the optical sky background

The Optical Sky Background

The Optical Sky. I: the V – I band

The Optical Sky. II: the R - I band

The Optical Sky. III: the I - Z band

The Optical Sky. IV: the z – J band

The Optical Sky. V

The Near-IR Sky Background

The Sky OH Emission Lines

The Sky OH Emission lines

The Near-IR Sky

The traditional J, H and K bands defined by the atmospheric absorption

At some wavelengths, the atmosphere is blocking the radiation. If those wavelengths are crucial, space observations are required

Observational tactics

Cosmic Volumes. I

Luminosity functionLuminosity function(L) = (L) = * * • • exp(-L/Lexp(-L/L**) ) • • (L/L(L/L**))

** ~ 10 ~ 10 -2-2 to 10 to 10 -3-3 Mpc Mpc-3-3

LL** ~ 10 ~ 101111 to 10 to 101212 L LO O or 10or 104444 to 10 to 104545 erg/sec erg/sec ~ -1.0 to -1.2 ~ -1.0 to -1.2 (optical)(optical);-1.4 to –1.8 ;-1.4 to –1.8 (UV)(UV)

Luminosity densityLuminosity density

LL = = ∫ ∫ dL dL • • L L • • * * • • exp(-L/Lexp(-L/L**) ) • • (L/L(L/L**))~ L~ L** • • * *

Cosmic Volumes. II

Maximum VolumeMaximum Volume VVmaxmax = A = A • • ∫∫ f(z) f(z) • • dV(z)/dz dV(z)/dz • • z z

A ~ survey areaA ~ survey areaf(z) survey redshift distribution functionf(z) survey redshift distribution function

Effective Volume: the volume visible to you using a galaxy Effective Volume: the volume visible to you using a galaxy with luminosity Lwith luminosity L VVeffeff(L) = V(L) = A (L) = V(L) = A • • ∫∫ f(z; L) f(z; L) • • dV(z)/dz dV(z)/dz • • z z

When measuring luminosity function: V(L)/ VWhen measuring luminosity function: V(L)/ Vmaxmax Always remember Always remember COSMIC VARIANCECOSMIC VARIANCE

Cosmic Volumes. III

At z = 3At z = 3 1 arcsec ~ 3 kpc (physical) ~ 10.2 kpc (comoving)1 arcsec ~ 3 kpc (physical) ~ 10.2 kpc (comoving) 10 arcmin ~ 1.8 Mpc (physical) ~ 7.2 Mpc 10 arcmin ~ 1.8 Mpc (physical) ~ 7.2 Mpc

(comoving)(comoving)

At z=3, dz ~ 1 At z=3, dz ~ 1 R(z=3) ~ 500 Mpc R(z=3) ~ 500 Mpc

FWHM = 100 FWHM = 100 ÅÅ corresponds to dz = 0.08 or corresponds to dz = 0.08 or R = 40 Mpc R = 40 Mpc

Survey Options

SpectroscopySpectroscopy Slit or slitless spectroscopySlit or slitless spectroscopy Objective prism imagingObjective prism imaging

Photometry (imaging)Photometry (imaging) Narrow band filtersNarrow band filters Tunable filtersTunable filters

A cost-benefit analysis of any survey designed A cost-benefit analysis of any survey designed should be done in light of the desired scientific should be done in light of the desired scientific goals to achievegoals to achieve

Emission Line SurveysLecture 3

Mauro GiavaliscoMauro Giavalisco

Space Telescope Science InstituteSpace Telescope Science Institute

University of Massachusetts, AmherstUniversity of Massachusetts, Amherst11

11From January 2007From January 2007

Observing Strategies: Slit Spectroscopy

dispersed images of targets through slit or slits. Can be blind or dispersed images of targets through slit or slits. Can be blind or targeted (targets pre-selected according to some selection criteria)targeted (targets pre-selected according to some selection criteria) PROs:PROs:

