Download Ay122a&Fall&2015+16& Background&subtrac9on,&calibra9on,& analysis& &

Ay122a&Fall&2015+16&
Background&subtrac9on,&calibra9on,&
analysis&
&
2014&November&23&
(Some&slides&from&L.&Hillenbrand)&
Sky&Subtrac9on&vs.&Flat&Fielding&
Flat&fielding&=&correc9ng&for&detector&QE&varia9on&w/&posi9on&
Sky&subtrac9on&=&removing&addi9ve&background&signal&
(One&oQen&judges&flaTielding&by&how&well&the&sky&appears&to&flaVen&out.)&
!
Although&the&background&responds&to&the&QE/flat&field,&it&may&and&oQen&will&
illuminate&the&detector&non+uniformly,&i.e.&differently&than&does&an&
astronomical&source&located&at&infinity.&&&Background&is&composed&of:&
•  Thermal&emission&from&telescope&components&
•  Stray&or&scaVered&light&from&out&of&field&
•  Spectral&dependence&of&flat&field&paVern&
•  Fringes&due&to&OH&sky&emission&lines&
•  Real&angular&structure&in&the&OH&airglow&
For&many&applica9ons,&1%&flat&fielding&uncertainty&is&quite&adequate,&and&is&not&a&
dominant&source&of&error&for&photometry.&
&
BUT:&&1%&sky&subtrac9on&errors&can&kill&you&(meaning,&dominate&one’s&ability&to&
detect&sources&and&measure&photometry)&when&the&sky&is&100x&brighter&than&
your&source&such&as&is&the&case&in&the&infrared!&
IR&Sky&Backgrounds&
OH&can&be&resolved&
atmosphere&
Zodiacal&light&
Standard MOSFIRE Mask Nod/ Interleaved Exposures
Slit
(4 degree tilt wrt detector columns)
A (+ nod distance from center)
nominal object position in mask
design
B (- nod distance from center)
Typical observing sequence: 15 nod pairs, integration 120s, mask nod
amplitude 1.5” (for 3” position separation):
ABABABABABABABABABABABABABABAB
= 3600s, 30x120s exposures.
4
MOSFIRE Background Subtraction
Y band (0.97-1.13 microns)
Interleaved 30s exposures over
30 minutes, 3 telescope nod)
5
2+D&Background&Subtrac9on
&
See&Kelson,&D.&D.&2003,&PASP,&115,&688&
“Background&Subtrac9on&for&the&21st&
Century”&
&
&
Bad Pixel Map
0.9% of pixels are “bad” (non-linear
response, no response, high dark
current, etc.)
Spectroscopic&Flat&Field&
Combined H-band Dome Flat
Normalized “Pixel Flat”
“Raw”&Night&Sky&Stack&(H+band)&
Used&to&fit&2+D&
wavelength&
solu9on&
Rec9fied&“Wavelength&Solu9on”&
Sanity&check&
for&2D&
wavelength&
solu9on&
Raw&A(stack)&–&B(stack)&interleaved&&
nega9ve&image&
posi9ve&image&
Background&Model&
Uses&“b+spline”&model&of&
first+pass&(A+B)&
background&residuals&
(mostly&OH&lines)&to&
improve&b.g.&subtrac9on&
Rec9fied&A+B&Image&&
Uses&slit&edge&traces&and&
2D&wavelength&solu9on&
to&transform&each&slit&
into&a&rec9linear&grid&of&
wavelength&(X&axis)&and&
spa9al&posi9on&along&alit&
(Y&axis).&
MOSFIRE&H+band&“A+B”&Image&&
A+(+B’),&where&B’&is&B&shiQed&into&registra9on&with&A&&
(in&this&case,&shiQ&=&3.0”&in&Y+direc9on)&
Zoomed+in&Region&of&
Rec9fied&Stack&& Targets&are&
there,&but&are&
2&nega9ve&images&
overwhelmed&
Posi9ve&image&
by&the&sky&
background&
Exposure&Time&Map&
Emission&Line&Source&
Gaussian&fits&to&emission&line&
profiles,&σ=99.0±0.5&km/s&
Con9nuum&Source&
Spectroscopy&&
•  For&your&science&you&might&want&to&measure:&
–  Con9nuum&shape&
–  Spectral&feature&presence&
–  Absorp9on/emission&line&strength&
–  Absorp9on/emission&line&broadening&and&shape&&
–  Radial&velocity&/&redshiQ&
