View Full Version : Arp 220: Dust and Molecular Gas

2014-Jul-24, 06:27 PM

1. Introduction to Arp 220.
2. Interstellar Dust.
2.1. Extinction.
2.2. Emission.
3. Molecular Gas.
3.1. Molecular Vibration.
3.2. Molecular Rotation.
4. Spectroscopic Investigations of Arp 220.
4.1. Continuum Emission in Arp 220.
4.2. Molecular Spectra in Arp 220.
5. Conclusions.
6. Acknowledgments.
7. References and Notes.

1. Introduction to Arp 220:

Arp 220 is an Ultraluminous Infrared Galaxy (http://en.wikipedia.org/wiki/Ultraluminous_infrared_galaxy) (ULIRG) located in the constellation of Serpens (http://en.wikipedia.org/wiki/Serpens). It is approximately 250 million light years distant from Earth with a redshift of z = 0.018126 and an apparent magnitude of 13.94 [1]. Its radiant energy output is strongest in the far-infrared (FIR) portion of the electromagnetic spectrum (λ = 15 to 1000 μm) with FIR luminosity [2] LFIR ≈ 2×1012 Lסּ. The high FIR luminosity results from starburst formation enshrouded in dense dust. The ultraviolet radiation from the young hot stars heats the dust by absorption. The hot dust ultimately radiates off a substantial portion of that energy at infrared wavelengths. The optical properties of interstellar dust are discussed in greater detail in Section 2 below.

As illustrated in Figures 1A and 1B from the Hubble Space Telescope (HST), Arp 220 appears as a single galaxy with peculiar morphology, the result of the dynamical merger of two spiral galaxies believed to have begun approximately 700 million years ago.


Fig. 1A. HST full-field image of Arp 220.
Image courtesy of NASA, ESA, the Hubble Heritage
(STScI/AURA)-ESA/Hubble Collaboration, and A. Evans
(University of Virginia/NRAO/Stony Brook University).

Fig. 1B. Infrared image of Arp 220 taken
by the HST Near Infrared Camera and Multi-Object
Spectrometer (NICMOS). Image courtesy of R. Thompson,
M. Rieke, G. Schneider (University of Arizona) and N. Scoville
(California Institute of Technology), and NASA.

The merger has strongly compressed both gas and dust into a compact region approximately 5,000 light years across as is clearly shown in Figure 2. The quantity of gas and dust [2] within this small region (1010 Mסּ) is believed to be equal to that within the entire Milky Way. This compression of material has triggered intense, rapid starburst activity yielding gigantic clusters of young hot stars. The star clusters are the bluish-white bright knots visible in the HST image. The HST images also resolve two bright sources 1,200 light-years apart assigned to the respective nuclei of the parent galaxies.


Fig. 2. Central region of Arp 220 revealing complex structure within
one arc second of the core. Image courtesy of E. Shaya, D. Dowling/U. of Maryland,
the WFPC Team, and NASA.

Investigations in other regions of the electromagnetic spectrum are consistent with the observational results and analyses summarized above. Figure 3A illustrates Arp 220 as seen in X-rays by the Chandra X-ray Observatory (http://chandra.harvard.edu/); while Figure 3B illustrates a composite of visible and microwave images of Arp 220 representing HST data and observations at 1.4204 GHz [3] obtained by the National Radio Astronomical Observatory (http://www.vla.nrao.edu/) (NRAO).


Fig. 3A. Chandra X-ray image of Arp 220. Image courtesy
of NASA/SAO/CXC/J.McDowell.

Fig. 3B. Composite image of Arp 220. Optical starlight is depicted
in green, yellow, and orange. Neutral atomic hydrogen gas is depicted in
blue. Image courtesy of NRAO/AUI, M. Yun and J. Hibbard.

The Chandra observations (Fig. 3A) served to pinpoint an X-ray source at the exact location of the visible nucleus of one of the pre-merger galaxies. The second, fainter X-ray source may well correspond to the nucleus of the other galaxy remnant. The bright central region is a glowing cloud of exceedingly hot (» 106 K) gas rushing out of the galaxy, driven by a "superwind" presumed to result from explosive activity generated by the formation of hundreds of millions of new stars. Giant lobes of hot gas extending 75,000 light years may be a remnant of the initial collisional impact.

The composite image (Fig. 3B) clearly indicates two core regions as well as an extended region of atomic hydrogen (HI) depicted in blue. The distribution of HI is consistent with a merger of two gas-rich spiral galaxies. Note that the apparent void in the HI distribution in the central region of Arp 220 is an observational artifact: HI is not observed in emission there because of the bright microwave continuum of the source object.

It remains unclear as to whether the lobes of hot gas seen in X-rays or the HI seen in the microwave will remain gravitationally bound to the galaxy or if they will continue to expand, eventually escaping the galaxy.

McDowell and coworkers [4] have simulated the dynamics of the galactic merger. Their results (http://iopscience.iop.org/0004-637X/591/1/154/55982.fg10.html) clearly illustrate both the plume regions and the lobes of the system.

