giuseppina battaglia eso
DESCRIPTION
The Chemo-Dynamical Structure of Galaxies: intermediate resolution spectroscopy of resolved stellar populations out to Virgo. Giuseppina Battaglia ESO. Simulations as part of the Design Reference Mission for E-ELT. Motivation: insights on galaxy formation and evolution. - PowerPoint PPT PresentationTRANSCRIPT
The Chemo-Dynamical Structure of Galaxies:
intermediate resolution spectroscopy of resolved stellar populations out to
Virgo
Giuseppina BattagliaESO
Simulations as part of the
Design Reference Mission for E-ELT
Motivation:insights on galaxy formation
and evolution
• Low mass stars can have lifetimes comparable to the age of the Universe -> record of changing galaxy properties
• By deriving ages, chemistry and kinematics of LARGE samples of individual stars we can gain insights on how galaxy properties were built up in time
These observations are needed for a variety of galaxy types (e.g. dwarfs, spirals, ellipticals) and in a variety of environments (e.g. groups, clusters)=> beyond the Local Group
The NIR CaII triplet (CaT)
• Well tested method for obtaining large numbers of metallicities and l.o.s. velocities of individual stars (currently used in studies of Local Group dwarf galaxies, M31, RAVE, GAIA in the future…)
• Red Giant Branch stars as targets (usually):
-> bright in the NIR -> cover wide age range (> 1 Gyr old)
• The CaT is a strong feature in the NIR spectra of late-type stars -> need only moderate resolution (R 6000) to obtain accurate l.o.s. vel and EW
• For individual RGB stars an empirical relation holds between [Fe/H] and CaT EW
-> extensive literature for stellar clusters -> in composite stellar population tested in
range -2.5 < [Fe/H] < -0.5
CaII triplet at 8600 Å
ESO/WFI Colour-Magnitude Diagram of the Fornax dSph (Battaglia et al. 2006)
Battaglia et al. 2008, MNRAS, 383, 183
[Fe/H]HR from about 60 Fe lines
[Fe/H] from CaT EW
Sculptor dSph: 93 stars* Fornax dSph: 36 stars
Example from VLT/FLAMES intermediate resolution (R=6500) studies of dSphs from DART team
coverage of Sculptor with VLT/FLAMES GIRAFFE
With 1h exposure time per pointingS/N=10/Å @ V=19.5. At this S/N:
-> l.o.s. velocities accurate ± 5 km/s
-> [Fe/H] accurate to ± 0.25 dex
20 pointings -> 670 individual members
The Sculptor dSph
(distance = 80 kpc)
Looking farther out
Main points to address:
• Larger distances
• Crowding: effect of stellar background in the spatial resolution element (spaxel) on the properties of target RGB star
-> to which extent will the CaT [Fe/H] and velocity derived from the integrated spectrum resemble the ones of the target RGB star?
=> Can we derive accurate [Fe/H] and line-of-sight velocities from the CaT lines for large numbers of individual RGB stars in a “reasonable” observing time?
Can we perform similar studies out to the distance of the Virgo cluster with the E-ELT?
Setting up the simulations
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Simulations: creating the stellar catalogue
1Re 2Re 5Re
Stellar population code developed by J.Liske & E.Tolstoy
• Stellar population: constant SF (14-12 Gyr ago); [Fe/H] = -1.8 (MP) or -1 (MR); solar [alpha/Fe]
• Target: RGB stars of different magnitude
• Stellar background:
- fix the distance and the surface brightness of the object to observe
- explore different projected radii (different crowding)
- at each radius, simulate a stellar catalogue over a large area (5” x 5”)
- include PSF effects (we use LTAO)
- find which stars contribute to the light in a 50mas spaxel, where the RGB star target is
The stellar catalogue contains all the stars that contribute a fraction of their light to the spaxel (target + stellar background)
• For each star in the catalogue find the appropriate synthetic spectrum (log g, Teff, [M/H], [alpha/Fe]) in the Munari et al. (2005) library at R=20˙000
• Rescale each spectrum according to the light contributed by the star to the spaxel
• Redshift individual spectra according to stellar velocities (we assume a Gaussian velocity distribution)
• Produce the integrated spectrum (R=20000) in the spaxel
• take into account effect of atmosphere, telescope, instrument, site, mirror coating
• Convolve to desired resolution (R=6000) and add noise
• Measure the line-of-sight velocity and CaT EWs from the integrated spectrum & compare to the ones of the target RGB star
Creating the simulated spectrum
Figures of merit (accuracies)
We consider that the simulations produce accurate enough velocities and ∑W if they
agree with the input values within- 0.7 Å -> 0.3 dex ( [Fe/H] = 0.44 x ∑W )- 30 km/s (CenA & M87); 5 km/s (NGC205)
(this is similar to accuracies of current studies)
• We perform 300 random realizations to account for varying stellar background and noise • We derive the distribution of (V_los_simulated - V_los_input) and (∑W_simulated - ∑W_input) where ∑W = EW2 + EW3
-> the systematics are quantified by the mean of the distributions-> the errors are quantified by the scatter of the distributions
SCIENTIFIC:
• STELLAR POPULATION: constant SFH (14-12 Gyr ago), [alpha/Fe]=0,
[Fe/H] = -1.8 (“metal poor”) and [Fe/H]= -1.0 (“metal rich”)
• DISTANCES: Local Group (800 kpc), Centaurus A (4 Mpc),
