Observations of High-Redshift Galaxies and What Can Be Inferred from Them. Alice Shapley (UCLA)

Transcription

1 Observations of High-Redshift Galaxies and What Can Be Inferred from Them Alice Shapley (UCLA)

2 The Big Picture How are stellar mass and structure assembled in galaxies? What are the physical processes driving star formation in individual galaxies? How do galaxies exchange gas and heavy elements with the intergalactic medium? What is the nature of the co-evolution of black holes and stellar populations?

3 Outline High-redshift context High-redshift galaxy selection High-redshift measurements and key results

4 Outline High-redshift context High-redshift galaxy selection High-redshift measurements and key results

5 What do we mean by high redshift? What do we mean by galaxies?

6 (Madau & Dickinson 2014) (Hopkins et al. 2007) z~2-3: Cosmic High Noon This epoch hosts the peak of both star formation and BH accretion activity. Qualitative imprints of local galaxy population (bimodal distribution of colors, strong clustering of red galaxies). Big differences as well: diversity among massive galaxies; absence of cold, quiescent disks; higher specific SFRs; ubiquitous galaxy outflows.

7 z~2 Galaxies Are More Actively Forming Stars (Wuyts et al. 2011) Correlation between stellar mass and SFR, the main sequence of galaxy evolution. The MS evolves towards higher redshift. The SFR for a M sun galaxy will be an order of magnitude higher at z~2. SFRs of 10s of M sun /year commonly observed at z~2, while such systems are rare at z~0.

8 z~2 Galaxies Are More Actively Forming Stars (Behroozi et al. 2013) Correlation between stellar mass and SFR, the main sequence of galaxy evolution. The MS evolves towards higher redshift. The SFR for a M sun galaxy will be an order of magnitude higher at z~2. SFRs of 10s of M sun /year commonly observed at z~2, while such systems are rare at z~0.

9 z~2 Galaxies Are More Actively Forming Stars (Muzzin et al. 2013) (Kriek et al. 2009) z>2 diversity of stellar populations in massive (>10 11 M sun ) galaxies. There is a red sequence, but SF galaxies comprise the majority at high (all?) masses. Contrast wrt z~0.

10 z~2 Galaxies Are Smaller (van der Wel 2014a) Galaxies of fixed stellar mass are smaller at z~2. Star-forming galaxies appear to be characterized by exponential profiles (i.e., disks). Ellipticity distributions suggest a significant fraction (M<10 10 M sun ) may be triaxial.

11 z~2 Galaxies Are Smaller (van der Wel 2014a) Galaxies of fixed stellar mass are smaller at z~2. Star-forming galaxies appear to be characterized by exponential profiles (i.e., disks). Ellipticity distributions suggest a significant fraction (M<10 10 M sun ) may be triaxial.

12 z~2 Disks are Turbulent (Wisnioski et al. 2015) Latest IFU dynamical studies suggests that majority of resolved SF galaxies at M>10 10 M sun are rotating disks (KMOS-3D). A lower disk fraction at lower masses. These disks are turbulent, unlike the Milky Way disk, with typical velocity dispersions of ~ km/s. Origin of turbulence?

13 z~2 Gas Flows are Ubiquitous (Steidel et al. 2010) <z>=2.27 Kinematic outflow signature (redshift offsets, broad Hα emission). Detected in almost all z>2 SF galaxies (in contrast to lower-z results). Plus no apparent correlation between v out and inclination. Suggests z>2 outflows are not collimated! More spherical in geometry? (Image credit: Subaru Observatory)

14 z~2 Gas Flows and the MZR z=0-1.6 z=2.3 z=3.5 (Zahid et al. 2014) (Sanders et al. 2015) (Troncoso et al. 2014) Slope, normalization, scatter, and evolution in mass-metallicity relation (MZR), and Fundamental Metallicity Relation (FMR), place constraints on models of gas outflow/inflow (e.g., Finlator & Dave 2008; Dave et al. 2012)! We are starting to collect larger samples at z>2, showing the evolution of the MZR. The results on the FMR are not very clear (much more at this meeting).

15 z~2 Gas Content is High (Tacconi et al. 2013) CO(3-2) observations of z~2 SF galaxies suggest molecular gas fractions of ~50%, and a linear relationship between molecular gas surface density and SF surface density. Molecular gas fractions of galaxies in the local universe in the same mass range are <10%. Gas fraction inferred at z~1 is ~33%.

