Harvard Astronomy 201b

Spitzer Infrared Nearby Galaxies Survey (SINGS)

1. A Brief Introduction to Spitzer Space Telescope

The Spitzer Space Telescope is one of the four NASA’s Great Observatories Program observatories, the others being the visible-light Hubble Space Telescope (HST), Compton Gamma-Ray Observatory (CGRO), and the Chandra X-Ray Observatory (CXO).

Before it was launched in August 2003, The Spitzer Space Telescope had been known as the Space Infrared Telescope Facility (SIRTF).

Spitzer is designed to detect infrared radiation, which is primarily heat radiation.

Spitzer has three instruments on-board:

  • IRS (Infrared Spectrograph) is an infrared spectrometer with four sub-modules operating at the wavelengths 5.3-14 µm (low resolution), 10-19.5 µm (high resolution), 14-40 µm (low resolution), and 19-37 µm (high resolution).
Here is a neat diagram illustrating the properties of the 3 instruments on Spitzer:

Fig.1: Properties of Spitzer Instrument (Image Credit: sings.stsci.edu/proposal/sings2mw.ppt)

2. Introduction to SINGS

It is a comprehensive imaging and spectroscopic study of 75 nearby galaxies within 30 Mega-parsecs. Here is the complete list of these 75 galaxies in Table 1 and Table 1 (continued) (Ref: Kennicutt et al 2003):

Table 1: SINGS Galaxy List (from Kennicutt et al 2003)

Table 1 (continued): SINGS Galaxy List (from Kennicutt et al 2003)

The primary products of SINGS are:
(1) A spectrophotometric library of IR-emitting galaxy components (nuclei, star forming regions, IR-selected objects) spanning the full range of accessible metallicities, densities, luminosities, and radiation fields present in the normal galaxies, as derived from spatially-sampled IRS (Infrared Spectrograph) spectra.
(2) A complete pixel-resolved Spectral Energy Distribution (SED) library of 75 representative galaxies (Table 1 and Table 1 (continued), Ref: Kennicutt et al 2003), extending from the visible to 160 µm (and in many cases from the UV to submillimeter), and covering the full range of physical properties found in the local universe, as derived from full-area IRAC and MIPS imaging, and large-area spectral maps with MIPS and IRS.
When the SINGS data of the 75 galaxies are combined with data at other wavelengths including visible, near-infrared, ultraviolet, and radio wavelengths, it will provide us with :
(1) New perspective on the connection between star formation processes and the ISM properties of galaxies.
(2) An important foundation of data, diagnostic tools, and astrophysical inputs for understanding Spitzer‘s observations of the distant universe and ultraluminous and active galaxies.
(3) Creating an integrated and self-consistent archive of visible, UV, IR, submillimeter studies of those 75 galaxies, so as to enable many follow-up investigations of star formation and the ISM.
Robert Kennicutt is the Principal Investigator of the SINGS Project. He is currently a Professor at the University of Cambridge.
Here is a link to a ppt file by Robert Kennicutt which gives a vivid illustration of SINGS:

3. Result of SINGS

(1) Spitzer Observations of the Supergiant Shell Region in IC 2574 (Cannon et al. 2005)
Please read the paper: (Cannon et al. 2005).
(1.1) Background Information of this paper
The first author of this paper, John M. Cannon, is currently an Assistant Professor at Macalester College.
IC 2574 is a dwaft spiral galaxy within the M81 Group. The M81 Group is a group of galaxies in the constellation Ursa Major that contains the well-known galaxies M81 and M82 as well as several other galaxies with high apparent brightnesses (Karachentsev 2005).The approximate center of the group is located at a distance of 3.6 Mpc, making it one of the nearest groups to the Local Group (Karachentsev 2005). The group is estimated to have a total mass of (1.03 ± 0.17)×1012 solar masses (Karachentsev et al. 2006). The M81 Group, the Local Group, and other nearby groups all lie within the Virgo Supercluster (i.e. the Local Supercluster) (Tully 1982).
Here is an image of IC 2574:

