Extended Star Formation in the Sgr B2 cloud

Transcription

1 Extended Star Formation in the Sgr B2 cloud Adam Ginsburg Adam Ginsburg, 1, 2 John Bally, 3 Ashley Barnes, 4 Nate Bastian, 4 Cara Battersby, 5, 6 Henrik Beuther, 7 Crystal Brogan, 8 Yanett Contreras, 9 Joanna Corby, 8, 10 Jeremy Darling, 3 Chris De Pree, 11 Roberto Galván-Madrid, 12 Guido Garay, 13 Jonathan Henshaw, 7 Todd Hunter, 8 J. M. Diederik Kruijssen, 14 Steven Longmore, 4 Xing Lu, 15 Fanyi Meng, 16 Elisabeth A.C. Mills, 17, 18 Juergen Ott, 19 Jaime E. Pineda, 20 Álvaro Sánchez-Monge, 16 Peter Schilke, 16 Anika Schmiedeke, 16, 20 Daniel Walker, 4, 21, 22 and David Wilner 5 Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

2 radio-astro-tools radio-astro-tools.github.io spectral-cube, pvextractor, radio-beam built for general users, not just blackbelts but for this audience, makes many tasks more convenient see also astroquery.alma, astroquery.nrao, astroquery.splatalogue

3 What gas conditions govern the star formation rate? What gas conditions control the stellar initial mass function?!

4 What gas conditions govern the star formation rate? What gas conditions control the stellar initial mass function?! Transient, pulsar folks: You care because event rates are N ~ (Ngalaxies) (SFR) ( IMF)! Also, dust & gas modify observability of events, so it s important where & when they occur

5 Star Formation in Galaxies log [Σ SFR (M year 1 kpc 2 )] SFR Surface Density M51 (Kennicutt et al. 2007) Apertures M51 (Schuster et al. 2007), NGC 4736 and NGC 5055 (Wong & Blitz 2002), and NGC 6946 (Crosthwaite & Turner 2007) Radial profiles Non-starburst spirals (Kennicutt 1998b) Global Starburst galaxies (Kennicutt 1998b) Global LSB galaxies (Wyder et al. 2009) Global 100% 10% 1% Kennicutt & Evans 2012 Bigiel log [Σ HI + H2 (M pc 2 )] Gas Surface Density Generally follows the Kennicutt-Schmidt relation, but this is empirical & much of the scatter comes from local physical effects.

6 Star Formation in the Galactic Center Our Galactic center has a low SFR per dense gas mass (a long apparent depletion timescale τdep ~ Mgas/SFR) Galaxy centers have shorter τdep (but higher variance in τdep) Galaxies (Gao & Solomon 2004) CMZ clouds (GCMS Paper I) Shorter τdep Milky Way clouds (Lada et al. 2010) CMZ averages (Longmore et al. 2013) reduction for flatter α 3 = 2.3 IMF in CMZ representative statistical uncertainty for CMZ clouds Kauffmann Relative Depletion Time Leroy et al 2013

7 CMZ averages (Longmore et al. 2013) CMZ clouds (this study) The discrepancy occurs on both global and local scales Milky Way clouds (Lada et al. 2010) factor 10 Kauffmann reduction for atter α 3 = 2.3 IMF in CMZ representative statistic uncertainty for CMZ clouds

8 Large-scale:! CMZ SFR is low because star formation is bursty! Gas is fed into the CMZ ring by the Galactic bar. Accreted gas is highly turbulent.! Once enough gas is in the ring, starbursts occur and radiation and supernovae blow out the gas Kruijssen+ 2014, 2015 Krumholz & Kruijssen 2015, 2017 Torrey+ 2016

9 Small Scale: This CMZ dark cloud (G0.253, Brick, Lima Bean) exhibits almost no star formation despite its high average density (>10 4 cm -3 ) Rathborne+ 2014, 2015! Lis & Carlstrom 1994, Lis & Menten 1998, Lis Longmore+ 2012, Kauffmann+ 2013, Rodriguez+ 2013, Bally+ 2014, Marsh+ 2016, Federrath+ 2016

10 CMZ averages (Longmore et al. 2013) CMZ clouds (this study) Milky Way clouds (Lada et al. 2010) factor 10 Kauffmann reduction for atter α 3 = 2.3 IMF in CMZ representative statistic uncertainty for CMZ clouds

