Star-forming Knots Research Articles

No supernovae detected in two long-duration gamma-ray bursts

There is strong evidence that long-duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. In the standard version of the collapsar model, a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies the GRB. This association has been confirmed in observations of several nearby GRBs. Recent observations show that some long-duration GRBs are different. No SN emission accompanied the long-duration GRBs 060505 and 060614 down to limits fainter than any known Type Ic SN and hundreds of times fainter than the archetypal SN 1998bw that accompanied GRB 980425. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration. Furthermore, the bursts originated in star-forming galaxies, and in the case of GRB 060505, the burst was localized to a compact star-forming knot in a spiral arm of its host galaxy. We find that the properties of the host galaxies, the long duration of the bursts and, in the case of GRB 060505, the location of the burst within its host, all imply a massive stellar origin. The absence of an SN to such deep limits therefore suggests a new phenomenological type of massive stellar death.

Star-forming knots and density wave in NGC 2997

Context. Many grand design spiral galaxies show strings of bright knots along their arms on near-infrared K-band images. The alignment of such knots suggests a relation to the spiral pattern and possibly to a large-scale, star-forming front associated with a density wave. Aims. Bright knots in the southern.arm of NGC 2997 were studied to determine their nature and evolutionary state. Methods. Low resolution near-infrared K- and J-band spectra of the knots were observed with ISAAC on the VLT. Results. Most of the knots show strong Hi Br γ emission with some also having He I and H 2 emission. A few knots show indications of 12 CO absorption. This suggests that the knots are very young stellar clusters with masses up to 5 X 10 4 M ⊙ . Conclusions. The knots azimuthal distance from the K-band spiral correlates well with their Bry strength, indicating that they are located inside the co-rotation of the density wave, which triggered them through a large-scale, star-forming front. These relative azimuthal distances suggest an age spread of more than 1.6 Myr, which is incompatible with standard models for an instantaneous star burst. This indicates a more complex star-formation history, such as several bursts or continuous formation.

A Deep Radio Survey of Abell 2125. III. The Cluster Core: Merging and Stripping

We use radio, near-IR, optical, and X-ray observations to examine dynamic processes in the central region of the rich galaxy cluster Abell 2125. In addition to the central triple of E and cD galaxies, including members of both major dynamical subsystems identified from a redshift survey, this region features a galaxy showing strong evidence for ongoing gas stripping during a high-velocity passage through the gas in the cluster core. The disk galaxy C153 exhibits a plume stretching toward the cluster center seen in soft X-rays by Chandra, parts of which are also seen in [O II] emission and near-UV continuum light. Hubble Space Telescope imaging shows a distorted disk with star-forming knots asymmetrically distributed and remnant spiral structure possibly defined by dust lanes. The stars and ionized gas in this disk are kinematically decoupled, demonstrating that pressure stripping (possibly turbulent, as well as ram) must be important, and that tidal disruption is not the only mechanism at work. Comparison of the gas properties seen in the X-ray and optical data on the plume highlight significant and poorly known features of the history of stripped gas in the intracluster medium that could be clarified through further observations of this system. The nucleus of C153 also hosts an active galactic nucleus, shown by the weak and distorted extended radio emission and a radio-compact core. The unusual strength of the stripping signatures in this instance is likely related to the very high relative velocity of the galaxy with respect to the intracluster medium during a cluster-cluster merger and its passage very near the core of the cluster. An additional sign of recent dynamical events is the diffuse starlight asymmetrically placed about the central triple in a cD envelope. Transient and extreme dynamical events as seen in A2125 may be important drivers of galaxy evolution in the cores of rich clusters.