Fig.2: Optical Image of IC 2574 (Image Credit: http://gallery.rcopticalsystems.com/gallery/ic2574.html)

As you can see in the image (Fig.2), this dwarf galaxy has gaseous knots (the blue clump) which are highly active star forming regions. These regions are known as the Super Giant Shells. Inside those rapidly expanding super giant shells are hot ionized H II gas, which is at high temperature and emits X-ray emission that has been detected by ROSAT. (Walter et al. 1998)
The star formation process creates holes and shells in the ISM. There are various mechanisms proposed to explain the formation of those shells, including feedback from massive stars (stellar winds and Type II supernovae) (Tenorio-Tagle & Bodenheimer 1988), high-velocity cloud impacts, disk instabilities, turbulence, ram pressure stripping, etc. (Sánchez-Salcedo, F. J. 2002)
(1.2) Discussion of Result
The following figure shows IC 2574 under 3 wavelengths:

Fig.3: (a) Optical V-band; (b) MIPS 70 mm; and (c) H I images of IC 2574. Note that the Super Giant Shell region dominates the far-IR luminosity of IC 2574, although emission is associated with other Star Formation regions. The box in each field denotes the area shown in the next figure and is ∼4.4 kpc * 3.8 kpc at the adopted distance. (Image Credit: Cannon et al. 2005)

The following figure shows the boxed region of previous Fig.3, which is the Super Giant Shell, under 9 wavelengths:

Fig.4: Images of the Super Giant Shell region at nine different wavelengths (see Fig.3 for location): (a) optical V-band; (b) and (c) IRAC 3.6 and 8 mm, respectively; (d)–(f) MIPS 24, 70, and 160 mm, respectively; (g) continuum-subtracted Ha; (h) 6 cm radio continuum; and (i) H I zeroth-moment image. The location of circular apertures super giant shell 1– 6 are overlaid and labeled in panel (d); the Super Giant Shell is shown as an ellipse in each frame. In panels (d)–( f ) and (i), the beam sizes (FWHM) are shown as boxed circles at bottom left.

Notice that in Fig.4(a), the optical image indicates that the progenitor stellar cluster (indicated by the arrow) which created the Super Giant Shell lies directly interior to the H I shell. This is one of the clearest examples of a kinematically distinct gaseous shell with the parent cluster still visible.

By 8 micron (see Fig.4(c)) the Spectral Energy Distribution (SED) of the progenitor cluster has fallen below the detection limit, and emission from hot dust and gas starts to dominate. Variations in emission in the MIPS bands (Figs.4 (d)–(f )) indicate a wide range of dust temperatures and Spectral Energy Distribution. Note that the shell morphology is still evident at 70 micron and 160 micron. The diffuse emission in the shell is most likely caused by the MIPS Point Spread Function profiles, which spread flux from high surface brightness regions onto arcminute scales (i.e., a few times the MIPS 160 micron PSF FWHM). Comparison of the MIPS, H-alpha, and radio continuum images (Figs.4 (g) and (h)) shows a wide variation in the relative ratios. Lastly, the H I distribution (Fig.4(i)) shows the H I shell very clearly; it is expanding into a nonuniform medium that may partially explain the variety of dust properties around the shell rim. (Cannon et al. 2005)

H-alpha () is a spectral line created by hydrogen atom when an electron falls from its 3rd to 2nd lowest energy level. It belongs to the Balmer series. The wavelength of H-alpha is 656.28 nm. H-alpha flux is an indicator of the star formation rate. The advantage of using H-alpha as the star formation tracer is that it is sensitive to low levels of star formation even in faint, low surface brightness galaxies, and that it traces high mass stars, and hence recent star formation. Given suitable assumptions, primarily about the extinction and the stellar initial mass function, this method yields quantitative measurements of the star formation rate. (Ref: H-alpha Galaxy Survey)

Therefore, from the comparison across the wavelengths, the authors found that the strong H-alpha sources are associated with regions of warm dust. Most of the active star formation took place at the rim of this Super Giant Shell. That is a strong indication that the expansion of the Super Giant Shell is compressing the surrounding ISM and triggering star formation.

