of the young population rise from ∼15% to 48%. These results illustrate the effectiveness of JWST in identifying and characterizing previously hidden star formation in the densest star-forming environments around active galactic nuclei (AGN).

Rings and arcs around evolved stars – III. Physical conditions of the ring-like structures in the planetary nebula IC 4406 revealed by MUSE

ABSTRACT We present the analysis of Very Large Telescope Multi Unit Spectroscopic Explorer (MUSE) observations of the planetary nebula (PN) IC 4406. MUSE images in key emission lines are used to unveil the presence of at least five ring-like structures north and south of the main nebula of IC 4406. MUSE spectra are extracted from the rings to unambiguously assess for the first time in a PN their physical conditions, electron density (ne), and temperature (Te). The rings are found to have similar Te as the rim of the main nebula, but smaller ne. Ratios between different ionic species suggest that the rings of IC 4406 have a lower ionization state than the main cavity, in contrast to what was suggested for the rings in NGC 6543, the Cat’s Eye Nebula.

Star formation in outer rings of S0 galaxies

Aims.Although S0 galaxies are usually thought to be “red and dead”, they often demonstrate star formation that is organized in ring structures. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. Two early-type galaxies with outer rings, NGC 6534 and MCG 11-22-015, were selected to be studied.Methods.After inspecting the gas excitation in the rings using the Baldwin–Phillips–Terlevich method, we estimated the star formation rates (SFR) in the two outer rings of our galaxies using several SFR indicators derived from narrow-band photometry in the Hαemission line and archival GALEX ultraviolet images of the galaxies.Results.The ionized gas is excited by young stars in the ring of NGC 6534 and partly by shocks in MCG 11-22-015. The oxygen abundances in the HII regions of the rings are close to solar. The derived SFRs allow us to qualitatively restore star formation histories (SFH) in the rings: in NGC 6534, the SFH has been flat during the last 100–200 Myr, and in MCG 11-22-015, the star formation has started only a few Myr ago. We suggest that the rings in NGC 6534 and MCG 11-22-015 have different natures: the former is a resonant ring supplied with gas perhaps through tidal effects, and the latter has been produced by a satellite accretion. Recent outer gas accretion is implied in both cases.

Double Nuclei in NGC 908 and NGC 1187

Abstract We analyze optical data cubes of the nuclear regions of two late-type galaxies, NGC 908 and NGC 1187, obtained with the Integral Field Unit of the Gemini Multi-Object Spectrograph. Both data cubes show stellar structures consistent with double nuclei. The morphology of the line-emitting areas in the central region of NGC 1187 is also of a double nucleus, while the spatial morphology of the line-emitting areas in the data cube of NGC 908 is consistent with a circumnuclear asymmetric ring. The emission-line ratios of the nuclear spectra (and, actually, along the entire field of view) of both galaxies are characteristic of H ii regions. In particular, based on its emission-line properties, the circumnuclear ring in NGC 908 can be identified as a star-forming ring. The observed spatial morphology of the stellar emission and also the differences in the properties of the stellar populations detected in the stellar nuclei of NGC 908 suggest that the most likely scenario to explain the double stellar nucleus in this object involves a minor merger, probably with a high mass ratio. On the other hand, considering the similar properties of the stellar populations in the stellar nuclei of NGC 1187, together with the stellar and gas kinematic properties, we conclude that the most likely scenario to explain the double stellar and gas nucleus in this galaxy involves the stellar and gas kinematics, in the form of a circular nuclear disk subject to perturbations.