•  Con9nuum&or&&flux&density&distribu9on&determined&at&
UV/O/IR&wavelengths&by&bound+free&and&free+free&
processes&(mostly&H,&H+)&
•  Lines&–&emission&or&absorp9on&
–  electronic&transi9ons&(bound+bound)&of&atoms,&ions,&or&
molecules&&
–  rota9onal&and/or&vibra9onal&transi9ons&of&molecules&
“Instrumental&Profile”&is&the&shape&of&an&unresolved&spectral&line.&In&
this&example,&the&line&has&FWHM=4.318&Å&centered&at&16030&Å,&so&
R=16030/4.318&=&3712,&or&σinst&=&(3e5/3712)/2.55&=&31.7&km&s+1&
&
Measurements&of&&line&widths&must&take&this&into&account&
Radial velocity precision/accuracy:
λ c
R≡
=
Δλ v
v=
c
R
•  R = 3700 (e.g., MOSFIRE): 81 km/s (FWHM), or
instrumental σ=FWHM/2.355 = 34.4 km/s
•  Centroid precision depends on S/N, but decent rule
€
of thumb is <1/20 resolution element (ignoring
wavelength calibration uncertainties)
•  HIRES at R = 50000 and 1/20th: 0.3 km/s=300 m/s
Line&Centroid&/&Velocity&
•  For&galaxies,&λobserved&/λintrinsic = (1+z)&&(redshiQ)&
•  For&galac9c&objects,&λobserved& λintrinsic => radial&
velocity&of&recession&(+)&or&approach&(+)&
–  Stellar&kinema9cs&in&galac9c&context&
–  Detailed&kinema9cs&of&circumstellar&and&interstellar&
material&
–  Binary&stars&and&brown&dwarfs&/&planets&
–  NOTE:&&veloci9es&are&typically&low&and&so&&&&&&&&&&&&&&&&&&&&&&&&&&
λ c
high&dispersion&spectroscopy&is&needed&
R≡
=
LRIS&R=2500&!&v&=&120&km/s&*&1/20&centroid&=&5&km/s&
HIRES&R=34,000&!&v&=&0.3&km/s&
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&(plus&calibra;on/systema;c&errors)&
Δλ
c
v=
R
v
Measuring&(Accurate)&Veloci9es&
•  Measure&by&cross+correla9on&with&radial&velocity&
standard&(need&to&observe&some!).&
•  First&need&to&correct&for&mo9on&of&the&observer&in&the&
direc9on&of&the&observa9on.&
–  Earth&rota9on&(diurnal&mo9on)&
–  Earth&center&mo9on&about&Earth+Moon&barycenter&
–  Earth+Moon&barycenter&mo9on&about&Sun&(annual&velocity)&
&&&&&above&three&correc9ons&put&you&in&heliocentric&frame&
–  Sun&center&mo9on&about&local&standard&of&rest&(solar&velocity)&
&&&&&above&four&correc9ons&put&you&in&LSR&frame&
3m/s
Precision With
An Iodine
Cell:
Butler et al. 1996,
PASP, 108, 500
(now down to ~1 m/s)
Timing&Correc9ons&
n.b.&for&planet&detec;on&signal&at&1&m/s&precision,&need&to&know&;me&to&within&30&seconds&&
Radia9ve&Transfer&
&
Op9cal&depth&τν (with&dτν&=&κν ρ ds)&accounts&for&
interac9on&between&maVer&and&the&radia9on&field.&
&
In&analyzing&spectra&need&to&consider&both&con9nuum&and&line&processes.&
Spectral&Line&Astrophysics&&
•  Spectroscopy&in&UV/O/IR&conveys&informa9on&
on:&
–  Gas&density,&abundance&
–  Gas&excita9on/ioniza9on,&temperature&
–  Gas&or&object&veloci9es&and&kinema9cs&
–  Dust&size,&abundance&(mid+infrared)&&
•  dτν&(line)&=&κν (line)&ρ ds&
Line&Forma9on&
Source&Func9on&
Func9on&describing&the&
shape&or&profile&of&the&line&
&&&&&&&&&&&&&&&&Line&
(also&describes&&
Pressure&broadening&&
and&&
Collisional!broadening)&
(also&describes&&
Rota/onal&broadening)&
Stellar&Rota9on&
(line&broadening)&
&
Magne9c&Fields&
(line&spliyng)&
&
(plus&spots)&
&
Binary&Star&Example&
Cross+correla9on&method&
&&&&&&&&Radial&velocity&curves&
[Kraus&et&al.&2011]&
Line&Shape&=>&Gas&Kinema9cs&&
Ellerbroek et al. (2011)
Galac9c&“Super+wind”&Signatures!