But also notice that the most luminous infrared and H-alpha sources are not necessarily cospatial.

The coolest dust (corresponding to longer infrared wavelengths) is found in the regions farthest from the rim of the shell. Those regions show the best agreement between star formation rates derived from H-alpha and from total infrared luminosities (although discrepancies at the factor of 3–4 level still exist). There is considerable variation in the radio–far-infrared correlation in different regions surrounding the shell (as shown in Fig.4). The data demonstrate that the expanding shell is dramatically affecting its surroundings by triggering star formation and altering the dust temperature. (Cannon et al. 2005)

(2) Two Early-Type Spiral Galaxies with Dust Rings (Bendo et al. 2006)

Please read the paper: (Bendo et al. 2006)

(2.1) Background Information of this paper

The author of this paper is George Bendo, who is currently working at at the Jodrell Bank Centre for Astrophysics at the University of Manchester in UK.
This is a short paper presenting the result of Spitzer‘s image of two galaxies, one is SB0/a galaxy NGC 1291, the other is SAa galaxy NGC 4594, which is M 104, also known as the famous Sombrero Galaxy, due to the presence of a very prominent circular dust ring in the mid-plane of this galaxy.

The NGC 4594 (Sombrero Galaxy) is located about 29 million light-years away. In the sky it lies just six degrees south of its equatorial plane. Spitzer detected infrared emission not only from the ring, but from the center of the galaxy too, where there is an Active Galactic Nucleus.

For Galaxy Morphology Classification, please see the following two references:

http://www.astr.ua.edu/keel/galaxies/classify.html

http://en.wikipedia.org/wiki/Galaxy_morphological_classification

(2.2) PAH

PAH (Polycyclic Aromatic Hydrocarbons (Fig.5) is an important component of Interstellar Dust. PAH molecules often contribute to emission features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7μm of the infrared emission spectra of spiral galaxies. (Ref: Bruce Draine)

Here is an illustration of PAH:

Fig.5: Typical PAH (Image Credit: wikipedia)

(2.3) Discussion of Result

Inside this paper (Bendo et al. 2006), the author presented Spitzer images of the SB0/a galaxy NGC 1291 (Fig.6) and the SAa galaxy NGC 4594 (Fig.7). Both galaxies contain dust rings. At 24 and 70 μm, the nuclei of both galaxies are the brightest sources in the galaxies, and the emission from the rings is relatively weak. At 160 μm, however, the dust rings are more prominent (See both Fig.6 & Fig.7). in NGC 4594, the dust ring is the source of virtually all of the 160 μm emission.

For NGC 1291, the author compared the dust emission to PAH (Polycyclic Aromatic Hydrocarbons) emission observed at 8 μm. For NGC 4594, the author also presented submillimeter data that show that the nucleus dominates the 850 μm emission. These results demonstrate that the 850 μm emission cannot come from the same cool dust that dominates the 160 μm emission. (Bendo et al. 2006)

Fig.6: NGC 1291. This figure shows NGC 1291 in the optical (from the Digitized Sky Survey), 8, 24, 70, and 160 μm bands. Each image is 16′ × 16′, with north up and east to the left. The nucleus is the dominant source of 8 μm PAH emission and 24 and 70 μm warm dust emission, but some emission can be seen from the outer ring. This ring is a relatively stronger source of cool dust emission at 160 μm. Interestingly, the 8 and 24 μm emission do correlate well with each other. (Image Credit: Bendo et al. 2006)

Fig.7: NGC 4594. This figure shows NGC 4594 in the optical (from the Digitized Sky Survey), 24, 70, 160, and 850 μm bands. Each image is 14′ × 6′, with north up and east to the left. The AGN nucleus is the most prominent source of emission at 24 and 70 μm, but it disappears at 160 μm, only to surprisingly reappear at 850 μm. Using these and other data, the authors have eliminated dust, bremsstrahlung, synchrotron, and CO emission as probable sources for the 850 μm emission. (Image Credit: Bendo et al. 2006)

(2.4) Composite Image of NGC 4594 (Sombrero Galaxy)

Here is a composite image (Fig.8) of NGC 4594 (Sombrero Galaxy) produced by combining the visible-wavelength image from Hubble Space Telescope and the infrared-wavelength image from Spitzer.