2. Interstellar Dust:

Dust is a vital component of the Interstellar Medium [5,6]. It can strongly influence the spectrum of an observed object via extinction and emission of radiation. It is involved directly in the thermodynamic energy balance of the interstellar gas and even provides catalytic surface sites for the formation and destruction of molecules. It also strongly influences the dynamics of star formation. Dust is a rather broad term in that it encompasses species as small as polyatomic molecules of the order of tens of Ångströms in size, to grains of the order of several micrometers in size. Figure 4 illustrates an interstellar grain captured in the stratosphere by NASA aircraft in April 2003 as Earth passed through the dust stream of comet 26P/Grigg-Skjellerup (http://en.wikipedia.org/wiki/26P/Grigg–Skjellerup) [7].


Fig. 4. Interstellar dust grain showing regions of local composition. Note the size
indicator to the right. Image courtesy of H. Busemann, University of Manchester.

With a dimensional range of four orders of magnitude, it is self-evident that the composition of dust varies greatly. Currently available evidence is consistent with grain compositions that include silicates of magnesium and iron; a wide variety of carbonaceous materials [8]; and small amounts of SiC granules and carbonates [5a]. Figure 5 illustrates an infrared spectrum of dust surrounding the massive protostar W33A (http://www.gemini.edu/node/11394) that clearly manifests features attributable to silicate materials [9].


Fig. 5. Infrared spectrum of interstellar dust surrounding W33A illustrating absorption features
assigned to solid state silicate materials. Image courtesy of E. Gibb, et. al. [9] and American Astronomical Society.

Here we shall endeavor to introduce some of the general optical properties of interstellar dust. Our focus will be upon extinction and emission of radiation in that those properties are directly significant to spectroscopic investigations of dusty galactic environments.

2.1. Extinction:

The term extinction is used to describe the aggregate effects resulting from absorption and scattering of incident radiation by intervening matter as that radiation propagates from some astronomical source to the observer. Given our existing understanding of galactic and stellar spectra, the phenomenon of absorption should not require further elaboration. Incident photons excite the material with concomitant annihilation of the incident photons. Some of the significant physical consequences of absorption are discussed in Section 2.2 below.

Scattering phenomena are perhaps newer concepts to many of us. In essence, scattering changes the direction of propagation of an incident photon. Scattering is said to be elastic if the frequency of the scattered photon is identical to that of the incident photon. Two important mechanisms of scattering are Rayleigh Scattering (http://en.wikipedia.org/wiki/Rayleigh_scattering) and Mie Scattering (http://en.wikipedia.org/wiki/Mie_scattering) [10]: both are elastic scattering mechanisms directly relevant to interstellar dust.

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2014-Jul-24, 06:28 PM
Rayleigh Scattering applies to scattering by particles having dimensions that are much smaller than the wavelength of the incident radiation. The efficiency of Rayleigh Scattering increases strongly as the ratio of the particle size to the wavelength increases. Importantly, Rayleigh Scattering is close to being isotropic in that the incident radiation is scattered in any direction with similar probability. Our sky appears blue because of Rayleigh Scattering by atmospheric particles.

Mie Scattering comes into play when the size of the scattering particle reaches approximately 10% of the incident wavelength. The efficiency of Mie Scattering is essentially independent of wavelength and it is much less isotropic than Rayleigh Scattering: the larger the scattering particle, the smaller the angle of scattering away from the initial path of the incident photon. A comparison of the scattering angular distributions can be found here (http://withfriendship.com/images/e/23905/Rayleigh-scattering-image.gif). Note however that at the extreme distances of astronomical objects, even scattering through very small angles has dramatic effects on the intensity of radiation reaching the observer.

Figure 6 depicts the wavelength dependence of extinction from scattering, extinction from absorption, and total extinction for visible and ultraviolet radiation. Note the steep increase in extinction from scattering and total extinction at shorter wavelengths. The bump at 220 nm is believed to be associated with small particles of graphite or other carbonaceous materials [5b]. The observed wavelength dependence is consistent with extinction arising from two populations of grain that differ in size [11].

http://t1.gstatic.com/images?q=tbn:ANd9GcT3pmnR7zpXZ4wIz0t72brL4HwudVrQV 2pPZWydh_me70VEb-_AIQ&t=1

Fig. 6. Extinction as a function of wavelength from the visible
to the ultraviolet. Image courtesy of The Open University.

All of this excessively pedantic verbiage can be summarized as follows: Extinction is much more pronounced at short wavelengths than at long wavelengths. Blue light is effected more strongly than red light which, as we already knew, leads to interstellar reddening by intervening dust and gas. We were also already aware that dense distributions of dust totally obscure the ultraviolet (UV) and visible radiation emanating from background stars and galaxies. Now perhaps, we have added to our basic understanding of why that is the case.

2.2. Emission:

Here we shall concentrate our attention on emission by interstellar dust and grains across the infrared (IR) continuum because of its direct relevance to our forthcoming discussions of Arp 220. It will be convenient to distinguish the excitation and relaxation dynamics pertinent to two domains of the IR continuum: the near- and mid-infrared domains (NIR/MIR) and the far-infrared (FIR) domain [6].

