Star Formation In NGC Research Articles

SPITZER OBSERVATIONS OF THE H II REGION NGC 2467: AN ANALYSIS OF TRIGGERED STAR FORMATION

We present new Spitzer Space Telescope observations of the region NGC 2467, and use these observations to determine how the environment of an H II region affects the process of star formation. Our observations comprise IRAC (3.6, 4.5, 5.8, and 8.0 μm) and MIPS (24 μm) maps of the region, covering approximately 400 arcmin2. The images show a region of ionized gas pushing out into the surrounding molecular cloud, powered by an O6V star and two clusters of massive stars in the region. We have identified as candidate young stellar objects (YSOs) 45 sources in NGC 2467 with infrared excesses in at least two mid-infrared colors. We have constructed color-color diagrams of these sources and have quantified their spatial distribution within the region. We find that the YSOs are not randomly distributed in NGC 2467; rather, over 75% of the sources are distributed at the edge of the H II region, along ionization fronts driven by the nearby massive stars. The high fraction of YSOs in NGC 2467 that are found in proximity to gas that has been compressed by ionization fronts supports the hypothesis that a significant fraction of the star formation in NGC 2467 is triggered by the massive stars and the expansion of the H II region. At the current rate of star formation, we estimate at least 25%-50% of the total population of YSOs formed by this process.

Off-Nuclear Star Formation and Obscured Activity in the Luminous Infrared Galaxy NGC 2623

New optical Hubble Space Telescope (HST), Spitzer Space Telescope, and XMM observations of the luminous infrared galaxy (LIRG) NGC 2623 are presented. This galaxy was observed as part of the Great Observatories All-sky LIRG Survey (GOALS). The prominent 3.2 kpc southern extension to the nucleus has been resolved by HST observations into ~100 star clusters, making it one of the richest off-nuclear concentrations of bright clusters observed in GOALS. The clusters have M_(F555W) ~-6.6 to -12.6 mag, which is within the magnitude range of Antennae galaxy clusters and in excess of 30 Doradus clusters at the high end. Their optical colors are primarily consistent with ages of ~1–100 Myr. Archival GALEX data show the off-nuclear region to be extremely bright in the far-ultraviolet, being equivalent in luminosity to the resolved nuclear region at 0.15 µm, but becoming less energetically significant at increasing wavelengths. In addition, [Ne v] 14.3 µm emission is detected with Spitzer IRS, confirming the inference from the X-ray and radio data that an active galactic nucleus (AGN) is present. Thus, the off-nuclear optical clusters are associated with a secondary burst of activity corresponding to a star formation rate ~0.1–0.2 M⊙ yr^(-1); the bulk of infrared (and thus bolometric) luminosity is generated via star formation and an AGN embedded behind dust within the inner kiloparsec of the system. If the infrared luminosity is primarily reprocessed starlight, the off-nuclear starburst accounts for <1% of the present star formation in NGC 2623.

Star Formation in NGC 5194 (M51a). II. The Spatially Resolved Star Formation Law

We have studied the relationship between the star formation rate (SFR) surface density and gas surface density in the spiral galaxy M51a (NGC 5194), using multi-wavelength data obtained as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS). We introduce a new SFR index based on a linear combination of H-alpha emission-line and 24 micron continuum luminosities, that provides reliable extinction-corrected ionizing fluxes and SFR densities over a wide range of dust attenuations. The combination of these extinction-corrected SFR densities with aperture synthesis HI and CO maps has allowed us to probe the form of the spatially-resolved star formation law on scales of 0.5 to 2 kpc. We find that the resolved SFR vs gas surface density relation is well represented by a Schmidt power law, which is similar in form and dispersion to the disk-averaged Schmidt law. We observe a comparably strong correlation of the SFR surface density with the molecular gas surface density, but no significant correlation with the surface density of atomic gas. The best-fitting slope of the Schmidt law varies from N = 1.37 to 1.56, with zeropoint and slope that change systematically with the spatial sampling scale. We tentatively attribute these variations to the effects of areal sampling and averaging of a nonlinear intrinsic star formation law. Our data can also be fitted by an alternative parametrization of the SFR surface density in terms of the ratio of gas surface density to local dynamical time, but with a considerable dispersion.