Fig.8: Composite Image of NGC 4594 (Sombrero Galaxy) (Image Credit: http://www.spitzer.caltech.edu/)

Spitzer’s full image of NGC 4594 (Sombrero Galaxy) shows that its disk and dust ring are warped a little bit, which is often the result of a gravitational encounter with another galaxy in the past, and clumpy areas spotted in the far edges of the ring indicate young star-forming regions.

The Spitzer picture is composed of four images taken at 3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red) microns. The contribution from starlight (measured at 3.6 microns) has been subtracted from the 5.8 and 8-micron images to enhance the visibility of the dust features. (Ref: http://www.spitzer.caltech.edu/)

(2.5) Comparison with dust ring feature observed in other galaxies

It seems that dust ring is a common feature shared by some spiral galaxies, including the NGC 4594 (Sombrero Galaxy) discussed in this paper as well as M31 (Andromeda Galaxy), which will be discussed as an interesting topic in the next section.

The dust ring in NGC 4594 seems to be warped as a result of gravitational interaction with another galaxy. Similarly, the dust ring in M31 seems asymmetric and the center of the ring is offset from the galactic nucleus, which is indicative of a past collision between M31 and its neighbor M32. So just based on the morphology of their dust rings, we could tell a lot about the evolutionary history of the galaxies.

4. Other Interesting results of Spitzer (NOT from SINGS, but very interesting)
The following are some interesting examples of the many results from Spitzer.

(1) Nebula Henize 206

Henize 206 is a star-formation region in LMC (Large Magellanic Cloud).  The distance of the nebula is 163,000 light-years. The age of the nebula is estimated to range from 2~10 million years. People are interested in studying this nebula because the heavy metal fractional content in LMC is typically 2~5 times smaller than in solar neighborhood. Therefore, LMC provides a nearby cosmic laboratory resembling distant universe in its composition.
Here is an image of Nebula Henize 206 (Fig.9):

Fig.9: Star Formation in Henize 206 in Large Magellanic Cloud (LMC) (Image Credit: http://www.spitzer.caltech.edu/)

The primary Spitzer image (Fig.9) on the left shows the filamentary structure of Henize 206, is a four-color composite mosaic created by combining data from IRAC at near-infrared wavelengths and MIPS from mid-infrared data. The blue color represents infrared light at wavelengths of 3.6 and 4.5 microns. Most of the stars in the field of view radiate primarily at these short infrared wavelengths (corresponding to the range of 3.6 and 4.5 microns).

Cyan color depicts emission at 5.8 microns, green color depicts emission at 8.0 microns, and red color depicts the thermal emission from dust at 24 microns. Three separate instrument images are shown as 3 small insets on the right.

A ring of emission dominates the central and upper regions of the composite image. This ring is the result of a supernova explosion millions of years ago. The shock waves from that explosion impacted nearby hydrogen gas, compressed it, and started a new generation of star formation. This is particularly evident in the MIPS inset, where the 24-micron emission peaks correspond to newly formed stars. The ultraviolet and visible-light photons from the new stars are absorbed by surrounding dust and re-radiated at longer infrared wavelengths, where it is detected by Spitzer. (Ref: http://www.spitzer.caltech.edu/)

More details could be found in the papers by (Gorjian et al 2004) and (Romita et al 2010).


(2) Dust Distribution in M31 Showing Evidence of a Galactic Collision

M31 (The Andromeda Galaxy) lies at 2.5 million light years away. It is the largest member of our Local Group, with the Milky Way being the second largest one.

