Massive Starburst Research Articles

Detection of 1.6 × 10 10 M ⊙ of Molecular Gas in the Host Galaxy of the z = 5.77 SDSS Quasar J0927+2001

We have detected emission by the CO(5-4) and (6-5) rotational transitions at z = 5.7722 ± 0.0006 from the host galaxy of the SDSS quasar J0927+2001 using the Plateau de Bure Interferometer. The peak line flux density for the CO(5-4) line is 0.72 ± 0.09 mJy, with a line FWHM = 610 ± 110 km s^(-1). The implied molecular gas mass is (1.6 ± 0.3) × 10^(10) M_⊙. We also detect the 90 GHz continuum at 0.12 ± 0.03 mJy, consistent with a 47 K dust spectrum extrapolated from higher frequencies. J0927+2001 is the second example of a huge molecular gas reservoir within the host galaxy of a quasar within 1 Gyr of the big bang. Observations of J0927+2001 are consistent with a massive starburst coeval with a bright quasar phase in the galaxy, suggesting the rapid formation of both a supermassive black hole through accretion, and the stellar host spheroid, at a time close to the end of cosmic reionization.

How many supernovae are we missing at high redshift?

Near-infrared and radio searches for core-collapse supernovae (CC SNe) in the local universe have shown that the vast majority of the events occurring in massive starburst are missed by the current optical searches as they explode in very dusty environments. Recent infrared observations have shown that the fraction of star formation activity that takes place in very luminous dusty starbursts sharply increases with redshift and becomes the dominant star formation component at z 0.5. As a consequence, an increasing fraction of SNe are expected to be missed by high-redshift optical searches. We estimate that 5‐10 per cent of the local CC SNe are out of reach of the optical searches. The fraction of missing events rises sharply towards z = 1, when about 30 per cent of the CC SNe will be undetected. At z = 2 the missing fraction will be about 60 per cent. Correspondingly, for Type Ia SNe, our computations provide missing fractions of 15 per cent at z = 1 and 35 per cent at z = 2. Such large corrections are crucially important to compare the observed SN rate with the expectations from the evolution of the cosmic star formation history, and to design the future SN searches at high redshifts.

An X‐Ray and Near‐Infrared Study of Young Stars in the Carina Nebula

We present a multiwavelength study of the central region of the Carina Nebula, including Trumpler 16 and part of Trumpler 14. Our analysis of the Chandra X-Ray Observatory archival data led to the identification of 454 X-ray sources. These sources were then cross-identified with optical photometric and spectroscopic information available from the literature and with newly obtained near-infrared (JHKs) imaging observations. A total of 38 known OB stars are found to be X-ray emitters. All the O stars and early-B stars follow the nominal relation between the X-ray and bolometric luminosities, LX ~ 10-7Lbol. A few mid- to late-B stars are found to be associated with X-ray emission, likely attributable to T Tauri companions. We discovered 16 OB star candidates that suffer a large extinction in the optical wave bands. Some 300 sources have the X-ray and infrared characteristics of late-type pre-main-sequence stars. Our sample represents the most comprehensive census of the young stellar population in the Carina Nebula so far and should be useful for the study of the star formation history of this massive starburst region. We also report the finding of a compact (2' × 4') group of 10 relatively bright X-ray sources, all of which are detected in the near-infrared wavelengths and are highly reddened. The group is spatially coincident with the dark V-shaped dust lane bisecting the Carina Nebula and may be part of an embedded association. The distribution of the young stellar groups surrounding the H II region associated with Trumpler 16 is consistent with the collect-and-collapse scenario of triggered star formation.

Initial Populations of Black Holes in Star Clusters

Using an updated population synthesis code we study the formation and evolution of black holes (BHs) in young star clusters following a massive starburst. This study continues and improves on the initial work described by Belczynski, Sadowski & Rasio (2004). In our new calculations we account for the possible ejections of BHs and their progenitors from clusters because of natal kicks imparted by supernovae and recoil following binary disruptions. The results indicate that the properties of both retained BHs in clusters and ejected BHs (forming a field population) depend sensitively on the depth of the cluster potential. In particular, most BHs ejected from binaries are also ejected from clusters with central escape speeds Vesc 50 km/s. BHs from single star evolution are also affected significantly: about half of the BHs originating from primordial single stars are ejected from clusters with Vesc < 50 km/s. Our results lay a foundation for theoretical studies of the formation of BH X-ray binaries in and around star clusters, including possible ultra-luminous sources, as well as merging BH--BH binaries detectable with future gravitational-wave observatories.

