South by High Redshift Conference

Conference website: Austin, TX.

(Day 1) Wednesday, 01 April 2015

Jump to: (Day 2) (Day 3)

Stacy – Influence of Minihalo Environment on the Growth of Pop III Clusters

  • Exploring ~10 Pop III star formation sites in a 1 Mpc/h box, focusing on multiplicity of Pop III clusters.
  • Total mass in sink particles are in the range 20-120 solar masses.
  • Binary fraction of 36%, flat IMF (alpha = -0.17) looking after 5000 years.
  • Some sink particles are ejected from the inner cluster and may have low masses (< 5 solar masses).  AGB stars?

Johnson – Have we found Population III?

  • (Yoshii 1981; Iben 1983; Frebel+ 2009; Komiya+ 2010) Has been long recognized that Pop III stars could accrete metal-enriched IGM, masking their true nature.
  • (Johnson 2014) Accretion onto a Pop III star. Dust grains can absorb more stellar radiation than gas. As gas accretes, dust can be expelled by radiation pressure.
  • Expected chemical signature would not have elements that were depleted into grains.  High Zn and low Ti abundances.
  • However, a stellar wind and magnetosphere could suppress accretion.

Yoshida – Formation of massive primordial stars and black holes

  • (Aoki+ 2014) Detection of a 2nd generation star? Low alpha/Fe, Co/Fe. Signatures of a pair-instability SN?
  • 100 First Stars (Hirano+ 2014): There are several wide binaries. Top-heavy IMF, favoring >100 solar masses. Highly rotating cores result in low-mass (40 solar masses) stars; low-spinning cores result in high-mass (300 solar masses) stars, rapidly accreting.
  • 1540 Primordial Stars (Hirano+ 2015): Pop III.2D stars are clouds that are exposed to a LW background
  • Direct Collapse BH through Burst Accretion: Possible to get direct collapse from the supermassive stars in rapid accretion events.

Schneider – Decoding the stellar fossils of dusty star formation at the highest redshifts

  • (Schneider+ 2014) Sources of dust: dust production versus progenitor masses.
  • (Ventura+ 2012ab, 2013, 2014): Dust yields from AGB stars between 1-8 solar masses at low metallicity.  Uncertainities from mass loss and convection borders.
  • AGB contribute up to 50% of the dust in 300 Myr.
  • (Schneider+ 2015) Inspecting the dust-to-gas ratio to metallicity relation, which has a jump from the effect of grain growth increases at some critical metallicity around [Z/H] ~ -1.5.
  • (Mancini+ in prep) Studying the dust mass in z ~ 7 “normal” star forming galaxies, especially the dust evolution and contributions from SN and AGB stars.
  • Dust mass depends sensitively on ISM density, so simulations with dust are required to shed further light on dust mass evolution.

Safranek-Shrader – Resolving Star Formation in the Early Universe

  • Coupled simulations, extracting a halo from a GADGET simulation and re-simulating with FLASH.
  • Gas cools down to the CMB temperature.  The dust and gas temperatures are coupled above 10^8 cm^-3.  The gas becomes optically thick around 10^10 cm^-3, suppressing fragmentation.
  • Accretion rates are variable in high mass sink particles.  Low-mass sinks are terminated by dynamical processes.
  • Hints at a power-law IMF?

Jeon – Formation of the First Galaxies under Stellar Feedback

  • Stellar feedback evacuates gas in halos with masses < 10^8 solar masses.
  • 20-30% of the volume is ionized by z ~ 10.  Most of the radiation has come from Pop III stars at this time.
  • Metallicity floor of [Z/H] ~ -4. No primordial gas exists in the target halo.
  • CC SNe are the main metal contributors in the target galaxy.  Pop II and III stars can coexist, but Pop II stars dominate.

