共查询到20条相似文献,搜索用时 406 毫秒
1.
C. H. Whaley J. A. Irwin S. C. Madden F. Galliano G. J. Bendo 《Monthly notices of the Royal Astronomical Society》2009,395(1):97-113
We present a multiwavlength infrared (IR) study of the nearby, edge-on, spiral galaxy NGC 891. We have examined 20 independent, spatially resolved IR images of this galaxy, 14 of which are newly reduced and/or previously unpublished images. These images span a wavelength regime from λ 1.2 μ m in which the emission is dominated by cool stars, through the mid-IR, in which emission is dominated by polycyclic aromatic hydrocarbons (PAHs), to λ 850 μm, in which emission is dominated by cold dust in thermal equilibrium with the radiation field. The changing morphology of the galaxy with wavelength illustrates the changing dominant components. We detect extraplanar dust emission in this galaxy, consistent with previously published results, but now show that PAH emission is also in the halo, to a vertical distance of z ≥ 2.5 kpc . We compare the vertical extents of various components and find that the PAHs (from λ 7.7 and 8 μm data) and warm dust (λ 24 μm) extend to smaller z heights than the cool dust (λ 450 μm). For six locations in the galaxy for which the signal-to-noise ratio was sufficient, we present spectral energy distributions (SEDs) of the IR emission, including two in the halo – the first time a halo SED in an external galaxy has been presented. We have modelled these SEDs and find that the PAH fraction, f PAH , is similar to Galactic values (within a factor of 2), with the lowest value at the galaxy's centre, consistent with independent results of other galaxies. In the halo environment, the fraction of dust exposed to a colder radiation field, f cold , is of the order of unity, consistent with an environment in which there is no star formation. The source of excitation is likely from photons escaping from the disc. 相似文献
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Alice C. Quillen Joss Bland-Hawthorn 《Monthly notices of the Royal Astronomical Society》2008,386(4):2227-2234
We introduce a differential equation for star formation in galaxies that incorporates negative feedback with a delay. When the feedback is instantaneous, solutions approach a self-limiting equilibrium state. When there is a delay, even though the feedback is negative, the solutions can exhibit cyclic and episodic solutions. We find that periodic or episodic star formation only occurs when two conditions are satisfied. First the delay time-scale must exceed a cloud consumption time-scale. Secondly, the feedback must be strong. This statement is quantitatively equivalent to requiring that the time-scale to approach equilibrium be greater than approximately twice the cloud consumption time-scale. The period of oscillations predicted is approximately four times the delay time-scale. The amplitude of the oscillations increases with both feedback strength and delay time.
We discuss applications of the delay differential equation (DDE) model to star formation in galaxies using the cloud density as a variable. The DDE model is most applicable to systems that recycle gas and only slowly remove gas from the system. We propose likely delay mechanisms based on the requirement that the delay time is related to the observationally estimated time between episodic events. The proposed delay time-scale accounting for episodic star formation in galaxy centres on periods similar to P ∼ 10 Myr , irregular galaxies with P ∼ 100 Myr , and the Milky Way disc with P ∼ 2 Gyr, could be that for exciting turbulence following creation of massive stars, that for gas pushed into the halo to return and interact with the disc and that for spiral density wave evolution, respectively. 相似文献
We discuss applications of the delay differential equation (DDE) model to star formation in galaxies using the cloud density as a variable. The DDE model is most applicable to systems that recycle gas and only slowly remove gas from the system. We propose likely delay mechanisms based on the requirement that the delay time is related to the observationally estimated time between episodic events. The proposed delay time-scale accounting for episodic star formation in galaxy centres on periods similar to P ∼ 10 Myr , irregular galaxies with P ∼ 100 Myr , and the Milky Way disc with P ∼ 2 Gyr, could be that for exciting turbulence following creation of massive stars, that for gas pushed into the halo to return and interact with the disc and that for spiral density wave evolution, respectively. 相似文献
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T. A. Bell S. Viti D. A. Williams 《Monthly notices of the Royal Astronomical Society》2007,378(3):983-994