16 Outline High-redshift context High-redshift galaxy selection High-redshift measurements and key results

17 A comment on high-z selection Until recently, various tricks were employed to cull high-z galaxies in selected redshift ranges from the masses of faint sources on the sky. These techniques primarily relied on the presence of UV/ optical spectral breaks, strong emission lines, or exceptional luminosities. It is important to understand how the selection techniques affect the sample completeness.

18 Rest-UV Selection (Adelberger et al. 2004) (Steidel et al. 2004) UGR criteria identify Lyman Break Galaxies (LBGs) at z~3. Different UGR criteria identify analogous SF galaxies at z~ Spectroscopic follow-up with, e.g., Keck I/LRIS-B. A few thousand galaxies with spectroscopic redshifts at z= LBG technique and its analogs applied at z>3.

19 Near-IR Selection: BzK Use B-z, z-k color criteria to select both star-forming galaxies and passive galaxies at z>1.4 Incomplete for fainter objects with small Balmer Breaks, weighted more towards fairly massive objects (Daddi et al. 2004) Significant overlap of BzK/ SF with UV-selected samples

20 Near-IR Selection: DRG J-K>2.3 (Vega) criterion meant to select evolved galaxies with significant Balmer/4000 Å breaks at z>2; turns out selection also yields dusty starbursts. (Franx et al. 2003)

21 Narrowband Selection: LAE, HAE (Gronwall et al. 2007) Broad and narrow filters used to identify high-ew emission lines. Objects with strong emission lines identified by red broadband minus narrowband colors (e.g., Lyα at z=3.1). Method for probing objects with significantly fainter UV continua. Technique used to find Hα emitters at high redshift as well. Objects with strong emission also culled from HST/WFC3 grism spectra. The_High-z_Emission_Line_Survey/HiZELS.html

22 Far/Mid-IR Galaxy Selection (Casey et al. 2014) (I. Smail) Originally found by SCUBA on JCMT (1998), Dusty Starforming Galaxies (DSFGs) are identified by their dust continua. Probe obscured SF and AGN activity in the most luminous systems. Also identified with Herschel/SPIRE and PACS, Spitzer/MIPS, WISE, SCUBA-2, others. Debate about origin of huge luminosity (>10 12 L sun ).

23 Photometric Redshift Selection (Skelton et al. 2014) 5 CANDELS Legacy fields have exquisite multi-wavelength datasets spanning from UV through radio, with excellent sampling in observed optical though mid-ir. Some recent works have relied on near-ir (rest-optical, observed HST/ F160W) or IRAC (rest-nir) selection, and photo-z s in a given range (e.g., 3D-HST survey). Use rest-frame UVJ colors to distinguish SF and quiescent galaxies. How complete are photo-z samples? E.g., consider objects without Balmer breaks.

24 Photometric Redshift Selection (Whitaker et al. 2011) 5 CANDELS Legacy fields have exquisite multi-wavelength datasets spanning from UV through radio, with excellent sampling in observed optical though mid-ir. Some recent works have relied on near-ir (rest-optical, observed HST/ F160W) or IRAC (rest-nir) selection, and photo-z s in a given range (e.g., 3D-HST survey). Use rest-frame UVJ colors to distinguish SF and quiescent galaxies. How complete are photo-z samples? E.g., consider objects without Balmer breaks.

25 The Zoo (Zeimann et al. 2014) Different selection techniques probe different swaths in the space of M *, SFR, extinction, age, clustering, with different incompleteness issues. The range of SFRs and number densities spanned by a given population is also magnitude-limit dependent (e.g., UV-selected galaxies are selected down to R=25.5 since much fainter is not practical for Keck spectroscopy).

26 The Zoo (Zeimann et al. 2014) Spanning the range of galaxy selection techniques: UV-selected galaxies have typical stellar mass~10 10 M sun and SFRs~few 10s/year. LAEs are typically more than an order of magnitude less massive, with lower SFRs, though ~25% of UV-selected galaxies are LAEs. DSFGs are slightly more massive than UV-selected objects with significantly higher typical SFRs.

27 The Zoo The situation is not as extreme as the blind men and the elephant. However, when high-redshift results are reported as representing star-forming galaxies at z~2, pay attention to selection technique.