Interacting galaxies and cosmological parameters

We propose a (physical)-geometrical method to measure and , the present rates of the density cosmological parameters for a Friedmann-Lemaître universe. The distribution of linear separations between two interacting galaxies, when both of them undergo a first massive starburst, is used as a standard of length. Statistical properties of the linear separations of such pairs of “interactivated” galaxies are estimated from the data in the Two Degree Field Galaxy Redshift Survey. Synthetic samples of interactivated pairs are generated with random orientations and a likely distribution of redshifts. The resolution of the inverse problem provides the probability densities of the retrieved cosmological parameters. The accuracies that can be achieved by that method on and are computed depending on the size of ongoing real samples. Observational prospects are investigated as the foreseeable surface densities on the sky and magnitudes of those objects.

Galaxy bimodality due to cold flows and shock heating

We address the origin of the robust bimodality observed in galaxy properties about a characteristic stellar mass ∼3 x 10 10 M ⊙ . Less massive galaxies tend to be ungrouped blue star forming discs, while more massive galaxies are typically grouped red old-star spheroids. Colour-magnitude data show a gap between the red and blue sequences, extremely red luminous galaxies already at z ∼ 1, a truncation of today's blue sequence above L*, and massive starbursts at z ∼ 2-4. We propose that these features are driven by the thermal properties of the inflowing gas and their interplay with the clustering and feedback processes, all functions of the dark matter halo mass and associated with a similar characteristic scale. In haloes below a critical shock-heating mass M shock ≤ 10 12 M ⊙ , discs are built by cold streams, not heated by a virial shock, yielding efficient early star formation. It is regulated by supernova feedback into a long sequence of bursts in blue galaxies constrained to a 'fundamental line'. Cold streams penetrating through hot media in M ≥ M shock haloes preferentially at z ≥ 2 lead to massive starbursts in L > L* galaxies. At z M shock haloes hosting groups, the gas is heated by a virial shock, and being dilute it becomes vulnerable to feedback from energetic sources such as active galactic nuclei. This shuts off gas supply and prevents further star formation, leading by passive evolution to 'red-and-dead' massive spheroids starting at z ∼ 1. A minimum in feedback efficiency near M shock explains the observed minimum in M/L and the qualitative features of the star formation history. The cold flows provide a hint for solving the angular momentum problem. When these processes are incorporated in simulations they recover the main bimodality features and solve other open puzzles.

Stellar populations in a complete sample of local radio galaxies

We investigate the nature of the continuum emission and stellar populations in the inner 1–3 kpc of a complete sample of 24 southern radio galaxies, and we compare the results with a control sample of 18 non-active early-type galaxies. 12 of the radio galaxies are classified as Fanaroff–Riley type I (FR I), eight as FR II and four as intermediate or undefined type (FR x). Optical long-slit spectra are used to perform spectral synthesis as a function of distance from the nucleus at an average sampling of 0.5–1.0 kpc and to quantify the relative contributions of a blue featureless continuum and stellar population components of different ages. Our main finding is a systematic difference between the stellar populations of the radio and control sample galaxies: the former have a larger contribution from an intermediate-age (1 Gyr) component, suggesting a connection between the present radio activity and a starburst which occurred ∼1 Gyr ago. In addition, we find a correlation between the contribution of the 1-Gyr component and the radio power, suggesting that more massive starbursts have led to more powerful radio emission. A similar relation is found between the radio power and the mean age of the stellar population, in the sense that stronger nuclear activity is found in younger galaxies. We also find that the stellar populations of FR I galaxies are, on average, older and more homogeneous than those of FR IIs. Significant population gradients were found in only four radio galaxies, which are also those with more than 10 per cent of their total flux at 4020 A contributed by age components younger than 100 Myr and/or a featureless continuum (indistinguishable from a 3-Myr-old stellar population).