The NIR/MIR continuum emission is associated with small, warm dust species. That is, species with dimensions of 5 to 50 Å and temperatures in the range of hundreds of Kelvins. Such dust species are excited by the absorption of one single photon of incident UV radiation which serves to transiently heat the particles. The structures of such species are sufficiently complex that they are characterized by an effectively continuous density of internal energy states. The excitation energy is very rapidly redistributed throughout those internal energy states with rapid dissipation of the excitation energy by continuous emission in the NIR/MIR and concomitant cooling of the dust particle to the equilibrium temperature of approximately 20 K.

The FIR continuum emission is associated with larger, cold grains. That is grains of dimension larger than 0.01 μm at temperatures of approximately 20 K. Such grains are heated by absorption of incident radiation just as a pebble sitting out in bright sunshine is heated. In this case, the excitation is cumulative resulting in macroscopic heating and steady emission of blackbody radiation (http://en.wikipedia.org/wiki/Blackbody).

The first mechanism dominates the continuum emission over the region 1 ≤ λ ≤ 60 μm while the second mechanism dominates the continuum emission at λ > 100 μm. Both mechanisms contribute at intermediate wavelengths.

These continuum infrared emission processes are important contributors to the energy balance of the Interstellar Medium (ISM). High energy radiation that cannot escape regions of dust and gas are transformed to low energy radiation that easily penetrates the region, thus dissipating energy and cooling the region. Such dissipation of excess energy facilitates gravitational contraction of material leading to more efficient star formation.

3. Molecular Gas:

Discussions of galactic and stellar spectra generally focus on atomic species, both neutrals and ions, that manifest electronic transitions within the near ultraviolet (UV) and visible portions of the electromagnetic spectrum. Given a basic understanding of the discrete, quantized nature of atomic structure and the consequential resultant discrete nature of atomic spectra as observed either in absorption or in emission, we can better understand what the spectrum of a galaxy or star reveals about the fundamental physical properties and dynamics of the observed object.

But what of molecules and the other frequency domains within the electromagnetic spectrum? It is after all well established that a wide and diverse variety of molecular species exist within the Interstellar Medium (http://en.wikipedia.org/wiki/Interstellar_medium) (ISM). Molecules detected within the ISM and/or circumstellar envelopes (http://en.wikipedia.org/wiki/List_of_molecules_in_interstellar_space) range from simple diatomics to highly exotic, large polyatomic molecules. Examples of simple diatomics include: C2, CH, CN, CO, OH, NH, NO, and SiO. The more exotic, large polyatomics are exemplified by HC7N, the polycyclic aromatic hydrocarbon (PAH) anthracene (http://en.wikipedia.org/wiki/Anthracene) (C14H10), and even Fullerene (http://en.wikipedia.org/wiki/Fullerene) (C60). A cautionary note is prudent at this juncture. The examples of simple diatomic molecules cited above include several species that would generally be considered highly reactive and therefore unstable within typical terrestrial environments. While those species are chemically reactive because of the presence of unpaired electrons, they are physically stable species under the rarefied environments of the ISM.

Here we shall endeavor to introduce some of the basic structural elements—that is the internal degrees of freedom—of simple molecules that are absent in atoms. We shall also endeavor to introduce the spectroscopic manifestations of those additional degrees of freedom and where the corresponding transitions generally fall within the electromagnetic spectrum. For simplicity, we shall focus upon the pertinent structural and spectroscopic features of diatomic molecules—that is molecules comprised of only two atoms. Having established the fundamentals, we will subsequently explore how such spectral features can be scrutinized to advantage to provide physical insight into astrophysical phenomena.

Figure 7 illustrates the basic static equilibrium geometrical structure of a diatomic molecule. The two atomic nuclei are depicted as circles. The nuclei are held together by a molecular bond depicted as a dashed line. That molecular bond is the result of co-sharing of one or more of the outer, valence electrons from each of the bound atoms. The actual bond structure depends upon the individual constituent atoms as described by Valence Bond Theory (http://en.wikipedia.org/wiki/Valence_bond_theory) or Molecular Orbital Theory (http://en.wikipedia.org/wiki/Molecular_orbital_theory). It is therefore more correct to consider the depicted dashed line as the bond axis, or cylindrical symmetry axis of the diatomic molecule rather than as the actual bond itself.


Fig. 7. Static equilibrium geometrical structure of a diatomic molecule.

Molecular bonds are not rigid. They are more like a classical coiled spring that can be stretched and compressed. Hooke's Law (http://en.wikipedia.org/wiki/Hooke's_law) describes the dynamical, oscillatory motion of two masses connected by a classical spring following displacement from the static equilibrium separation. This classical behavior corresponds quite closely to the vibrational motion of the nuclei of a diatomic molecule relative to the center of mass. This symmetric stretch vibration is illustrated phenomenologically in Figure 8A below.

Diatomic molecules are also free to rotate—that is to tumble end over end—about the center of mass. This motion is analogous to that of twirling a pencil between one's fingers. Rotational motion can occur about any axis that is perpendicular to the bond axis and that passes through the center of mass as illustrated in Figure 8B.