Zackrisson – Searching for Population III Galaxies at High Redshifts

  • (Zackrisson+ 2015) Extreme lensing (µ ~ 1000) will allow for the detection of tiny stellar systems of M ~ 10^4 solar masses.  Possible to get IMF constraints.
  • (Zackrisson+ 2011) Yggdrasil code. Population synthesis code, including Pop III stars.
  • (Rydberg+ 2015ab) Inspecting the CLASH field for Pop III galaxy candidates.

Griffen – The Caterpillar Project

  • Focusing on 24 MW-like halos at z=0 with a mass resolution of 3 x 10^4 solar masses.
  • (Snyder+ 2015) There is some morphology dependence (bulge-dominated vs. disks) on merger history.  Not all major mergers are created equally, depends on the cold gas fraction and orientation of the merger.
  • Interactive tool to create initial conditions and submit them to clusters.
  • Convergence down to V_max ~ 5 km/s, resolving down to 10^6 solar mass halos.
  • Infall times and merger history – delineates between being quenched before or after infall.

Natarajan – Formation of SMBHs at High Redshift

  • Stressing that an SMBH outlier at z = 6 isn’t a problem with LCDM because it may be a rare object.  Once you find a large population of SMBHs, then it becomes a problem.
  • (Natarajan+ 2015) Timing challenge on rapid accretion to grow the biggest SMBHs within 1 Gyr.
  • (Alexander & Natarajan 2014) Super-Eddington accretion when stellar-mass BHs are orbiting in a dense gas cloud.
  • (Volonteri & Natarajan 2012; Natarajan+ 2015) Lower BH occupation fraction in low-mass galaxies; dramatic differences with respect to seed masses.

Glover – Forming black holes by direct collapse: what determines the critical UV flux?

  • Astrophysical uncertainty for J_crit: Strength of the X-ray background, SED of the background, halo-to-halo scatter.
  • Modeling uncertainties: Composition of chemical network, rate coefficients, treatment of H_2 self-shielding
  • (Glover+ 2015) Determined the minimal chemical network for direct collapses.  A key reaction that was missing was the collisional ionization of H by H:
    • H + H -> H+ + e- + H
    • Dominates when the electron fraction is small.  Omitting this reaction changes J_crit by a factor of two.
  • (Sugimura+ 2014) Investigated different H2 self-shielding prescriptions.  Draine & Bertoldi (1996) assume rotationally cold H2; Wollcott-Green+ 11 assumes LTE.  How to calculate the H2 column density?
  • Open questions: Influence of incident SEDs? Treatment of self-shielding?

Agawaral – Current State of Affairs: The Direct Collapse BH Scenario

  • J_crit depends on the SED of the background – determines the photo-detactment of H- and photo-dissociation of H2.
  • Parameter study of J_crit by exploring different rate coefficients, searching for the rates that can produce a DCBH.  The galaxy that’s providing the incident radiation 5 kpc away.
  • Exploring different SEDs, finds that a galaxy with 10^8.5 solar masses with an age of 1 Myr can provide enough radiation to prevent H2 cooling (J_crit > 700 for Starburst99).
  • No “unique” value of J_crit: depends on the age, metallicity, SFH of the nearby galaxy.

Whalen – The Evolution of Supermassive Black Holes Over Cosmic Time

  • (Whalen & Fryer 2012) Low-mass BHs (core-collapse) often have kicks from an asymmetric explosions, further increasing the difficulty of subsequent accretion.
  • Using a (100 Mpc)^3, identified a high-sigma peak (10^9 solar mass halo) at z=18. Considered X-ray feedback with a monochromatic spectrum of 1 keV.  10 AMR levels (300 pc resolution). Subgrid alpha-disk model of accretion.
  • X-ray feedback suppresses mass accretion by a factor of six.

