background image: ngc 4911, hst expo time: 28 hours ...akhlaghi/research/spiralouter.pdf · • in...

Background image: NGC 4911, HST expo time: 28 hours, release: 2010. http://hubblesite.org/newscenter/archive/releases/2010/24/

• Brief history

• Observational studiesDetailed classification of the outer regions

(Pohlen & Trujillo 2006) & (Erwin et al. 2008)Color and mass gradients in the outer regions (Bakos, et al. 2008)

• Physical significance and interpretationOuter spiral disks as clues to galaxy formation and evolution

(Vlajic, 2010)In efficient star formation (Bigiel, et al. 2010)

Physical Interpretation

Observation

Brief historyObservational• Patterson (1940) and de Vaucouleurs (1958), showed that

the disks of spiral galaxies generally follow an exponential decrease in their radial surface brightness profile.

• Freeman (1970) pointed out that not all disks are simple exponentials.

• van der Kruit (1979) and van der Kruit & Searle (1981a, 1981b) first drew attention to the truncation phenomenon.

• Erwin et al. (2005) showed that some galaxies suggest a third class known as Antitruncated galaxies.

Theoretical Physical Interpretations:Truncations: 1. The initial collapsing cloud (van der Kruit 1987)2. The effects of a star formation threshold at large radii

(Kennicutt 1989; Schaye 2004; Elmegreen & Hunter 2006).

Antitruncation:1. Star Formation in initial cloud Elmegreen & Hunter

(2006)2. Minor mergers (Younger et al. 2007)

Observational Studies(Pohlen & Trujillo 2006)

• In a previous study (Pohlen, et al. 2002), the luminosity profile of face-on galaxies was put in focus. While most studies prior to it used edge-on galaxies.

Pohlen, M. Trujillo, I. 2006. The structure of galactic disks, studying late-type spiral galaxies using SDSS. Astronomy and Astrophysics 454, 759-772.

Edge on Luminosity Profiles:• Facilitates the discovery of truncation and Luminosity profiles• Severe problems caused by line of sight and radial integration.

Truncation wasn’t evident in all galaxies:• Bland-Howthorn et al. 2005 found that NGC

300’s exponential Luminosity profile simply continues down to ~10 radial scale lengths.

The main goal of the present study is to conduct a complete census of the outer disk

structure of late type galaxies.

Sample and DataData Set:• LIDA online galaxy catalogue (Initial: 557 galaxies;

http://leda.univ-lyon1.fr)• SDSS DR2 (98 galaxies; http://www.sdss.org)

To characterize the environment galaxies just about the primary galaxy were studied:

• The difference in velocity with respect to the targeted galaxy is less than 350km/s

• Their absolute magnitude is brighter than -16 r’

The u', i' and z' bands images are less sensitive and, consequently less useful to study the profile towards the outer disk.

Transforming SDSS filters to the Johnson B & R.B=g'+0.47(g'-r')+0.17 R=g'-1.14(g'-r')-0.14


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Observational Studies(Erwin, Pohlen & Beckman 2008)

• This study is a continuation of the Pohlen, et al. 2006 study. Focusing on Barred late type galaxies including S0s.

• Their sample is composed of 66 early type S0-Sb galaxies.• Initially imaged in B and R with the 3.5 m WIYN Telescope.• Subsequently B and R with the Andalucia Faint Object

Spectrograph and Camera (ALFOSC) on the 2.56m Nordic Optical Telescope (NOT), problems:

1. The small field of view and 2. Light scattering problem, making problems with the Sky

subtraction.• To remedy these: the Wide Field Camera (WFC) of the

2.5 m Isaac Newton Telescope (INT) was used. • Finally, images were found for about three quarters of the

samples in the Sloan Digital Sky Survey (York et al. 2000), including Data Release 5.

Erwin, P. Pohlen, M. Beckman, J.E. 2008. The outer disks of early type galaxies. I. Surface-Brightness profiles of Barred Galaxies. The Astronomical Journal, 135, 20-54

Measurement of Sky value• SDSS original value (0.2%) is not enough and a more elaborate value is

required. Depending on the area of the galaxy and its position on the chip. So the sky value will be different for every galaxy in an image.

• Common procedure: the median of the pixel distribution in random manually placed boxes around the galaxy. SDSS data restrict this method

• In this study: drawing graphs of luminosity in free ellipse fits using IRAF to find the boundary and averaging the sky.

• It is confirmed that SDSS profiles can be obtained reliably down to -27.0 r'-mag/arcsec2.