Multi-wavelength campaign on NGC 7469

We investigate the physical structure of the active galactic nucleus (AGN) wind in the Seyfert-1 galaxy NGC 7469 through high-resolution X-ray spectroscopy with Chandra HETGS and photoionisation modelling. Contemporaneous data from Chandra, HST, and Swift are used to model the optical-UV-X-ray continuum and determine the spectral energy distribution (SED) at two epochs, 13 yr apart. For our investigation we use new observations taken in December 2015–January 2016, and historical ones taken in December 2002. We study the impact of a change in the SED shape, seen between the two epochs, on the photoionisation of the wind. The HETGS spectroscopy shows that the AGN wind in NGC 7469 consists of four ionisation components, with their outflow velocities ranging from − 400 to − 1800 km s-1. From our modelling we find that the change in the ionising continuum shape between the two epochs results in some variation in the ionisation state of the wind components. However, for the main ions detected in X-rays, the sum of their column densities over all components remains in practice unchanged. For two of the four components, which are found to be thermally unstable in both epochs, we obtain 2 &lt; r &lt; 31 pc and 12 &lt; r &lt; 29 pc using the cooling and recombination timescales. For the other two thermally stable components, we obtain r &lt; 31 pc and r &lt; 80 pc from the recombination timescale. The results of our photoionisation modelling and thermal stability analysis suggest that the absorber components in NGC 7469 are consistent with being a thermally driven wind from the AGN torus. Finally, from analysis of the zeroth-order ACIS/HETG data, we discover that the X-ray emission in the range 0.2–1 keV is spatially extended over 1.5–12′′. This diffuse soft X-ray emission is explained by coronal emission from the nuclear starburst ring in NGC 7469. The star formation rate inferred from this diffuse soft X-ray emission is consistent with those found by far-infrared studies of NGC 7469.

MUSE observations of the counter-rotating nuclear ring in NGC 7742

Aims. We present results from MUSE observations of the nearly face-on disk galaxy NGC 7742. This galaxy hosts a spectacular nuclear ring of enhanced star formation, which is unusual in that it is hosted by a non-barred galaxy, and because this star formation is most likely fuelled by externally accreted gas that counter-rotates with respect to its main stellar body. Methods. We used the MUSE data to derive the star-formation history (SFH) and accurately measure the stellar and ionized-gas kinematics of NGC 7742 in its nuclear, bulge, ring, and disk regions. Results. We have mapped the previously known gas counter-rotation well outside the ring region and deduce the presence of a slightly warped inner disk, which is inclined at approximately 6° compared to the outer disk. The gas-disk inclination is well constrained from the kinematics; the derived inclination 13.7° ± 0.4° agrees well with that derived from photometry and from what one expects using the inverse Tully-Fisher relation. We find a prolonged SFH in the ring with stellar populations as old as 2–3 Gyr and an indication that the star formation triggered by the minor merger event was delayed in the disk compared to the ring. There are two separate stellar components: an old population that counter-rotates with the gas, and a young one, concentrated to the ring, that co-rotates with the gas. We recover the kinematics of the old stars from a two-component fit, and show that combining the old and young stellar populations results in the erroneous average velocity of nearly zero found from a one-component fit. Conclusions. The spatial resolution and field of view of MUSE allow us to establish the kinematics and SFH of the nuclear ring in NGC 7742. We show further evidence that this ring has its origin in a minor merger event, possibly 2–3 Gyr ago.

Multiwavelength study of the radio emission from a tight galaxy pair Arp 143

We present results of the recent low-frequency radio observations of a tight galaxy pair Arp 143 at 234 and 612 MHz. These data are analysed together with the archive data at 1490, 4860, 8440, and 14940 MHz. From the analysis of the radio emission we derive constraints on the age of the radio emitting structures as well as on the properties of their magnetic field. We show that the collisional ring of NGC 2445 hosts strong magnetic fields (reaching 12 $\mu$G in its northwestern part) manifesting as a steep--spectrum, nonthermal radiation at radio frequencies. The spectral age of this structure is higher than estimates derived for the star-forming regions from the H$\alpha$ distribution, suggesting that the radio emission might have a different origin. The galactic core is of a very young spectral age, suggesting an ongoing starburst activity. Additionally we identify a possible ridge of emission between the ring galaxy and its elliptical companion NGC 2444.

