Efficient Star Formation Research Articles

Observability of the virialization phase of spheroidal galaxies with radio arrays

In the standard galaxy formation scenario plasma clouds with a high thermal energy content must exist at high redshifts since the protogalactic gas is shock heated to the virial temperature, and extensive cooling, leading to efficient star formation, must await the collapse of massive haloes (as indicated by the massive body of evidence, referred to as downsizing). Massive plasma clouds are potentially observable through the thermal and kinetic Sunyaev–Zel'dovich effects and their free–free emission. We find that the detection of substantial numbers of galaxy-scale thermal Sunyaev–Zel'dovich signals is achievable by blind surveys with next generation radio telescope arrays such as EVLA, ALMA and SKA. This population is even detectable with the 10 per cent SKA, and wide field of view options at high frequency on any of these arrays would greatly increase survey speed. An analysis of confusion effects and of the contamination by radio and dust emissions shows that the optimal frequencies are those in the range 10–35 GHz. Predictions for the redshift distributions of detected sources are also worked out.

High‐excitation OH and H2O Lines in Markarian 231: The Molecular Signatures of Compact Far‐infrared Continuum Sources

The ISO LWS far-infrared spectrum of the ultraluminous galaxy Mrk 231 shows OH and H2O lines in absorption from energy levels up to 300 K above the ground state, and emission in the [O I] 63 μm and [C II] 158 μm lines. Our analysis shows that OH and H2O are radiatively pumped by the far-infrared continuum emission of the galaxy. The absorptions in the high-excitation lines require high far-infrared radiation densities, allowing us to constrain the properties of the underlying continuum source. The bulk of the far-infrared continuum arises from a warm (Tdust = 70–100 K), optically thick (τ100μ m = 1–2) medium of effective diameter 200-400 pc. In our best-fit model of total luminosity LIR, the observed OH and H2O high-lying lines arise from a luminous (L/LIR ~ 0.56) region with radius ~100 pc. The high surface brightness of this component suggests that its infrared emission is dominated by the AGN. The derived column densities N(OH) 1017 cm−2 and N(H2O) 6 × 1016 cm−2 may indicate X-ray dominated region (XDR) chemistry, although significant starburst chemistry cannot be ruled out. The lower-lying OH, [C II] 158 μm, and [O I] 63 μm lines arise from a more extended (~350 pc) starburst region. We show that the [C II] deficit in Mrk 231 is compatible with a high average abundance of C+ because of an extreme overall luminosity to gas mass ratio. Therefore, a [C II] deficit may indicate a significant contribution to the luminosity by an AGN, and/or by extremely efficient star formation.

On the morphologies, gas fractions, and star formation rates of small galaxies

We use a series of N-body/smoothed particle hydrodynamics simulations and analytic arguments to show that the presence of an effective temperature floor in the interstellar medium at T_F ~ 10^4 K naturally explains the tendency for low-mass galaxies to be more spheroidal, more gas rich, and less efficient in converting baryons into stars than larger galaxies. The trend arises because gas pressure support becomes important compared to angular momentum support in small dark matter haloes. We suggest that dwarf galaxies with rotational velocities ~ 40 km/s do not originate as thin discs, but rather are born as thick, puffy systems. If accreted on to larger haloes, tenuous dwarfs of this kind will be more susceptible to gas loss or tidal transformation than scaled-down versions of larger spirals. For a constant temperature floor, pressure support becomes less important in large haloes, and this produces a tendency for massive isolated galaxies to have thinner discs and more efficient star formation than their less massive counterparts, as observed.