Fig. 8A. Illustration of molecular vibration.

Fig. 8B. Illustration of molecular rotation.

Given our understanding of the quantized nature of the electronic energy levels of atoms, it should be no surprise that the electronic energy levels of molecules are also quantized. In the case of a diatomic molecule, the total electron orbital angular momentum , and if applicable the total electron spin and total nuclear spin angular momenta are quantized with respect to the bond axis. Nor should we be chagrined to learn that the vibrational and rotational degrees of freedom of a diatomic molecule are also rigorously quantized.

We now turn our attention to the quantized nature of the pure vibrational states and the pure rotational states of a diatomic molecule. The spectroscopic significance of these quantized energy levels will also be introduced.

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2014-Jul-24, 06:30 PM
3.1. Molecular Vibration:

To first approximation, we can develop the vibrational states of a diatomic molecule within the framework of the Quantum Harmonic Oscillator (http://en.wikipedia.org/wiki/Quantum_harmonic_oscillator). This level of approximation is similar to that of the Bohr Approximation (http://en.wikipedia.org/wiki/Bohr_model) for the electronic states of an atom. So doing, we find that the vibrational energies Ev are given by:

{E}_{v } = \left( v+\frac{1}{2 } \right)\hbar\omega
Eq. 1

\omega = \sqrt{k/\mu}
Eq. 2

\mu = {m}_{1 }{m}_{ 2}/\left( {m}_{1 }+{m}_{ 2} \right)
Eq. 3

where v is the vibrational quantum number; ℏ is the reduced Planck's constant; ω is the vibrational frequency in radians per second; k is the force constant of the molecular bond; μ is the reduced mass of the system; and m1 and m2 are the masses of the respective nuclei. The force constant k reflects the strength of the molecular bond. The selection rule for dipole-allowed transitions between pure vibrational states is given by:

\Delta\left( v \right) = \pm1
Eq. 4

Figure 9 illustrates a generic energy level diagram for a few of the lower vibrational states given by Equation 1 and compares them to those obtained by a more sophisticated treatment, the Morse Potential (http://en.wikipedia.org/wiki/Morse_potential), that includes the effects of anharmonicity (http://en.wikipedia.org/wiki/Anharmonicity). Application of Equations 1 and 4 demonstrate that at the level of the Harmonic Oscillator approximation, the pure vibrational spectrum will consist of a series of lines equally spaced by ℏω. Note that the Morse Potential yields energy separations that decrease with increasing vibrational quantum number.


Fig. 9. Comparison of the vibrational energy levels of a diatomic molecule as given by the Harmonic Oscillator approximation and by a more rigorous
Morse Potential treatment. Image courtesy of Mark Somoza, via Wikipedia.

The vibrational constants of diatomic molecules typically fall within the range of 500 to 3000 cm-1 [12] corresponding to transitions within the mid-infrared (MIR) portion of the electromagnetic spectrum (λ = 3 to 50 μm). Note the correspondence of this frequency domain to that shown in the spectrum of interstellar dust (Fig. 5) above.

3.2. Molecular Rotation:

To first approximation, we can develop the rotational states of a diatomic molecule within the framework of the Rigid Rotor (http://en.wikipedia.org/wiki/Rigid_rotor). This level of approximation is again similar to that of the Bohr Approximation for the electronic states of an atom. So doing, we find that the rotational energies EJ are given by:

{E}_{ J} = B\left[ J\left( J+1 \right) \right]
Eq. 5

B = {\hbar}^{ 2}/2I
Eq. 6

I = \mu{r}^{2 }
Eq. 7

where B is the rotational constant; J is the rotational quantum number [13]; I is the moment of inertia; μ is the reduced mass of the system; and r is the internuclear separation. The selection rule for dipole-allowed transitions between pure rotational states is given by:

\Delta\left( J \right) = \pm1
Eq. 8

Figure 10 illustrates a schematic energy level diagram for a few of the lower rotational states given by Equation 5 for arbitrary B as well as the dipole-allowed transitions as given by Equation 8. Application of Equations 5 and 8 demonstrate that at the level of the Rigid Rotor approximation, the rotational energy levels are separated by:

\Delta\left( {E}_{ J} \right) = 2B\left( {J}^{\prime\prime } + 1 \right)
Eq. 9

where J″ is the quantum number of the lower energy state resulting in a spectrum of lines equally spaced by 2B. From our discussions of molecular vibration above, we recognize fully that the bond is in fact not rigid. Thus a more sophisticated analysis allows for a non-rigid bond by incorporating the effects of centrifugal distortions that increase quadratically with increasing J and slightly lower the rotational energy levels from those predicted by the Rigid Rotor approximation.


Fig. 10. Schematic energy level diagram of the low rotational states
of a diatomic molecule under the Rigid Rotor approximation. The red
vertical lines indicate the dipole-allowed pure rotational transitions.

The rotational constants of diatomic molecules typically fall within the range of 1 to 10 cm-1 [12] corresponding to transitions within the microwave to far-infrared (FIR) portion of the electromagnetic spectrum.