Study of star formation in NGC 1084

ABSTRACT We present UBVRI broad band, Hα narrow band photometry of the star formingcomplexes in the infra-red bright galaxy NGC 1084. Results of medium resolutionspectroscopy of some of the brighter complexes are also discussed. Spectroscopic datais used to better estimate the internal reddening within the galaxy which is found to behighly variable and to calculate metallicity which is close to the solar value. Diagnosticdiagramidentifies the shockedregionswithin this galaxy.The narrowbandHα fluxandits equivalent width are used to determine the star formation rates of the complexesand the distribution of ages. Star formation rates for a few of the complexes are foundto be as high as 0.5 M ⊙ yr −1 . The star forming complexes lie in the age range 3 Myrto 6.5 Myr. U − B vs V − I colour-colour mixed population model created usingthe Starburst99 model colours is used to estimate the ages of the stellar populationspresent within these regions. Using this technique, it is found that the star formationin NGC 1084 has taken place in a series of short bursts over the last 40 Myr or so. Itis proposed that the likely trigger for enhanced star formation is merger with a gasrich dwarf galaxy.Key words: star forming regions; general - galaxies: individual: NGC 1084

NGC 5253 and ESO 269-G058: Dwarf Galaxies with a Past

Deep r' and i' images obtained with GMOS on Gemini South are used to probe the bright stellar content in the outer regions of the Centaurus Group dwarf galaxies NGC 5253 and ESO 269-G058. Red giant branch (RGB) stars are traced out to a distance of 8 kpc along the major axis of NGC 5253 and 6 kpc in ESO 269-G058. The outer regions of both galaxies are metal-poor; RGB stars located between projected major axis distances of 2 and 4 kpc in NGC 5253 have [Fe/H] ∼ -1, whereas RGB stars in the corresponding portion of ESO 269-G058 have [Fe/H] ∼ -1.8. Stars with metallicities that differ from the mean by more than a few tenths of a dex make only a modest contribution to the stellar content in the outer regions of both galaxies. A population of bright asymptotic giant branch (AGB) stars is seen in both galaxies. In the particular case of NGC 5253, roughly 1%-10% of the stellar mass may have formed during the past few hundred million years, and it is suggested that the progenitors of the two recent Type Ia supernovae in this galaxy may have formed at this time. That NGC 5253 has experienced either episodic or continuing elevated levels of star formation during the past few hundred million years is reminiscent of what is seen in other dwarf starburst galaxies, such as NGC 3077, and it is argued that the current episodes of star formation in NGC 5253 and ESO 269-G058 may have been triggered up to ∼1 Gyr in the past. Finally, a distance modulus is computed for each galaxy based on the brightness of the RGB tip, and the results are μ0 = 27.48 ± 0.14 for NGC 5253 and 27.93 ± 0.18 for ESO 269-G058.

Champagne flow and triggered star formation in NGC 1893

Using Hα grism spectroscopy and optical and Two-Micron All-Sky Survey photometry of the fields containing NGC 1893 and two emission nebulae, Sim 129 and Sim 130, we show the presence of a number of Hα and near-infrared excess sources towards the region between HD 242935 and the two emission nebulae. Among them a large majority of these pre-main-sequence candidate sources are concentrated towards regions closer to Sim 129 and Sim 130. Age distribution of these sources (viz. main-sequence stars in the cluster, including HD 242935, young sources located in the region between HD 242935 and two nebulae and those located inside Sim 129) obtained from their positions in optical V, V−I colour–magnitude diagram strongly suggest ‘small-scale sequential star formation’ along the axes of the clouds caused by the outward advance of ionization/shock front from the H ii region. The IRAS source, IRAS 05198+3325, identified with CPM 16 is a Herbig Be (B3) type star showing Hα and Ca ii triplet lines in emission. From the orientation of the two emission nebulae it is suggested that HD 242935 is most likely responsible for the cometary morphology of the two nebulae and the trigger of star formation in the region. From the global distribution of the interstellar material, it is also suggested that the same source is responsible for the champagne flow causing the entire molecular cloud to have a cometary appearance.