Bar pattern speed and position of the circumnuclear ring in NGC 1097

We present the first galactic-scale model of the gas dynamics of the prototype barred Seyfert 1 galaxy NGC1097. We use large scale FaNTOmM Fabry-Perot interferometric data covering the entire galactic disc and combine the distribution and kinematics maps with high resolution two-dimensional spectroscopy from the Gemini telescope. We build a dynamical model for the gravitational potential by applying the analytic solution to the equations of motion, within the epicyclic approximation. Our model reproduces all the significant kinematic and structural signatures of this galaxy. We find that the primary bar is 7.9+/-0.6 kpc long and has a pattern speed of 36 +/- 2 km s^-1 kpc^-1. This places the corotation radius at 8.6 +/-0.5 kpc, the outer Lindblad resonance at 14.9+/-0.9 kpc and two inner Lindblad resonances at 60+/-5 pc and 2.9+/-0.1 kpc. These derivations lead to a ratio of the corotation radius over bar length of 1.0--1.2, which is in agreement with the predictions of simulations for fast galaxy bars. Our model presents evidence that the circumnuclear ring in this galaxy is not located near any of the resonance radii in this galaxy. The ring might have once formed at the outer inner Lindblad resonance radius, and it has been migrating inward, toward the centre of the galactic gravitational potential.

Spectropolarimetry with the Giant Metrewave Radio Telescope at 610 MHz: a case study of two southern compact group fields

We present 610 MHz spectropolarimetric images from the Giant Metrewave Radio Telescope (GMRT). We discuss the properties of the GMRT's full-polarisation mode in detail and provide a technical characterisation of the instrument's polarisation performance. We show that the telescope can be used for wide-field spectropolarimetry. The application of Rotation Measure Synthesis results in a sensitivity at the level of tens of micro-Jy in Faraday space. As a demonstration of the instrument's capabilities, we present observations of two Southern Compact Groups - the Grus Quartet and USCG S063. The observations are compared with other radio and infra-red data to constrain the group members' spectral index, polarisation fraction, and Faraday depth. Radio continuum emission is detected from all four galaxies in the Grus Quartet and one galaxy in USCG S063. Linear polarisation is detected from a circumnuclear starburst ring in NGC 7552, the active nucleus of NGC 7582, and three extended radio galaxies in the background of the Grus Quartet field. These background sources allow for the classification of an FR-I and an X-shaped radio galaxy. No evidence is found for interaction with the intragroup medium in either galaxy group.

Explaining two circumnuclear star forming rings in NGC 5248

The distribution of gas in the central kiloparsec of a galaxy has a dynamically rapid evolution. Nonaxisymmetries in the gravitational potential of the galactic disk, such as a large scale stellar bar or spiral, can lead to significant radial motion of gaseous material from larger radii to the central region. The large influx of gas and the subsequent star formation keep the central region constantly changing. However, the ability of gas to reach the nucleus proper to fuel an AGN phase is not guaranteed. Gas inflow can be halted at a circumnuclear star forming ring several hundred parsec away. The nearby galaxy NGC5248 is especially interesting in this sense since it is said to host 2 circumnuclear star forming rings at 100pc and 370pc from its quiescent nucleus. Here we present new subarcsecond PdBI+30m CO(2-1) emission line observations of the central region. For the first time the molecular gas distribution at the smallest stellar ring is resolved into a gas ring, consistent with the presence of a quiescent nucleus. However, the molecular gas shows no ring structure at the larger ring. We combine analyses of the gaseous and stellar content in the central kiloparsec of this galaxy to understand the gas distribution and dynamics of this star forming central region. We discuss the probability of two scenarios leading to the current observations, given our full understanding of this system, and discuss whether there are really two circumnuclear star forming rings in this galaxy.

How does star formation proceed in the circumnuclear starburst ring of NGC 6951?

