Direct Gas-Phase Metallicities and its Dependence on Star Formation for Strongly Star-forming Dwarf Galaxies

Transcription

1 Direct Gas-Phase Metallicities and its Dependence on Star Formation for Strongly Star-forming Dwarf Galaxies Chun Ly (Goddard Space Flight Center) NASA Postdoctoral Fellow Credit: DEEP2 4,000 spectra from the DEEP2 Survey [OII] H! H! H! [OIII] Jane Rigby (NASA/GSFC), Sangeeta Malhotra (ASU), Matt Malkan (UCLA), Michael Cooper (UCI), Tohru Nagao (Kyoto), Nobunari Kashikawa (NAOJ), et al. Galaxies of Many Colours Conference June 3, 2015

2 The gas-phase metallicity of galaxies traces what has happened physically in and around galaxies: gas inflows, star formation, and feedback from supernova SDSS: Tremonti+ (2004) See also: Zaritsky+ (1994), Garnet (2002), Erb+ (2006), Zahid+ (2011) 2

3 Does the fundamental metallicity relation holds for low mass, starburst galaxies at z = ? WFIRST JWST 3

4 Mass, Gas Metallicity, and SFR for z < 2: relation or fundamental plane Ellison+ (2008) found that higher SFR galaxies have lower metallicity at fixed stellar mass Mannucci+ (2010) and Lara-Lopez+ (2010) extended this result, argue for a tighter relation Interpretation: SFR dependence due to accretion of cold gas (Dave+ 2011) or feedback from star formation Mannucci+ (2010) There are studies that support this claim: e.g., Richard+ (2011), Hunt+ (2012), Belli+ (2013), Andrews & Martini (2013), Henry+ (2013a,b), de los Reyes,Ly+ (2014) But others that oppose it or find evidence for evolution: e.g., Yates+ (2012), Yabe+ (2012, 2014), Zahid+ (2013), Stott+ (2013), Sanchez+(2013), Hughes+ (2013), Ly+ (2014,2015), Salim+ (2014), Sanders+ (2014), Steidel et al. (2014) 4

5 [Aside]: Limitations of Strong-Line Metallicity Estimates Strong lines: or Calibrated empirically (observations of nearby H II regions, local galaxies) or with theoretical photo-ionization models (e.g., Dopita 2002) Discrepancies at ~1 dex! Can only use in a relative sense, but problematic if the physical conditions of the ISM evolves! Kewley & Ellison (2008) 5

6 [O III] 4363: The Holy Grail of Metallicity Ø Most direct measures of metallicity require detections of [O III] 4363 [O III] 4363/ 5007 exp(kte) Te log(o/h) -1 Ø Problem: Intrinsically weak (~1% of [O III] 5007 flux) Ø SDSS: One in 1700 spectra has >2 detection (Izotov+ 2006) Ø Greater chance of detection in lower metallicity regime Ø Detecting at large cosmological distances is very difficult but some have succeeded (Hoyos+ 2005; Hu+ 2009; Atek+ 2011; Amorin+ 2014) Ø Require large spectroscopic sample (DEEP2) or techniques to identify high-ew galaxies (SDF) 6

7 The Subaru Deep Field (SDF) See: Kashikawa+ (2004), Ly+ (2007) SDF = 160 HDF's SDF = 5.7 GOODS-N's Imaging in over 28 bands! Optical imaging depth comparable to HDF s Combination of area coverage, depth, and pan-chromatic imaging makes the SDF unique Deeper by avoiding 27 OH sky lines 34 GOODS-N HDF Field-of-View of Subaru's Suprime-Cam 7

8 The SDF Narrow-band and Intermediate-band Survey Published in Ly et al. (2007, 2012a, 2012b) At z < 1.03, accurate redshift for 67% of co-moving volume space! Volume is 3.8x larger than previous z < 1 NB survey (e.g., Hu et al.) Deeper NB imaging by ~1.5 mag! SDF is the field to look for high-ew galaxies at z ~ Keck/DEIMOS and MMT/Hectospec spectra detected [O III] 4363 in ~65 galaxies at z ~ from ~1185 targets (~1400 spectra)! 8