4. Spectroscopic Investigations of Arp 220:

Rangwala and colleagues have recently conducted an elegant series of observations of Arp 220 utilizing the European Space Agency's Herschel Space Observatory (http://sci.esa.int/science-e/www/area/index.cfm?fareaid=16) [2]. Exploiting the Spectral and Photometric Imaging Receiver (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=34691&fbodylongid=1593) (SPIRE) imaging Fourier Transform Spectrometer (FTS) and imaging photometer, the authors were able to observe Arp 220 over the continuous range of 190 to 670 μm. The observed spectrum provided quality measurements not only of the emission continuum, but also of discrete spectral features assigned to several molecular and atomic species.

Fortified by the above discussions of some fundamentals, we venture to look more closely at Arp 220 itself.

4.1. Continuum Emission in Arp 220:

Using their recent observations in conjunction with existing literature data, Rangwala, et. al. [2] employed a modified blackbody expression to model the spectral energy distribution of the dust emission continuum in Arp 220 over a broad range of wavelengths. Their analysis yields a dust temperature of 66 K; a total FIR luminosity of LFIR ≈ 2×1012 Lסּ; a total dust mass of Mdust ≈ 108 Mסּ; a total molecular gas mass of Mgas ≈ 1010 Mסּ; and total hydrogen column density of ≈ 1025 cm-2. It is interesting to note that their analysis suggests that the dust emission in Arp 220 is associated with only one of the two galactic nuclei.

4.2. Molecular Spectra in Arp 220:

The SPIRE-FTS spectrum of Arp 220 [2] observed over the range of 200 to 700 μm (1500 to 427 GHz) shows strong emission features from pure rotational transitions of CO and H2O; high rotational transitions of HCN in absorption; rotational transitions of the rare molecules OH+, H2O+, and HF in absorption; as well as electric-quadrupole allowed, fine structure transitions within the ground electronic states of the atomic species [CI] and [NII]. Figures 11 and 12 reproduce the spectrum and assignments given by Rangwala, et. al. [2]. Note the designations of a rotational transition of a diatomic molecule, for example CO(5-4). By convention, the notation gives the molecular species followed by (J′-J″) where J′ is the rotational quantum number of the higher energy state and J″ is the rotational quantum number of the lower energy state. This notation is utilized whether the transition is observed in emission or in absorption.


Fig. 11. Herschel SPIRE-FTS spectrum of Arp 220 illustrating
molecular features in the range of 450 to 950 GHz. Image courtesy of N. Rangwala, et. al. [2].


Fig. 12. Herschel SPIRE-FTS spectrum of Arp 220 illustrating
molecular features in the range of 950 to 1550 GHz. Image courtesy of N. Rangwala, et. al. [2].

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2014-Jul-24, 06:32 PM
Here we shall focus on the rotational transitions of CO. Rangwala, et. al. [2] analyzed the observed rotational state population distribution—corrected for extinction—using a model that did not assume local thermodynamic equilibrium (LTE). Their fits are consistent with the coexistence of two spatially distinct temperature components. One component corresponds to cold molecular gas at approximately 50 K that dominates the low-J CO spectrum. Another component corresponds to warm molecular gas at approximately 1350 K that dominates the high-J CO spectrum. While the warm gas is the principal source of CO luminosity, it represents only 10% of the CO mass.

Figures 13 and 14 depict simulated pure rotational spectra of CO calculated assuming rotational temperatures of 50 K and 1350 K respectively. Comparison of the simulated spectra clearly reveals the marked dependence of the rotational spectrum on temperature.


Fig. 13. Simulated rotational spectrum [14] of 12C16O at 50 K.


Fig. 14. Simulated rotational spectrum [14] of 12C16O at 1350 K.

Rangwala, et. al. [2] considered a number of mechanisms that might be responsible for heating the region of warm CO gas. Their analyses ruled out excitation by UV radiation, X-rays, and cosmic rays. They concluded that heating is the result of mechanical energy transfer, possibly driven by supernovae and strong stellar winds.

P-Cygni line profiles (http://astro1.panet.utoledo.edu/~gthomps/PCyg.html) were also observed for OH+, H2O+, and H2O. Such line profiles are composites of both emission (to the red) and absorption (to the blue) features that are indicative of massive molecular outflows. These observations suggest the existence of an X-ray luminous Active Galactic Nucleus (AGN) within Arp 220 [2]. It is perhaps noteworthy that an OH mega-MASER has also been observed in Arp 220 [15].

5. Conclusions:

Some of the fundamental optical properties of interstellar dust have been introduced including absorption, scattering, and emission. The vibrational and rotational motions of diatomic molecules have also been introduced, as have the spectroscopic ramifications of these internal degrees of freedom. The significance of these concepts with respect to observations of galaxies have been illustrated via discussions of recent spectroscopic observations of Arp 220 in the infrared. Throughout, our intent has been to demonstrate conclusively the importance of molecules, dust, and observations outside of the visible spectral domain to detailed investigations of astronomical objects.