Particularly efficient star formation in M 33

The Star Formation (SF) rate in galaxies is an important parameter at all redshifts and evolutionary stages of galaxies. In order to understand the increased SF rates in intermediate redshift galaxies one possibility is to study star formation in local galaxies with properties frequently found at this earlier epoch like low metallicity and small size. We present sensitive observations of the molecular gas in M 33, a small Local Group spiral at a distance of 840 kpc which shares many of the characteristics of the intermediate redshift galaxies. The observations were carried out in the CO(2–1) line with the HERA heterodyne array on the IRAM 30 m telescope. A region in the northern part of M 33 was observed, reaching a detection threshold of a few 103 . The correlation in this field between the CO emission and tracers of SF (8 μm, 24 μm, Hα, FUV) is excellent and CO is detected very far North, showing that molecular gas forms far out in the disk even in a small spiral with a subsolar metallicity. One major molecular cloud was discovered in an interarm region with no HI peak and little if any signs of SF – without a complete survey this cloud would never have been found. The radial dependence of the CO emission has a scale length similar to the dust emission, less extended than the Hα or FUV. If, however, the ratio varies inversely with metallicity, then the scale length of the H2 becomes similar to that of the Hα or FUV. Comparing the SF rate to the H2 mass shows that M 33, like the intermediate redshift galaxies it resembles, has a significantly higher SF efficiency than large local universe spirals. The data presented here also provide an ideal test for theories of molecular cloud formation and cover a new region in parameter space, where . We find that a simple pressure-based prescription for estimating the molecular to atomic gas fraction does not perform well for M 33, at least in the outer parts. On the other hand, we show that the molecular gas fraction is influenced by (i) the total Hydrogen column density, dominated in M 33 by the HI, and (ii) the galactocentric distance.

Natural downsizing in hierarchical galaxy formation

Stellar-population analyses of today's galaxies show ‘downsizing’, where the stars in more massive galaxies tend to have formed earlier and over a shorter time-span. We show that this phenomenon is not necessarily ‘antihierarchical’ but rather has its natural roots in the bottom-up clustering process of dark-matter haloes. While the main progenitor does indeed show an opposite effect, the integrated mass in all the progenitors down to a given minimum mass shows a robust downsizing that is qualitatively similar to what has been observed. These results are derived analytically from the standard extended Press–Schechter (EPS) theory, and are confirmed by merger trees based on EPS or drawn from N-body simulations. The downsizing is valid for any minimum mass, as long as it is the same for all haloes at any given time, but the effect is weaker for smaller minimum mass. If efficient star formation is triggered by atomic cooling, then a minimum halo mass arises naturally from the minimum virial temperature for cooling, T≃ 104 K, though for such a small minimum mass the effect is weaker than observed. Baryonic feedback effects, which are expected to stretch the duration of star formation in small galaxies and shut it down in massive haloes at late epochs, are likely to play a subsequent role in shaping up the final downsizing behaviour. Other appearances of downsizing, such as the decline with time of the typical mass of star-forming galaxies, may not be attributed to the gravitational clustering process but rather arise from the gas processes.

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.

On the Structural Differences between Disk and Dwarf Galaxies

Gas-rich dwarf and disk galaxies overlap in numerous physical quantities, which makes their classification subjective. We report the discovery of a separation of dwarfs and disks into two unique sequences in the mass (luminosity) versus scale length plane. This provides an objective classification scheme for late-type galaxies that only requires optical or near-IR surface photometry of a galaxy. Since the baryonic Tully-Fisher relation for these samples produces a continuous relation between baryonic mass and rotational velocity, we conclude that the difference between dwarfs and disks must be due to their distribution of stellar light such that dwarfs are more diffuse than disk galaxies. This structural separation may be due to a primordial difference between low- and high-mass galaxies or produced by hierarchical mergers in which disks are built up from dwarfs. Structural differences between dwarf and disk galaxies may also be driven by the underlying kinematics, where the strong rotation in disks produces an axially symmetric object that undergoes highly efficient star formation in contrast to the more weakly rotating, more disordered motion of dwarfs that produces a diffuse, triaxial object with a history of inefficient star formation.

Formation of First Stars Triggered by Collisions and Shock Waves: Prospect for High Star Formation Efficiency and High Ionizing Photon Escape Fraction

We show that large, high-redshift (z > 10) galaxies with virial temperature in excess of 104 K may be composed mostly of cold atomic clouds that were formerly minihalos. These clouds move at a speed of ~15-30 km s-1 and may collide with one another on an average time interval of ~107 yr. The supersonic collisions may result in spatially distributed and efficient star formation. Most of the subsequent star formation in cold atomic clouds may be triggered by shock waves launched from the first stars formed in colliding clouds. Those shock-wave-compressed clouds may be more widespread spatially, because of the large imparted velocities, and some may escape into the intergalactic medium. The resultant widespread star formation would allow the possibility of a much higher ionizing photon escape fraction. These favorable conditions may form a physical basis to enable the standard cosmological model to produce a reasonably high Thomson optical depth τe = 0.11-0.14. Furthermore, a chain reaction of star formation in minihalos in the intergalactic space may be triggered by explosions in intergalactic medium, if minihalos are strongly clustered. In this case, a still higher τe would be achievable.