High sensitivityHigh sensitivity Large radial velocity/redshift coverageLarge radial velocity/redshift coverage Easy selection of spectral coverageEasy selection of spectral coverage Easy trade off b/w spectral resolution, sensitivity and coverageEasy trade off b/w spectral resolution, sensitivity and coverage

CONs:CONs: Very limited spatial coverageVery limited spatial coverage Data acquisition of medium complexityData acquisition of medium complexity Data reduction and analysis of medium complexityData reduction and analysis of medium complexity Very costly if large volumes of space need to be covered: cost Very costly if large volumes of space need to be covered: cost

driven by number of individual slit(s)-mask exposuresdriven by number of individual slit(s)-mask exposures

Observing Strategies: Slitless Spectroscopy

dispersed images through a dispersive spectral element (prism, dispersed images through a dispersive spectral element (prism, grism, grating). Blind surveysgrism, grating). Blind surveys PROs:PROs:

Large spatial coverageLarge spatial coverage Large radial velocity/redshift coverageLarge radial velocity/redshift coverage Relatively large spectral coverageRelatively large spectral coverage Trade off b/w spectral resolution and coverageTrade off b/w spectral resolution and coverage Easy data acquisitionEasy data acquisition

CONs:CONs: Low sensitivity (high background)Low sensitivity (high background) Complex data reduction and analysisComplex data reduction and analysis Some spatial coverage losses due to spectra overlappingSome spatial coverage losses due to spectra overlapping Exposure times longer than slit spectroscopyExposure times longer than slit spectroscopy

Observing Strategies: Narrow-Band Imaging

Photometry or “narrow” band imaging: images through a set of filters Photometry or “narrow” band imaging: images through a set of filters selected to measure emission line flux and continuum flux densityselected to measure emission line flux and continuum flux density PROs:PROs:

Large spatial coverageLarge spatial coverage Spatial mapping of emission line regions Spatial mapping of emission line regions Easy data acquisitionEasy data acquisition Easy data reduction and analysisEasy data reduction and analysis

CONs:CONs: Limited spectral (radial velocity/redshift) coverageLimited spectral (radial velocity/redshift) coverage Increasing spectral coverage (broader filters) decreases Increasing spectral coverage (broader filters) decreases

sensitivitysensitivity Exposure times longer than slit spectroscopy (but shorter than Exposure times longer than slit spectroscopy (but shorter than

slitless one)slitless one) Accurate measure of continuum costly (several bands needed)Accurate measure of continuum costly (several bands needed)

Emission-line surveys

Targeted surveysTargeted surveys: “a-priory” knowledge of : “a-priory” knowledge of redshift or radial velocity:redshift or radial velocity: Narrow-band imaging: Narrow-band imaging:

Best option if coverage of area of sky Best option if coverage of area of sky required. Examples:required. Examples:

• Galaxy cluster, supercluster candidatesGalaxy cluster, supercluster candidates Slit spectroscopy:Slit spectroscopy:

Best sensitivity. Examples:Best sensitivity. Examples:• Galaxies causing QSO or GRB absorption Galaxies causing QSO or GRB absorption

systemssystems

Strategy of Emission-line surveys

Blind surveysBlind surveys: no “a-priory” knowledge of redshift or radial velocity:: no “a-priory” knowledge of redshift or radial velocity: Narrow-band imaging: Narrow-band imaging:

Useful if large volume density of sources suspected and if Useful if large volume density of sources suspected and if large sensitivity can be achieved. Examples:large sensitivity can be achieved. Examples:

• Distant galaxy searchesDistant galaxy searches Slitless spectroscopy:Slitless spectroscopy:

Good option if high sensitivity can be achieved (e.g. from Good option if high sensitivity can be achieved (e.g. from space). Examples:space). Examples:

• Distant galaxies searchesDistant galaxies searches Slit spectroscopy:Slit spectroscopy:

