GALAXIES 626 Spring 2007 Introduction: What do we want to learn?
The light of the universe is in discrete chunks: Why?
How did we get from the tiny density fluctuations at recombination
to the beautifully ordered structures we see today M87 M110 E1 E6
Galaxies are complex Made up of: Dark matter halos Old and Young stars Gas and Dust Supermassive black holes
so how galaxies form and how present galaxies work M96 M91 M100 SBb Sa Sc
Feedback is the key element The structure of a galaxy is a complex function of the interaction between these components. Understanding galaxies requires us to understand how dynamical and energetic feedback processes work against gravity
We also need to study galaxies at many wavelengths Appearance of galaxies is strongly dependent on which wavelength the observations are made in. e.g. the nearby galaxy M81 X-ray UV Visible Near-IR Far-IR Note: large change in appearance between the UV and the infrared images. In the UV/Optical we see the star formation directly. In the Far infrared we see heated dust. X-rays are produced by the nucleus and a small number of binaries
Sllva et al. 1998 Different galaxies emit most of their light at different wavelengths ULIG LIG OPT FIR spiral Elliptical
Galaxy Bolometry To understand the star formation and accretion history of the universe we need to have a complete census of galaxies at all wavelengths so we see all of the energy production
So the goals of the class are.. To understand the physical processes which govern the formation of galaxies To relate these physical processes to the observations of galaxy evolution at all wavelengths To understand the structure of present day galaxies and what this tells us about the universe
The universe of galaxies In the first two lectures we will describe the universe of galaxies The extragalactic background light produced by all the galaxies over their lifetime The number counts of galaxies The galaxy luminosity functions
GALAXIES 626 Spring 2007 Lecture 1: The extragalactic background light
Extragalactic Background Light (EBL) The first thing we can look at is the light produced by all the galaxies.
What does the EBL measure? The EBL is an integral of the total light production BUT. Early light production contributes less to the EBL because of photon adiabatic expansion losses o. 0 Convert light density to a surface brightness by multiplying by the rate of photons crossing the unit surface (c ) and dividing by 4 pi to get it per unit solid angle 1 nw m -2 sr -1 = 3000/λ(µm) MJy sr -1 where 1 Jansky = 10-26 W m -2 Hz -1
Cosmic Background Radiation (CBR) CMB is not from radiative processes but rather from recombination of the gas Big Bang Galaxies (+some AGN) AGN Rest of backgrounds are consequences of radiative processes of various sorts, mostly star formation but also some contributions from accretion onto supermassive black holes
What do we want from the EBL? The EBL tells us the complete integrated history of galaxies: In some cases we have known of the background before we found the sources that give rise to it; e.g., the X-ray background (0.5 8 kev) was unidentified for many years and has now been resolved into AGN with Chandra In other cases the determination of the background has paralleled the resolution of the light into discrete sources, e.g., the far-infrared background (0.1 1 mm) was determined from COBE and nearly simultaneously resolved into bright star-forming galaxies with SCUBA on the JCMT In the optical/near-ir our understanding of the resolved sources is much better than our EBL information and always has been, but in other cases (cm radio, mid-ir) we have essentially no EBL information at all, but we know a lot about the individual source counts
Riccardo Giacconi Nobel Prize in Physics 2002 for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources
The First Background Glow In 1962, Giacconi s rocket mission to study X-rays from the Moon found a diffuse uniform X-ray glow from the sky X-ray telescopes can now resolve the glow into individual sources
Only a small fraction of the total energy produced in the universe emerges in X-rays Star formation does not produce much X-ray radiation, so most of the XRB is produced by Active Galactic Nuclei (supermassive black holes radiating as material accretes onto them)