Did most present-day spirals form during the last 8 Gyr?

Studies of distant galaxies have shown that ellipticals and large spirals (Schade et al. [CITE], ApJ, 525, 31; Lilly et al. [CITE], ApJ, 500, 75) were already in place 8 Gyr ago, leading to a very modest recent star formation (Brinchmann & Ellis [CITE], ApJ, 536, L77) in intermediate mass galaxies ( ). This is challenged by a recent analysis (Heavens et al. [CITE], Nature, 428, 625) of the fossil record of the stellar populations of ~105 nearby galaxies, which shows that intermediate mass galaxies formed or assembled the bulk of their stars 4 to 8 Gyr ago. Here we present direct observational evidence supporting this findings from a long term, multi-wavelength study of 195 intermediate mass galaxies, mostly selected from the Canada France Redshift Survey (CFRS). We show that recent and efficient star formation is revealed at IR wavelengths since ~15% of intermediate mass galaxies at are indeed luminous IR galaxies (LIRGs), a phenomenon far more common than in the local Universe. The star formation in LIRGs is sufficient in itself to produce 38% of the total stellar mass of intermediate mass galaxies and then to account for most of the reported stellar mass formation since . Observations of distant galaxies have also the potential to resolve their star formation and mass assembly histories. The high occurrence of LIRGs is easily understood only if they correspond to episodic peaks of star formation, during which galaxies are reddened through short IREs (infrared episodes). We estimate that each galaxy should experience 4 to IREs from to , being the characteristic timescale. An efficient and episodic star formation is further supported by the luminosity-metallicity relation of emission line galaxies, which we find to be on average metal deficient by a factor of ~2 when compared to those of local spirals. We then examine how galaxy IREs can be related to the emergence at high redshift of the abundant population of galaxies with small size (but not with small mass), blue core and many irregularities. We show that recent merging and gas infall naturally explain both morphological changes and episodic star formation history in a hierarchical galaxy formation frame. We propose a simple scenario in which % of intermediate mass spirals have recently experienced their last major merger event, leading to a drastic reshaping of their bulges and disks during the last 8 Gyr. It links in a simple manner distant and local galaxies, and gives account of the simultaneous decreases during that period, of the cosmic star formation density, of the merger rate, and of the number densities of LIRGs, compact and irregular galaxies, while the densities of ellipticals and large spirals are essentially unaffected. It predicts that 42, 22 and 36% of the IR (episodic) star formation density is related to major mergers, minor mergers and gas infall, respectively.

Baryonic conversion tree: the global assembly of stars and dark matter in galaxies from the Sloan Digital Sky Survey

Using the spectroscopic sample of the Sloan Digital Sky Survey Data Release 1 (SDSS DR1), we measure how gas was transformed into stars as a function of time and stellar mass: the baryonic conversion tree (BCT). There is a clear correlation between early star formation activity and present-day stellar mass: the more massive galaxies have formed approximately 80 per cent of their stars at z > 1, while for the less massive ones the value is only approximately 20 per cent. By comparing the BCT with the dark matter merger tree, we find indications that star formation efficiency at z > 1 had to be approximately a factor of two higher than today (∼10 per cent) in galaxies with present-day stellar mass larger than 2 × 1011M⊙, if this early star formation occurred in the main progenitor. Therefore, the λ cold dark matter (LCDM) paradigm can accommodate a large number of red objects. On the other hand, in galaxies with present-day stellar mass less than 1011M⊙, efficient star formation seems to have been triggered at z∼ 0.2. We show that there is a characteristic mass (M*∼ 1010M⊙) for feedback efficiency (or lack of star formation). For galaxies with masses lower than this, feedback (or star formation suppression) is very efficient while for higher masses it is not. The BCT, determined here for the first time, should be an important observable with which to confront theoretical models of galaxy formation.

Starbursts from the Strong Compression of Galactic Molecular Clouds due to the High Pressure of the Intracluster Medium

We demonstrate that the high pressure of the hot intracluster medium (ICM) can trigger the collapse of molecular clouds in a spiral galaxy, leading to a burst of star formation in the clouds. Our hydrodynamical simulations show that the high gaseous (ram pressure and static thermal) pressure of the ICM strongly compresses a self-gravitating gas cloud within a short timescale (~107 yr), dramatically increasing the central gas density and consequently causing efficient star formation within the cloud. The stars developed in the cloud form a compact, gravitationally bound star cluster. The star formation efficiency within such a cloud is found to depend on the temperature and the density of the ICM and the relative velocity of the galaxy with respect to it. Based on these results, we discuss the origin of starburst/post-starburst populations observed in distant clusters, the enhancement of star formation for galaxies in merging clusters, and the isolated compact H II regions recently discovered in the Virgo Cluster.