The giant Hα/X-ray filament in the cluster of galaxies A1795

The cluster of galaxies A1795 hosts a 46 kpc long filament at its core, which is clearly visible in the light of Hα and X-ray emission. We present optical slit spectroscopy and deeper Chandra X-ray data of the filament. The optical spectra reveal that the bulk of the filament is quiescent (with σ< 130 km s −1 ), although considerable velocity structure is apparent around the powerful radio source in the central cluster galaxy, where a direct interaction is occurring between the radio plasma and the surrounding intracluster medium. The filament contains a clump of ultraviolet/blue continuum halfway along its length, which we resolve into a chain of at least five distinct knots using archival Hubble Space Telescope images; the optical spectrum of this clump confirms it to be mostly composed of O stars. It is well-removed from the central galaxy and radio source, and is most likely an example of a group of young star clusters condensing directly from the cooling gas in the filament. The observed spatial offset between these knots of star formation and the peak in the optical line emission confirms that the massive star formation is most unlikely to be responsible for the bulk of the observed emission-line luminosity in the filament. Some other (as yet undetermined) source of energy is required to power and maintain the optical line emission, yet it must not completely impede the cooling of the X-ray gas within the filament to form the star clusters. Ke yw ords: galaxies: clusters: individual: A1795 ‐ X-rays: galaxies: clusters.

Spectrophotometry of Star-forming Regions in HiiGalaxies

We present spectrophotometric observations of 111 H II galaxies selected from various surveys. Apart from the integrated spectra, we present emission-line fluxes and equivalent widths of different star-forming knots for 33 galaxies for which the spatial distribution of physical properties can be assessed. Most of the objects have been observed previously. We reobserved these galaxies with uniform instrumentation, and data reduction was performed with homogeneous methods. Our analysis of the quality of the data indicates that our observations reach good signal-to-noise ratio over the whole spectral range, allowing the measurement of—and the inclusion of additional—low-intensity emission lines.

The Unusual Tidal Dwarf Candidate in the Merger System NGC 3227/3226: Star Formation in a Tidal Shock?

We report the discovery of active star formation in the H I cloud associated with the interacting Seyfert system NGC 3227/3226 that was originally identified as a candidate tidal dwarf galaxy (TDG) by Mundell et al. and that we name J1023+1952. We present broadband optical B, R, I (from the Isaac Newton Telescope), and ultraviolet images (from XMM-Newton) that show that the H I cloud is associated with massive ongoing star formation seen as a cluster of blue knots (MB -15.5 mag) surrounded by a diffuse ultraviolet halo and cospatial with a ridge of high neutral hydrogen column density (NH ~ 3.7 × 1021 cm-2) in the southern half of the cloud. We also detect Hα emission from the knots with a flux density of FHα ~ 2.55 × 10-14 ergs s-1 cm-2 corresponding to a star formation rate of SFR(Hα) ~ 10.6 × 10-3 M☉ yr-1. J1023+1952 lies at the base of the northern tidal tail, and, although it spatially overlaps the edge of the disk of NGC 3227, Mundell et al. showed that the H I cloud is kinematically distinct with an H I mean velocity 150 km s-1 higher than that of NGC 3227. Comparison of ionized (Hα) and neutral (H I) gas kinematics of the cloud shows closely matched recessional velocities, providing strong evidence that the star-forming knots are embedded in J1023+1952 and are not merely optical knots in the background disk of NGC 3227, thus confirming J1023+1952 as a gas-rich (MH/LB > 1.5) dwarf galaxy. No star formation is detected in the northern half of the cloud, despite similar H I column densities; instead, our new high-resolution H I image shows a ridge of high column density coincident with the reddest structures evident in our B - I image. We suggest that these structures are caused by the background stellar continuum from the disk of NGC 3227 being absorbed by dust intrinsic to J1023+1952, thus placing J1023+1952 in front of NGC 3227 along the line of sight. We discuss two scenarios for the origin of J1023+1952: as a third, preexisting dwarf galaxy involved in the interaction with NGC 3227 and NGC 3226, or as a newly forming dwarf galaxy condensing out of the tidal debris removed from the gaseous disk of NGC 3227. The first scenario is feasible given that NGC 3227 is the brightest member of a galaxy group, an environment in which preexisting dwarf galaxies are expected to be common. However, the lack of a detectable old stellar population in J1023+1952 makes a tidal origin more likely. If J1023+1952 is a bound object forming from returning gaseous tidal tail material, its unusual location at the base of the northern tail implies a dynamically young age similar to its star formation age, and suggests it is in the earliest stages of TDG evolution. Whatever the origin of J1023+1952, we suggest that its star formation is shock-triggered by collapsing tidal debris.

