1. Extra-galactic spectral lines – SWG SKA Town Hall2017
Rob Beswick &
Francoise Combes
On behalf of the ex-gal spectral line SWG

2. SWG update New SWG co-chair – Francoise Combes (Obs. Paris)
John Conway stepped down as SWG co-chair effective Mar
John has made numerous contributions to the founding and development of this SWG, and continues to play a major role.
Continued activity and growth of SWG membership.
Strong recognition of science area in the community and the potential SKA capabilities

3. Background to SWG science areas
This SWG:
Many important astrophysical line tracers beyond HI (21-cm)
Uniquely trace different material/environment = different science
From high-redshifted lines to local universe probe of physics
Success of instruments like ALMA underline the crucial nature of such traces
SKA-MID Bands 2 & 5 provide critical science capabilities in this area.. This SWG is a strong advocate of increased frequency range/capability
Strongly supports the band priority order assigned in 2015 Re-baseline process:
‘Receiver bands 2, 5 and 1 should be constructed for all SKA1-Mid dishes with their priority order as written.’

4. Science areas Our aims to facilitate the science development:
focus on ‘key science concepts (KSCs)’ rather than projects -
Commensal/complementary science goal synergies with other SWG areas
Broad range of science areas/drivers…
Galactic-type masers in local universe
Local galaxies
Dense molecular gas traces [thermal & masers lines]
RRLs
Wide-area line Maser searches (OH/H20 etc)
High-z dense gas tracers
CO/CS/HCN etc, lensed lines [follow-up]
Combination of commensal observing as well as specific programmes

5. Frequency coverage of main lines*
19.2 [GHz]
9.6
4.8
2.4
1.2
0.6
0.3
SKA1-MID
Band 1
GHz
Band 2
GHz
Band 5
GHz
ALMA
38.4
19.2
9.6
4.8
2.4
1.2
0.6
0.3
76.8 [GHz] 1 2 3
OH masers
H2O maser
formaldehyde
maser
OH* masers
methanol maser
methanol* maser
*NOTE: the SKA rebaseline process Band 5 ( GHz) or now band 5a/b/[c]) was prioritized to second highest science priority.
Science priority order Band 2, 5, 1
View supported by this SWG.

6. Frequency coverage of main lines
19.2 [GHz]
9.6
4.8
2.4
1.2
0.6
0.3
SKA1-MID
Band 1
GHz
Band 2
GHz
Band 5
GHz
ALMA
38.4
19.2
9.6
4.8
2.4
1.2
0.6
0.3
76.8 [GHz] 1 2 3
Radio recombination
lines
Radio recombination
lines
Local dense gas tracers – HCN, HCO+, HNC etc

7. Frequency coverage of main lines
19.2 [GHz]
9.6
4.8
2.4
1.2
0.6
0.3
SKA1-MID
Band 1
GHz
Band 2
GHz
Band 5
GHz
ALMA
38.4
19.2
9.6
4.8
2.4
1.2
0.6
0.3
76.8 [GHz] 1 2 3
HCN[1-0]
HCO+[1-0]
CS[1-0]
CO[1-0]
redshift
HCN[1-0]
HCO+[1-0]
CS[1-0]
CO[1-0]

8. Cost Control project This SWG:
On-going assessment of implications. Time constraints mean this review is not complete yet.
Short preliminary assessments presented here

9. Cost Control project Preliminary headline science conclusions:
[MID] Band 5 capabilities critically important to SWG.
Band 5 to 2.5GHz B/w [5.13.2] – if frequency-slicing/zoom modes can be spread over full tuneable range for lines – OKAY, but still effects line search parameter space.
Reduce MID band 5: 2.5 to 1.4GHz  Effect on simultaneous line observations, and line searches. -> loss of commensality, efficiency and loss of science
Cuts half (13067) RXs elements deployed detrimental to capabilities [5.5.2] – loss in sensitivity & capability
Full loss of Band 5 – Devastating science implications/losses
(SWG’s view is that there is a disproportionate number of cuts to band 5, cf. ‘lower priority’ band 1)
[MID] Baseline length reductions [5.24.] – ‘initially incremental downgrade’ SWG consensus opinion prefers removal core dishes over further erosion of resolution.
Longer integrations can re-gain sensitivity on already well sampled short spacings. But reducing baseline lengths cannot be mitigated for by operational/observational adaptations – should be guarded against.