The Formation of Stellar Clusters: Mass Spectra from Turbulent Molecular Cloud Fragmentation

Turbulent fragmentation determines where and when protostellar cores form, and how they contract and grow in mass from the surrounding cloud material. This process is investigated, using numerical models of self-gravitating molecular cloud dynamics. Molecular cloud regions without turbulent driving sources, or where turbulence is driven on large scales, exhibit rapid and efficient star formation in a clustered mode, whereas interstellar turbulence that carries most energy on small scales results in isolated star formation with low efficiency. The clump mass spectrum of shock-generated density fluctuations in pure hydrodynamic, supersonic turbulence is not well fit by a power law, and it is too steep at the high-mass end to be in agreement with the observational data. When gravity is included in the turbulence models, local collapse occurs, and the spectrum extends towards larger masses as clumps merge together, a power-law description dN/dM ~ M^nu becomes possible with slope nu < -2. In the case of pure gravitational contraction, i.e. in regions without turbulent support, the clump mass spectrum is shallower with nu = -3/2. The mass spectrum of protostellar cores in regions without turbulent support and where turbulence is replenished on large-scales, however, is well described by a log-normal or by multiple power laws, similar to the stellar IMF at low and intermediate masses. In the case of small-scale turbulence, the core mass spectrum is too flat compared to the IMF for all masses.

The Accretion of Lyα Clouds onto Gas‐rich Protogalaxies: A Scenario for the Formation of Globular Star Clusters

A satisfactory theory for the formation of globular star clusters (GCs) has long been elusive, perhaps because their true progenitors had not yet been guessed. In this paper, I propose a causal relationship between the strongly decreasing densities of Lyα clouds at high redshift and the formation of GCs—namely, that GCs were created by the accretion of Lyα clouds onto protogalaxies. I describe a scenario that involves an inherently stable and orderly cycling of compression and cooling in the central cores of these clouds during the extended period of dissipation in the outer regions of gas-rich protogalaxies, culminating in a burst of efficient star formation. I demonstrate that the comoving density of GCs is comparable to that of Lyα clouds at high redshift, that the energetic requirements for compression to core GC densities can be met, and that the timescale for cooling is within obvious limits imposed by dynamical stability. This dissipative process requires there to be a large column of dissipated gas about the attractor in order to form GCs. In addition, the energy requirements of compression to GC central densities requires attractor masses greater than that capable of sustaining circular velocities of ~40 km s-1. If this scenario is supported by numerical simulations, then by implication the GCs were formed at modest redshifts of z~1-3. This knowledge could help break the troublesome degeneracy between look-back time and redshift. This model is consistent with a picture of hierarchical galaxy growth over timescales of many billions of years.

The Age Spread Among Galaxies and the Origin of the UV Radiation from Elliptical Galaxies

Recent UV observations of elliptical galaxies are interpreted as evidence for the global second parameter phenomenon of horizontal-branch (HB) morphology within, as well as between, these galaxies. In this picture, the origin of the UV radiation is mostly due to hot HB stars and their post-HB progeny produced by the metal-poor tail of the wide metallicity distribution expected to be present in these systems. The attractive feature of this model is that the bimodal temperature distributions of HB stars (and their progeny), required to generate the 2000 Å dip of the spectral energy distribution (SED), can naturally be reproduced from the standard HB population models with large range of metal abundance (see Lee 1994, ApJ, 430, L113). Detailed population synthesis models are presented, which reproduce the systematic variation of UV upturn among elliptical galaxies (Fig 1). If age is the major second parameter, as suggested by the fossil record in our Galaxy, the observed UV color gradient and the UV upturn-total mass (mean metallicity) correlation, within and between the early-type systems, would imply, respectively, (1) that most galaxies formed from the inside out, and (2) that there is age spread among galaxies, in the sense that more massive galaxies are older (and more metal-rich in the mean) than less massive galaxies as a result of more efficient star formation (and metal enrichment) in denser environments.