Looking Closely at Medusa: Star-forming Knots at the Center of NGC 4194

We report high-resolution ultraviolet and visible-wavelength imaging of the blue compact galaxy NGC 4194 (the Medusa) using the Hubble Space Telescope. A complete sample of 38 UV-bright knots is identified. These knots produce 20% of the ultraviolet emission from the area of the galaxy observed. The bright end of the knot flux distribution can be fitted by a power law with exponent α = -1.47 ± 0.27. The complete flux distribution is Gaussian. Comparison with starburst evolutionary tracks indicates that two-thirds of the knots are younger than 20 Myr. There is one concentration of very young knots less than 5 Myr old. The knot masses are large, comparable to those found in other major merger galaxies. The mass distribution is a power law with exponent -1.85 ± 0.24. The difference between the far-ultraviolet flux distribution and the mass distribution can be understood by the rapid decrease in a knot's far-ultraviolet flux due to stellar evolution. The star formation rate calculated from knots younger than 20 Myr is ~6 M⊙ yr-1, suggesting that the star formation rate throughout the galaxy may be as high as 30 M⊙ yr-1. Estimates of knot stability suggest that at least half the knots are bound protoclusters and will become globular clusters in several gigayears. The location and masses of individual star-forming regions in NGC 4194 and the rate of star formation are typical of major merger galaxies.

Spatial Analysis of the Hα Emission in the Local Star‐forming UCM Galaxies

We present a photometric study of the Hα emission in the Universidad Complutense de Madrid (UCM) Survey galaxies. This work complements our previously published spectroscopic data. We study the location of the star-forming knots, their intensity and concentration, and the relationship of these properties with those of the host galaxy. We also estimate that the amount of Hα emission that arises from the diffuse ionized gas is about 15%-30% of the total Hα flux for a typical UCM galaxy. This percentage seems to be independent of the Hubble type. Conversely, we find that an "average" UCM galaxy harbors a star formation event with 30% of its Hα luminosity arising from a nuclear component. The implications of these results for higher redshift studies are discussed, including the effects of galaxy size and the depth of the observations. A correlation between the star formation rate and the Balmer decrement is observed, but such correlation breaks down for large values of the extinction. Finally, we recalculate the Hα luminosity function and star formation rate density of the local universe using the new imaging data. Our results point out that, on average, spectroscopic observations detected about one-third of the total emission-line flux of a typical UCM galaxy. The new values obtained for the Hα luminosity density and the star formation rate density of the local universe are 1039.3±0.2 ergs s-1 Mpc-3 and ρSFR = 0.016 M☉ yr-1 Mpc-3 (H0 = 50 km s-1 Mpc-1, ΩM = 1.0, Λ = 0). The corresponding values for the "concordance cosmology" (H0 = 70 km s-1 Mpc-1, ΩM = 0.3, Λ = 0.7) are 1039.5±0.2 ergs s-1 Mpc-3 and ρSFR = 0.029 M☉ yr-1 Mpc-3.

Spectrophotometric Observations of Blue Compact Dwarf Galaxies: Markarian 370

We present results from a detailed spectrophotometric analysis of the blue compact dwarf galaxy (BCD) Mrk 370, based on deep UBVRI broadband and Hα narrowband observations, and on long-slit and two-dimensional spectroscopy of its brightest knots. The spectroscopic data are used to derive the internal extinction and to compute metallicities, electronic density, and temperature in the knots. By subtracting the contribution of the underlying older stellar population, modeled by an exponential function, removing the contribution from emission lines, and correcting for extinction, we can measure the true colors of the young star-forming knots. We show that the colors obtained this way differ significantly from those derived without the above corrections and lead to different estimates of the ages and star-forming history of the knots. Using predictions of evolutionary synthesis models, we estimate the ages of both the starburst regions and the underlying stellar component. We found that we can reproduce the colors of all the knots with an instantaneous burst of star formation and the Salpeter initial mass function (IMF) with an upper mass limit of 100 M☉. The resulting ages range between 3 and 6 Myr. The colors of the low surface brightness component are consistent with ages larger than 5 Gyr. The kinematic results suggest ordered motion around the major axis of the galaxy.

