South Korea 2013 The signatures of long-lived spirals in disk galaxies The signatures of long-lived spirals in disk galaxies The signatures of long-lived spirals in disk galaxies The signatures of long-lived spirals in disk galaxies Eric E. Martínez-García (INAOE, Mexico) Collaborators: Rosa Amelia González (CRyA, UNAM) Gustavo Bruzual (CRyA) Gilberto Gómez (CRyA) Ivanio Puerari (INAOE)
What is the origin of the spiral arms?
Early studies (60's) of spiral patterns propose density waves. • Attempt to explain spiral structure, avoiding the winding dilemma : Linblad, Lin & Shu , Roberts, Toomre, Kalnajs, Bertin, Contopoulos, inter alia.
“Chaotic” spirals Kaufmann & Contopoulos (1996) invoke for the first time the need of chaotic orbits as building blocks of spirals near the end bar. Part of the spirals attached to the bar are due to chaotic orbits (Patsis, 2006).
“Invariant manifolds” theory • Romero-Gómez et al. (2006, 2007) Athanassoula et al. (2009a,b, 2010) consider a continuous flow of orbits along the manifolds emanating from L1 or L2. “Lyapunov tube model” . ● Voglis et al. (2006a,b); Tsoutsis et al. (2008, 2009) consider the apsidal sections of the manifolds.
“Invariant manifolds” theory • Romero-Gómez et al. (2006, 2007) Athanassoula et al. (2009a,b, 2010) consider a continuous flow of orbits along the manifolds emanating from L1 or L2. “Lyapunov tube model” . -> Corroborated for galaxies in which the spiral arms maintain a logarithmic shape al least for 70° in azimuth (Martinez-Garcia 2012).
Large scale shocks in spiral arm regions. Gittins & Clarke (2004) Roberts (1969) Suggests that star formation is triggered near spiral arms.
Shocks may extend to high z (the height above the galactic midplane) above the arm (Martos et al. 1999, Alfaro et al. 2001) Density contours and the velocity field.
Expected SF sequence across spiral arms. -> Photodissociated (Louie et al. 2013) see e.g., Foyle et al. (2011) Martinez-Garcia & Puerari (in preparation)
Expected SF sequence across spiral arms.
Azimuthal gradients (age, color ). Spiral pattern: Ω p ≅ constant Stars and gas: Differential rotation
The Ω p may vary as a function of radii for some objects. Meidt et al. (2009); M101: Radial Tremaine– Weinberg (TWR) method. The kinematic tracer (CO and HI) is assumed to obey the continuity equation. Must orbit in a single plane. Black and blue curves:
Pattern Speed Variation with Radii. An effect of non-circular motions? (Martinez-Garcia et al. 2009, 2013) MHD simulations of gas orbits. 2-arm potential of Pichardo et al. 2003. Gas response: 4 arms (Martos et al. 2004).
Pattern Speed Variation with Radii. An effect of non-circular motions? (Martinez-Garcia et al. 2009, 2013) May be an artifact produced by the non-circular streaming motions of shocked material. Besides, shocks may extend to high z (Martos et al. 1999)
Have azimuthal gradients been observed? • Gónzalez & Graham (1996), M99, first extragalactic reliable gradient~ 50´´ (4 kpc) . Avoid HII regions. Photometric index “Q”, reddening-free and star formation tracer.
GG96 method. Photometric index “Q” (Mihalas & Binney, 1981 ): − E ( U B ) = − − − Q ( UBV ) ( U B ) ( B V ) − E ( B V ) Reddening free (for screen model): − − E ( U B ) E ( U B ) − − − = − − − ( U B ) ( B V ) ( U B ) ( B V ) − − 0 0 E ( B V ) E ( B V ) Defined in r, J, g, i : − E ( r J ) = − − − Q ( r Jgi ) ( r J ) ( g i ) s − s s E ( g i ) Do not confuse with Toomre's Q!
“Index Q”, GG96 method. star formation tracer: I I 2 . 05 2 . 50 = Q ( r Jgi ) log g J s 10 I I 2 . 50 2 . 05 r i s
Q predicted by stellar population synthesis models (Bruzual & Charlot). Q Young stars fraction between 0.5 y 2%, in agreement with Schweizer (1976).
CB07 2007, IMF limit: 100 Msun: “Index Q”, for a mixture of dust and stars. Witt et al. (1992) models: CB07 2007, IMF limit: 10 Msun: Two-component dust model of Charlot & Fall (2000): τ v < 2 for face-on galaxies (e.g. Peletier 1995)
Structural type of the spiral arms. From the point of view of their spiral arms, there are three different types of galaxies (e.g., Efremov 2011 ). (2) Multi-armed or (3) Flocculent spiral (1) Symmetric grand- “knee-like” spirals galaxies (e.g., NGC 2841) design spirals (e.g., M81) (e.g.,M101) Mass arms maybe explained by DWs.