Sky Subtraction, ProfilesClassic Method of Sky Subtraction:• Advantages 1. Avoid places with extra light, 2. Enables bootstrap re-sampling• Disadvantages: Underestimate the sky for the galaxy its self. Ellipse method of Sky Subtraction:• Concentric fixed ellipses are used to determine the intensity level as a function of

radius from the galaxy center.


Profiles:FREE ellipse tool: Position Angle

(PA) and ellipticity are allowed to vary. (Dashed line)

FIXED ellipse too: All parameters are held fixed (Solid line)

In this figure the difference between these two tools is drawn

Comparison of telescopes and exposure

• Surface-brightness profiles for NGC 7280. The thick profile is from a 10,200s observation with the 2.5 m Isaac Newton Telescope’sWide Field Camera, while the thin line is the profile from a 300 s observation with the 3.5 m WIYN Telescope:


SDSS data comparisonSDSS data were found to be reliable due to these three reasons:1. Images were always taken at dark times,2. Scan Drift mode (excellent for flat-fielding)3. Excellent telescope and camera designOther telescope images (with significantly more exposure) were only marginally

deeper! Only three SDSS samples (thick) diverged at fain surface brightness:


Profile extractionThe IRAF ellipse task (STSDAS package) was used on the sky subtracted images

to produce our final surface brightness profiles.

Two kinds of ellipse fits in IRAF:• FREE: Tends to follow morphological features like bars, spirals, or asymmetries (not ideal)• FIXED: Predefined ellipses

Image: Final surface brightness profile (circles) over-layed by the four profiles obtained by either over- or under-subtracting the sky by ±1σ(triangles) or by ±3σ (squares).


Classification• Type I profiles are single-exponential profiles, with the profile

continuing out to the limit set by our sky-background uncertainty• Type II profiles contain a break, where the profile bends “down”—

that is, the profile becomes steeper outside the break.• Type III profiles are similar to type II profiles, except that the

profile bends “up” beyond the break


Subdivisions of Type II• Based on whether the break is an “inner” break or an “outer” break,

compared to the bar radius.


Subdivisions of Type II.oIt is observed that many type II.o profiles have a break which coincides with an

outer ring or occurs at ∼2–3 times the bar radius.Outer rings are generally understood to be linked to the Outer Lindblad

Resonance (OLR) of barsIn two galaxies with type II.o profiles (NGC 2273 and NGC 2950), the break

occurs well outside what appears to be the outer ring, these are classified as “Classical Truncations or Type II.o-CT


Subdivision of Type III

Based on whether the evidence indicates that the outer part of the profile, beyond the break, is still part of the disk (type III-d) or whether it is due to a more spheroidal component (type III-s).


Complicated Samples• There are at least 4 galaxies whose profiles are more complicated,

combining elements of more than one type. Two examples of multiple classifications:


ResultsFrequency of profile types in relation to the

Hubble type:• Type I: Open Circles• Type II-CT: Filled Squares• Type II.o-OLR: Open squares• Type III: Open TrianglesClassical truncations are more frequent in later

types while the fraction of upbendingbreakes rise towards earlier types.

Histogram of surface brightness (r band) of the break radius

• Type II-CT: Solid lines• Type III: Dashed lines• Type II.o-OLR Dotted linesClassical truncations are more frequent in

later types while the fraction of upbending breakes rise towards earlier types.


Measuring the Break RadiusClassical approach: Exponential functions are fit to portions of the profiles that

appear approximately linear; outside the bar region and inside the limiting surface brightness level.

A more precise and rigorous approach is using a “broken Exponential function:

I0: Central intensity of inner exponentialγ and β are the inner and outer exponential scale lengthsRb: Break Radius α parameterizes the sharpness of the break


Measuring the Break Radius

Example of a broken-exponential fit to the type II.o profile of NGC 3729


Results• About 90% of our sample can be classified into one of the classes described;

remaining show two breaks


• About the table: Frequency of disk types: listed are the number of galaxies per profile typeSurprisingly only <15% of all galaxies have a normal purely exponential disk down to our noise limit.

• For more than 60% of Type III galaxies we find good indication that the outer up-bending part is a disk like structure.

• Type III: Close Physical neighbors and slightly disturbed morphology suggest in several cases interaction as a possible origin.

Results• Four galaxies (6% of the sample) combine type II and type III

characteristics

• The type II.i profiles are rare (only 6% of the sample): Similar to simulations of bar formation

• Type II.o profiles are common (42% of the sample): OLR: 35%, CT: 5%

• Type III galaxies: 36%: III-d: 18%, III-s: 12%, 4 galaxies un certain.