6. Acknowledgments:

The author is greatly indebted to Dr. Henner Busemann, School of Earth, Atmospheric and Environmental Sciences, University of Manchester for permission to reproduce Figure 4; to Professor Erika Gibb, Department of Physics and Astronomy, University of Missouri-St. Louis and the American Astronomical Society (AAS) for permission to reproduce Figure 5; to Professor Jason Glenn, Center for Astrophysics and Space Astronomy, University of Colorado Boulder for permission to reproduce Figures 11 and 12; and to Professor William Keel, Department of Physics and Astronomy, University of Alabama at Tuscaloosa for constructive discussions.

7. References and Notes:

1. NASA/IPAC Extragalactic Database (NED) (http://ned.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=Arp+220&extend=no&hconst=73&omegam=0.27&omegav=0.73&corr_z=1&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES). Position(J2000): RA = 15h34m57.29s Dec = +23d30m09.5s. Alternative designations: IC 1127, VV 540, KPG 470, UGC 09913.

2. N. Rangwala, P. R. Maloney, J. Glenn, C. D. Wilson, A. Rykala, K. Isaak, M. Baes, G. J. Bendo, A. Boselli, C. M. Bradford, D. L. Clements, A. Cooray, T. Fulton, P. Imhof, J. Kamenetzky, S. C. Madden, E. Mentuch, N. Sacchi, M. Sauvage, M. R. P. Schirm, M. W. L. Smith, L. Spinoglio, and M. Wolfire, Observations of Arp 220 using Herschel-SPIRE: An Unprecedented View of the Molecular Gas in an Extreme Star Formation Environment (http://arxiv.org/abs/1106.5054), The Astrophysical Journal, submitted for publication (13 June 2011).

3. 21 cm emission from the hyperfine structure levels of the ground electronic state of atomic hydrogen (HI).

4. J. C. McDowell, D. L. Clements, S. A. Lamb, S. Shaked, N. C. Hearn, L. Colina, C. Mundell, K. Borne, A. C. Baker, and S. Arribas, Chandra Observations of Extended X-Ray Emission in Arp 220 (http://arxiv.org/abs/astro-ph/0303316), The Astrophysical Journal, 591, 154-166 (2003).

5a. B.T. Draine, Interstellar Dust Grains (http://arxiv.org/abs/astro-ph/0304489), Annual Review of Astronomy and Astrophysics, 41, 241-289 (2003); 5b. B. T. Draine, Interstellar Grains (http://ned.ipac.caltech.edu/level5/March01/Draine/Draine.html), Encyclopedia of Astronomy and Astrophysics, 1266-1273 (2001).

6. J. S. Mathis, Interstellar Dust and Extinction (http://articles.adsabs.harvard.edu//full/1990ARA&A..28...37M/0000037.000.html), Annual Review of Astronomy and Astrophysics, 28, 37-70 (1990).

7. JPL Small-Body Database Browser: 26P/Grigg-Skjellerup (http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=26P;orb=0;cov=0;log=0;cad=0#cad).

8. Carbonaceous materials are defined as materials that are predominantly composed of carbon by mass. They include graphite, amorphous carbon, polycyclic aromatic hydrocarbons (PAH), and long-chain aliphatic hydrocarbons.

9. E. L. Gibb, D. C. B. Whittet, W. A. Schutte, A. C. A. Boogert, J. E. Chiar, P. Ehrenfreund, P. A. Gerakines, J. V. Keane, A. G. G. M. Tielens, E. F. van Dishoeck, and O. Kerkhof, An Inventory of Interstellar Ices toward the Embedded Protostar W33A (http://iopscience.iop.org/0004-637X/536/1/347/fulltext), The Astrophysical Journal, 536, 347-356 (2000).

10. Also consult Hyperphysics (http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c2).

11. A. Li, Dust in Active Galactic Nuclei (http://ned.ipac.caltech.edu/level5/Sept07/Li2/Li2.html), In “The Central Engine of Active Galactic Nuclei", ASP Conference Series, Vol. 373, Xi'an, China, 16-21 October, 2007, Eds. L. C. Ho and J.-M. Wang.

12. Wavenumbers (cm-1) are a common unit used by spectroscopists. It is nothing more than the reciprocal of the wavelength. Note that wavenumbers are directly proportional to energy.

13. Rigorously, J is the quantum number for the total angular momentum exclusive of nuclear spin.

14. Rotational spectra of CO were simulated with the PGOPHER (http://pgopher.chm.bris.ac.uk/) spectral package by Colin Western, School of Chemistry, University of Bristol, UK using a rotational constant of 55.3 GHz and an arbitrary Lorentzian linewidth of 0.5 cm-1.

15. E. Rovilos, P. J. Diamond, C. J. Lonsdale, C. J. Lonsdale, H. E. Smith, Continuum and spectral line observations of the OH Megamaser galaxy Arp 220 (http://arxiv.org/abs/astro-ph/0302416), Monthly Notices of the Royal Astronomical Society, 342, 373 (2003); C. J. Lonsdale, OH megamasers, Cosmic Masers: From Proto-Stars to Black Holes (http://adsabs.harvard.edu/full/2002IAUS..206..413L), IAU Symposium #206, 413, Rio de Janeiro, Brazil, 5-10 March 2001, Eds. V. Mineese and M. Reid, (2002).