Rest-frame optical and far-infrared observations of extremely bright Lyman-break galaxy candidates atz∼ 2.5

We have investigated the rest-frame optical and far-infrar ed properties of a sample of extremely bright candidate Lyman-break galaxies (LBG) identified in the Sloan Digital Sky Survey. Their high ultraviolet luminosities and lack of str ong ultraviolet emission lines are suggestive of massive starbursts, although it is possible t hat they are more typical luminosity LBGs which have been highly magnified by strong gravitationa l lensing. Alternatively, they may be an unusual class of weak-lined quasars. If the ultraviolet and submillimetre properties of these objects mirror those of less luminous, starbur st LBGs, then they should have detectable rest-frame far-infrared emission. However, our submm photometry fails to detect such emission, indicating that these systems are not merely scaled-up (either intrinsically or as a result of lensing) examples of typical LBGs. In addition we have searched for the morphological signatures of strong lensing, using high-resol ution, near-infrared imaging, but we find none. Instead, near-infrared spectroscopy reveals tha t these systems are, in fact, a rare class of broad absorption-line (BAL) quasars.

Near‐Infrared Photometry of the High‐Redshift Quasar RD J030117+002025: Evidence for a Massive Starburst atz= 5.5

With a redshift of z = 5.5 and an optical blue magnitude MB ~ -24.2 mag (~4.5 × 1012 L☉), RD J030117+002025 is the most distant, optically faint (MB > -26 mag) quasar known. Max-Planck Millimeter Bolometer (MAMBO) continuum observations at λ = 1.2 mm (185 μm rest frame) showed that this quasar has a far-infrared (FIR) luminosity comparable to its optical luminosity. We present near-infrared J- and K-band photometry obtained with the Near-Infrared Camera (NIRC) on the Keck I telescope, tracing the slope of the rest-frame ultraviolet (UV) spectrum of this quasar. The observed spectral index is close to the value of αν ~ -0.44 measured in composite spectra of optically bright SDSS quasars. It thus appears that the quasar does not suffer from strong dust extinction, which further implies that its low rest-frame UV luminosity is due to an intrinsically faint active galactic nucleus (AGN). The FIR to optical luminosity ratio is then much larger than that observed for the more luminous quasars, supporting the suggestion that the FIR emission is not powered by the AGN but by a massive starburst.

XMM‐NewtonSpectroscopy of the Starburst‐Dominated Ultraluminous Infrared Galaxy NGC 6240

We present a new XMM-Newton observation of the ultraluminous infrared galaxy (ULIRG) NGC 6240. We analyze the reflecting grating spectrometer (RGS) data, as well as data from the other instruments, and find a starburst-dominated 0.5-3 keV spectrum with global properties resembling those observed in M82 but with a much higher luminosity. We show that the starburst region can be divided into an outer zone, beyond a radius of about 2.1 kpc, with a gas temperature of about 107 K, and a central region with temperatures in the range (2-6) × 107 K. The gas in the outer region emits most of the observed O VIII Lyα line, and the gas in the inner region the emission lines of higher ionization ions, including a strong Fe XXV line. We also identify a small inner part, very close to the active nuclei, with typical Seyfert 2 properties, including a large amount of photoionized gas producing a strong Fe Kα 6.4 keV line. The combined abundance, temperature, and emission measure analysis indicates supersolar Ne/O, Mg/O, Si/O, S/O, and possibly also Fe/O. The analysis suggests densities in the range of (0.07-0.28)-1/2 cm-3 and a total thermal gas mass of ~4 × 1081/2 M☉, where is the volume filling factor. We used a simple model to argue that a massive starburst with an age of 2 × 107 yr can explain most of the observed properties of the source. NGC 6240 is perhaps the clearest case of an X-ray-bright luminous AGN that is in a merger and whose soft X-ray spectrum is dominated by a powerful starburst.

Further Wolf-Rayet stars in the starburst cluster Westerlund 1

We present new low and intermediate-resolution spectroscopic observations of the Wolf Rayet (WR) star population in the massive starburst cluster Westerlund 1. Finding charts are presented for five new WRs - four WNL and one WCL - raising the current total of known WRs in the cluster to 19. We also present new spectra and correct identifications for the majority of the 14 WR stars previously known, notably confirming the presence of two WNVL stars. Finally we briefly discuss the massive star population of Westerlund 1 in comparison to other massive young galactic clusters.