Gas inflowing along stellar bars is often stalled at the location of circumnuclear rings, that form an effective reservoir for massive star formation and thus shape the central regions of galaxies. However, how exactly star formation is proceeding within these circumnuclear starburst rings is subject of debate. Two main scenarios for this process have been put forward: In the first the onset of star formation is regulated by the total amount of gas present in the ring with star forming starting once a mass threshold has reached in a `random' position within the ring like `popcorn'. In the second star formation preferentially takes place near the locations where the gas enters the ring. This scenario has been dubbed `pearls-on-a-string'. Here we combine new optical IFU data covering the full stellar bar with existing multi-wavelength data to study in detail the 580 pc radius circumnuclear starburst ring in the nearby spiral galaxy NGC 6951. Using HST archival data together with Sauron and Oasis IFU data, we derive the ages and stellar masses of star clusters as well as the total stellar content of the central region. Adding information on the molecular gas distribution, stellar and gaseous dynamics and extinction, we find that the circumnuclear ring in NGC 6951 is ~1-1.5 Gyr old and has been forming stars for most of that time. We see evidence for preferred sites of star formation within the ring, consistent with the `pearls-on-a-string' scenario, when focusing on the youngest stellar populations. Due to the ring's longevity this signature is washed out when older stellar populations are included in the analysis.

The star formation history of the Large Magellanic Cloud star clusters NGC 1846 and NGC 1783★

NGC1846 and NGC1783 are two massive star clusters in the Large Magellanic Cloud, hosting both an extended main sequence turn-off and a dual clump of red giants. They present similar masses but differ mainly in angular size. Starting from their high-quality ACS data in the F435W, F555W and F814W filters, and updated sets of stellar evolutionary tracks, we derive their star formation rates as a function of age, SFR(t), by means of the classical method of CMD reconstruction which is usually applied to nearby galaxies. The method confirms the extended periods of star formation derived from previous analysis of the same data. When the analysis is performed for a finer resolution in age, we find clear evidence for a 50-Myr long hiatus between the oldest peak in the SFR(t), and a second prolonged period of star formation, in both clusters. For the more compact cluster NGC1846, there seems to be no significant difference between the SFR(t) in the cluster centre and in an annulus with radii between 20 and 60 arcsec (from 4.8 to 15.4 pc). The same does not occur in the more extended NGC1783 cluster, where the outer ring (between 33 and 107 arcsec, from 8.0 to 25.9 pc) is found to be slightly younger than the centre. We also explore the best-fitting slope of the present-day mass function and binary fraction for the different cluster regions, finding hints of a varying mass function between centre and outer ring in NGC1783. These findings are discussed within the present scenarios for the formation of clusters with multiple turn-offs.

INTEGRAL FIELD SPECTROSCOPY AND MULTI-WAVELENGTH IMAGING OF THE NEARBY SPIRAL GALAXY NGC 5668: AN UNUSUAL FLATTENING IN METALLICITY GRADIENT

We present the analysis of the full bi-dimensional optical spectral cube of the nearby spiral galaxy NGC 5668, observed with the PPAK IFU at the Calar Alto observatory 3.5m telescope. We make use of broad-band imaging to provide further constraints on the evolutionary history of the galaxy. This dataset will allow us to improve our understanding of the mechanisms that drive the evolution of disks. We investigated the properties of 62 H II regions and concentric rings in NGC 5668 and derived maps in ionized-gas attenuation and chemical (oxygen) abundances. We find that, while inwards of r\,$\sim\,36",\sim$\,4.4kpc\,$\sim$\,0.36\,$(\frac {D_{25}}{2})$ the derived O/H ratio follows the radial gradient typical of spiral galaxies, the abundance gradient beyond r$\sim36"$ flattens out. The analysis of the multi-wavelength surface brightness profiles of NGC 5668 is performed by fitting these profiles with those predicted by chemo-spectrophotometric evolutionary models of galaxy disks. From this, we infer a spin and circular velocity of $\lambda$=0.053 and v$_{c}$=167\,km\,s$^{-1}$, respectively. The metallicity gradient and rotation curve predicted by this best-fitting galaxy model nicely match the values derived from the IFU observations, especially within r\,$\sim36\arcsec$. The same is true for the colors despite of some small offsets and a reddening in the bluest colors beyond that radius. On the other hand, deviations of some of these properties in the outer disk indicate that a secondary mechanism, possibly gas transfer induced by the presence of a young bar, must have played a role in shaping the recent chemical and star formation histories of NGC 5668.