We present theoretically established values of the CO-to-H2 and C-to-H2 conversion factors that may be used to estimate the gas masses of external galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946, M82 and SMC N27. The physical parameters that best represent the conditions within the molecular clouds in each of the galaxy types are estimated using a χ2 analysis of several observed atomic fine structure and CO rotational lines. This analysis is explored over a wide range of density, radiation field, extinction and other relevant parameters. Using these estimated physical conditions in methods that we have previously established, CO-to-H2 conversion factors are then computed for CO transitions up to J = 9 → 8. For the conventional CO(1–0) transition, the computed conversion factor varies significantly below and above the canonical value for the Milky Way in the four galaxy types considered. Since atomic carbon emission is now frequently used as a probe of external galaxies, we also present, for the first time, the C-to-H2 conversion factor for this emission in the four galaxy types considered. 相似文献
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P. Andreani M. Magliocchetti G. De Zotti 《Monthly notices of the Royal Astronomical Society》2010,401(1):15-22
Optically very faint ( R > 25.5) sources detected by the Spitzer Space Telescope at 24 μm represent a very interesting population at redshift z ∼ (1.5–3) . They exhibit strong clustering properties, implying that they are hosted by very massive haloes, and their mid-infrared emission could be powered by either dust-enshrouded star formation and/or by an obscured active galactic nucleus (AGN). We report observations carried out with the Max Planck Millimetre Bolometer (MAMBO) array at the IRAM 30-m antenna on Pico Veleta of a candidate protocluster with five optically obscured sources selected from the 24-μm Spitzer sample of the First-Look Survey. Interestingly, these sources appear to lie on a high-density filament aligned with the two radio jets of an AGN. Four out of five of the observed sources were detected. We combine these measurements with optical, infrared and radio observations to probe the nature of the candidate protocluster members. Our preliminary conclusions can be summarized as follows: the spectral energy distributions (SEDs) of all sources include both AGN and starburst contributions; the AGN contribution to the bolometric luminosities ranges between 14 and 26 per cent of the total. Such a contribution is enough for the AGN to dominate the emission at 5.8, 8 and 24 μm, while the stellar component, inferred from SED fitting, prevails at 1.25 mm and at λ < 4.5 μ m. The present analysis suggests a coherent interplay at high z between extended radio activity and the development of filamentary large-scale structures. 相似文献
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T. A. Bell E. Roueff S. Viti D. A. Williams 《Monthly notices of the Royal Astronomical Society》2006,371(4):1865-1872
A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star-forming properties. However, H2 , the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the 12 CO( J = 1 → 0) emission line, using a CO-to-H2 conversion factor, X . Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon-dominated region (PDR) time-dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time-dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor. 相似文献
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The dense gas tracers HNC and CN prove to be detectable in sensitive single dish studies of infrared luminous starburst galaxies.
Their line intensity with respect to both the standard tracer of molecular gas, CO, and the dense, starforming component,
HCN, varies significantly. This opens a new, extinction-free window into the investigation of the properties and the evolutionary
stage of starbursts.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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Chanda J. Jog Chaitra A. Narayan 《Monthly notices of the Royal Astronomical Society》2001,327(3):1021-1028
We investigate the dynamical effects of a molecular cloud complex with a mass ∼ 107 M⊙ and a size ∼ a few 100 pc on the vertical distribution of stars and atomic hydrogen gas in a spiral galactic disc. Such massive complexes have now been observed in a number of spiral galaxies. The extended mass distribution in a complex, with an average mass density 6 times higher than the Oort limit, is shown to dominate the local gravitational field. This results in a significant redistribution or clustering of the surrounding disc components towards the mid-plane, with a resulting decrease in their vertical scaleheights.
