1. Galactic Diffuse Emission for DC2
Gamma-ray Large Area Space Telescope
Galactic Diffuse Emission for DC2
S. W. Digel
Stanford Linear Accelerator Center

2. Outline
Motivation: Diffuse gamma-ray emission from the Milky Way is an important component of the sky model, of course, and also for likelihood analysis (model fitting) of the LAT gamma rays
Rest of talk: The current state of the LAT diffuse emission model and improvements possible for DC2 and Science Tools Checkout 3
~60% of EGRET g-rays were diffuse emission from the Milky Way
(~30% isotropic emission, and ~10% from detected point sources)
EGRET
>100 MeV, Phase 1-5

3. Current state of the LAT diffuse emission model
Aside: ‘diffuse emission model’ means the same thing as ‘interstellar emission model’ and ‘Galactic diffuse emission model’. Galaxy with a capital ‘G’ means the Milky Way.
For observation simulation and likelihood analysis we have the MapCube source (by Jim Chiang) for specifying a diffuse emission model in as much detail as anyone could want
For Science Tools Checkout 2 (March-April this year), we used a gamma-ray intensity map calculated by GALPROP using the cosmic-ray distributions of Strong, Moskalenko, & Reimer (2004)
This is the latest comprehensive study with GALPROP and involved optimizing (in some sense) the model for different regions of the sky
The MapCube model for Checkout 2 was 720×360×32 (grid spacing 0.5° in Galactic coordinates and ~30 MeV – 980 GeV in steps of factors of 1.4 in E)

4. Current state (2) Example Checkout 2 GALPROP intensity map for 1.2 GeV
Many of the DC2 goals related to the diffuse emission model are evident here
From

5. Current state (3)
Gamma-ray intensity maps from GALPROP have not been used for before for quantitative analyses like source detection (to my knowledge)
Details that were not particularly important for studies of cosmic-ray propagation or diffuse emission on large angular scales now become important
Within 10-15° longitude of the Galactic center and anticenter, the model is essentially interpolated.
At low latitudes in particular this is because the distribution of gas along these lines of sight is not well known
Hard edges for Taurus and Ophiuchus clouds are artifacts of this interpolation, too
Some of the large-scale, low-level striping is probably due to calibration artifacts of the old H I surveys used (see later); the rest may be due to what (IC model? line-of-sight integration effects?)
Calibration of absolute intensity is more important than small scale stripes

6. What’s new (or newly available)
The best-yet composite H I survey of the sky has been released (although it is not yet available on Vizier) Leiden/Argentine/Bonn (Kalberla et al. 2005)
Other developments for inputs – new CO surveys, evidence for ‘dark’ molecular gas, a new model for the ISRF, and new g-ray production functions (see )

7. What’s new (2)
For current work on GALPROP, please see the presentation by Igor Moskalenko at the Catalogs and Diffuse Emission workshop last month at SLAC

8. Issues to be resolved for DC2
Can we resolve kinematic distance ambiguities sufficiently well to make it worth doing?
How will we incorporate the ‘dark’ molecular gas, how will we deal with cold atomic hydrogen (seen in self absorption) and determining N(H I) in general, and how significant are the dark and cold components relative to the ISM traced by CO and H I emission.
Modeling the interstellar medium toward the Galactic center and anticenter
Grenier, Casandjian, & Terrier (2004)

9. Issues to be resolved for DC2 (2)
What angular resolution is appropriate for the diffuse emission model? Do we need a full MapCube representation or just a 2-dimensional map × one spectrum?
Spectral variations in current GALPROP model are not very great
Average spectra for 10° ranges of Galactic latitude
-10-0° and 0-10°
Artifact (reached end of IC model, I think)
From

10. Issues (3)
Yes, as Julie has pointed out, we don’t necessarily want the model provided for DC2 analyses to be perfect
Analysis of the diffuse emission per se is not a formal goal of DC2, as far as I know, but we know that our model won’t be perfect, even after tuning

11. Who does what?
The interstellar emission model is a deliverable from the LAT team and the Diffuse Emissions science working group has been chartered to develop it (surprise)
If analysis of the Galactic diffuse emission itself is not also a goal of DC2, then the Diffuse Emissions group can be asked to provide both the sky model version (secret) and the distributed model (used for analyses)