9 Examples of our SDF [O III] 4363 Detections Ly et al. (2015), in prep H [O III] MMT: 16 at at , 16 at Keck: 9 at at , 11 at

10 The DEEP2 Survey Magnitude-limited (R < 24 mag) Keck spectroscopic survey at z = Covers 3 deg 2 in four different fields; 53,000 galaxies Require accurate redshift (N=37,396) Spectral coverage of rest-frame Å (N=4,140; z = ) Require >100 of [O III]5007 and >3 detections of [O III]4363 Sample size: 28 galaxies (Ly et al. 2015, ApJ, 805, 45) 10

11 SDF + DEEP2: ~93 galaxies Largest z ~ sample with direct oxygen abundance measurements 11

12 Not Surprisingly: High-EW Emission Lines 12

13 All Star-Forming: No evidence of AGN or LINERs Turnover in [OIII]/H!! 13

14 Analogs to High-z Galaxies Ionization Parameter Metallicity 14

15 Specific SFRs: ~1 dex above the star-forming main sequence The average mass doubling time is ~100 Myr Instantaneous dust-corrected SFRs 15

16 Significant scatter Inconsistent with FMR predictions RMS relative to M Z relation : 0.34 (SDF), 0.23 (DEEP2), 0.31 dex (Both) RMS relative to local FMR : 0.42 (SDF), 0.29 (DEEP2), 0.38 dex (Both) Galaxies are not in equilibrium, stochastic star formation Note: Andrews & Martini (2013) stacked spectra for [OIII]4363 detections, thus the amount of true scatter is underestimated 16

17 But is there an inverse dependence between SFR and Z? Bin samples based on specific SFRs Included robust non-detections to account for selection bias 17

18 98.7% confidence that the distributions are different. But in the opposite direction of FMR predictions! Dist. in 12+log(O/H) - Z(M) low-sfr high-sfr Interpretation: positive Z-SFR dependence due to recent SF that enriched the gas, but SN feedback has not significantly driven the metal-enriched gas out Z 18

19 Why such extremely high SFRs?within 100 kpc! Two-thirds of our galaxies have companion(s) MMT01 NB816 MMT03 NB973 MMT06 NB921 MMT07 NB921 MMT10 NB704 MMT11 IA598 MMT12 NB816 MMT13 NB711 Keck1 IA679 Keck2 NB816 Keck3 IA679 Keck5 NB = 130 kpc (z = 0.4) 190 kpc (z = 0.85) 19

20 Two-thirds of our galaxies have companion(s) within 100 kpc! MMT01 NB816 MMT03 NB973 MMT06 NB921 MMT07 NB921 MMT10 NB704 MMT1 MMT12 For typical galaxies atmmt13 z = star-forming NB816 NB711 IA598 (H!) and 0.85 ([O III]), the nearest neighbor is on average 350 kpc (projected) away, and only 12% has a companion within 100 kpc Keck1 IA = 130 kpc (z = 0.4) 190 kpc (z = 0.85) Keck2 NB816 Keck3 IA679 Keck5 NB921 20

21 Conclusions Ø Identified ~93 metal-poor galaxies with [OIII]4363 in DEEP2 and SDF Ø Samples consistent with the local FMR; significant dispersion suggests weak SFR Z dependence due to stochastic star formation (FMR breaking down) Ø K-S test does show a SFR Z dependence at 98.7% confidence, but in the opposite direction of FMR predictions Ø Most of our SDF galaxies have 1 4 nearby companions within 100 kpc Ø We believe that galaxy interactions are driving inflows of metal-poor gas in these starburst galaxies, much like local interacting galaxies (e.g., Rupke et al. 2010) Stay tuned: (1) Efforts to re-calibrate metallicity diagnostics at z ~ 1 (2) Keck/MOSFIRE time granted for DEEP2 sample: H, [NII], and [SII] 21