Structural type of the spiral arms. In order to determine whether the arms are indeed mass DWs it is important to disentangle the contribution of young stars and clusters at longer wavelengths. Such contribution can reach up to 20%–30% in the NIR (e.g., Rix & Rieke 1993; GG96; Rhoads 1998; James & Seigar 1999; Patsis et al. 2001; Grosbøl et al. 2006).
Resolved maps of stellar mass. Method of Zibetti et al. (2009). Monte Carlo library of 50,000 stellar population spectra. Constructed from the SPS models of Bruzual & Charlot (2003 ), and Charlot & Bruzual (2007). Each spectrum is computed by randomly drawing the model parameters: ➢ Star formation history ➢ Metallicity ➢ Dust attenuation (dust model of Charlot & Fall (2000).
Resolved maps of stellar mass. Method of Zibetti et al. (2009). Less degeneracy in M/L: g & i optical bands + 1 NIR
Data sample. ***) 19 nearly face-on SA and SAB spirals. Angular diameters between 4' y 6'. Deep images (30-60 minutes of exposure time). Taken during 1992-1995 Observatories: Lick, Kitt Peak, CTIO Optical data: bands g, r, i (near) IR: bands J y Ks (or H)
Sample of objects & mass maps.
By visual inspection, the following objects were rejected prior to analysis: NGC 3162 NGC 3938 NGC 5371 NGC 7083 NGC 7126 NGC 7753 Remaining 13 objects analyzed with Fourier techniques in search for azimuthal color/age gradients.
− E ( r J ) = − − − Q ( r Jgi ) ( r J ) ( g i ) s − s s E ( g i ) FOURIER detection of color gradients. Patterns with amplitudes between 0.02-0.06 mag in Q(rJgi). CB 2007 models Objects analyzed in rings for all radii.
− E ( r J ) = − − − Q ( r Jgi ) ( r J ) ( g i ) s − s s E ( g i ) FOURIER detection of color gradients. CB 2007 models
Expected result from MHD simulation, with a constant pattern speed for all radii. The data diverge towards corotation ~0.6
Azimuthal phases method of Puerari & Dottori (1997 ) Based on computing the radial Fourier transform: Where I R is the intensity of radiation with phase: For m=2: π radians symmetry assumption.
Azimuthal phases of the g, r, i, and J Bands Cautions: ➢ Effect of dust obscuration? ➢ Emission of the old stellar arm (DW)? ➢ π radian symmetry?
Results NGC 3338 and NGC 4254 Suggests a constant pattern speed for all radii.
NGC 4603 Can't suggest a constant pattern speed for all radii.
Fourier results for detection of color gradients 7 of 13 (54%) objects may present a constant pattern speed for all radii. For the remaining objects color gradients may be difficult to detect because of: 1) Dust extinction (phases method) 2) “Infant mortality” of star clusters (Lada & Lada 2003) 3) Overlapping radiation of HII regions 4) Physical conditions (magnetic field or pitch angle; Efremov 2010) 5) Not a constant pattern speed?
Pitch angle test ➔ Complementary test of DW presence. ➔ Examine the pitch angles of the spiral arms in different wavebands (g, r, i, and J) and in “resolved mass maps”. Differences in wavelength should exist if age/color gradients are present across spiral arms!
(e.g., Considere & Athanassoula Pitch angles Fourier method 1988; Puerari & Dottori 1992; Seigar et al. 2006) ✔ It is assumed again that the arms can be represented by logarithmic spirals. ✔ Determines the “average” pitch angle inside a range of radii.
Pitch angle test results ✔ Negative differences indicate that the pitch angles are larger in the NIR than in the optical. ✔ Agreement with Grosbøl & Patsis (1998), who find tighter arms in bluer colors in images of four spirals, suggesting the presence of DWs. ✔ Mass arms are statistically more open than the arms in NIR light.
Other evidence; age gradients H α to far-UV flux ratio method: Sánchez-Gil et al. (2011) found age gradients across the spiral arms of the grand-design spirals M74 and M100 .
Other evidence Scarano & Lepine (2011,2013) found "breaks" in the radial metallicity distribution near corotation (CR). Implies that spiral arms must be long-lived structures. A dominant pattern speed must exist with a unique CR. Or otherwise any discontinuities in the radial metallicity profiles would be smoothed out (Scarano & Lepine 2013).
Relationship between Spiral arm pitch angle - Supermassive black hole mass Berrier et al. (2013) Seigar et al. (2008) Suggests the presence of density waves in disk galaxies
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