2014-Jul-24, 08:19 PM
González-Alfonso and colleagues [1] have recently reported Herschel/PACS observations of the ultraluminous infrared galaxies Arp 220 and NGC 4418 [2]. Both galaxies show rotational-state resolved absorption features from the molecules H2O, OH, HCN, and NH3. Excitation within both objects is sufficient to afford observations of extended rotational ladders [3] as summarized in Figure 2 of Reference 1 in the cases of both ortho-H2O and para-H2O [4].

The analyses of González-Alfonso, et. al. are consistent with prior investigations of Arp 220 indicating an outflow of material (vide supra). Interestingly, redshifts in the observed rotational spectra of NGC 4418 are interpreted by the authors as indicative of molecular inflow of the order of 12 Mסּ per year.

The distribution of relative H2O, OH, and HCN abundances is interpreted as evidence indicating distinct regions in which different chemistry prevails. The high abundance of HCN and H2O observed in the cores of both galaxies are consistent with hot, evolved cores in which evaporation from grain mantles is important. The high abundance of OH in core regions appears to require discrete regions in which the chemistry is driven by X-ray and/or cosmic ray excitation.

NGC 4418 and Arp 220 show 16O/18O ratios in both H2O and OH that differ by a factor of approximately 5. This ratio decreases with increasing generations of star formation, thus suggesting that Arp 220 is in a later stage of evolution than is NGC 4418.

The aggregate data are discussed relative to NGC 4418, Arp 220, and Markarian 231 (http://en.wikipedia.org/wiki/Markarian_231). Despite substantive differences in morphology, “preliminary evidence is found for an evolutionary sequence from infall, hot-core like chemistry, and solar oxygen isotope ratio to high velocity outflow, disruption of the hot core chemistry and cumulative high mass stellar processing of 18O” [1].

References and Notes:

1. E. González-Alfonso, J. Fischer, J. Graciá-Carpio, E. Sturm, S. Hailey-Dunsheath, D. Lutz, A. Poglitsch, A. Contursi, H. Feuchtgruber, S. Veilleux, H. W. W. Spoon, A. Verma, N. Christopher, R. Davies, A. Sternberg, R. Genzel, and L. Tacconi, Herschel/PACS spectroscopy of NGC 4418 and Arp 220: H2O, H218O, OH, 18OH, O0, HCN and NH3 (http://arxiv.org/abs/1109.1118), Astronomy and Astrophysics, submitted for publication.

2. NCG 4418 is also designated NGC 4355. The former designation is retained for consistency with that of Reference 1.

3. The rotational energy level structure of polyatomic molecules is more complex than that of the generic diatomic molecule introduced previously. For example, H2O is an asymmetric top. The interested reader is referred to C. H. Townes and A. L. Schawlow, “Microwave Spectroscopy”, Chapters 1-4, Dover Publications, New York (1975) for comprehensive treatments.

4. Like molecular hydrogen (http://en.wikipedia.org/wiki/Spin_isomers_of_hydrogen), water has two isomeric forms defined by the nuclear spin angular momenta of the constituent protons. In the case of ortho-H2O, the nuclear spin angular momenta of the protons are parallel corresponding to a triplet nuclear spin state with I=1. In the case of para-H2O, the nuclear spin angular momenta of the protons are anti-parallel corresponding to a singlet nuclear spin state with I=0.

2014-Jul-24, 10:08 PM

One might well ask what the lengthy tome posted above is all about.

It could be argued that it is about an introduction to the significance of molecular gas and dust within galaxies; an introduction to molecular spectroscopy, particularly to vibrational and rotational transitions falling within the infrared frequency domain; an introduction to the photophysics of molecular gas and dust; and an introduction to the significance of those species in revealing the physical and dynamical properties of galaxies.

I do so argue.

Yet it is intended to be more. It is intended to provide a stimulus to the CosmoQuest readership--member and visitor alike--a stimulus to embrace the fascinating field of galaxies.

Best regards,

Ara Pacis
2014-Jul-24, 11:24 PM
tome not tomb

2014-Jul-25, 12:09 AM

Thank you.

Jean Tate
2014-Jul-25, 07:37 AM
Very cool, Eigen State! :)

This looks like Sunday, 24 July 2011: Arp 220: Dust and Molecular Gas (http://www.galaxyzooforum.org/index.php?topic=279339.0), an Object of the Day (OotD) in the (now closed) Galaxy Zoo forum (http://www.galaxyzooforum.org/index.php) [1]. There are some interesting posts in that thread on scattering due to dust. And there's a link to an older OotD in which Arp 220 is describes as a Bump DOG (Dust Obscured Galaxy, with a bump in its spectrum).

I have some questions about molecular gas, and what's observed (and what's not) in the spectrum of Arp 220, if I may.