A dust component ~2 kpc above the plane in NGC 891

Context. The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in Hα emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time. Aims. We suggest that NGC 891 is intrinsically almost symmetric and the large asymmetry in Hα emission is mostly due to dust attenuation. We will quantify the additional optical depth needed to cause the observed asymmetry in this model. Methods. By comparing large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. Results. From the 24 µm emission we find that the intrinsic asymmetry in star formation in NGC 891 is small i.e., ∼30%. The additional asymmetry in Hα is modeled as additional symmetric dust attenuation which extends up to ∼40 �� (1.9 kpc) above the plane of the galaxy with a mid-plane value of τ = 0.8 and a scale height of 0.5 kpc

An XMM-Newton view of the young open cluster NGC 6231 - III. Optically faint X-ray sources

We discuss the properties of the X-ray sources with faint optical counterparts in the very young open cluster NGC 6231. From their positions in the Hertzsprung–Russell diagram, we find that the bulk of these objects probably consists of low-mass pre-main-sequence (PMS) stars with masses in the range 0.3–3.0 M⊙. The age distribution of these objects indicates that low-mass star formation in NGC 6231 started more than 10 Myr ago and culminated in a starburst-like event about 1–4 Myr ago when the bulk of the low-mass PMS stars as well as the massive cluster members formed. We find no evidence for a spatial age gradient that could point towards a sequential star formation process. Only a few X-ray sources have counterparts with a reddening exceeding the average value of the cluster or with infrared colours indicating the presence of a moderate near-IR excess. The X-ray spectra of the brightest PMS sources are best fitted by rather hard thermal plasma models and a significant fraction of these sources display flares in their light curve. The X-ray brightest flaring sources have decay times between 2 and 16 ks. The X-ray selected PMS stars in NGC 6231 have log LX/Lbol values that increase strongly with decreasing bolometric luminosity and can reach a saturation level (log LX/Lbol∼−2.4) for non-flaring sources and even more extreme values during flares.

Polychromatic view of intergalactic star formation in NGC 5291

Context.Star formation (SF) takes place in unusual places such as way out in the intergalactic medium out of material expelled from parent galaxies.

Arp 116: interacting system or chance alignment?

Using high spatial resolution Hubble Space Telescope /Advanced Camera for Surveys archival imaging observations of Arp 116, centred on the elliptical galaxy NGC 4649, we explore the novel technique of pixel-by-pixel analysis of the galaxy's colour–magnitude diagram to search for any evidence of recent enhanced star formation due to the apparent tidal interaction with its spiral companion, NGC 4647. From a detailed analysis of the system's geometry, and based on additional circumstantial evidence from extant multi-wavelength observations, we conclude that, while there may be grounds for the tidal-interaction assumption for this system, any interaction has thus far been of insufficient strength to trigger an enhanced level of recent star formation in the elliptical component, although close inspection of our colour images shows a faint excess of bluer pixels (a ~0.20 mag bluer “loop”) in the elliptical galaxy on the side of the spiral companion. Given that there appears to be a moderate reservoir of available gas for ongoing star formation (although at low column density), this suggests that we are currently witnessing the onset of the tidal interaction between NGC 4647 and NGC 4649. In addition, the triggering of new star formation in NGC 4649 may be significantly impeded due to the much lower mass of the spiral component.