The Physical Connections among Infrared QSOs, Palomar‐Green QSOs, and Narrow‐Line Seyfert 1 Galaxies

We study the properties of infrared-selected QSOs (IR QSOs), optically selected Palomar-Green QSOs (PG QSOs), and narrow-line Seyfert 1 galaxies (NLS1s). We compare their properties from the IR to the optical and examine various correlations among the black hole mass, accretion rate, star formation rate, and optical and IR luminosities. We find that the IR excess in IR QSOs is mostly in the far-IR, and their IR spectral indices suggest that the excess emission is from low-temperature dust heated by starbursts rather than active galactic nuclei (AGNs). The IR excess is therefore a useful criterion to separate the relative contributions of starbursts and AGNs. We further find a tight correlation between the star formation rate and the accretion rate of central AGNs for IR QSOs. The ratio of the star formation rate and the accretion rate is about several hundred for IR QSOs but decreases with the central black hole mass. This shows that the tight correlation between the stellar mass and the central black hole mass is preserved in massive starbursts during violent mergers. We suggest that the higher Eddington ratios of NLS1s and IR QSOs imply that they are in the early stage of evolution toward classical Seyfert 1 galaxies and QSOs, respectively.

AChandraX-ray survey of nearby dwarf starburst galaxies - II. Starburst properties and outflows

We present a comprehensive comparison of the X-ray properties of a sample of eight dwarf starburst galaxies observed with Chandra (IZw18, VIIZw403, NGC1569, NGC3077, NGC4214, NGC4449, NGC5253, He2–10). In PaperI we presented in detail the data reduction and analysis of the individual galaxies. For the unresolved X-ray sources we find the following: point sources are in general located close to bright H ii regions, rims of superbubbles, or young stellar clusters. The number of X-ray point sources appears to be a function of the current star formation rate and the blue luminosity of the hosts. Ultraluminous X-ray sources are only found in those dwarf galaxies which are currently interacting. The power law index of the combined cumulative X-ray point source luminosity function is α = 0.24 ± 0.06, shallower than that of more massive starburst galaxies (α = 0.4 − 0.8) and of non-starburst galaxies (α ∼ 1.2). For those galaxies showing extended X-ray emission (6 out of the 8 galaxies), we derive the following: Superwinds develop along the steepest gradient of the H i distribution with volume densities of 0.02 −0.06cm 3 , pressures of 1 −3 ×10 5 Kcm 3 , thermal energies of 2 − 30 × 10 54 erg, and hot gas masses of 2 − 20 × 10 6 M⊙ (∼ 1 per cent of the H i masses. On global scales, the distribution of the X-ray emission looks remarkably similar to that seen in Hα (comparing azimuthal averages); locally however their distribution is clearly distinct in many cases – this can be explained by the different emission mechanisms (forward vs. reverse shocks). Mass-loading of order 1 to 5 is required to explain the differences between the amount of hot gas and and the modelled mass-loss from massive stars. The metallicity of the dwarf galaxies correlates with the diffuse X-ray luminosity and anti-correlates with the cooling time of the hot gas. The diffuse X-ray luminosity is also a function of the current star formation rate. The mechanical luminosities of the developing superwinds are energetic enough to overcome the gravitational potentials of their host galaxies. This scenario is supported by the overpressures of the hot gas compared to the ambient ISM. Extended H i envelopes such as tidal tails, however, may delay outflows on timescales exceeding those of the cooling time of the hot gas.

Rapid Large-Scale Metal Enrichment in the Starbursts of an Interacting Galaxy System

By obtaining integral field spectra of the interacting galaxy system NGC 6090, we have found a kiloparsec-scale region where active star formation is currently increasing the fraction of heavy elements. Young massive starbursts are occurring in regions offset from the galactic centers, highlighting the present epoch of metal enrichment over the previous ones usually seen at galactic centers. The short timescale of ~107 yr for the starbursts, inferred from the galactic rotation, provides a strong constraint on the origins of the metallicity/abundance enhancements. While oxygen is considered to originate in supernovae, the observed nitrogen enhancement is likely to be caused by winds/mass loss from massive stars rather than being a product of intermediate-mass stars.