High resolution IR observations of the starburst ring in NGC 7552

We observed the ring galaxy NGC 7552 with the mid-infrared (MIR) instrument VISIR at an angular resolution of 0.3"- 0.4" and with the near-infrared (NIR) integral-field spectrograph SINFONI on the VLT, and complement these observations with data from ISO and Spitzer. The starburst ring is clearly detected at MIR wavelengths at the location of the dust-extincted, dark ring seen in HST observations. This "ring", however, is a rather complex annular region of more than 100 parsec width. We find a large fraction of diffuse [Ne II] and PAH emission in the central region that is not associated with the MIR peaks on spatial scales of \sim30 pc. We do not detect MIR emission from the nucleus of NGC 7552, which is very prominent at optical and NIR continuum wavelengths. However, we have identified nine unresolved MIR peaks within the ring. The average extinction of these peaks is A(V)=7.4 and their total infrared luminosity is L(IR) = 2.1*10^10 Lo. The properties of these peaks are typical for MIR-selected massive clusters found in other galaxies. The ages of the MIR-selected clusters are in the range of 5.9\pm0.3 Myr. The age spread among the clusters of 0.8 Myr is small compared to the travel time of \sim5.6 Myr for half an orbit within the starburst ring. We find no strong evidence for a scenario where the continuous inflow of gas leads to the ongoing formation of massive clusters at the contact points between galactic bar and starburst ring. Instead, it appears more likely that the gas density build up more gradually over larger ring segments, and that the local physical conditions govern cluster formation. We note that the fundamental limitation on the accurate derivation of cluster age, mass and IMF slope is the lack of higher angular resolution.

Far-infrared line imaging of the starburst ring in NGC 1097 with theHerschel/PACS spectrometer

NGC 1097 is a nearby SBb galaxy with a Seyfert nucleus and a bright starburst ring. We study the physical properties of the interstellar medium (ISM) in the ring using spatially resolved far-infrared spectral maps of the circumnuclear starburst ring of NGC 1097, obtained with the PACS spectrometer on board the <i>Herschel<i/> Space Observatory. In particular, we map the important ISM cooling and diagnostic emission lines of [OI] 63 <i>μ<i/>m, [OIII] 88 <i>μ<i/>m, [NII] 122 <i>μ<i/>m, [CII] 158 <i>μ<i/>m and [NII] 205 <i>μ<i/>m. We observe that in the [OI] 63 <i>μ<i/>m, [OIII] 88 <i>μ<i/>m, and [NII] 122 <i>μ<i/>m line maps, the emission is enhanced in clumps along the NE part of the ring. We observe evidence of rapid rotation in the circumnuclear ring, with a rotation velocity of ~220 km s<sup>-1<sup/> (inclination uncorrected) measured in all lines. The [OI] 63 <i>μ<i/>m/[CII] 158 <i>μ<i/>m ratio varies smoothly throughout the central region, and is enhanced on the northeastern part of the ring, which may indicate a stronger radiation field. This enhancement coincides with peaks in the [OI] 63 <i>μ<i/>m and [OIII] 88 <i>μ<i/>m maps. Variations of the [NII] 122 <i>μ<i/>m/[NII] 205 <i>μ<i/>m ratio correspond to a range in the ionized gas density between 150 and 400 cm<sup>-3<sup/>.

Rings and haloes in the mid-infrared: the planetary nebulae NGC 7354 and NGC 3242