Combined ConclusionHere “late types” is used to refer to the barred, Sbc–Sm subset of the Pohlen

& Trujillo sample (47 of the 85 galaxies in their sample with disk-profile classifications).

• Type I profiles are much rarer in late type spirals

• Type II profiles are significantly more common in late types

• Type III are slightly rarer in late types

• By far, early spirals display OLR properties in type II

• Classical Truncation is found in the late types (half of them)

• Type II-i are rare in both early and late types


Observational Studies(Bakos, Trujillo, Azzollini, Pohlen, 2008)

This study was conducted on the same sample as the Pohlen-Trujillo 2006.The data in that study were averaged based on their type.

Bakos, J. Trujillo, I. Azzollini, R. Pohlen, M. 2008. Color profiles of spiral galaxies: clues on outer-disk formation scenarios. The Astrophysical Journal, 683: L103-106

Results• For all three types we find that the brighter the galaxy,

the redder the color at the break. ρ is Spearman’s correlation coefficient

• The behavior of the surface brightness profile outside the break is basically due to a change in the ingredients of the stellar population.

• Azzollini et al (2008) have shown that a similar minimum in the color profile can be found in Type II disks at high z. So we can say that this behavior is long lived.

• For Type III galaxies, the situation is less clear (lower data and lack of theoretical explanations).

Bakos, J. Trujillo, I. Azzollini, R. Pohlen, M. 2008. Color profiles of spiral galaxies: clues on outer-disk formation scenarios. The Astrophysical Journal, 683: L103-106

Over view of Observational studies(Vlajic 2010)

• Surface photometry suffers from many technical difficulties at levels below μR ≈27 mag arcsec−2:

• difficulties of data flatfielding (Pohlen et al. 2002), • zodiacal light (Bernstein et al. 2002), • diffuse Galactic emission (Reynolds 1992; Haikala et al. 1995), • light scattered by the HI disk (Bland-Hawthorn et al. 2005), and • effects of extended tails of the Point Spread Function (de Jong 2008).

Vlajic, M. 2010. Outer spiral disks as clues to galaxy formation and evolution. Publications of the Astronomical Society of Australia 27, 252-255.

By resolving individual stars and calculating the effective surface brightness from star counts, it is possible to reach 3-5 mag/arcsec2 deeper than possible with surface photometry:

Inefficient star formation in the outer regions

Conditions in the outer disks of spirals compared to the typical inner parts:• Low metallicities and dust abundances, • relatively high shear, • low total gas column densities spread over significant scale heights, • preponderance of HI compared to H2, and • comparatively weak stellar potential

Importance of Star formation in outer region environmental studies for galaxy evolution:

• It affects how chemical enrichment varies across a galaxy (Gil de Paz et al. 2007b)

• Plays critical role in form and location of break (Pohlen, Trujillo 2006)• Helps determine the consumption of the outer disks gas reservoir to find it’s

role in the galaxy formation on cosmological time scales (e.g., shlosman et al, 1989, Blitz 1996, Bauermeister et al. 2010).

Bigiel, F. Leroy, A. Walter, F. Blitz, L. Brinks, E. de Block W.J.G, Madore, B. 2010. Extremely inefficient star formation in the outer disks of nearby galaxies. The Astronomical Journal. 140:1194-1213.

Inefficient Star formation• (left)HI and FUV emission

exhibit different radial behaviors

• (bottom) HI scale length as a function of FUV scale length

Bigiel, F. Leroy, A. Walter, F. Blitz, L. Brinks, E. de Block W.J.G, Madore, B. 2010. Extremely inefficient star formation in the outer disks of nearby galaxies. The Astronomical Journal. 140:1194-1213.

Structure of outer disksThirty years after the discovery of breaks in the Luminosity Profiles , the origin

of their diversity is unknown;• Type III: Interactions and minor mergers have been proposed to explain

galaxies; Supported by simulation (Penarrubia et al. 2006; Younger et al. 2007) and the observation that Type III galaxies lie in high density regions (Pohlen & Trujillo 2007)

• Type II: Two models proposed:1.Break is seeded during the process of galaxy formation (minimum angular momentum; van der Kruit 1987)2. A possible star formation threshold and/or secular evolution (Kennicutt

1989, Sunchez-Bluzquez et al. 2009)Combination; the distribution of cold gas gas is truncated to reflect the

maximum angular momentum of the collapsed cloud, while the break in the stellar light profiles points to the presence of a star formation threshold(van den Bosch, 2001)

Vlajic, M. 2010. Outer spiral disks as clues to galaxy formation and evolution. Publications of the Astronomical Society of Australia 27, 252-255.

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