28 Outline High-redshift context High-redshift galaxy selection High-redshift measurements and key results

29 Rest-frame UV Spectra: History (Steidel et al. 1996) First spectral window into high-redshift star-forming galaxies ~20 years ago from Keck/LRIS. Rest-frame UV. z~3.

30 Rest-frame UV Spectra UV cont: O & B stars Stellar: photospheric and wind Outflow-related: Lyα (Δv=+360 km/s), low and high ions (Δv=-170 km/s) Nebular emission: CIII], OIII] Fine-structure emission (Shapley et al. 2003)

31 Rest-frame UV Spectra UV cont: O & B stars Stellar: photospheric and wind Outflow-related: Lyα (Δv=+360 km/s), low and high ions (Δv=-170 km/s) Nebular emission: CIII], OIII] Fine-structure emission (Shapley et al. 2003)

32 Rest-frame UV Spectra UV cont: O & B stars Stellar: photospheric and wind Outflow-related: Lyα (Δv=+360 km/s), low and high ions (Δv=-170 km/s) Nebular emission: CIII], OIII] Fine-structure emission (Shapley et al. 2003)

33 Rest-frame UV Spectra UV cont: O & B stars Stellar: photospheric and wind Outflow-related: Lyα (Δv=+360 km/s), low and high ions (Δv=-170 km/s) Nebular emission: CIII], OIII] Fine-structure emission (Shapley et al. 2003)

34 Rest-frame UV Spectra (Erb et al. 2010) BX418, z=2.30, M*~10 9 M sun Use OIII] 1661,1665/ [OIII] 5007 to infer T e, and direct Oxygen abundance. Use CIII]/OIII] to infer C/O vs. O/H. Uncertainties in dust extinction. See also Stark et al. (2014).

35 Rest-frame UV Spectra UV cont: O & B stars Stellar: photospheric and wind Outflow-related: Lyα (Δv=+360 km/s), low and high ions (Δv=-170 km/s) Nebular emission: CIII], OIII] Fine-structure emission (Shapley et al. 2003)

36 Rest-frame Optical Spectra Emission-line set: [OII], Hβ, [OIII], Hα, [NII], [SII] Ratios of emission lines used to infer a wide range of physical conditions: SFR {Balmer lines} Metallicity (oxygen) {R 23, N2, O3N2, others} Electron density {[OII] and [SII] doublet ratios} Ionization parameter {[OIII]/[OII]} (Kennicutt 1998) Electron temperature {[OIII] ratios} Dust extinction {Balmer line ratios}

37 Rest-frame Optical Spectra At z > 1.4, [OII] moves past 9000Å. Becomes a near-ir problem.

38 Ground-based Limitations Atmospheric absorption limits the accessible redshift ranges for various rest-frame optical features. z~2 is great for getting [OII], [OIII]+Hβ, Hα+[NII]+[SII] simultaneously. z~1.8 and z~2.8. OUT OF LUCK! [OII] is the only strong rest-frame optical line accessible beyond z~3.7.

39 Ground-based Limitations Significant fraction of JHK bands is affected by strong sky emission lines, leading to severe systematics at affected wavelengths. Beyond λ~2.35 µm, thermal background becomes very high. With Keck/NIRSPEC (R~1500), ~1/3 of each bandpass (JHK) is affected by strong sky-lines. The nominal resolution of Keck/ MOSFIRE is R~3600. Much better!! But still an issue. Keck/MOSFIRE Sky spectrum

40 Ground-based Limitations Keck/NIRSPEC H-band Sky Spectrum Significant fraction of JHK bands is affected by strong sky emission lines, leading to severe systematics at affected wavelengths. Beyond λ~2.35 µm, thermal background becomes very high. With Keck/NIRSPEC (R~1500), ~1/3 of each bandpass (JHK) is affected by strong sky-lines. The nominal resolution of Keck/ MOSFIRE is R~3600. Much better!! But still an issue. Keck/MOSFIRE Sky spectrum

41 Rest-frame Optical Spectra: History (Pettini et al. 1998) First near-ir spectra of z>2 galaxies published in 1998, based on UKIRT/CGS4 long-slit spectra. Sample of 5 LBGs. Detections of [OIII] and Hβ. Really need 8-10 meter class telescopes.