The Afterglow and Complex Environment of the Optically Dim Burst GRB 980613

We report the identification of the optical afterglow of GRB 980613 in R- and I-band images obtained between 16 and 48 hours after the gamma-ray burst. Early near-infrared (NIR) H and K' observations are also reported. The afterglow was optically faint (R ~ 23) at discovery but did not exhibit an unusually rapid decay (power-law decay slope alpha < 1.8 at 2 sigma). The optical/NIR spectral index (beta_RH < 1.1) was consistent with the optical-to-X-ray spectral index (beta_RX ~ 0.6), indicating a maximal reddening of the afterglow of ~0.45 mag in R. Hence the dimness of the optical afterglow was mainly due to the fairly flat spectral shape rather than internal reddening in the host galaxy. We also present late-time HST/STIS images of the field in which GRB 980613 occurred, obtained 799 days after the burst. These images show that GRB 980613 was located close to a very compact, blue V = 26.1 object inside a complex region consisting of star-forming knots and/or interacting galaxy fragments. Therefore, GRB 980613 constitutes a strong case for the association of cosmological gamma-ray bursts with star-forming regions.

On the Recent History of Star Formation in the Blue Compact Dwarf Galaxy VII Z[CLC]w[/CLC] 403

Here we attempt to infer the recent history of star formation in the BCD galaxy VII Zw 403, based on an analysis that accounts for the dynamics of the remnant generated either by an instantaneous burst or by a continuous star formation event. The models are restricted by the size of the diffuse X-ray–emitting region, the Hα luminosity from the star-forming region, and the superbubble diffuse X-ray luminosity. We have reobserved VII Zw 403 with a better sensitivity corresponding to the threshold Hα flux 8.15 × 10-17 ergs cm-2 s-1. The total Hα luminosity derived from our data is much larger than reported before and presents a variety of ionized filaments and incomplete shells superimposed on the diffuse Hα emission. This result has a profound impact on the predicted properties of the starburst-blown superbubble. Numerical calculations based on the Hubble Space Telescope Hα data predict two different scenarios of star formation able to match simultaneously all observed parameters. These are an instantaneous burst of star formation with a total mass of 5 × 105 M⊙ and a star-forming event with a constant star formation rate SFR = 4 × 10-3 M⊙ yr-1, which lasts for 35 Myr. The numerical calculations based on the energy input rate derived from our observations predict a short episode of star formation lasting less than 10 Myr with a total star cluster mass of ~(1–3) × 106 M⊙. However, the five main star-forming knots are sufficiently distant to form a coherent shell in a short timescale and still keep their energies blocked within local, spatially separated bubbles. The X-ray luminosities of these are here shown to be consistent with the ROSAT PSPC diffuse X-ray emission.

Multiband Analysis of a Sample of Blue Compact Dwarf Galaxies. II. Spatially Resolved and Integrated Photometry

This is the second in a series of papers devoted to the study of a sample of 28 blue compact dwarf galaxies (BCDs). In the first paper, we presented the broadband observations in B, V, R, and I. Deep contour maps, surface-brightness profiles, and color profiles were used to analyze and discuss their morphology and structure. Here, we present new U-band and Hα observations. We compute isophotal and asymptotic magnitudes of the sample galaxies and total Hα fluxes. Comparison with previous works is done to assess the quality of the data available for BCDs. Finally, we produce an atlas of detailed color and Hα maps, the first systematic atlas of this kind published so far. The high quality of the data allows us to identify the different star-forming knots in the maps and to discriminate between them and the regions where star formation activity has already ceased. We supplement the atlas with a morphological description of each galaxy and group the galaxies in four classes according to the distribution of their star-forming regions.

ChandraObservations of “The Antennae” Galaxies (NGC 4038/9)

We report the results of a deep Chandra ACIS pointing at the merging system NGC 4038/9. We detect an extraordinarily luminous population of X-ray sources, with luminosity well above that of XRBs in M31 and the Milky Way. If these sources are unbeamed XRBs, our observations may point to them being 10-(a few)100 M☉ black hole counterparts. We detect an X-ray bright hot ISM, with features including bright superbubbles associated with the actively star-forming knots, regions where hot and warm (Hα) ISM intermingle, and a large-scale outflow.