The modified, self-consistent stellar density distribution is obtained by solving the combined Poisson equation and the force equation along the z -direction for an isothermal stellar disc on which the complex is imposed. The effect of the complex is strongest at its centre, where the stellar mid-plane density increases by a factor of 2.6 and the vertical scaleheight decreases by a factor of 3.4 compared with the undisturbed stellar disc. A surprising result is the large radial distance of ∼ 500 pc from the complex centre over which the complex influences the disc; this is due to the extended mass distribution in a complex. The complex has a comparable effect on the vertical distribution of the atomic hydrogen gas in the galactic disc. This 'pinching' or constraining effect should be detectable in the nearby spiral galaxies, as for example has been done for NGC 2403 by Sicking. Thus the gravitational field of a complex results in local corrugations of the stellar and H i vertical scaleheights, and the galactic disc potential is highly non-uniform on scales of the intercomplex separation of ∼ 1 kpc. 相似文献
The modified, self-consistent stellar density distribution is obtained by solving the combined Poisson equation and the force equation along the z -direction for an isothermal stellar disc on which the complex is imposed. The effect of the complex is strongest at its centre, where the stellar mid-plane density increases by a factor of 2.6 and the vertical scaleheight decreases by a factor of 3.4 compared with the undisturbed stellar disc. A surprising result is the large radial distance of ∼ 500 pc from the complex centre over which the complex influences the disc; this is due to the extended mass distribution in a complex. The complex has a comparable effect on the vertical distribution of the atomic hydrogen gas in the galactic disc. This 'pinching' or constraining effect should be detectable in the nearby spiral galaxies, as for example has been done for NGC 2403 by Sicking. Thus the gravitational field of a complex results in local corrugations of the stellar and H i vertical scaleheights, and the galactic disc potential is highly non-uniform on scales of the intercomplex separation of ∼ 1 kpc. 相似文献
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Steven L. Finkelstein Sangeeta Malhotra James E. Rhoads Nimish P. Hathi Norbert Pirzkal 《Monthly notices of the Royal Astronomical Society》2009,393(4):1174-1182
Recent results have shown that a substantial fraction of high-redshift Lyman α (Lyα) galaxies contain considerable amounts of dust. This implies that Lyα galaxies are not primordial, as has been thought in the past. However, this dust has not been directly detected in emission; rather it has been inferred based on extinction estimates from rest-frame ultraviolet (UV) and optical observations. This can be tricky, as both dust and old stars redden galactic spectra at the wavelengths used to infer dust. Measuring dust emission directly from these galaxies is thus a more accurate way to estimate the total dust mass, giving us real physical information on the stellar populations and interstellar medium enrichment. New generation instruments, such as the Atacama Large Millimeter Array and Sub-Millimeter Array, should be able to detect dust emission from some of these galaxies in the submillimeter. Using measurements of the UV spectral slopes, we derive far-infrared flux predictions for of a sample of 23 z ≥ 4 Lyα galaxies. We find that in only a few hours, we can detect dust emission from 39 ± 22 per cent of our Lyα galaxies. Comparing these results to those found from a sample of 21 Lyman break galaxies (LBGs), we find that LBGs are on average 60 per cent more likely to be detected than Lyα galaxies, implying that they are more dusty, and thus indicating an evolutionary difference between these objects. These observations will provide better constraints on dust in these galaxies than those derived from their UV and optical fluxes alone. Undeniable proof of dust in these galaxies could explain the larger than expected Lyα equivalent widths seen in many Lyα galaxies today. 相似文献
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M. den Heijer G. I. G. Jzsa J. Kerp P. Serra Th. van der Hulst G. Kauffmann J. Wang F. Bigiel J. Brinchmann D. Carton D. Cormier M.‐L. Huang 《Astronomische Nachrichten》2015,336(3):284-311
We examine the H I kinematics of the “Bluedisk” ensemble of 48 galaxies selected from the Sloan Digital Sky Survey and observed in H I with the Westerbork Synthesis Radio Telescope. The sample consists of 25 galaxies with a high H I mass fraction and a comparatively large control sample comprising 23 galaxies of comparable stellar mass, stellar mass surface density, redshift, and inclination. By studying the H I velocity fields of these galaxies, we investigate whether there are signatures of ongoing gas accretion: i.e. global asymmetries and indications for warping and kinematical lopsidedness. We find no enhanced kinematical asymmetries between the H I‐rich sample and the control sample galaxies, indicating no significant difference in kinematical signatures such as warping and lopsidedness. Furthermore, we find no difference in position angle and systemic velocity offset with respect to the optical between both sub‐samples. We therefore do not find compelling evidence for enhanced global asymmetry of the H I‐excess galaxies ensemble properties in comparison to the control sample galaxies. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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Susanne Aalto 《Astrophysics and Space Science》2005,295(1-2):143-153
The molecular phase of the ISM constitutes the main source of fuel for the activity in starburst and AGNs. The physical conditions and chemical constitution of the molecular gas will change with, and respond to, the evolution of the activity. This paper includes a short discussion of the 12CO/13CO 1–0 line intensity ratio as a diagnostic tool of the molecular gas properties of luminous galaxies – paired with examples of high-resolution studies of how the line ratio varies within galaxies. A possible connection between the OH megamasers and galaxies with unusually high 12CO/13CO 1–0 line intensity ratios are also briefly discussed.The relative intensities of the dense gas tracers HNC, HCN, HCO+ and CN are a result of both chemistry and starburst evolution. The discussion on the interpretation of HNC 1–0 emission includes the importance of ion-neutral chemistry in a luminous starburst region. Finally, simple cartoon ISM models and how they can be applied to LIRGs and ULIRGs, are presented. 相似文献
17.