Obscured Star Formation in the Central Region of the Dwarf Galaxy NGC 5253

We present Hubble Space Telescope (HST) NICMOS observations (1.1-2.2 μm) and 1.9-4.1 μm spectroscopy of the central region of the dwarf galaxy NGC 5253. The HST NICMOS observations reveal the presence of a nuclear double star cluster separated by 03-04, or 6-8 pc (for a distance d = 4.1 Mpc). The double star cluster, also a bright double source of Paα emission, appears to be coincident with the double radio nebula detected at 1.3 cm. The eastern near-infrared star cluster (C1) is identified with the youngest optical cluster, whereas the western star cluster (C2), although it is almost completely obscured in the optical, becomes the brightest star cluster in the central region of NGC 5253 at wavelengths longer than 2 μm. Both clusters are extremely young, with ages of approximately 3.5 Myr. C2 is more massive than C1 by a factor of 6-20 (MC2 = 7.7 × 105-2.6 × 106 M☉, for a Salpeter initial mass function [IMF] in the mass range 0.1-100 M☉). Analysis of the circumnuclear spectrum excluding C1 and C2, as well as of a number of other near-infrared-selected clusters with a range of (young) ages, suggests that the star formation was triggered across the central regions of the galaxy. We have also modeled the nuclear UV to mid-infrared spectral energy distribution (SED) of NGC 5253 and found that the infrared part is well modeled by a highly obscured (AV = 17 mag) young starburst with a stellar mass consistent with our photometric estimates for C1 and C2. The SED model predicts a moderately bright polycyclic aromatic hydrocarbon (PAH) feature at 3.3 μm that is not detected in our nuclear L-band spectrum. NGC 5253's low metallicity and a top-heavy IMF likely combine to suppress the 3.3 μm PAH emission that is commonly seen in more massive starburst systems.

The Secrets of the Nearest Starburst Cluster. I. Very Large Telescope/ISAAC Photometry of NGC 3603

VLT/ISAAC JHKL photometry with subarcsecond resolution of the dense, massive starburst cluster NGC 3603 YC forming the core of the NGC 3603 giant molecular cloud is analyzed to reveal characteristics of the stellar population in unprecedented detail. The color-magnitude plane features a strong pre–main-sequence/main-sequence (PMS/MS) transition region, including the PMS/MS transition point, and reveals a secondary sequence for the first time in a nearby young starburst cluster. Arguments for a possible binary nature of this sequence are given. The resolved PMS/MS transition region allows isochrone fitting below the hydrogen-burning turn-on in NGC 3603 YC, yielding an independent estimate of global cluster parameters. A distance modulus of 13.9 mag, equivalent to d = 6.0 ± 0.3 kpc, is derived, as well as a line-of-sight extinction of AV = 4.5 ± 0.6 toward PMS stars in the cluster center. The interpretation of a binary candidate sequence suggests a single age of 1 Myr for NGC 3603 YC, providing evidence for a single burst of star formation without the need to employ an age spread in the PMS population, as argued for in earlier studies. Disk fractions are derived from L-band excesses, indicating a radial increase in the disk frequency from 20% to 40% from the core to the cluster outskirts. The low disk fraction in the cluster core, as compared to the 42% L-band excess fraction found for massive stars in the Trapezium cluster of a comparably young age, indicates strong photoevaporation in the cluster center. The estimated binary fraction of 30%, as well as the low disk fraction, suggest strong impacts on low-mass star formation due to stellar interactions in the dense starburst. The significant differences between NGC 3603 YC and less dense and massive young star clusters in the Milky Way reveal the importance of using local starbursts as templates for massive extragalactic star formation.

Discovery of a Dwarf Poststarburst Galaxy near a High Column Density Local Lyα Absorber