ChandraObservations of Circumnuclear Star Formation in NGC 3351

ThenearbySB(r)bgalaxy NGC3351 (M95)displays an � 600 pcdiameter star-forming circumnuclearringfueled bygas accretedthrough astellar bar.TheX-ray emission from this region is composedof numerous pointlikesources embedded in hot (kT ¼ 0:5 � 0:1 keV), nonuniform diffuse gas. Most of the pointlike sources are likely themselves concentrated regions of hot gas. The morphology of the X-ray emission is similar to, but not identical with, UVand Hhot spots in the circumnuclear ring. The emission morphology can be understood if star formation is occurring through a series of instantaneous starbursts at various locations around the ring and the UV, H� , and X-ray emission trace successively later stages in the evolution of the underlying stellar populations. X-ray emission extends at least 500 pc beyond the circumnuclear ring. This emission is interpreted as the result of gas outflow from the star-forming ring into the disk and halo of NGC 3351. There is evidence that the outflow is confined by cold ambient gas near the plane of the galaxy but thatflow out of the plane is not restricted. The nucleusof the galaxy lacks both large amounts of hot gas and any pointlike source above an estimated X-ray luminosity of 10 37 ergs s� 1 in the 0.5-8.0 keV band. This luminosity is orders of magnitude too low to account for the observed nuclear Hluminosity, 3:5 ; 10 38 ergs s � 1 , by X-ray photoionization. Subject headingg galaxies: individual (NGC 3351) — galaxies: starburst — galaxies: structure — X-rays: galaxies

Mapping and Mass Measurement of the Cold Dust in NGC 205 withSpitzer

We present observations at 3.6, 4.5, 5.8, 8, 24, 70 & 160um of NGC 205, the dwarf elliptical companion of M31, obtained with the Spitzer Space Telescope. The point sources subtracted images at 8 and 24um display a complex and fragmented infrared emission coming from both very small dust particles and larger grains. The extended dust emission is spatially concentrated in three main emission regions, seen at all wavelengths from 8 to 160um. These regions lie approximately along NGC 205's semi-major axis and range from ~100 to 300 pc in size. Based on our mid-to-far infrared flux density measurements alone, we derive a total dust mass estimate of the order of 3.2e4 solar masses, mainly at a temperature of ~20K. The gas mass associated with this component matches the predicted mass returned by the dying stars from the last burst of star formation in NGC 205 (~0.5 Gyr ago). Analysis of the Spitzer data combined with previous 1.1mm observations over a small central region or Core'' (18 diameter), suggest the presence of very cold (~12K) dust and a dust mass about sixteen times higher than is estimated from the Spitzer measurements alone. Assuming a gas to dust mass ratio of 100, these two datasets, i.e.with and without the millimeter observations, suggest a total gas mass range of 3.2e6 to 5e7 solar masses.

Distribution of extinction and star formation in NGC 1569

We investigate spatial the distribution of the intrinsic extinction in the starburst dwarf galaxy NGC1569 creating an extinction map of the whole galaxy derived from the Halpha/Hbeta emission line ratio. We differentiate the extinction in the HII regions from the extinction of the diffuse gas. The intrinsic extinction shows considerable variations over the plane of the galaxy, from negligible extinction up to highest values of A(V)=0.8mag. The extinction map shows small scale clumpy structures possibly due to a clumpy dust distribution. We also identify in this map a shell structure, for which we establish a causal relation with the expanding gas structure produced by the stellar winds coming from the Super Star Clusters (SSC) in the center of the galaxy. The comparison of the spatial profiles of the extinction, dust and gaseous emissions crossing the border of the shell shows a layered structure; the peak of this Halpha distribution lying closest to the SSC A, followed outwards by the peak of the extinction and at a still larger distances by the bulk of the atomic gas. We suggest that the extinction shell has been produced by the SSCs and that it can be explained by the accumulation of dust at the border of this ionized gas structure.

The Virgo High-Resolution CO Survey: VI. Gas Dynamics and Star Formation along the Bar in NGC 4303

Abstract We present CO interferometer observations of the barred galaxy NGC 4303 (M 61). This galaxy has a strong gas concentration at the central region and prominent offset ridges at the leading sides of the bar. Sharp velocity gradients are apparent across the ridges. The brightness temperature in the ridges is low, indicating the existence of unresolved molecular clouds. Additionally, an analysis of the newborn stellar clusters revealed in HST images suggests that the parent molecular clouds had masses of $10^{4-6} \,{{{M}_{\odot}}}$. The observed shear velocity gradient across the ridges is too small to break up giant molecular clouds. Therefore, the clouds are likely to survive passage through the ridges. We discuss a cloud orbit model in a bar potential for understanding the gas distribution, dynamics, and star formation in NGC 4303. The model reproduces the narrow offset ridges and sharp velocity gradients across the ridges, although no shock is associated with the ridges. We discuss cloud–cloud collisions (and close interactions) as a possible triggering mechanism for star formation. The newborn stellar clusters in NGC 4303 are located predominantly at the leading sides of the offset ridges. Cloud orbits are densely populated in the region where the clusters are found, suggesting a high collisional frequency and possibly a high rate of triggered star formation. Cloud-based dynamics is less dissipative than smooth hydrodynamic models, possibly extending the timescales of gas dynamical evolution and gas fueling to central regions in barred galaxies.

