Possible Masters Projects

Project 1: 

The Dynamical Structure and Population of the Extremely Metal-Rich "Knot" at the center of the Milky Way 

Background

The Milky Way's formation history is encoded in the distribution of stellar orbits -- ages -- and chemical abundances. They are the key to: how many stars formed when from what material? Gaia and spectroscopic surveys such as SDSS/APOGEE make it now possible to draw up such map for the full galaxy. 

Given that the element abundances are permanent 'birth tags' it makes sense to ask what the spatial (or orbit) distribution of 'mono-abundance' populations is. In recent work, we have found/discovered that the extremely metal rich stars in our Galaxy mostly form an extremely metal-rich (EMR) knot at the center of the Milky Way.

All-sky maps of the stellar density for very metal-rich stars in the inner Galaxy (from Rix+2024). Note that the extremely metal-rich stars (EMR; bottom panel) are largely confined to a central "knot".

In light of this finding, some questions are:

• did the stars form there?
• on what orbits are they? radial, rotating, etc..
• how old are they? (did they form in several episodes)

We have initial kinematics, that point to radial, centrally confined orbits.

The current analyses are limited by a) dust (when considering radial velocities from Gaia), and b) by modest sample size, when considering SDSS IV/APOGEE spectra.

Goal

In the context of SDSS-V we are getting (and have gotten) many more spectra towards the central 1.5 kpc. The central goal of the masters project is to take these, potentially do some post-processing on them, and build a kinematic/dynamical model for the extremely metal-rich central knot.

(possible) steps

• collect all data (velocities, metallicities) of existing and new SDSS/APOGEE spectra in the inner 1.5 kpc of the Milky Way
• find the very metal rich ones
• get the best possible distances of these stars (Gaia and spectroscopic information)
• combine SDSS/APOGEE information with Gaia to determine orbits
• determine orbit distribution, mostly the distribution in binding energy (or apocenter) and eccentricity.
• build a simple dynamical model.
• determine the spatial and orbit distribution, and (optional) compare it to TNG50 Milky Way formation simulations.

Tools

• working with the sloan data base
• working with python dynamics packages such as galpy
• writing a set of jupyter notebooks (or other forms of python code) to do further analysis and make plots.

Hoped-For Outcome

Leading a refereed population on this analysis

Project 2: 

Mapping the Metallicity of Young Stars across the Milky Way Disk

or: how homogeneous is the birth material of stars at a given time and radius?

Background

We have reason to believe that the interstellar medium -- from which stars form -- is nearly homogeneous in azimuth, at a given radius and time in the life of a galaxy: at any given epoch, a star's chemical abundances only depend on the radius at which it was born. It would be important to test this hypothesis, as it is a starting point for understanding many evolutionary mechanisms in disk galaxies (e.g. radial migration). The way one could do this is to find young stars (say, less than an orbital period, or 250Mio yrs) luminous to be seen across the disk) and measure their abundances, to see whether this important assumption about "chemical homogeneity" is true.

What to take for young luminous stars: the easiest would be to take hot young stars (OB stars); but they have few metal lines, so it is hard to measure [Fe/H]. But all stars (>Mio years) have a red giant phase, where they are cool enough to yield metallicities. 

Goal

The goal is to find (among the 10 Mio) the red giants with [M/H] from Gaia the ones that are <200Mio old, and map their metallicities: are there azimuthal variations?

How: the two plots below show that the temperatures and luminosities of giants depend on age, and metallicity. If one know the metallicity, one gets the age:

CMD positions of red giants with solar metallicity, but different ages

CMD positions of stars of 10^9 years age, but of different
metallicities. Age and metallicity are covariant.

We have developed a piece of code (for application in the LMC) that takes the distances, magnitudes, metallicities and temperatures of giants and determines their ages.How: the two plots below show that the temperatures and luminosities of giants depend on age, and metallicity. If one know the metallicity, one gets the ages.

Goal (possible) steps

• take intellectual ownership of the age fitting code (with some possible tweaks/checks)
• collect the data (from Gaia; catalogs exist) of the stellar parameters for all "Gaia giants with spectroscopy". find the subset with good distances, and apply age-fitting code.
• find the young giants
• make metallicity maps
• take it from there..

Tools

• working with Gaia and zenodo data base
• learn and adopt a piece of existing code
• writing a set of jupyter notebooks (or other forms of python code) to do further analysis and make plots.

Hoped-For Outcome

Leading a refereed population on this analysis

Thoughts on Wide-Field Slitless Spectroscopy from Space

 Slit-less Survey Spectroscopy from Space

This reflects some rambling thoughts that HWR has harboured over the last years on the question of what's the ultimate all-sky spectroscopic survey. Given that there is much (MEGAMAPPER, MSE, ..) pondering about the ground-based options, this is about space (inspired by the Gaia, JWST and Euclid slitless data).

