High-Redshift Galaxies - Exploring Galaxy Evolution - Populations - Current Redshift Frontier

Transcription

1 Lecture 20; Nov 06, 2017 High-Redshift Galaxies - Exploring Galaxy Evolution - Populations - Current Redshift Frontier Pick up PE #20 Reading: Chapter 9 of textbook I will hand back HW#7 Wednesday: Second 30-min test Continue working on your final project (presentations due Nov 15/20). If you have questions, let us know.

2 A few general notes on the final project Make sure to focus on clear explanations, in language that other audience members can clearly understand and follows. Only include references you actually read and use them to back up what you say. Be selective. Make sure to explain all terms, equations, figures etc. that you use, to make sure your audience can follow your presentation.

3 History of the universe

4 Starting Point: the present-day galaxy population Different properties of individual galaxies are strongly correlated formerly known as the Hubble sequence M *, L, M/L, t age, SFR, size, shape, [Fe/H], s, v circ Mass is the decisive parameter in setting properties M * or M halo Most stars live in massive galaxies ( M sun ) Most massive galaxies don t form stars anymore ( early types ) 1000 M sun < M * (galaxy) < M sun Galaxies and the DM Cosmogony can match galaxies ß à halos by abundance or clustering àvastly different efficiencies in turning baryons into stars, peaking at M halo ~ M sun LCDM: massive halos (+ galaxies) preferentially found in dense & early-collapsed regions.

5 Exploring Galaxy Evolution: Approaches How did the present-day galaxy population come into being? Evolution of individual galaxies is not observable! Evolution of population properties is observable. Experiment: Look-back observations (high redshift) Fine enough time/redshift-resolution to see gradual population changes Modeling: how well can galaxy population properties be explained by: Initial conditions: density fluctuations and cosmological parameters Non-linear (hydro-) dynamical simulations + sub-grid-physics Semi-Analytical Models (SAMs, 1990s): Merger-driven galaxy formation in a dark matter-dominated universe SAMs give a plausible quantitative description of the end-product of the galaxy population at z~0, under the assumption of efficient feedback (stellar & AGN)

6 Why detailed empirical data are needed Good ab-initio cosmological models exist, describing: Initial fluctuation spectrum W tot, L, W b, H 0 Growth of structure But the baryonic component is trouble: sets observable galaxy properties physics on 1 M sun and M sun scales strongly coupled models barely getting good at post-diction

7 To study galaxies at early epochs (=high redshift), one has to find them first Distant galaxies are faint à deep fields (Hubble Deep Field, Chandra Deep Field South/H-UDF, COSMOS, UDS, ) Foregrounds dominate à Need pre-selection technique i<22.5 mag i<24 mag Le Fevre et al. 2003

8 Boris s 3-fold image

9 Techniques & Classifications The last decade+: in-situ observations allow direct (and even spatially resolved) studies of galaxies during their formation epoch Lyman-α: 1215 Å Lyman Alpha Forest (LAF) Gunn-Peterson effect (IGM absorption) Lyman Alpha Emitters (LAEs) Ly-a line Lyman Alpha Blobs (LABs) extended Ly-a emission regions Dropouts /color selection Lyman Break Galaxies (LBGs) BzK, BX/BM, etc. Special objects Exceptionally luminous objects (e.g.: radio galaxies, QSOs, ULIRGs, submillimeter galaxies etc.) Hosts of Gamma Ray Bursts (GRBs) Gravitationally lensed galaxies See, e.g., Ellis 2007, Saas Fee lecture, for an introductory review (on the arxiv)

10 Lyman Alpha Forest

11 The first steps in finding high-z galaxies Lyman Break Technique (Steidel 1996) (ionizing photons) LyC Ly-limit Ly-a Ly-break galaxy (LBG) massive star massive star+ism Identified by colors of (rest frame) FUV around 912 Å Lyman continuum discontinuity Star-forming but otherwise normal galaxies at z > 2.5 Ly-a forest massive star+ism+igm UGR filters From the ground, we have access to the redshift range z=2.5-6 in the µm range