42 Rest-frame Optical Spectra: Instruments Long-slit: Keck/NIRSPEC, VLT/ ISAAC and X-shooter, LBT/ LUCIFER, Gemini/GNIRS and NIRI, Magellan/FIRE IFU: VLT/SINFONI Keck/ OSIRIS, Gemini/NIFS From space: HST/WFC3 grism. Multi-object: Keck/MOSFIRE, Subaru/MOIRCS and FMOS, VLT/ KMOS, Magellan/MMIRS. State of the art (seeing-limited) from the ground: MOSFIRE and KMOS Entering into an era of statistical samples of high-redshift galaxies with rest-frame optical spectra.

43 Rest-frame Optical Spectra (Kriek et al. 2015) z~2 MOSFIRE spectra. [OII] in J, [OIII]+Hb in H, Ha+ [NII]+[SII] in K. Stellar continuum is very faint, even for 2 hour exposures with H~22-23 AB. Characteristic sky residuals due to differencing of dithered images. Most spectra have limited spatial information (seeing ~ few to several kpc). Compare to SDSS fiber spectra, measuring innermost (highest- SB) regions of SF galaxies. Calibration across filters is crucial.

44 Rest-frame Optical Spectra (Wuyts et al. 2011) (Shivaei et al. 2015) vs. Star formation and Dust Extinction Balmer line ratios provide estimate of dust extinction (assuming Cardelli curve). Dust-corrected Hα provides estimate of instantaneous SFR. Countless SFR-related investigations possible with robust sample of dust-corrected Hα fluxes. (E.g., scatter in SFR-M* relationship, Fundamental Metallicity Relation). Ultimately combine Hα and IR luminosity to capture completely obscured SF.

45 Rest-frame Optical Spectra Physical conditions Small samples of objects, z>1 star-forming galaxies are offset in the BPT excitation diagram used to separate star-forming galaxies from AGNs. Determine electron density ([OII], [SII]), ionization parameter (O 32 plus models). (Brinchmann et al. 2008) What is the cause of this offset? Implications for abundance determinations. Nature of star formation at high redshift. Stacked J-band spectrum (Steidel et al. 2014)

46 Rest-frame Optical Spectra (Sanders et al. 2015) (Wuyts et al. 2014) Metallicities Typically, O/H at high redshift is determined from strong-line ratios (N2, O3N2, R23, etc.). BPT offset raises red flags. Apparent consensus that galaxies at fixed mass have lower oxygen abundances. No consensus on existence/nature of local Fundamental Metallicity Relation. Do galaxies follow the same if any relationship among M*, Z, and SFR? Z=1.7 Rare, direct O/H measurement from lensed galaxy. (Yuan & Kewley 2009)

47 Spatially-Resolved Measurements IFU (few-several kpc) (Wisnioski et al. 2015) IFU+AO (~1 kpc) Seeing-limited and AOassisted near-ir IFU spectrographs on Keck, VLT, Gemini. WFC3/IR grism. Emission-line intensity and ratio maps. Kinematics (virial, turbulent, outflow). (Law et al. 2009) IFU+AO+lensing (~100 pc) (Jones et al. 2010)

48 Spatially-Resolved Emission Fluxes Hα maps are clumpy, reveal kpc-scale star-forming complexes. Spatially-resolved line ratio maps suggest BPT offset holds on sub-kpc scales. (Forster Schreiber et al. 2011b) Measurements of metallicity gradient slopes are inconclusive. (Jones et al. 2013) (Jones et al. 2010)

49 Spatially-Resolved Dynamics Emission-line maps can be used to study the velocity fields in high redshift galaxies. (Wisnioski et al. 2015) c Virial motions (rotation, dispersion) and dynamical masses. Turbulence (dispersion). Outflows (broad components, clumps, nucleus, off-nucleus). ZC (z=2.19), Hα map (Newman et al. 2012a)

50 Summary Rest-UV and optical spectra of high-redshift galaxies offer rich probes of stars, gas, and dust. To date, most spectroscopic observations represent spatially integrated quantities, or quantities with few to several kpc resolution. IFU, IFU+AO, IFU+AO+lensing (or WFC3 or WFC3+lensing) offer spatially-resolved probes of the stars and gas emission and dynamics. Significantly larger samples of both rest-optical and rest-uv spectra should provide powerful insights into the detailed nature of high redshift galaxies.