The Star Formation Threshold in NGC 6822

We investigate the star formation threshold in NGC 6822, a nearby Local Group dwarf galaxy, on subkiloparsec scales using high-resolution, wide-field, deep H I, Hα, and optical data. In a study of the H I velocity profiles we identify cool and warm neutral components in the interstellar medium of NGC 6822. We show that the velocity dispersion of the cool component (~4 km s-1) when used with a Toomre Q-criterion gives an optimal description of ongoing star formation in NGC 6822 superior to that using the more conventional dispersion value of 6 km s-1. However, a simple constant surface density criterion for star formation gives an equally superior description. We also investigate the two-dimensional distribution of Q and the star formation threshold and find that these results also hold locally. The range in gas density in NGC 6822 is much larger than the range in critical density, and we argue that the conditions for star formation in NGC 6822 are fully driven by this density criterion. Star formation is local, and in NGC 6822 global rotational or shear parameters are apparently not important.

Star-formation in NGC 4038/4039 from broad and narrow band photometry: cluster destruction?

Accurately determining the star formation history in NGC 4038/4039 -- ``The Antennae'' is hampered by extinction. We therefore used near infrared images obtained with ISAAC at the VLT and with SOFI at the NTT to determine the recent star formation history in this merger. In combination with archival HST data, we determined ages, extinction and other parameters for single star clusters, and properties of the cluster population as a whole. About 70% of the K_s-band detected star clusters with masses >= 10^5 M_sun are younger than 10 Myrs (approximately an e-folding time for cluster ages), which we interpret as evidence for rapid dissolution but not free expansion. The total mass of K-band selected clusters is about 5-10x10^8 M_sun and represents about 3-6% of the total molecular gas. This takes into account only the detected clusters and in view of the rapid dissolution means that this is only a lower limit to the total mass of stars produced in clusters during the burst. Studies of cluster formation in other galaxies recently suggested short cluster dissolution timescales, too, which means that star formation rates may have been severely underestimated in the past. Extinction is strongly variable and very high in some regions, but around A_V=1.3 mag on average. Even though most clusters are detected at least in I-band, only the information about individual cluster ages and extinction allows to avoid uncertainties of orders of magnitude in star formation rate estimates determined from optical fluxes. From the distribution of individual cluster extinction vs. age, which is significantly higher for clusters below 8-9 Myr than for older clusters, we infer that this is the time by which a typical cluster blows free of its native dust cocoon.

Near-IR photometry of asymptotic giant branch stars in the dwarf elliptical galaxy NGC 185

J , H and K' images obtained with the Canada-France-Hawaii Telescope were used to investigate the stellar contents of the asymptotic giant branch (AGB) in the dwarf elliptical galaxy NGC 185. The bright parts of and color-magnitude diagrams consist of a group of bright blue stars, a dominant population of M-giants and a red C star population. There were 73 C stars with a mean magnitude of , corresponding to , and mean colors of and . The number ratio of C stars to M-giants was estimated to be without any radial gradient from the center of NGC 185. The and color distributions of AGB stars showed an M-giant peak and blue and red tails, where the latter two correspond to AGB stars younger than those along M-giant peak and C stars. The bolometric luminosity functions of M-giants and C stars indicate that the M-giant AGB sequence has terminated at , while the most luminous C star has . The bolometric luminosity function of C stars in NGC 185 is very similar to that of recent literature values derived from Vi band photometry. The logarithmic slope of the luminosity function for bright M-giant stars was estimated to be in K band. Theoretical isochrone models, compared with the observed near-infrared photometric properties of AGB stars, indicate that star formation in NGC 185 has a wide range of ages with possibly two different epochs of star formation.