10. Cost Control project – details #1 specific SWG comments
Essential that Zoom modes be preserved

11. Cost Control project – details #2 specific SWG comments
Implications for this SWG. Reduction in availability of highest data rate projects reduced may commensality with other programmes
Significant effect on band 5 continuum science – but less critical for this SWGs specific work
Targeted line work – Okay as long and B/W can be split to accommodate multiple simultaneous lines separated by >2.5GHz
Line searches – e.g. redshifted CO etc. loss of search parameter space.
Reduced resolution:
Prefer to see dishes removed from core rather than arms. But cost implications noted.

12. Cost Control project – details #3 specific SWG comments
Significant effect on band 5 science – multiple SWGs.
Major reduction in sensitivity – inc high res capabilities (re-weighting withstanding)
Core baselines required for VLBI (inc work in this SWG).
Significant reduction in competiveness c.f jVLA (sensitivity) or e-MERLIN (resolution). Compounded for continuum science when bandwidth reductions are included

13. Aside: Band 5 deployment locations
Locations of Band 5 systems only to be deployed on half RXs
[option a] ‘core+arms’
[option b] ‘deploy on arms only’
[option c] ‘deploy core only’
SWG preference for options b/a
- As standalone instrument option b is preferable.
But – Major loss and compromise to ability to phase-core up in VLBI-mode. – large science potential loss for this SWG (e.g. maser follow-ups).
Logical compromise is a blend of options b and a.
If option B, needs enough feeds (~20++) deployed on inner part b~<10km for phasing-up for VLBI
- Option c strongly disfavoured

14. Cost Control project – details #3 specific SWG comments
Further reductions on band 5 B/W – multiple SWGs.
Major reduction in sensitivity for continuum
Simultaneous line capabilities removed – detriment to this SWG

15. Cost Control project – details #4 specific SWG comments
Implications for this SWG less clear. Reduction in availability of highest data rate projects/reduced commensality of programmes

16. Cost Control project – details #5 specific SWG comments

17. Transformational(?) and comments/ordering
The two highest priority bands for this SWG are MID bands 2 & 5
Band 5:
Cuts even above the line and especially to Band 5 risks the science in a number of areas (inc. this SWG).
Combined 50% cut in feeds deployed, B/w limitations have a large cumulative effect
Biggest effect is on continuum science objectives but line work also hit hard.
Ratio of cuts between band 5 and band 1 is disproportionate – no direct cuts proposed to band 1 until the worst-case scenario below the cost cap. This is at complete odds to the re-baseline priority order.
Competitive edge compared to jVLA & e-MERLIN is eroded in this band, albeit less of an erosion for line work than for continuum science
Other SWGs will have different opinions of science priorities, ability to reinstate elements is important to remember – ‘and should be some comfort’.
Band 2:
Minimal cuts specific to band 2.
For this SWG it remains essential that the Band 2 top frequency remains at 1750MHz or greater so that OH lines at low redshift can be accesses.
Band 1: Not a top priority for this group (compared other bands at least)

18. Cost Control project Preliminary headline science conclusions:
[MID] Band 5 capabilities critically important to SWG.
Band 5 to 2.5GHz B/w [5.13.2] – if frequency-slicing/zoom modes can be spread over full tuneable range for lines – OKAY, but still effects line search parameter space.
Reduce MID band 5: 2.5 to 1.4GHz  Effect on simultaneous line observations, and line searches. -> loss of commensality, efficiency and loss of science
Cuts half (13067) RXs elements deployed detrimental to capabilities [5.5.2] – loss in sensitivity & capability
Full loss of Band 5 – Devastating science implications/losses
(SWG’s view is that there is a disproportionate number of cuts to band 5, cf. ‘lower priority’ band 1)
[MID] Baseline length reductions [5.24.] – ‘initially incremental downgrade’ SWG consensus opinion prefers removal core dishes over further erosion of resolution.
Longer integrations can re-gain sensitivity on already well sampled short spacings. But reducing baseline lengths cannot be mitigated for by operational/observational adaptations – should be guarded against.