Quasar Formation in Hierarchical Structure Formation Models

Hierarchical cosmogonies can consistently explain the evolution of the quasar population if quasars are short-lived and supermassive black holes form fast in the newly-formed nuclei of dark-matter haloes. Here we investigate the relevant physical processes and show that such a fast formation is plausible. The angular-momentum and the gas-supply problem for the formation/feeding of a supermassive black hole are strongly alleviated compared to a scenario in which gas is transported to the centre by tidal interaction of ready-assembled galaxies. The baryonic component of the newly-formed nucleus will cool catastrophically and settle into a self-gravitating angular momentum-supported disc of radius ∼ 100 pc. Gravitational instabilities and/or supernovae-induced turbulence will transport the gas further to the centre within less than 108 yr. In nuclei of very massive dark matter haloes with sufficiently deep potential well to retain the gas against feedback processes from massive stars and supernovae, concentration of a major fraction of the gas component of the nucleus within the central 1 pc and subsequent formation of a black hole seem unavoidable. A coeval short phase of efficient star formation could explain the observed high metallicities of quasars.

Millimeter interferometry of the molecular gas in ARP 20

The Owens Valley Millimeter Wave Interferometer has been used to map the 2.6-mm CO emission in the ultraluminous infrared galaxy Arp 220. Approximately 70 percent of the CO emission from the galaxy originates from an unresolved region less than 4 arcsec x 6 arcsec in size (corresponding to 1500 pc diameter) centered on the near-infrared nucleus. The mass of gas within this region is 10 to the 10th solar mass, which is about 30 times greater than that in an equivalent area of the Galaxy. This concentration could result in efficient star formation via cloud-cloud collisions and provide a significant accretion flow onto a compact, central object.

Star formation within OB subgroups - Implosion by multiple sources

We present the results of new detailed two-dimensional radiation-hydrodynamical calculations of the effects of radiation-driven shock waves from two O stars on inhomogeneities embedded in molecular clouds. The calculations indicate the neutral primordial clumps of gas with 84 M/sub sun/ can be highly compressed in 3 x 10/sup 4/ yr with density enhancements greater than 170 over ambient densities and 40 M/sub sun/ remaining. Inhomogeneities that are compressed in this manner by stars in the range O7--B0 survive ionization evaporation and may rapidly form new stars. Low-mass objects would not survive, and there would be a natural cutoff of low-mass and high-mass stars. We present a scenario for hierarchical radiation-driven implosion as a potential, new highly efficient mechanismfor star formation that may explain aspects of recent observations of new star formation in ultracompact H II regions.

H I in early-type galaxies. II - Mass loss and galactic winds

view Abstract Citations (425) References (75) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS H I in early-type galaxies. II. Mass loss and galactic winds. Faber, S. M. ; Gallagher, J. S. Abstract Implications of the low upper limits to the neutral-hydrogen content observed in E and S0 galaxies are examined. Evidence for mass loss in these systems is reviewed, mass loss by dying stars in them is analyzed, and it is shown that the gas must either be hidden in some undetectable form or be completely removed from the interstellar medium. Several ways to conceal the gas are considered and found to fail on one or more grounds, indicating that a removal mechanism is likely. Possible candidates investigated for such a mechanism are efficient star formation, sweeping by nuclear explosions, and hot supernova-driven galactic winds. It is shown that the presence of galactic winds in ellipticals not only can explain the observed lack of hydrogen, but also is apparently in agreement with other observations, including the lack of reddening in early-type galaxies and the preferential association of RF activity with massive ellipticals. The possibility that hot winds may provide internal removal mechanisms in the bulges of disk galaxies is briefly discussed. Publication: The Astrophysical Journal Pub Date: March 1976 DOI: 10.1086/154180 Bibcode: 1976ApJ...204..365F Keywords: Cosmic Gases; Galactic Evolution; Hydrogen Clouds; Intergalactic Media; Astrophysics; Cosmology; Elliptical Galaxies; Mass Transfer; Radio Sources (Astronomy); Spiral Galaxies; Stellar Evolution; Stellar Winds; Supernovae; Astrophysics full text sources ADS | data products SIMBAD (5) NED (3) Related Materials (4) Part 1: 1975ApJ...202....7G Part 3: 1976LicOB.709....1F Part 4: 1976LicOB.726....1B Part 5: 1976ApJ...209..710B