To cut long story short. One dream-option could be:

• let's presume a 6.5m (warm?) telescope could be designed [credit to Roger Angel here] with a near diffraction limited  0.25 (or 1) sqdeg FOV in a TESS-like or L2 orbit ; 
• and if one could then implement slitless spectroscopy with a resolution R (say R=1000, or 2000?), and a bandpass filter that picks out NR (say NR=1000, or 2000?) resolution elements
• the actual wavelength requires a great deal of thought, but let's take here 0.8mum-1.6mum
• Notes:
• a 1sqdeg FOV would require about 20 Gpixels (same as imaging at the same resolution and FOV); so, thinking about undersampling, or 0.25sqdeg may make this idea less pie-in-the-sky
• note that the number of pixels needed is the same as for direct imaging; it's just imaging with every source being a short streak 
• How long is the slitless spectral streak in the focal plane?  for NR resolution elements, the streak is NR * FWHM(PSF)  = 47" at NR=1000; lambda=1.5mum, D=6.5m
• then obvious science include (see section below the S/N estimates for more/growing detail). Brief quip:  that MSE, SpecTel, Roman, etc.. just much better.
• stellar physics
• Galactic history, structure and dynamics
• redshift surveys
• AGN finding
• good angular resolution
• spectral "follow-up" on LISA GW sources
• to cover a good portion of the sky "in a reasonable period" (few years?), exposure times per pointing 1000s-ish?

Why would that be a dream? 

The S/N estimates written out below illustrate the power that arise from combining:

• slitless (you get everything)
• observations from space (low background)
• a large and diffraction limited telescope
• compact sources (the last two boost the source/background contrast)

• 

Survey speed (to a given depth) for faint, compact sources scales with telescope size as D^4.

[This can do (more) in one year than Roman (slitless) in 50 years]

In addition, the probability of source confusion (at given R, and NR) goes down as D^-2.

Here are some plots that show an initial S/N estimate exercise. Given that the background tends to kill you in slitless spectroscopy, compact sources (PSF) are great.

Anyone who wants to play with S/N matters, go to the collab notebook here

This shows at R=1000, 1.5mum the continuum S/N (per resolution element) in a 1000 second exposure for sources of different spatial extent, as a function of their size (source diameter)

This shows the S/N (per resolution element) for a continuum point source, as a function of telescope diameter. For reference, Euclid~1m, Roman~2.4m

This is an analogous plot, but for a (spectrally) unresolved emission line on top of negligible source continuum. The envisioned line-flux sensitivity for ground-based "stage 5" redshift experiments (0.5e-16) is indicated (see https://arxiv.org/pdf/2209.03585.pdf )

This is a first attempt at mapping the S/N to some physical input, such as a star-forming region/galaxy as a function of of their size and SFR. Yes, spatial extendedness is a killer.

=======

Reminder 1: what is the physical resolution as a function of redshift

=======

Just as background, here's the sky values from Rigby+2023

Science Cases with such a Survey:

This deserves simulations and asking what range of lambda,R,NR, etc.. is optimal, and which acceptable

Spectra of Stars:

• 2-5 abundances of cool stars
• are there any zero-metallicity stars in the Milky Way?
• chemical identification of streams (as small-scale DM probes)
• find the fastest stars (in the bulge): BH dynamics
• spectra of every O stars within 5 Mpc
• free-floating (semi-young) planets and stuff

Spectra of AGNs:

• AGN as LSS probes to z=7(?)
• earliest AGN (z~12) ==> BH growth; seed BHs

Spectra of Galaxies 

What can we expect for emission line spectroscopy of galaxies?

Let's take the Yung, Somerville+2022 SC-SAM simulations, and the Kennicutt

conversion of SFR --> Halpha; and request a 7 sigma Halpha detection, given line flux and disk-size of the galaxy. Consider a total area of 10.000 sqdeg on the sky. Quite staggering galaxy numbers ....

• ?? <what are the most interesting things>
• host galaxy diagnostics of BH GW events with LISA

Cosmology:

• probes of inflationary signatures (by stage 5+ spectroscopy)

• kinematic lensing on steroids
• 
  

(inadvertent) Spectra of Transients

• way too many gravitational lenses with spectra
• serendipidous (single epoch) SN spectra to faint levels
• GRB hosts
• tidal disruption events in AGN
• <you name it>

Low-mass objects

• there are ATMO2020 models from Phillips+2020 and newer (JWST-oriented) models Legget&Tremblin 2024
  
  
  

from Theissen, Burgasser et al 2023.  LTY dwarf spectral library

How many more stars (for stellar streams) does one get going below the MS turn-off
(from Bellazini+ https://arxiv.org/abs/1203.3024) 

going from absmag 2 (in I) to 3 is 20x more stars

Notes on crowding:

I downloaded Gaia data in Baade's window, and did number counts, which resulted in an estimate of how many stars there are per spectral streak area (code at getBaadesWindow.ipynb) in 

/Users/rix/Science/Projects/SlitlessSpectroscopySpace/SpectraSims

The plots looks like this, and implies that the crowding is unproblematic to 21st magnitude

Next steps would be to 

a) get deeper data

b) calculate the Poisson probability of being uncontaminated and add it to that plot.

More science application ideas

The universe through a looking-glass

Strong (>30) lensing magnification (size and flux) happens, by is rare. When it happens, it opens up a new regime of spatial resolution.

Takahashi et al 2011 have calculated statistics. The plot below shows magnification averaged over 3kpc

For compact sources there should be much more magnification.

Extreme magnification probabilities have been discussed in Diego (2019)

So, there is a 1 in a million chance to get magnification of more than a 1000. A linear magnification of 20 leads to a physical resolution (6.5m at 0.75mum) of 9 pc at z=5.