Trump – Revealing Black Hole Seeds by the Fossil Record in Local Dwarfs

  • Did the M-sigma relation form at high-z or through mergers (mean limit; Janske+ 2011)?
  • (Sun+ 2015) The relationship is already in place by z=2 in massive galaxies.  QSO duty cycle ~10%.
  • (Wilcott+ 2015) M-sigma is mostly in place at z=6 with a large scatter in massive galaxies (V_rms > 100 km/s).
  • (Trump+ 2015) Using line ratio AGN selection techniques to search for AGN in low-mass galaxies in SDSS (320,000 galaxies).
  • Coeval SMBH growth and star formation in massive systems.
  • Below a stellar mass of 10^10 solar masses, the SMBH occupation fraction drops from unity to ~10%.
  • (Trump+ 2011, 2014) Use spatially-resolved BPT to separate the nuclear AGN from the extended HII regions.
  • Q&A: M_BH – M_star relation may form before M_BH – sigma.

Ferrara – Ultra Faint Galaxies

  • (Ali+ 2015) Paper-64 result: constraints that reionization is not cold (must be photo-heated and photo-ionized).
  • (Mesinger+ 2013) Three peaks in the 21-cm power spectrum at k = 0.1 Mpc/h, corresponding to the peaks in absorption, strong absorption, and emission.
  • (Mesinger+ 2015) Can use the number density evolution of Lyman-α emitters to constrain reionization; however, one must consider the co-evolution of galaxy properties and reionization.
  • (Yue+ 2014) Estimated number of galaxies in Abell 2744 with lensing. 80% of the ionizing photons coming from halos <10^9 solar masses. May have already detected 20-30 of these galaxies in the Frontier Fields.
  • Cosmic IR background provides another constraint on the first galaxies.

Becker – Patchy Reionization and the Ionizing Photon Budget

  • (Roberston+ 2015) Reconciles lower τ with galaxies forming later.
  • Extending a simple reionization model to z = 2.5, one finds 19 LyC photons / atom / Gyr from stars only, however observations only show 4 LyC photons / atom / Gyr from stars and AGN.  Suggests that galaxies are evolving.
  • To count photons in the background, need to measure the opacity (Becker+ 2013), temperature (Becker+ 2011) and mean free path (Worseck+ 2014) of the IGM.
  • (Becker & Bolton 2013) Ionizing photon density doesn’t evolve from z= 2.5 to 5.0.  However, the SFRD is decreasing with redshift.  However, pushing this to z=6, then the UVB drops by a factor of ten.
  • At z ~ 6, there is large scatter in Lyman-alpha opacity from the density field alone because it’s biased toward overdense regions with QSOs.
  • (Becker+ 2015) ULAS J0148+0600 (z=5.98): Huge GP trough between z = 5.52 to 5.88 (110 Mpc/h). But there is Lyβ transmission.  Mainly this shows that there is great variation in the UVB at this time. Can this originate from density variations alone? No! Need a factor of 3 variations.
  • With a fixed mean free path and galaxy sources in a 100 Mpc/h simulation, there is a ratio between UVB with different mfps. No simple model fits the data.
  • Upper limit of the mfp at z=5.5 is ~40 Mpc/h with 4 photons / atom / Gyr.  To explain the opaque QSO lines of sight, a mfp of 10 Mpc/h fits the data but with a higher ionizing photon density (19 photons / atom / Gyr).

Bullock – Did faint galaxies reionize the universe?

  • (Boylan-Kolchin+ 2014) High-z galaxy LF must break at M_UV ≥ -14 from near-field constraints.
  • Through abundance matching, one can recover a relationship between UV magnitudes and halo mass. Simulations (Wise+; Finlator+) lie on these relationships.
  • (BK+ 2014) ~170 atomic cooling halos at z=8 in a LG Lagrangian volume. Over-predict galaxies with >2 x 10^5 solar masses of ancient stars.
  • Note: z=8 progenitors of MW/M31 have M_UV ~ –16 from abundance matching.

Dressler – The Contribution of Faint LAEs to Reionization

  • LAEs alone appear to be supplying a large fraction of the LyC photons in reionization. LyC escape = 20% (Gronke+, Garel+ 2015).

