Alessandro Romeo

Transcription

1 A simple and accurate approximation for the Q stability parameter and characteristic instability wavelength in multi-component and realistically thick discs Alessandro Romeo romeo@chalmers.se

2 Toomre s (1964) stability criterion Q κσ πgσ 1

3 Why is a multi-component analysis needed? Stars and cold interstellar gas have an important interplay in the gravitational instability of galactic discs (e.g., Lin & Shu 1966; Jog & Solomon 1984)

4 Why is a multi-component analysis needed? Stars and cold interstellar gas have an important interplay in the gravitational instability of galactic discs (e.g., Lin & Shu 1966; Jog & Solomon 1984) Atomic and molecular gas play a fundamentally different role in star-gas instabilities:

5 THINGS Spirals Romeo & Falstad (2013), MNRAS, 433, 1389

6 The challenges of a multi-component analysis Highly non-trivial problem: N components up to N minima no analytical solution for N > 1 Find a convenient parametrization non-trivial even for N = 2 Output: the Q stability parameter the characteristic instability wavelength λc

7 So what can you do? Use my approximations, which follow from rigorous stability analyses!

8 So what can you do? Use my approximations, which follow from rigorous stability analyses! Romeo & Wiegert (2011), MNRAS, 416, 1191: stars + gas, 0.5 σz /σr 1

9 So what can you do? Use my approximations, which follow from rigorous stability analyses! Romeo & Wiegert (2011), MNRAS, 416, 1191: stars + gas, 0.5 σz /σr 1 Romeo & Falstad (2013), MNRAS, 433, 1389: stars + HI + H2 +..., 0 σz /σr 1 More general and as easy to use!

10 QRF is simple: 1 = Q RF N i=1 W i T i Q i

11 QRF What is Qi? Q i = κσ i πgσ i )

12 QRF What is Ti? ( σz ) 2 T i = σ R ( σz i ) if 0 (σ z /σ R ) i if 0.5 (σ z /σ R ) i 1 σ R i

13 ) QRF ( ) What is Wi? ( ) W i = 2σ mσ i σ 2 m + σ 2 i And m? T m Q m =min{t i Q i }

14 QRF is accurate: Q RF 0.03 N Q RF

15 The 2-component (2D) case Romeo & Falstad (2013), MNRAS, 433, 1389

16 QRF predicts the local stability level of the disc

17 QRF predicts the local stability level of the disc which component dominates the (in)stability process

18 THINGS Spirals Toomre s Q Our 2-component Q Romeo & Wiegert (2011), MNRAS, 416, 1191

19 THINGS Spirals Our 2-component Q Our 3-component Q Romeo & Falstad (2013), MNRAS, 433, 1389

20 λrf Q is also simple: λ RF =2π σ m κ Do you remember what m is?

21 λrf Q is also simple: ( ) λ RF =2π σ m κ Do you remember what m is? T m Q m =min{t i Q i }

22 λrf accurately predicts the sizes of star-forming complexes

23 NGC 7469 Fathi, Izumi, Romeo et al. (2015), ApJL, in press (arxiv: )

24 λrf accurately predicts the sizes of star-forming complexes

25 λrf accurately predicts the sizes of star-forming complexes the sizes of starbursts and bars within bars

26 NGC 6946 Romeo & Fathi (2015), MNRAS, in press (arxiv: )

27 CONCLUSION QRF and λrf are state-of-the-art diagnostics for detecting gravitational instabilities in galaxy discs. Use them!

28 Extra Slides... if I had more time :-( Inaccuracy of the Wang-Silk (1994) approximation A double molecular disc in the triple-barred starburst galaxy NGC 6946 Our stability map of turbulence... and what it can predict

29 Inaccuracy of the Wang-Silk (1994) approximation: 1/QWS = 1/Qstars + 1/Qgas Romeo & Wiegert (2011), MNRAS, 416, 1191

30 A double molecular disc in the triple-barred starburst galaxy NGC 6946 Romeo & Fathi (2015), MNRAS, in press (arxiv: )

31 Our stability map of turbulence: how Σ ^a and σ ^b affect disc instability Regime C Regime B Regime A Romeo, Burkert & Agertz (2010), MNRAS, 407, 1223 (see also Hoffmann & Romeo 2012; Romeo & Agertz 2014)

32 ... and what it can predict: simulations with similar Q, but in different stability regimes! Regime C Regime A Agertz, Romeo & Grisdale (2015), MNRAS, 449, 2156