Most of the galaxy/agn energy produced in the universe emerges in the FIR and optical Big Bang Galaxies (+some AGN) AGN Comparable amounts of light in the FIR and optical backgrounds mean comparable amounts of star formation seen directly and obscured by dust, if the two backgrounds are formed at the same redshifts (note that if FIR were formed earlier, would have to have more star formation to make it because of the larger 1+z suppression)
EBL Measurements EBL measurements are notoriously difficult to make The problem is the absolute calibration and removal of foregrounds. Many of the claims remain controversial Early work in the optical, as an example: Mattila (1976): dark Galactic clouds as zero level extinguishing everything behind them (but they reflect light themselves) Dube et al (1979): first attempt to remove foregrounds (but from the ground, which does not work well)
Contaminating Optical/IR Foregrounds Airglow: emission in upper atmosphere (90km), which has timedependent (time of day, activity in solar wind) and spatiallydependent (intrinsic fine-scale & due to propagation through the lower atmosphere) structure Once outside atmosphere, still zodiacal light: scattering of sunlight by interplanetary dust that mostly lies in the ecliptic, so varies due to motion of Earth on ecliptic plane, extends to β~30 It is also spatially complex with internal structure At longer wavelengths, the dust radiates giving direct emission (quite warm because fairly close to Sun)
Contaminating Optical/IR Foregrounds total background see from ground thermal Zodi (huge!) reflected Cirrus all reflected Kashlinsky 2005
The Zodiacal Light Ecliptic (1) Airglow varies continuously; can get upper limits on zodi light from gaps between the lines (2) Modeling the zodi light requires both a 3-D interplanetary model and a scattering model so that time dependence can be evaluated (complicated)
Contaminating Optical/IR Foregrounds Moving outwards, next contribution is faint galactic stars: in optical, fairly easy to remove them since can just count them in mid-ir, it is more of a problem because the low angular resolution observations make it difficult to pick out the stars
Contaminating Optical/IR Foregrounds Diffuse Galactic light (`cirrus ): reflected light from the dust and gas clouds illuminated by starlight very complicated structure, but is a function of galactic latitude, such that the farther out of the plane you look, the less cirrus there is can find areas with low cirrus backgrounds, since some regions do not have much material along the line-of-sight at longer wavelengths, get thermal emission from those dust clouds
Contaminating Optical/IR Foregrounds Can attempt to remove by careful design of EBL experiment: - Use of HST avoids airglow (though HST itself glows when the particle backgrounds strike it) - Zodiacal background should show seasonal variation & its spectrum in optical/near-ir is consistent with solar (since it is basically a reflected solar spectrum) - Galactic cirrus can be minimized/monitored via Galactic latitude of fields (not a big deal for optical, just do not want to point into a dense cloud!))
Case Study - I: Bernstein et al (2002) Claim a significant optical detection in excess of known sources at λ = 300, 550 and 800nm Experiment design: - Fields observed with HST WFPC-2 and FOS (avoiding airglow) - Galactic stars easily identified by HST
Contribution of Resolved Sources WFPC-2 counts WFPC-2 image Nominal EBL is quoted excluding V AB <23 sources but the likely contribution of 23<V AB <28.5 sources is estimated [strictly speaking, they should not have taken the galaxy sources out]
Case Study - I: Bernstein et al (2002) Claim a significant optical detection in excess of known sources at λ = 300, 550 and 800nm Experiment design: - Fields observed with HST WFPC-2 and FOS (avoiding airglow) - Sources removed to V AB ~23 (Galactic stars easily identified by HST) - Zodi spectrum measured simultaneously with ground-based telescope & iteratively subtracted according to model assuming solar spectrum (pointed in same direction as HST measurements, but Zodi light has propagated through the atmosphere so may be averaging over things not quite coming from same position)