We report the discovery of a dwarf (MB = -13.9) poststarburst galaxy coincident in recession velocity (within uncertainties) with the highest column density absorber (NH i = 1015.85 cm-2 at cz = 1586 km s-1) in the 3C 273 sight line. This galaxy is by far the closest galaxy to this absorber, projected just 71 h kpc on the sky from the sight line. The mean properties of the stellar populations in this galaxy are consistent with a massive starburst ≈3.5 Gyr ago, whose attendant supernovae, we argue, could have driven sufficient gas from this galaxy to explain the nearby absorber. Beyond its proximity on the sky and in recession velocity, the further evidence in favor of this conclusion includes both a match in the metallicities of absorber and galaxy and the fact that the absorber has an overabundance of Si/C, suggesting recent Type II supernova enrichment. Thus, this galaxy and its ejecta are in the expected intermediate stage in the fading dwarf evolutionary sequence envisioned by Babul & Rees to explain the abundance of faint blue galaxies at intermediate redshifts. While this one instance of a QSO metal-line absorber and a nearby dwarf galaxy is not proof of a trend, a similar dwarf galaxy would be too faint to be observed by galaxy surveys around more distant metal-line absorbers. Thus, we cannot exclude the possibility that dwarf galaxies are primarily responsible for weak (NH i = 1014-1017 cm-2) metal-line absorption systems in general. If a large fraction of the dwarf galaxies expected to exist at high redshift had a similar history (i.e., they had a massive starburst that removed all or most of their gas), these galaxies could account for at least several hundred high-z metal-line absorbers along the line of sight to a high-z QSO. The volume-filling factor for this gas, however, would be less than 1%.

The FORS Deep Field spectroscopic survey

We present a catalogue and atlas of low-resolution spectra of a well defined sample of 341 objects in the FORS Deep Field. All spectra were obtained with the FORS instruments at the ESO VLT with essentially the same spectroscopic set-up. The observed extragalactic objects cover the redshift range 0. 1t o 5.0. 98 objects are starburst galaxies and QSOs at z > 2. Using this data set we investigated the evolution of the characteristic spectral properties of bright starburst galaxies and their mutual relations as a function of redshift. Significant evolutionary effects were found for redshifts 2 < z < 4. Most conspicuous are the increase of the average C IV absorption strength, of the dust reddening, and of the intrinsic UV luminosity, and the decrease of the average Lyα emission strength with decreasing redshift. In part the observed evolutionary effects can be attributed to an increase of the metallicity of the galaxies with cosmic age. Moreover, the increase of the total star-formation rates and the stronger obscuration of the starburst cores by dusty gas clouds suggest the occurrence of more massive starbursts at later cosmic epochs.

First results from SPIFFI, II: The luminous infrared galaxy NGC 6240 and the luminous sub‐millimeter galaxy SMMJ 14011+0252

AbstractThis is the second of two papers (I: Horrobin et al. 2003) on the first scientific results from the SPIFFI integral field spectrometer at the VLT. Here we discuss the observations and properties of the prototypical luminous infrared galaxy NGC 6240 and the luminous sub‐millimeter galaxy SMMJ 14011+0252. Taking full advantage of the excellent seeing conditions of 0.27″, our integral field spectroscopy data allow us for the first time to study in detail the stellar and gas dynamics in NGC 6240 on scales of 125 pc, and to establish a galactic shock as the origin of the strong emission from molecular hydrogen. Our observations of SMMJ 14011+0252 provide us with deep, spatially resolved near infrared spectra of the SCUBA selected luminous submillimeter galaxy at a redshift of z = 2.565, revealing a remarkably old, massive and metal‐rich starburst galaxy for the early epoch at which it is observed. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The essential signature of a massive starburst in a distant quasar.

Observations of carbon monoxide emission in high-redshift (zeta > 2) galaxies indicate the presence of large amounts of molecular gas. Many of these galaxies contain an active galactic nucleus powered by accretion of gas onto a supermassive black hole, and a key question is whether their extremely high infrared luminosities result from the active galactic nucleus, from bursts of massive star formation (associated with the molecular gas), or both. In the Milky Way, high-mass stars form in the dense cores of interstellar molecular clouds, where gas densities are n(H2) > 10(5) cm(-3) (refs 1, 2). Recent surveys show that virtually all galactic sites of high-mass star formation have similarly high densities. The bulk of the cloud material traced by CO observations, however, is at a much lower density. For galaxies in the local Universe, the HCN molecule is an effective tracer of high-density molecular gas. Here we report observations of HCN emission from the infrared-luminous 'Cloverleaf' quasar (at a redshift zeta = 2.5579). The HCN line luminosity indicates the presence of 10 billion solar masses of very dense gas, an essential feature of an immense starburst, which contributes, together with the active galactic nucleus it harbours, to its high infrared luminosity.