12 High Redshift Galaxies: K correction

13 Redshifted spectra For a set of objects of known spectral characteristics: Precise photometric redshifts are possible

14 Photometric Cuts: Predictions and Practice Expectations Real Data (10 field) Spectral energy distributions allow us to predict where distant SF galaxies lie in color-color diagrams such as (U-G vs G-R) (Steidel et al. 1996)

15 LBGs vs. BX/BM galaxies Fine-tuning of LBG technique: Different UGR colors for different redshifts LBG LBG BX BM classical LBG: z~3 and higher BX: z~ BM: z~ Þ Tuned to fill the classical redshift desert where few galaxies were known Þ LBG and BX/BM are often lumped together as a population Steidel et al. 2004

16 LBGs: Spectroscopic Confirmation

17 What kind of galaxies are LBGs? Population synthesis modeling & spectra: data fit continuous star formation models with range of ages ( Myrs), stellar masses ( M sun ), and metallicities (0.3 to >1 solar), IMF~Salpeter/Chabrier for >10 M sun Pettini et al. 2000, Shapley et al. 2003, 2005, Erb et al. 2006abc

18 Properties of Lyman Break Galaxies (z~3) <age> = 320 z = 3 <M * > = ~2 x M <E(B-V)> =0.15 A UV ~1.7 ~5 <SFR> ~ 45 M yr -1 Extinction correlates with age young galaxies are much dustier SFR for youngest galaxies average 275 M yr -1 ; oldest average 30 M yr -1 Objects with the highest SFRs are the dustiest objects Shapley et al ApJ 562, 95

19 Composite Spectra: Young versus Old Young LBGs have much weaker Lya emission, stronger interstellar absorption lines and redder spectral continua Þ dustier Galaxy-scale outflows ( superwinds ), with velocities ~500 km s -1, are present in essentially every case examined in sufficient detail Shapley et al ApJ 562, 95

20 Lyman Break Galaxies: Summary Period of elevated star formation (~100 s M yr -1 ) for ~50 Myr with large dust opacity Superwinds drive out both gas and dust, resulting in more quiescent star formation (10s M yr -1 ) and smaller UV extinction later on Quiescent star formation phase lasts for at least a few hundred Myr; by end at least a few M of stars have formed All phases are observable because of near-constant far-uv luminosity (decreasingly dusty towards older age/lower SFR)

21 Lyman-a emitters (LAEs) (broad-band)-(narrow-band) Spectroscopic follow-up of candidates Þ Tend to be less massive, fainter subpopulation of LBGs [contaminants: lower-z emission line galaxies] 5007Å 3727Å 1216Å Compare signal in narrow-band filter with broad-band signal

22 Lyman-a Blobs (LABs) Giant blobs of Ly-a emission Ly-a Blob of Hydrogen gas X-ray: AGN Commonly tens of kpc or more across Winds/outflows driven by star-forming galaxies X-Ray+optical+IR Lyman-a continuum

23 LBGs: Extended Lyman-a Emission UV continuum Lyman-alpha Lyman-alpha blobs (rare) Steidel et al.: Stacking of z=2-4 LBGs Lyman-alpha shows evidence emission in z=2 to 4 that galaxies extended extended, Ly-alpha emission common is to common all galaxy types. Þ galactic-scale outflows are common at high z!

24 Passively-Evolving Galaxies? LBGs/LAEs are star-forming galaxies Availability of panoramic IR cameras opens possibility of locating non-sf galaxies at high z Termed variously: Extremely Red Objects Distant Red Galaxies depending on selection criterion A break at z=2.5: at 1.4µm Such objects would not be seen in Lyman-break samples for z ~ 1-2: select on I-H color for z > 2: select on J-K color

25 Objects with J-K > 2.3 Surprisingly high surface density: ~0.8/arcmin 2 to K=21 (two fields) ~2/arcmin 2 to K=22 (HDF-S) ~3/arcmin 2 to K=23 (HDF-S) van Dokkum, Franx, Rix et al.

26 Characteristic Properties of Distant Red Galaxies (Franx et al. 2003, van Dokkum et al. 2004, Foerster-Schreiber et al. 2005, Labbe et al. 2005) Epoch: z~2.5 SED fitting to get M*, SFR,t dust M * ~5x x10 11 M sun Nearly as massive as most massive galaxies today Contain the bulk of stars at those epochs Star-formation rate ~ M sun /yr Dust extinction important A V ~2 mag SFR cross-checked with thermal-ir For SFR ~ e -t/ t à tfit ~500Myr à Mass build-up: SFR x t ~ M sun DRGs: Massive, but often not passive, but dusty.