Leonid Pilyugin 《Astrophysics and Space Science》2003,284(2):775-778
Spiral galaxies with a reported bend in the slope of the oxygen abundance O/HR
23, derived with the traditionally used R23-method, are examined. It is shown that the artificial origin of the reported bends can be naturally explained. Two causes
of the false bend in the slope of O/HR
23 are indicated.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
18.
T. G.Robinson C. N.Tadhunter J. E.Dyson 《Monthly notices of the Royal Astronomical Society》2002,331(2):L13-L18
We present accurate measurements of the physical conditions in five powerful radio galaxies, as derived from deep, long-slit spectroscopic observations. All five objects show prominent extended line emission, and have X-ray luminosities similar to those of isolated elliptical galaxies. The data are high enough quality that the electron density and temperature can be measured at several positions across the emission-line nebulae.
We subtract a model continuum comprising a combination of a 15-Gyr stellar template, a young stellar template and a power law, so as to be better able to measure faint diagnostic lines. Electron temperatures measured from the [O iii ](4959+5007)/4363 line ratio are in the range 10 000< Te <20 000 K , whilst [S ii ](6716/6731) densities fall between 100–500 cm-3 . Using these values, we find pressures within the line-emitting clouds a factor of 10–100 times higher than expected for pressure balance with the hot X-ray haloes of the host galaxies.
Previous studies of sources that show significant evidence of jet–cloud interactions, both in terms of their kinematics and ionization, have concluded that the overpressure is a result of the warm, line-emitting gas being compressed by the radio cocoon; however, there is no evidence that the radio jet is influencing the emission-line regions in four of our five objects.
We suggest that it is plausible that the line-emitting clouds have not yet relaxed into pressure equilibrium from their initial photoionization by the central active galactic nucleus. 相似文献
We subtract a model continuum comprising a combination of a 15-Gyr stellar template, a young stellar template and a power law, so as to be better able to measure faint diagnostic lines. Electron temperatures measured from the [O iii ](4959+5007)/4363 line ratio are in the range 10 000< T
Previous studies of sources that show significant evidence of jet–cloud interactions, both in terms of their kinematics and ionization, have concluded that the overpressure is a result of the warm, line-emitting gas being compressed by the radio cocoon; however, there is no evidence that the radio jet is influencing the emission-line regions in four of our five objects.
We suggest that it is plausible that the line-emitting clouds have not yet relaxed into pressure equilibrium from their initial photoionization by the central active galactic nucleus. 相似文献
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A. Schurer F. Calura L. Silva A. Pipino G. L. Granato F. Matteucci R. Maiolino 《Monthly notices of the Royal Astronomical Society》2009,394(4):2001-2021
We present photometric evolution models of galaxies, in which, in addition to the stellar component, the effects of an evolving dusty interstellar medium have been included with particular care. Starting from the work of Calura et al., in which chemical evolution models have been used to study the evolution of both the gas and dust components of the interstellar medium in the solar neighbourhood, elliptical and irregular galaxies, it has been possible to combine these models with a spectrophotometric stellar code that includes dust reprocessing ( grasil ) to analyse the evolution of the spectral energy distributions (SEDs) of these galaxies. We test our models against observed SEDs both in the local universe and at high redshift, and use them to predict how the percentage of reprocessed starlight evolves for each type of galaxy. The importance of following the dust evolution is investigated by comparing our results with those obtained by adopting simple assumptions to treat this component. 相似文献