Merging Process and Tidal‐induced Star Formation in the Ultraluminous Infrared Galaxy IRAS 08572+3915

Integral field optical spectroscopy using the INTEGRAL system, complemented with HST imaging, has been used to characterize the merging process giving rise to the ultraluminous infrared galaxy IRAS 08572+3915, the star formation along the tidal tails of the galaxy, and the nature of the nuclear ionizing sources deep within the galaxy. The overall morphology with two well-identified nuclei, the widespread star formation, and the unperturbed two-dimensional gas kinematics indicate that IRAS 08572+3915 is a dynamically young system formed by two disks' galaxies which are in the process of merging. The galaxies have a mass ratio of about 5, the brighter being a ~0.5L* galaxy. The ionized gas distribution traces the presence of young (6 Myr) dust-enshrouded massive nuclear starbursts of 2 × 107 to 10M☉. Contrary to previous claims, and based on the two-dimensional extinction-corrected optical emission line ratios, there is no evidence for a LINER or Seyfert-like nucleus in either of the galaxies. This is unusual for a warm, ultraluminous, infrared galaxy like IRAS 08572+3915. Tidal-induced, star-forming knots, located at distances of about 7 kpc from the nuclei and along the tidal tails, are traced by the presence of bright [O III]-emitting regions. These knots, with ages of 5.5-6 Myr and masses of ~106 M☉, seem to represent a common phenomenon of the merging process; they are already detected in many other ultraluminous infrared galaxies, and tidal dwarf galaxies could be just the more massive manifestation of the same phenomenon.

Near-infrared spectroscopy of the circumnuclear star formation regions in M100: evidence for sequential triggering

We present low resolution (R~450) K-band spectroscopy for 16 of the 43 circumnuclear star-forming knots in M100 identified by Ryder & Knapen (1999). We compare our measurements of equivalent widths for the Br-gamma emission line and CO 2.29 micron absorption band in each knot with the predictions of starburst models from the literature, and derive ages and burst parameters for the knots. The majority of these knots are best explained by the result of short, localised bursts of star formation between 8 and 10 Myr ago. By examining both radial and azimuthal trends in the age distribution, we present a case for sequential triggering of star formation, most likely due to the action of a large-scale shock. In an appendix, we draw attention to the fact that the growth in the CO spectroscopic index with decreasing temperature in supergiant stars is not as regular as is commonly assumed.

Stellar Populations in the Host Galaxies of Markarian 1014, IRAS 07598+6508, and Markarian 231

We present deep spectroscopic and imaging data of the host galaxies of Mrk 1014, IRAS 07598+6508, and Mrk 231. These objects form part of both the QSO and the ultraluminous infrared galaxy (ULIG) families and may represent a transition stage in an evolutionary scenario. Our imaging shows that all three objects have highly perturbed hosts with tidal tails and destroyed disks and appear to be in the final stages of major mergers. The host galaxies of the three objects have spectra typical of E+A galaxies, showing simultaneously features from an old and a young stellar component. We model spectra from different regions of the host galaxies using Bruzual & Charlot spectral synthesis models using two-component models including an old underlying population and recent superposed starbursts. Mrk 1014 has intense star formation concentrated in a large knot less than 2 kpc from the nucleus, along the leading edge of the tidal tail, and in several knots scattered around the host. The starburst ages in these regions range from 180 to 290 Myr. IRAS 07598+6508 has multiple knots of star formation concentrated in two regions within 16 kpc of the QSO nucleus, with ages ranging from 30 to 70 Myr; the host galaxy shows an older population in other regions. Mrk 231 shows a wider range of starburst ages, ranging from 42 Myr in the arc 3 kpc south of the nucleus to ~300 Myr spread on a plateau ~20 kpc across around the nucleus, as well as a UV-bright region 12 kpc south of the nucleus, which is apparently a region of currently active star formation. Our results indicate a strong connection between interactions and vigorous bursts of star formation in these objects. We propose that the starburst ages found are indicative of young ages for the QSO activity. The young starburst ages found are also consistent with the intermediate position of these objects in the far infrared color-color diagram.