11

12 Sgr B2 is the most massive protocluster cloud in the galaxy Sgr B2 Brick Contreras+ 2016

13 ALMA 3 mm continuum! (mix of free-free & dust) Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

14 near some very faint and di use 3 mm emission; it is unclear why the 3 mm is so weak here, but it hints that there are MYSOs with 3 mm emission below our detection limit. H2 O masers Water masers are generally associated with young, accreting stars. We matched our catalog with the McGrath et al. (2004) water maser catalog, finding that 23 of our sources have a water maser within 100. These sources are likely to contain YSOs, but not necessarily MYSOs based on their H2 O maser detections alone. There are 14 masers from their catalog that do not have associated sources in our catalog, though not all of these maser spots are spatially distinct. Most of these unassociated masers are seen outside of Sgr B2 N and Sgr B2 S and may be associated with outflows. X-ray sources Some young stars exhibit X-ray emis- sion, including some MYSOs (e.g., Townsley et al. 2014), so we searched for X-ray emission from our sources. 3 of the sources have X-ray counterparts in the Muno et al. (2009) Chandra point source catalog within 100. The Muno et al. (2009) catalog covers our entire observed area. The X-ray associated sources most likely contain YSOs. There are 102 X-ray sources in the field of view that do not have associated 3 mm sources. Spitzer mid-infrared sources We searched the Yusef- Ginsburg et Zadeh et al. (2009) catalogs of 4.5 µm excess sources and YSO candidates and found only one source association, though there are 5 and 14, respectively, of these sources in our field of view. Two of the 4.5 µm excess sources and one of the YSO candidates are associated with extended H ii regions (which we do not catalog); the single association is of a 4.5 µm source with the central region of Sgr B2 M. By-eye comparison of the Spitzer maps and the ALMA images suggests that the lack of associations is at least in part because of the high extinction in al the2018: regionstinyurl.com/sgrb2alma-2 containing the 3 mm cores; there are overall nisms include free-free and thermal dust emission, so in this section we explore whether the sources could be different classes of dust or free-free sources. We examine whether they are dusty prestellar cores ( 3.3.1), externally ionized globules ( 3.3.2), H ii regions from an extended population of OB stars ( 3.3.3), or H ii regions around young massive stars ( 3.3.4). After determining that the above alternatives do not readily explain the whole sample, we conclude that the sources are primarily dense gas and dust cores with internal heating sources ( 3.3.5). ALMA enables YSO counting in massive SFRs Discovered 271 tra extracted from the full line cubes, and no lines are depoint(ish) tected peaking toward most of the sources, sources (most sources have emission in some lines, such as HC N 10-9, but this most of which are new emission is clearly extended and not associated with the A lack of line emission We visually inspected the spec- 3 compact source). Given the relatively poor line sensitivity (RMS 6 K), the dearth of detections is not very surprising. We therefore cannot use spectral lines to classify most sources.

15 Class 0/I YSO: an accreting star with a gas & dust envelope may have a disk and an outflow

16 Class 0/I YSO SED Robitaille F F [relative] [relative] mm flux density µm] [µm] Fig. 2. A subset of 2000 SEDs for each model set, normalized to the total luminosity of each SED. [µm] Fig. 2. A subset of 2000 SEDs for each model set, normalized to the total luminosity of each SED A11, page 9 of 16 each model set, normalized to the total luminosity of each SED. A11, page 9 of 16 A11, page 9

17 3 mm source classification Most are HMYSOs. Some are HCHIIs. All will likely form massive stars. (YSOs in Orion) L~2000 L

18 Estimate total (proto)stellar mass using an assumed IMF each observed ~10 M source implies the presence of ~100 M of lower-mass stars { Inferred SFR ~ M /yr up from M /yr, assuming t~0.74 Myr >half CMZ total Observed {

19 YSO counts -> density Compare to gas mass N(H2) AK 2x x x x x

20 Local Cloud Comparison N(H2) AK 2x x x California Molecular Cloud (d~450 pc) AK 2 pc Lada+ 2017

21 Each source represents ~100 M Local Cloud Comparison N(H2) AK 2x x x California Molecular Cloud (d~450 pc) AK 2 pc Each source represents ~0.5 M Lada+ 2017

22 Local SF Laws within clouds: Lada California D ~ 400 pc, M~10 5 M Protostellar vs Gas Surface Density 10 2 M pc M pc -2 Gutermuth D ~ pc, M~ M

23 Σstar based on Gutermuth Σgas Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

24 Lombardi Lada California Cloud Orion A and B based on Gutermuth Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

25 Lombardi Lada California Cloud Orion A and B based on Gutermuth Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

26 Sgr B2 does not fit on Σgas-Σstar relations extrapolated from local clouds!! A linear relation! fits the Sgr B2 data, but not the local based on Gutermuth Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

27 Sgr B2 does not 0.74 Myr α = Myr fit on Σgas-Σstar relations extrapolated from local clouds!! A linear relation! [ ( 0.1 Myr 0.74 Myr 0.1 Myr fits the Sgr B2 data, but not the local α = Myr Σ (x,y,t) t = ckσ gas (x,y,t) α Σ (x,y,t) = cσ gas (x,y,0) α kt. based on Gutermuth Ginsburg et al 2018: tinyurl.com/sgrb2alma-2

28 Why is Sgr B2 different? Higher Multiplicity YSOs actually more massive Incomplete Multiple clouds along LOS Non-uniform IMF (primordial mass segregation) Lower local SFE Higher density threshold

29 SF in the CMZ is lower than predicted and allows us to test SF relations. Sgr B2 is vigorously forming stars, and not just in the main protoclusters Surface-density star formation relations can t fit both Sgr B2 and local clouds. he critical density for SF is higher in CMZ clouds

30 Immediate objectives of the ALMA-IMF Large Progra ALMA-IMF (PI Motte, Ginsburg, Sanhueza, Louvet) ultimate objective of ALMA-IMF is to push forward the understanding of the IMF origin will repeat this experiment on about a dozen ulate improvements to star formation models. To this aim, we will image with ALMA 15 ma high-mass star (cluster?) regions oclusters and apply the robust analysis steps we haveforming identified with our earlier projects.

31 Several ALMA programs to dig in to Sgr B2 at higher resolution & sensitivity to directly probe the lower-mass regime

32 1. Age estimate for the distributed population in DS Gas σv ~ 10 km/s! Ridge width r~0.5 pc! Most sources within 0.5 pc of the ridge! Diffusion (dispersion) timescale t = r / σv = 5x10 4 yr! (sims suggest t = 5 (r/σv) = 2.5x10 5 yr Offner+2009)

33 2. Spectral Shape Dust >2 Synchrotron: <0 Free-free (HII regions) 0-2

34 3. Rule out alternatives freggs: Too compact, wrong locations Prestellar Cores: Too bright, implied mass too large Compact HII regions: Can explain some, but not most Sahai+ 2012

35 Young, Dust-dominated: Most are HMYSOs. Some are HCHIIs. All will likely form massive stars. L~2000 L