UV&from&hot,&
massive&stars&
astronomer&
Nebular&emission&
Resonance&Lyman&α&photons&
scaVered&from&“back”&side&of&
flow+&acquire&redshiQ&with&
respect&to&stars&
Nebular&emission&lines&
from&gas&around&
forming&stars+&at&rest&
with&respect&to&galaxy&
redshiQ&
Photons&absorbed&by&
gas&moving&toward&
observer,&acquire&
blueshiQ&&&
v&
astronomer&
The&View&“Down&the&Barrel”&to&a&Forming&Galaxy&(tLB~11&Gyr)&&
• 
• 
• 
• 
• 
• 
SFR&≈&50+100&M"/yr&
M*&≈&1010&M" ; MDM&≈&1012&M"
Vc&≈&125&km/s&from&nebular&emission&
Mgas&>&M*&(gas+dominated)&
Gas+phase&metallicity&≈0.3&solar&
OuTlow:&&
•  extends&to&vmax&≈&800&km/s&
•  dMout/dt&&>&SFR&
•  gone&forever,&&coming&back,&or&in&
between?&
Where&is&the&accre9on&
signature?&
λobs=&λrest(1+z)&
High+resolu9on&Quasar&
Spectrum,&zQ=2.720&
λ=1215.7(1+zQ)&
λobs=&1215.7(1+zabs)&
“Lyman&α&forest”&
Metal&lines&
Voigt profile fits to ~6000 HI
absorbers
all w/ Lyα and Lyβ (at least)
Rudie, CS, Trainor +12
QSO!
101&kpc&
156&kpc&
90&kpc&
154&kpc&
Nebular&Diagnos9cs&
Line&intensity&ra9os&provide&
informa9on&on:&
•  chemical&abundances&
•  Gas&density&
•  Ioniza9on&sources&
•  electron&temperatures&
•  star&forma9on&rates&&
•  ex9nc9on&
•  etc.&&
Composite&Spectra&of&30&star+forming&galaxies&at&z=2.40&
Figure 1 from Narrowband Imaging of Escaping Lyman-continuum Emission in the SSA22 Field
Daniel B. Nestor et al. 2011 ApJ 736 18 doi:10.1088/0004-637X/736/1/18
Narrow-Band Imaging: to isolate emission in a
spectral line or absorption over short wavelength intervals
Broad-band V Image (Keck/LRIS)
Narrow-band 4670/80 Image (Keck/LRIS)
Color-Selection of Targets for follow-up Spectroscopy
Ratio of fluxes in NB filter to that in
nearby continuum
Leiden August 2012
NB4670/80 - V (Lyman alpha) Image (Keck/LRIS)
Spectra&of&NB+selected&Objects&
IFU&Spectroscopy&
•  Clearly,&a&hybrid&between&NB&imaging&and&spectroscopy&
•  Challenge&is&to&cover&a&reasonably&large&field&(e.g.,&to&cover&1’&
x&1’&on&the&sky,&the&MUSE&instrument&at&the&VLT&needs&24&
spectrographs&!&)&