2H, deuterium (D), is among the top hundred isotopes, in terms of cosmic abundance [2]. Given its huge difference in mass, with 1H (protium), the spectra of HD and D2 should be quite distinct from that of H2. Ditto those of D2O, DCN, OD+, etc. Do you know if any deuterated molecular species have been observed, in spectra of Arp 220?

[1] formatting changes aside, and sans references to Budgieye and NGC3314 ;)
[2] I need a cite

2014-Jul-25, 01:51 PM
This is a great depthy introduction to the optical properties of dust and gas. I did a lot of development work involving particle size distributions. There is nothing simple about this science, as individual particles have so many physical characteristics that get totally lost in the shuffle when the particle stream is more than a handful of molecules thick.

Stellar work!

Amber Robot
2014-Jul-25, 05:55 PM
2H, deuterium (D), is among the top hundred isotopes, in terms of cosmic abundance [2]. Given its huge difference in mass, with 1H (protium), the spectra of HD and D2 should be quite distinct from that of H2. Ditto those of D2O, DCN, OD+, etc. Do you know if any deuterated molecular species have been observed, in spectra of Arp 220?

I don't believe there are any lines of H2 or HD in the bandpass of Herschel SPIRE-FTS, so they wouldn't have been observed. The brightest rotational H2 lines are in the mid-infrared 17-30 microns or so. I'm not sure where the HD lines are. One would think that because of their dipole moment they could be bright, but the abundance is quite low too. Do you know what the HD abundance is expected to be in molecular clouds?

2014-Jul-25, 06:36 PM

I have some questions about molecular gas, and what's observed (and what's not) in the spectrum of Arp 220, if I may.

2H, deuterium (D), is among the top hundred isotopes, in terms of cosmic abundance [2]. Given its huge difference in mass, with 1H (protium), the spectra of HD and D2 should be quite distinct from that of H2. Ditto those of D2O, DCN, OD+, etc. Do you know if any deuterated molecular species have been observed, in spectra of Arp 220?

Vibrational, rotational, and rovibrational transitions from 1H2, 1H2H, and 2H2 are indeed well resolved because of the isotope shifts. This is evident from the known vibrational and rotational constants [1] which differ by approximately 600 cm-1 and 15 cm-1 between each of the isotopic variants.

As pointed out by Amber Robot, molecular hydrogen is out of the frequency window probed within the investigations discussed above. I did not see any data on hydrogen isotope containing species in a quick check of the manuscripts.

A search of Google Scholar utilizing the term Arp 220 deuterium does however afford a lot of citations.

Best regards,

[1]. K. P. Huber and G. Herzberg, Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules, Van Nostrand Reinhold (1979).

Jean Tate
2014-Jul-26, 02:17 PM
Thanks Eigen State, Amber Robot.

While perhaps not the most active of topics, it seems there is some interest in deuterated species in extra-galactic sources, and some interesting physics and chemistry concerning the formation of gas-phase deuterated species. For example, T. J. Millar's 2005 "Deuterium in interstellar clouds" (http://adsabs.harvard.edu/doi/10.1111/j.1468-4004.2005.46229.x) and Bayet+ 2010 (http://adsabs.harvard.edu/abs/2010ApJ...725..214B) "Deuterated Species in Extragalactic Star-forming Regions".

I also discovered The UMIST database for astrochemistry 2012 (http://adsabs.harvard.edu/abs/2013A%26A...550A..36M):

We present the fifth release of the UMIST Database for Astrochemistry (UDfA). The new reaction network contains 6173 gas-phase reactions, involving 467 species, 47 of which are new to this release. We have updated rate coefficients across all reaction types. We have included 1171 new anion reactions and updated and reviewed all photorates. In addition to the usual reaction network, we also now include, for download, state-specific deuterated rate coefficients, deuterium exchange reactions and a list of surface binding energies for many neutral species. Where possible, we have referenced the original source of all new and existing data. We have tested the main reaction network using a dark cloud model and a carbon-rich circumstellar envelope model. We present and briefly discuss the results of these models.

All codes, along with reaction networks and data files, are accessible at http://www.udfa.net.

Over 6k gas-phase reactions! :eek:

2014-Jul-26, 05:31 PM

While perhaps not the most active of topics, it seems there is some interest in deuterated species in extra-galactic sources, and some interesting physics and chemistry concerning the formation of gas-phase deuterated species. For example, T. J. Millar's 2005 "Deuterium in interstellar clouds" (http://adsabs.harvard.edu/doi/10.1111/j.1468-4004.2005.46229.x) and Bayet+ 2010 (http://adsabs.harvard.edu/abs/2010ApJ...725..214B) "Deuterated Species in Extragalactic Star-forming Regions".

The problem with 2H is that it is I=1! I admit that is merely a personal thing.

I also discovered The UMIST database for astrochemistry 2012 (http://adsabs.harvard.edu/abs/2013A%26A...550A..36M):

Good find--bookmarked--along with the actual database itself. Note that only gas phase reactions are discussed leaving out a wealth of significant surface phenomena involving grains. It is important to read the manuscript to understand the physical significance of the rate parameters (\alpha, \beta, \gamma) given in the database for different reaction processes: two-body reactions, cosmic-ray induced reactions.

Best regards,