Here is an image of M31 at 24-micron. An outer ring of dust is prominent. This picture is constructed from 11,000 separate snapshots by MIPS. Asymmetrical features are seen in the prominent ring of star formation. The ring appears to be split into two pieces, forming the hole to the lower right. These features may have been caused by interactions with satellite galaxies (such as M32 and M110) around M31 (The Andromeda Galaxy) as they plunge through its disk. This image also reveals tracings of spiral arms within this ring that reach into the very center of the galaxy.

Fig.10: The Andromeda Galaxy (M31) at 24-micron by MIPS (Image Credit: http://www.spitzer.caltech.edu/)

With a further imaging using IRAC onboard Spitzer, new evidence was found that the Andromeda Galaxy was probably involved in a violent head-on collision with the neighboring dwarf galaxy Messier 32 (M32) more than 200 million years ago. The following image (Fig.11) reveals an inner dust ring deep within the Andromeda galaxy in addition to the outer prominent dust ring visible in the 24-micron image (Fig.10). When combined with a previously observed outer ring, the presence of both dust rings suggests that M32 plunged through the disk of the Andromeda Galaxy along Andromeda’s polar axis approximately 210 million years ago. (Ref: http://www.spitzer.caltech.edu/)

Fig.11: M31 Inner Dust Ring at 8-micron by IRAC (Image Credit: http://www.spitzer.caltech.edu/)

What is most interesting is that the centers of those rings (outer ring center is number 1 with red color, inner ring center is number 2 with green color) are offset from the galactic center (number 3 with purple color in the upper diagram (Fig.11)). The outer ring of star formation is offset from the galactic center by about 1kpc, which is about 10% of the radius of the outer ring. The inner ring of star formation is offset from the galactic center by about 0.5kpc, which is about 40% of the radius of the inner ring.

More details could be found in the papers by (Block et al 2006) and (Hammer et al 2010).

(3) Detection of Buckyballs in SMC

In July 2010, astronomers confirmed the detection of buckyballs (C60 and C70) in nearby Small Magellanic Cloud (SMC). SMC lies at a distance of about 200,000 light-years away from us. Those buckyballs were detected in a planetary nebula called Tc 1 within SMC. Tc 1 is a young, low-excitation planetary nebula where the white dwarf is still enshrouded by the dense stellar ejecta. The Spitzer IRS spectrum of Tc 1 shows emission features from cold and neutral C60 and C70.

The Buckyball C60 is also called Buckminsterfullerene. It was first synthesized in a lab on Earth in 1985 by Richard Smalley, Robert Smalley, Robert Curl, James Heath, Sean O’Brien, and Harold Kroto at Rice University. Richard Smalley, Robert Curl and Harold Kroto were awarded the 1996 Nobel Prize in Chemistry for their contribution to the discovery of Buckminsterfullerene and a related class of molecules called Fullerene, which is basically molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. For more info, please check out this link: http://www.bristol.ac.uk/Depts/Chemistry/MOTM/buckyball/c60a.htm

The structure of C60 is a truncated icosahedron made of 20 hexagons and 12 pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge. Here is a picture of C60:

Fig.12: C60 (Image Credit: wikipedia)

The structure of C70 is similar, but just a little bit elongated in one direction, giving it an ellipsoid shape, as shown below:

Fig.13: C70 (Image Credit: eskimo.force9.co.uk)


The following is an artist’s illustration (Fig.14) of an infrared photo of the Small Magellanic Cloud taken by Spitzer with two callouts. The middle callout shows a magnified view of an example of a planetary nebula, and the right callout shows an even further magnified depiction of C60 buckyballs, which consist of 60 carbon atoms arranged like soccer balls:

Fig.14: Detection of Buckyball in SMC (Image Credit: http://www.spitzer.caltech.edu/)