High‐Resolution Optical/Near‐Infrared Imaging of Cool Ultraluminous Infrared Galaxies

We present here new multiwavelength observations with 1.5 and 4x the spatial resolution of previous ground-based observations at optical and near-infrared wavelengths; despite being ground-based, they allow us to isolate interesting features such as the star-forming knots detected in the warm ULIG sample.

Nuclear Bar, Star Formation, and Gas Fueling in the Active Galaxy NGC 4303

A combination of Hubble Space Telescope (HST) WFPC2 and NICMOS images are used to investigate the gas/dust and stellar structure inside the central 300 pc of the nearby active galaxy NGC 4303. The NICMOS H-band (F160W) image reveals a bright core and a nuclear elongated bar-like structure of 250 pc in diameter. The bar is centered on the bright core, and its major axis is oriented in proyection along the spin axis of the nuclear gaseous rotating disk recently detected (Colina & Arribas 1999). The V-H (F606W - F160W) image reveals a complex gas/dust distribution with a two-arm spiral structure of about 225 pc in radius. The southwestern arm is traced by young star-forming knots while the northeastern arm is detected by the presence of dust lanes. These spirals do not have a smooth structure but rather they are made of smaller flocculent spirals or filament-like structures. The magnitudes and colors of the star-forming knots are typical of clusters of young stars with masses of 0.5 to 1 x $10^5 M_{solar}, and ages of 5 to 25 million years. The overall structure of the nuclear spirals as well as the size, number and masses of the star-forming knots are explained in the context of a massive gaseous nuclear disk subject to self-gravitational instabilities and to the gravitational field created by the nuclear bar. According to the model, the gaseous disk has a mass of about 5 x 10^7 M_{solar} inside a radius of 400 pc, the bar has a radius of 150 pc and a pattern speed of about 0.5 Myr^{-1}, and the average mass accretion rate into the core (R < 8 pc) is about 0.01 M_{solar}$ yr^{-1} for about 80 Myr.

Imaging and Spectrophotometry of Markarian 1094: Implications for the Recent Star Formation

Narrowband Hα imaging and intermediate- and high-resolution optical spectrophotometry have been obtained for the blue compact dwarf Wolf-Rayet galaxy Mrk 1094. The galaxy presents a bar-shaped structure and is currently undergoing a strong star formation burst distributed in several spatially separated knots. Analysis of the properties of the individual knots indicates that the brightest ones have sizes and luminosities similar to those of giant H II regions in Local Group galaxies. The Hα equivalent width map of Mrk 1094 indicates that star formation has not been strictly coeval all along the bar. We discuss the different processes that could explain these observational facts and conclude that interaction with a companion H I cloud could be the reason for the present enhanced star-forming process and the bar-shaped distribution of the different star-forming knots.

High‐Resolution Tip/Tilt Near‐Infrared Imaging of Warm Ultraluminous Infrared Galaxies

We present high spatial resolution H and K'-band images of a complete sample of ultraluminous infrared galaxies selected to have warm mid-infrared colors known to be characteristic of active galaxies. The extended underlying galaxy is detected in each system, as are tidal features and many of the star-forming knots seen at optical wavelengths. While some of these knots have a considerable near-infrared excess, we show that they are likely to have bolometric luminosities more similar to that of the extended starbursts with similar optical morphology seen in less luminous interacting systems. We find that each ULIG has increasing contributions at long wavelengths by a very compact source which we identify as an active galactic nucleus. We show that the optical/near-infrared colors of these putative nuclei are much more extreme than the most active starburst IRAS galaxies, yet are identical to far-infrared loud quasars which are in turn similar to optical quasars combined with large quantities of hot dust. Half of the ULIGs have nuclei with dereddened near-infrared luminosities comparable to those of QSOs, while the other half have dereddened luminosities more similar to Seyferts, although this may be an effect of patchy extinction and scattering.