Alavi – Understanding The Likely Sources of Reionization by Studying The Faint Lensed Star-forming Galaxies at 1<z<3

  • Investigating the number density of dwarf galaxies at z = 1-2. The faint-end slope is around -1.5 with no turnover down to M_UV = -12.
  • There is some general trend to bluer spectra (beta – UV slope) in fainter galaxies. The intrinsic scatter still exists in low-luminosity galaxies at z ~ 1.
  • (Alavi+ in prep) Using the FIRE simulations, they calculate the UV slope versus M_UV for several galaxies. The scatter is created by bursty star formation (i.e. redder/bluer during quiescent/active periods) even without dust attenuation.

Tilvi – Probing the Epoch of Reionization

  • Studying the evolution of Lyman-alpha equivalent width. Missing high EW galaxies at z ≥ 7. Neutral IGM absorption? Need to move to other probes, such as C III].
  • Only ~20% of galaxies have Lyα EW > 25Å at z > 6.
  • (Treu+ 2012, 2013) Using toy models, the data favors models number density evolution, i.e. only the brightest sources can penetrate through a neutral IGM.

(Day 2) Thursday, 02 April 2015

Jump to: (Day 1) (Day 3)

Shapiro – Simulating Cosmic Reionization and Its Observable Consequences

  • Now using the CLUES simulations to conduct rad-hydro runs. Questions: Does reionization help with the missing satellites problem? Was the Local Group ionized internally or externally?
  • Using RAMSES, coupled with ATON to perform the simulations.
  • Reionization occurs at z ~ 5, which only includes radiating sources within the LG.  However, this goes against the previous work of Ocvirk+.

Park – Impact of Cosmic Reionization on Small-Scale Structures : Clumping Factor

  • (Park+ in prep) Examining the clumping factors on scales smaller than a comoving Mpc.
  • Use a 200 kpc/h box with 50 Msun DM resolution to study the clumping factor (cf. Emberson+ 2013) with an external UVB with J_21 = 0.1, 1, turned on at z = 10, 9, and 8.
  • After 200 Myr, the clumping factor decreases from 10 to 3.
  • Use the number of recombinations (relative to the mean rate) as a function of density as a subgrid model in large-scale reionization simulations.

Dijkstra – Constraints on Reionization at z=6-7 from Lya Emitting Galaxies

  • The Lya fraction and LAE luminosity function both have reductions at z ≥ 6. Why?
  • Lya transfer is a multi-scale problem (100 Mpc -> galaxy scales -> scattering in a dusty medium). The gas kinematics are very important in Lya transmission. Must consider neutral absorbers within large-scale HII regions.
  • To explain the reductions, models need an ionized fraction of 50% at z=7.
  • (Gronke+ 2015) Calculated the faint-end slopes for LAEs and LBGs, and they differ by 0.4.  The LAEs are probing fainter galaxies.
  • Are Lya and LyC escape fractions correlated? The slope of the LAE LF could indicate the emissivity of LyC photons.

Pober – A Lower Limit on the z = 8.4 IGM Temperature From 21 cm Power Spectrum Observations

  • In the cold reionization scenario, a small spin temperature can dominate the power spectrum.
  • Using 21cmFAST simulation to explore spin temperature and ionized fraction parameter space. Rules out a cold reionization scenario.
  • PAPER has finished observations and work has started on HERA.

Mutch – Modelling galaxy formation and the EoR with DRAGONS

  • Coupling a semi-analytic galaxy formation model with a semi-numeric model (21cmFAST) of reionization.
  • Calibrate the SA model against star mass functions at z=5-7 (Duncan+ 2014)
  • Reionization at z = 8 ± 1, given a factor of two uncertainty in escape fraction.
  • Matches galaxy LFs very well at z=6 and z=8, even though it wasn’t calibrated to do so.
  • Will include AGN feedback in the near future.