C60 molecule has exactly 174 vibrational modes . Four of these vibrational modes cause the molecules to either absorb or emit infrared light. All four modes were identified in the spectrum, indicated by the red arrows in the following diagram (Fig.15). Likewise, Spitzer identified several vibrational modes of C70, shown by the blue arrows. (Ref: http://www.spitzer.caltech.edu/)

Fig.15: These data from NASAs Spitzer Space Telescope show the signatures of buckyballs in space. Spectra taken by IRS. (Image Credit: http://www.spitzer.caltech.edu/)

About 5.8*10^-8 solar masses of pure C60 and 4.7*10^-8 solar masses of pure C70 are required to reproduce the emission bands. The two molecules amount to a few percent of the available cosmic carbon in this region. This detection suggests that if the conditions are right, fullerenes can and do form efficiently in space.

More details could be found in the papers by (Cami et al. 2010) and (Ehrenfreund et al. 2010).

5. Acknowledgement

I would like to give great Thanks to Christopher Beaumont and Alyssa Goodman for their gracious teaching and help in Ay201b ISM class. Through the process of contructing the WordPress webpage and in-class presentation, I learnt a lot.

6. Reference

Physics of the Interstellar and Intergalactic Medium by Bruce T. Draine.

http://www.eskimo.force9.co.uk/fullerene/html/reactivity.htm

Bendo, G.J., & the SINGS Team: Spitzer Observations of Two Early-Type Spiral Galaxies with Dust Rings. The Spitzer Space Telescope:  New Views of the Cosmos, ed. L. Armus & W.T. Reach (San Francisco: ASP), 192 (2006)

http://www.ing.iac.es/PR/SH/SH2008/hasurvey.html

A New Look at the Holes of IC 2574, Sanchez-Salcedo, F. J. 2002, Rev. Mex. AA, 38, 39

Large-scale expanding superstructures in galaxies, Tenorio-Tagle, G., & Bodenheimer, P. 1988, ARA&A, 26, 145

X-Ray Emission from an Expanding Supergiant Shell in IC 2574, Fabian Walter et al 1998 ApJ 502 L143

http://gallery.rcopticalsystems.com/gallery/ic2574.html

The Local Supercluster, Tully, R.B.,Astrophysical Journal, Part 1, vol. 257, June 15, 1982, p. 389-422

The Local Group and Other Neighboring Galaxy Groups, D. Karachentsev 2005, The Astronomical Journal 129 178

Masses of the local group and of the M81 group estimated from distortions in the local velocity field, I. D. Karachentsev and O. G. Kashibadze 2006,, ASTROPHYSICS, Volume 49, Number 1, 3-18

http://en.wikipedia.org/wiki/M81_Group

http://www.daviddarling.info/encyclopedia/C/Coddingtons_Nebula.html

Spitzer Observations of the Supergiant Shell Region in IC 2574, , Cannon et al., ApJ, 630, L37 (2005)

sings.stsci.edu/proposal/sings2mw.ppt

http://www.ast.cam.ac.uk/~robk/

http://www.aerospaceguide.net/telescope/sirtf.html

SINGS: The SIRTF Nearby Galaxies Survey, Kennicutt et al., Publications of the Astronomical Society of the Pacific, 115:928–952, 2003 August

Infrared Imaging of the Large Magellanic Cloud Star-forming Region Henize 206, Gorjian et al. 2004

Young Stellar Objects in the Large Magellanic Cloud Star-forming Region N206, Romita et al. 2010

Does M31 Result from an Ancient Major Merger?, Hammer et al. 2010

An almost head-on collision as the origin of two off-centre rings in the Andromeda galaxy, Block et al. 2010

Fullerenes and Cosmic Carbon, Ehrenfreund, et al. Science 329, 1159 (2010)

Detection of C60 and C70 in a Young Planetary Nebula, Jan Cami, et al. Science 329, 1180 (2010)

http://sings.stsci.edu/

http://www.spitzer.caltech.edu/

wikipedia

http://www.bristol.ac.uk/Depts/Chemistry/MOTM/buckyball/c60a.htm

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