The Anatomy of Star Formation in NGC 300

The Spitzer Space Telescope was used to study the mid-infrared to far-infrared properties of NGC 300, and to compare dust emission to Halpha to elucidate the heating of the ISM and the star formation cycle at scales < 100 pc. The new data allow us to discern clear differences in the spatial distribution of 8 micron dust emission with respect to 24 micron dust and to HII regions traced by the Halpha light. The 8 micron emission highlights the rims of HII regions, and the 24 micron emission is more strongly peaked in star forming regions than at 8 microns. We confirm the existence and approximate amplitude of interstellar dust emission at 4.5 microns, detected statistically in Infrared Space Observatory (ISO) data, and conclude it arises in star forming regions. When averaging over regions larger than ~ 1 kpc, the ratio of Halpha to Aromatic Feature emission in NGC 300 is consistent with the values observed in disks of spiral galaxies. The mid-to-far-infrared spectral energy distribution of dust emission is generally consistent with pre-Spitzer models.

Ionized gas kinematics and massive star formation in NGC 1530

We present emission line mapping of the strongly barred galaxy NGC 1530 obtained using Fabry-Perot interferom- etry in Hα, at significantly enhanced angular resolution compared with previously published studies. The main point of the work is to examine in detail the non-circular components of the velocity field of the gas, presumably induced by the strongly non-axisymmetric gravitational potential of the bar. To do this we first derive a model rotation curve making minimum assump- tions about kinematic symmetry, and go on to measure the non-circular component of the full radial velocity field. This clearly reveals the streaming motions associated with the spiral density wave producing the arms, and the quasi-elliptical motions with speeds of order 100 km s −1 aligned with the bar. It also shows in some detail how these flows swing in towards and around the nucleus as they cross a circumnuclear resonance, from the dominant x1 orbits outside the resonance to x2 orbits within it. Comparing cross-sections of this residual velocity map along and across the bar with the surface brightness map in Hα indicates a systematic offset between regions of high non-circular velocity and massive star formation. To investigate further we produce maps of velocity gradient along and across the bar. These illustrate very nicely the shear compression of the gas, revealed by the location of the dust lanes along loci of maximum velocity gradient perpendicular to the bar. They also show clearly how shear, seen in our data as velocity gradient perpendicular to the flow, acts to inhibit massive star formation, whereas shocks, seen as strong velocity gradients along the flow vector, act to enhance it. Although the inhibiting effect of gas shear flow on star formation has long been predicted, this is the clearest observational illustration so far of the effect, thanks to the strong shock-induced counterflow system in the bar. It is also the clearest evidence that dust picks out shock-induced inflow along bars. These observations should be of considerable interest to those modelling massive star formation in general.

The Asymptotic Giant Branch of NGC 205: The Characteristics of Carbon Stars and M Giants Identified fromJHK′ Images

J, H, and K' images are used to investigate the asymptotic giant branch (AGB) content of the Local Group dwarf elliptical galaxy NGC 205. The AGB on the (K, H-K) and (K, J-K) color-magnitude diagrams consists of two sequences: a near-vertical plume of giants with spectral types K and M and a red arm containing C stars. There are 320 C stars with Mbol 1.5 within 2' of the nucleus. C stars account for 10% of the integrated luminosity of AGB stars brighter than Mbol = -3.75 near the center of NGC 205, and this is in excellent agreement with what is measured in intermediate-age clusters in the LMC. The most luminous AGB star has Mbol = -6.5, although variability introduces an uncertainty of a few tenths of a magnitude when using this as an estimate of the AGB tip brightness. Comparisons with models suggest that the brightest AGB stars formed within the past 0.1 Gyr and that the previous episode of star formation occurred a few tenths of a Gyr earlier. These results are consistent with star formation in NGC 205 being triggered by interactions with M31. These data also demonstrate that near-infrared imaging provides an efficient means of identifying C stars in nearby galaxies. The techniques used here to identify C stars and probe the AGB are well suited to studies of galaxies outside of the Local Group using data obtained with adaptive optics systems on large ground-based telescopes.