Finkelstein – A Review of Observational Galaxy Studies at z ≥ 4

  • Current space programs: HUDF, (S-)CANDELS, CLASH, Frontier Fields, Spitzer, GOODS, SEDS
  • What have we learned?
    • The good news: 2-sigma agreement between different groups on LFs (Finkelstein+ 2014)
    • A surprise! The knee in the Schechter function does not evolve, but the faint-end slope becomes steeper and the number densities decrease with increasing redshift.
    • Very little data at the bright end at z=7 and z=8. Cannot determine whether it is a power-law or Schechter function.
  • (Finkelstein+ 2015) Same luminosity galaxies exist in smaller halos at higher redshift. Why?
    • Increased gas density; more cold gas; effects of feedback lesser at high-z.
    • Halo masses = 10^11.4 (z=7) and 10^11.9 (z=4) solar masses
  • Full analysis of the first-year Frontier Fields data does not show a sudden drop-off in cosmic SFR density (McLeod+ in prep)

Bowler – Rapid evolution in the bright end of the galaxy luminosity function between z = 5-7

  • Using a survey field that is 8x the CANDELS field down to AB magnitudes of 25-27, depending on the filter.
  • There has been disagreement at the bright-end between space- and ground-based surveys.
  • This was caused by cosmic variance between the fields by a factor of three.  Showing the need of >degree-scale fields to probe the bright-end of the LF.
  • Models indicate strong dust obscuration may be necessary to reproduce the observations (Bowler+ 2014).

Curtis-Lake – Can we tell if Lyman Break Galaxies are getting bigger between redshifts 8 and 4?

  • (Ono+ 2013; Shibuya+ 2015) Size evolution of LBGs from z = 2 to 8 from 1 kpc by a factor of ~3.
  • (Curtis-Lake+ 2014) Looks that size scales somewhere between the halo circular velocity and halo mass.  The peak of the size distribution doesn’t change with redshift, but the large-end tail builds up over time.
  • However, using a null hypothesis of redshifting the z=4 galaxies to greater distances, a size evolution of (1+z)^n of both n=-1 and 0 cannot be ruled out.

Bradač – Pushing the Frontiers of Galaxy Formation with Spitzer and Cluster Lenses as Cosmic Telescopes

  • (Wang+ in prep) There can be some discrepancies between photometric and spectroscopic redshifts in lensed multi-imaged systems.
  • Use Spitzer at z ~ 8 to break degeneracies in ages to measure the Balmer break.  New survey called SURFS-UP covering 10 clusters.  Spitzer observations are important for preparation for JWST.
  • (Hoag+ 2015) z = 9.5 galaxy with an estimated stellar mass of 10^8.8 Msun, SFR = 1 Msun/yr, ~350 Myr ages. There are several other detections with stellar masses between 10^8 and 10^9 solar masses. Also found sSFRs around 10 Gyr^-1 with spread (and errors) of an order of magnitude.  No trend with SFR.

Livermore – Pushing the Frontiers: uncovering the earliest galaxies in the Hubble Frontier Fields

  • Use wavelet decomposition to remove the light from the clusters.
    • Convolve the image with a large wavelet, subtract it, and repeat with progressively smaller wavelets.
  • Found 95 galaxies in the Abell 2744 between z = 6-9, mostly focusing on z=7 galaxies.
  • Finding a possible faint-end turnover at M_1500 == -16 at z=7 in one cluster. Warned that there are many uncertainties at these scales because of incompleteness and magnification errors.

Song – Probing stellar mass build-up in galaxies at z=4-8 with CANDELS and S-CANDELS

  • Using HST + Spitzer data (sample size ~ 7000 with ~300 at z=7) to constrain the stellar mass function.
  • Finds a constant scatter of 0.4-0.5 dex in the stellar mass – UV luminosity relation. No evolution in slope with redshift.
  • At z=4, the stellar mass function is depleted from the halo mass function, but at higher (z ≥ 6) redshifts, it’s closer to the halo mass function. Feedback in action?
  • Stellar mass function is in agreement with the cosmic SFRD.