Good option if very high sensitivity required and small Good option if very high sensitivity required and small volumes OK (esp. from ground). Examples: volumes OK (esp. from ground). Examples:

• DLA galaxies (redshift can be highly unconstrained)DLA galaxies (redshift can be highly unconstrained)• host galaxies of faded GRBshost galaxies of faded GRBs

Survey Design: Narrow-Band Imaging Emission line detected as excess flux in in-band images compared to Emission line detected as excess flux in in-band images compared to

off-band images, which measure continuum flux density (essentially, off-band images, which measure continuum flux density (essentially, color selection)color selection)

In-band images generally obtained through narrow-band spectral In-band images generally obtained through narrow-band spectral elements (solid-state or Fabry-Perot tunable filters). For broad lines elements (solid-state or Fabry-Perot tunable filters). For broad lines and/or large Wand/or large W, medium band elements OK., medium band elements OK.

Off-band images can be either through narrow-band elements (one Off-band images can be either through narrow-band elements (one required; two preferable) or medium and broad-band onesrequired; two preferable) or medium and broad-band ones

Best photometric accuracy reached using multiple narrow-band Best photometric accuracy reached using multiple narrow-band elements. Usually costlyelements. Usually costly

Final sensitivity of the survey is the ability to detect excess flux, not Final sensitivity of the survey is the ability to detect excess flux, not just S/N in in-band images: need to achieve accurate continuum just S/N in in-band images: need to achieve accurate continuum measure to have sensitivity to lines with weak Wmeasure to have sensitivity to lines with weak W..

Uncertainty on continuum flux density (due to SED scatter and Uncertainty on continuum flux density (due to SED scatter and limited “spectral resolution” of using filters, not just S/N of narrow-limited “spectral resolution” of using filters, not just S/N of narrow-band image is crucial, especially for weak Wband image is crucial, especially for weak W

Observational Strategies: How to Choose Filters

Matsuda et al.

Narrow-Band Imaging: Blind Surveys

Rhoads et al.

Slitless Spectroscopy (space): Blind Surveys

Rhoads et al.

McCarthy et al. (1999)

Slitless Spectroscopy (space): Blind Surveys


Slit Spectroscopy: Targeted Surveys


DLAs in the

spectrum ofQSOs

Slit Spectroscopy: Targeted Surveys

DLAs in the

spectrum ofQSOs

Ly Surveys: Early Galaxies Originally designed to find star-forming galaxies at very high Originally designed to find star-forming galaxies at very high

redshifts (Partridge Peebles 1967)redshifts (Partridge Peebles 1967) ready my ARAA paper (Giavalisco 2002)ready my ARAA paper (Giavalisco 2002)

Early surveys essentially unsuccesfulEarly surveys essentially unsuccesful Koo & Kron (1980)Koo & Kron (1980) Djorgovski et al. (1985)Djorgovski et al. (1985) Lowenthal et al (1990); Thompson et al. (1995)Lowenthal et al (1990); Thompson et al. (1995)

First to be found by LyFirst to be found by Ly were (steep spectrum) radio galaxies were (steep spectrum) radio galaxies Spinrad & Djorgovski 1984a,b; Spinrad et al.\ 1985Spinrad & Djorgovski 1984a,b; Spinrad et al.\ 1985

Significant results came with advent of 8-m class telescopesSignificant results came with advent of 8-m class telescopes Rhoads et al. (2000)Rhoads et al. (2000) Taniguchi et al., Ouchi et al. , Matsuda et al. Shimasaku et al.Taniguchi et al., Ouchi et al. , Matsuda et al. Shimasaku et al.

Ly Surveys

Today, LyToday, Ly surveys mostly useful to complement surveys mostly useful to complement continuum-based searches:continuum-based searches: Fainter continuum levelsFainter continuum levels Trace LSS, clustering, clustersTrace LSS, clustering, clusters Spatial mapping of emitting regionsSpatial mapping of emitting regions Constraints on reionizationConstraints on reionization

the optical sky background

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