Virgo (17 Mpc)
• GALAXIES: NGC205 (dwarf, internal dispersion used = 35
km/s), CenA, M87 (internal dispersion used = 100 km/s). Surface brightness profiles from Mateo 1998, van den Bergh 1976, goldmine web database, respectively
• RADII: 1, 2, 3.5, 5 Re (depending on the galaxy)
Explored parameters
TECHNICAL:
• TELESCOPE DIAMETER = 42 m
• SPAXEL SIZE= 50mas
• PSF: LTAO in I band (to minimize the loss of flux from the spaxel)
• AIRMASS = 1.0 & SEEING = 0.8”
• INSTRUMENT EFFICIENCY: 0.28
• WAVELENGTH RANGE: 8000-9000 Å
• RESOLVING POWER: R 6000 (3 pixels per resolution element)
• MIRRORS COATING: bare Al; Ag/Al
• SITE: Paranal-like; High&Dry
• EXPOSURE TIME: from 20min to 50h, depending on the object
Explored parameters
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Results
Results for the Local Group: NGC205• Down to 1 mag below the
tip (I 21.4)• Exp. times: 20m, 30m, 1h,
2h• Stars resolved with a 50
mas spaxel at every radius • 20min, Paranal-like site,
bare Al coating
• 20m exposure time x pointing =>
accuracy: ± 2 km/s accuracy: ± 0.2 dex
Velocity
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EW
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Tip of the RGB
1 mag below the tip
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Velocity
* MP
MR
Centaurus A: example of spectra
• Targets: MR and MP RGB at the tip and 0.5 mag below tip (MR: I= 23.9, 24.4; MP: I= 23.7, 24.2)
• R/Re = 1, 2, 3.5, 5 -> R 6, 12, 22, 31 kpc (crowding is not a problem at these radii, for the considered spaxel size)
• Exp. time: 1h, 2h, 5h
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MR, Paranal, Ag/Al, 5h, tip of the RGB
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Tip of the RGB
0.5 mag below the tip
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Tip of the RGB
0.5 mag below the tip
Example of
different crowding
Centaurus A: velocity (0.5 mag below the tip)
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5h 2h 1h
High&Dry+
Ag/Al
Paranal-like +
Ag/Al
Paranal-like +
Bare Al
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Centaurus A: EW (0.5 mag below the tip)
5h 2h 1h
High&Dry+
Ag/Al
Paranal-like +
Ag/Al
Paranal-like +
Bare Al
Results for M87 (Virgo): R= 2Re• Targets: MR and MP RGB at the tip ( I = 27.2, I = 27.0)
• R/Re = 2 (a lot of crowding), 5 (not so much crowding) -> R 18, 45 kpc
• Exp. time: 20h, 50h
• Integration on: 1 spaxel, 9 spaxels, 25 spaxelsParanal, Ag/Al, 50h, tip of the RGB
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+312 km/s
-integrated spectrum on 25 spaxels
- integrated spectrum on 1 spaxel
- rescaled spectrum of the target RGB star
Integrated spectrum on 25 spaxels dominated by stellar background
Results for M87 (Virgo): R= 5Re• Targets: MR and MP RGB at the tip ( I = 27.2, I = 27.0)
• R/Re = 2 (a lot of crowding), 5 (not so much crowding) -> R 18, 45 kpc
• Exp. time: 20h, 50h
• Integration on: 1 spaxel, 9 spaxels, 25 spaxels
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Paranal, Ag/Al, 50h, tip of the RGB
MR VRGB= -79km/s MP VRGB= +312 km/s
-integrated spectrum on 25 spaxels
- integrated spectrum on 1 spaxel
- rescaled spectrum of the target RGB star
Integrated spectrum on 25 spaxels considerable improvement
Results for M87 (Virgo): summary
• Integrated spectrum on 1 spaxel has too low a s/n both at 2Re and 5Re, in all of the explored cases
=> too much flux is lost because of small spaxel size + PSF effect
• At 2Re, integrated spectrum on 25 spaxels is dominated by stellar background
=> at this and smallest projected radii there is too much crowding
• At 5Re: - metallicities are usually underestimated of about 1
dex - the Paranal-like + bare Al case yields very
discrepant velocities - velocities can be measured within the desired
accuracy for very bright targets (at the tip of the RGB, MP) with 20h exposure time when integrating on 25 spaxels (50h for 9 spaxels).
Observing time to collect samples of 1000 targets
M87 (10” = 0.8 kpc):
At the tip of the RGB, in the outer parts (around 5 Re), about 6 pointings would be needed:-20h exp. time per pointing -> 120h on source-50h exp. time per pointing -> 300h on source
ASSUMPTION: single IFU with 10”x10” field-of-view (spaxel size= 50mas)
NGC205 (10” = 40pc):
Down to 1 mag below the tip, with 20m exp. time per pointing
=> 25 pointings (within 2Re), I.e. 8h exposure time on sourceCentaurus A (10” = 180pc):
Down to 0.5 mag below the tip:
-5h exp. time per pointing (Paranal-like, bare Al), 25 pointings within 5Re => 130h on source
-2h exp. time per pointing (Paranal-like, Ag/Al), 25 pointings within 5Re => 50h on source
±
Sensitivity to input parameters
INSTRUMENT EFFICIENCY:
M87 case appears to be “border-line” -> important to have values as realistic as possible
SITE: negligible influence
MIRRORS COATING:
Appears to be important for
-CenA => reduced exp. time per pointing from 5h to 2h
-M87 => when considering 20h exp. time, velocities are not recovered
with a bare Al coating
FIELD-OF-VIEW:
A 10”x10” field-of-view is sufficient in most cases.
Given the goal of acquiring large number of targets over large areas,
a larger field-of-view (or multi-IFUs) would allow shorter exposure times