Estimating Zodiacal Contribution Simultaneous long-slit spectrum in WFPC field Solar spectrum Cross-correlation of night sky spectrum with solar template used to estimate absolute Zodi contribution at Earth Method disputed due to unclear correction for extinction and atmospheric scattering (Zodi background larger at HST) Normalize solar spectrum to point where remove features & subtract; part left due to airglow normalization Well-defined minimum
Case Study - I: Bernstein et al (2002) Illustration of difficulty of measurement at λ = 550nm Total WFPC-2 background: 105.7 ± 0.3 ( 10-9 cgs sr -1 Å -1 ) Zodi background: 102.2 ± 0.6 Diffuse Galactic: 0.8 Galaxies V<23: 0.5 Claimed excess: 2.7 ± 1.4 (1σ) If you make a 1% error in your zodiacal background, you are in trouble! Excess is only 2-3% of the background, which is almost all zodiacal light
Case Study - II: COBE detections DIRBE: 10-channel photometer 1.25<λ<240µm with 0.7 deg (!!) resolution chopping at 32Hz onto an internal zero flux surface for instrumental background [goal was to measure MIR/FIR backgrounds] FIRAS: Absolute spectrometer with 7 deg resolution in 100µm< λ< 5mm [primary purpose was to measure CMB]
Early detections (>3σ) in selected fields at high Galactic & ecliptic latitudes at 140 and 240µm only, using elaborate time-dependent Zodi model (Kelsall et al 1998, Hauser et al 1998) Schlegel et al (1998) combined with higher resolution IRAS 100µm maps to improve removal of Galactic cirrus and dust emission,100/240µm ratio used to give spatial distribution of dust temp Finkbeiner et al (2000) extended the DIRBE Zodi model to provide first detections at 60 and 100µm Wright (2000) used 2MASS to improve Galactic source removal and adopted the DIRBE Zodi model to claim the first detection at 2.2µm Key issue in all of the above is the reliability of the Zodi model
COBE Results IRTS DIRBE DIRBE FIRAS Galaxy counts λ (µm) Big excess in NIR claimed!
Case Study - III: TeV Gamma-Rays Different way of approaching the problem Instead of measuring the background, measure the column densities of IR photons along the line-of-sight to get the light density and hence the EBL Completely orthogonal way of making the measurement Relies on gamma-rays moving through the intergalactic medium and interacting with IR photons; if they hit an IR photon of the right energy, an electron-positron pair is produced; thus, there is spectral information in the opacity, which is proportional to the column densities of IR photons Problem: need a TeV source with a well-known intrinsic spectrum and sophisticated detectors; a number of these are coming online just now
Case Study - III: HESS High Energy Stereoscopic System (HESS) Windhoek, Namibia Array of 4 telescopes detecting Cerenkov radiation (particles moving faster than speed of light in air) Gamma ray ~ 10 km Ch er en ko vl igh t Particle shower ~ 1o ~ 250 m High-energy gamma rays are absorbed and converted into secondary particles forming an air shower. Cerenkov light is generated, a faint beam of blue light, which on the ground illuminates an area of about 250 m in diameter. The faint flash lasts a few billionths of a second Gamma rays interact with 1-10 µm IR photons via pair creation process producing absorption features in distant sources (e.g. blazars). Strength of the absorption indicates the ambient IR photon background (EBL)
New Results from HESS Team ----HESS---- HESS team assume 3 model EBLs with different normalizations and see if any are consistent with their TeV spectra of z~0.2 blazars
New Results from HESS Team (z=0.186) corrected (take out opacity for that model) acceptable Γ observed Strong NIR background models make spectrum too steep compared to lower energy spectra (latter not shown)
New Results from HESS Team Big excess not real ----HESS----
Summary We have made astonishing progress in understanding the extragalactic populations across the spectrum However, the EBL observations remain challenging and controversial because of instrumental effects and dominant foregrounds and determining how much light remains unidentified (the UBL) will be the covered in the next lecture There may be small (10%-ish) residual excesses at many wavelengths but the 1-10µm region is the only place where a large UBL has been claimed but. Ultra high energy TeV spectra of distant blazars may give the most sensitive upper limit to the infrared background at 1-10µm and suggest there is again only a small UBL at these wavelengths.