27 Distant Red Galaxies: Spectroscopy z=2.43 z=2.43 z=2.43 z=2.71 z=3.52 van Dokkum et al.

28 Redshifted spectra B, z, K bands at z=

29 BzK selection of passive and SF z>1.4 galaxies New apparently less-biased technique for finding all galaxies 1.4<z<2.5 sbzk: star forming galaxies pbzk: passive galaxies (z-k) overlap between different samples is fairly high at same Ks criteria >90% of BX/BM at bright levels (~10 11 M sun, Ks<20) are s-bzk BX/BM are low-obscuration subset of s-bzk less overlap at fainter levels DRGs are more of a mixed bag, include passive galaxies and appear to frequently select AGNs (B-z) Daddi et al ApJ 617, 746

30 Lyman breaks or dropouts at higher z z-dropout Stanway et al. (2003) Traditional dropout technique poorly-suited for z > 6 galaxies: - significant contamination (cool stars, z~2 passive galaxies) - spectroscopic verification impractical below ~few L* i-drop volumes: UDF ( ), GOODS-N/S ( ), Subaru (10 6 ) Mpc 3 flux limits: UDF z<28.5, GOODS z<25.6, Subaru z<25.4

31 Contamination from z~2 Passive Galaxies Addition of a precise opticalinfrared color (z - J) can, in addition to the (i - z) dropout cut, assist in rejecting z~2 passive galaxy contaminants. (i z) 5.7 < z < 6.5 z~2 passive galaxies This contamination is ~10% at z~25.6 but is negligible at UDF limit (z~28.5) (z J)

32 Contamination by Galactic dwarfs - more worrisome UDF z<25.6 L dwarfs E/S0 HST half-light radius R h more effective than broad-band colors Contamination at bright end (z<25.6) is significant (30-40%)

33 Keck spectroscopy of i-drops: 10.5 hrs z AB <25.6 z=5.83 Ly-a L-dwarfs contaminate at bright end

34 Spectroscopy: The Current Frontier Finkelstein et al. 2013: LBG with Ly-a emission line ID-ed at z=7.51 (6 th at z>7) Corresponds to an epoch 700 million years after the Big Bang Looked at 43 candidate z~8 galaxies from HST, only confirmed this one Þ difficult, but possible endeavor with new near-ir multi-object spectrographs Nature N&V; Riechers 2013

35 Spectroscopy: The Current Frontier Oesch et al. 2015: z spec = 7.73 galaxy identified Zitrin et al. 2015: z spec = 8.68 galaxy identified From same sample (4 galaxies), also confirmed one at z spec = 7.47 All are very bright, why sudden, high confirmation rate?

36 Spectroscopy: The Current Frontier Roberts-Borsani et al. 2015: selection based on bright, red Spitzer/IRAC colors (3.6 vs. 4.5 µm) Þ implies strong Ha and [OIII]+Hb emission lines Þ selects bright, intensely star-forming galaxies à perhaps also high Ly-a escape fraction??

37 The Future: z=10, and beyond H-UDF Ellis et al. 2013: revised photo-z to z~12 Bouwens et al. 2010, submitted Three z~10 candidates at >5s Brammer et al. 2013: Possible 2.7s line Would be more consistent with z~2.2 interloper Bouwens et al. 2010, final version One different z~10 candidate (after including more data) Capak et al. 2013: No line seen as bright as Brammer, but possible faint 2.2s line

38 Strongly Lensed Candidates CLASH (Cluster lensing) z=9.6; Zheng et al z=10.7; Coe et al. 2013; JD1 3: Lensed images JD1+JD2 Gonzalez, Riechers et al. 2014: no [CII] at z~11

39 A galaxy at redshift 11? Oesch et al. 2016: z spec = galaxy candidate 12 orbits of HST time for spectrum 0.6 +/- 0.3 kpc across M * ~10 9 M sun SFR: 24 M sun /yr Much brighter than expected Þ Possible rare bright outlier Þ Requires confirmation: JWST!