We present images of the planetary nebulae NGC 7354 and NGC 3242 in four mid-infrared (MIR) photometric bands centred at 3.6, 4.5, 5.8 and 8.0 μm, the results of observations undertaken using the Spitzer Space Telescope. The resulting images show the presence of a halo and rings in NGC 3242, as previously observed through narrow-band imaging at visual wavelengths, as well as evidence for a comparable halo and ring system in NGC 7354. This is the first time that a halo and rings have been observed in the latter source. Similarly, whilst partial rings may have been previously detected in the MIR (for the source NGC 3132), the present observations appear to constitute the first detections of complete ring systems outside of the visual wavelength regime. The halo/core emission ratios appear to be preferentially higher at MIR wavelengths than is the case in the visible, and show less steep fall-offs than those observed in the [O iii]λλ4959+ 5007 A transitions. The variation in surface brightness S with radial distance R, where this is approximated by the power-law relation S∝R−β, implies exponents 1.7 < β < 4 in the inner portions of the haloes, and 4.5 < β < 12 towards the outer limits of these structures. The value of β is also, in most cases, somewhat smaller at longer wavelengths. It is additionally noted that the 3.6 μm/4.5 μm, 5.8 μm/4.5 μm and 8.0 μm/4.5 μm flux ratios increase markedly away from the nuclei, and reach their maximum values in the haloes, where much of the longer wave flux may derive from dust band and continuum emission. An analysis of the rings suggests that some of them, at least, are likely to be associated with higher densities of dust particles, and that the gaseous and particle ring systems are likely to be spatially in register. Finally, we have analysed the formation of haloes as a result of radiatively accelerated mass loss in the AGB progenitors. Although the models assume that dust formation occurs in C-rich environments, we note that qualitatively similar results would be expected for O-rich progenitors as well. The model fall-offs in halo density are found to result in gradients in halo surface brightness which are similar to those observed in the visible and MIR.

A MULTIWAVELENGTH VIEW OF STAR FORMATION IN INTERACTING GALAXIES IN THE PAVO GROUP

ABSTRACT We combine Spitzer Infrared Array Camera mid-infrared and Chandra X-ray observations of the dominant galaxies NGC 6872 and NGC 6876 in the Pavo group with archival optical and H i data to study interaction-induced star formation. In the spiral galaxy NGC 6872, 8.0 μm and 5.8 μm nonstellar emission having colors consistent with polycyclic aromatic hydrocarbons (PAHs) is concentrated primarily in clumps in three regions: in a 5 kpc radius outer ring about the center of the spiral galaxy, in a bridge of emission connecting NGC 6872's northern spiral arm to IC 4970, and along the full extent of both NGC 6872's tidal arms. PAH emission is correlated with young star clusters and dense H i regions. We find no strong differences in the mid-infrared colors of the star-forming regions in the spiral galaxy NGC 6872 as a function of position relative to the tidally interacting companion galaxy IC 4970. We find 11 very luminous X-ray sources (≳ (0.5–5) ×1039 erg s−1 ) clustered to the southwest in NGC 6872, near bright star-forming regions. In NGC 6872's tidal features, young star clusters form at the boundaries of diffuse X-ray gas, suggesting that stars form as gas stripped by the interactions cools. The nucleus of NGC 6872 is a weak X-ray point source with 0.5–8 keV luminosity of 8.5 × 1039 erg s−1 , but there is little evidence in the inner 1 kpc of NGC 6872 for PAH emission from recent star formation or nuclear activity. However, a 4 kpc “stream,” leading from the outer ring of NGC 6872 to the nucleus, may signal the transport of interstellar matter into NGC 6872's nuclear region. Nonstellar emission, consistent with PAH emission, is also found in the central region of elliptical galaxy NGC 6877, companion to the dominant Pavo group elliptical galaxy NGC 6876. However, in the central region of NGC 6876, the dust emission is more likely due to silicate emission from old asymptotic giant branch stars.

A SINFONI view of the nuclear star formation ring in NGC 613

We present preliminary results from a near-infrared, integral-field study of the central 500 pc in the nearby Sbc spiral NGC613. More specifically, we present maps of various gaseous emission lines in which the ring structure can be identified. The ring consists of at least seven “hot spots” that are actively forming stars and thus can be identified in recombination lines of hydrogen (H) and Helium (He). The molecular gas (traced by emission from the H2 molecule) is more evenly distributed over the entire ring. We also present Hand K-band spectra of one of the ring hot spots and the nucleus of NGC613 (i.e. the central 1′′). The nucleus emits little or no flux in H or He recombination lines. Instead, its spectrum is dominated by H2 and [FeII] emission, suggesting that NGC613 is currently in a phase in which gas is accumulating within the central 50 pc. Moreover, we find clear evidence that the ring structure is disturbed by an energetic outflow from the nucleus, supporting evidence that NGC613 harbors an AGN.