Salmon – The SFR and Stellar Mass evolution at z>4 from CANDELS

  • UV faint, high-mass objects with significant reddening.
  • The SFR – stellar mass relation evolves little in slope and declines in scale at z > 5.
  • The scatter in SFR at a given mass is small at all redshifts (0.2-0.3 dex). Favors smooth, cosmological accretion into galaxies.
  • The SFR function steepens with redshift, becoming increasingly like a power law at z ≥ 5. This could possibly mean the mode of SFR is changing going into reionization.

Sajina – Massive galaxies at z~1-6 as seen in Spitzer-SERVS and Vista-VIDEO data

  • Most of the detected galaxies are dusty and actively forming stars.
  • They find 8 z=4-5 galaxies with stellar masses >10^11 solar masses. Need spectroscopic confirmation of all of the detected galaxies. Where are the dusty galaxies at these earlier times because they exist at z = 2-4.

Greiner – Exploring hidden star formation over 3<z<5 with Gamma-Ray Burst Host Galaxies

  • There are over 400 GRBs with a known redshift with 65 in z = 3-5 and 21 with a host galaxy detected.
  • The UV LF is inconsistent with GRB data. They find no evidence for selection effects, such as metallicity bias. To match the LF, need a model with little metallicity dependence.

Note: Missed talks by Rigby (The James Webb Space Telescope and the High Redshift Universe) and Illingworth (The First Billion Years: The Growth of Galaxies in the Reionization Epoch).

Oesch – Stellar Mass Measurements at the Cosmic Dawn: Insights from Ultra-Deep HST and Spitzer Observations

  • (Gonzalez+ 2011, in prep) Stellar Mass – M_UV relationship: shallow when considering a simple SED fit.  Highest mass systems are not the brightest in the UV because the dust saturates the UV light.
  • Recently awarded GREATS survey with 733-hour exposure time.
  • (Oesch+ 2015) Spec. confirmed galaxy at z = 7.7 with Lya.
  • New survey (HDUV) to provide rest-frame UV for z=0.5-3 galaxies, possibly analogs of high-z galaxies.

Bouwens – Rest-frame Optical Nebular Emission Lines in z~4-8 Galaxies

  • (van der Wal+ 2011) There have been extreme emission line galaxies with EWs ~1000Å at z ~ 1.
  • With H-alpha, H-beta, [OIII] fluxes, you can measure SFR, metallicities, and dust extinction.
  • See Capak+ (2015) for recent data on dust corrections.
  • Found 11 bright z=6-9 galaxies in the CANDELS field to follow up spectroscopically. Looked at one, and it was, lo and behold, a z=7.7 galaxy (mentioned in Oesch’s talk).

Schmidt – GLASS: Probing the state of the reionization epoch through HST spectroscopy

  • Select dropouts at z ≥ 7 in six of the Frontier Fields and classified the objects according to several selection rules. Using the Treu+ (2012) framework with equivalent widths.
  • NIR flux limits are around 10^-18 erg/s/cm^2, resulting in 100 EW limits at z ≥ 7 from GLASS. This is evidence for strong evolution as the universe is reionized.

Sobral – Exploring the z~6-9 Universe with the largest Lyman-alpha narrow-band surveys

  • (Sobral+ 2015; Matthee+ 2015) Detection of HeII 1640Å at z = 6.6, and the emission is very compact. HeII / Lya = 0.3 with Lya EW = 230Å, however there is no CIII] or OIII]. No other lines!
  • The Lya/HeII is coming from one blue clump, and there is a redder separate component.

(Day 3) Friday, 03 April 2015

Sorry, but I had to go back to Atlanta due to other commitments!

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