Application No.
Time Allocation Committee for
MPG time at the ESO 2.2m-telescope
c/o MPI für Astronomie
Königstuhl 17
D-69117 Heidelberg / Germany
Observing period
Received
October 2016
APPLICATION FOR OBSERVING TIME
from
1.
X
MPIA
MPG institute
Telescope:
2.2-m
X
Hans-Walter
2.1 Applicant
2.2 Collaborators
other
Rix
MPIA
Name
Institute
Knigstuhl 17
69120 Heidelberg
street
ZIP code - city
ESO User Portal username
e-mail
Brani Sesar, Nicolas Martin
MPIA/Uni Strasbourg
name(s)
institute(s)
name(s)
institute(s)
Nicolas Martin
MPIA
name
name
2.3 Observers
By specifying the names under item 2.3 it is obligatory to also send out these observers to
La Silla, if required. Correspondence on the rating of this application will be sent to the
applicant (P.I.) as quoted under 2.1 above.
3.
Category: C
Observing programme:
Title
:
Abstract :
A New Way of Finding Ultra-faint Distant Satellite Galaxies
We know that there are galaxies that only contain a few hundred stars in total. But we know
them only as Milky Way satellites to distances of ∼ 25 kpc because they are so hard to find.
Yet, most ultra-faint galaxies contain two or more RR Lyrae (RRL) stars; this makes close
pairs of distant RR Lyrae halo stars a great starting point for a search. Using PS1 data, we
have just compiled a clean (90% pure, 80% complete), deep (to ∼ 120 kpc) sample of 35.000
RRL. Among them there are about 10 close, equidistant pairs, at distances beyond 45 kpc
from us, unlikely to be mere coincidences. We plan deep V & I photometry (to 1 mag below
a presumed MS turn-off) towards four RRL pairs, to see where any of them are signpost for
ultra-faint galaxies, at yet unexplored distances (≥ 50 kpc) in the halo.
X
4.
Instrument:
WFI
FEROS
GROND
5.
Brightness range of objects to be observed:
6.
Number of hours:
18
from
applied for
10
no restriction
7.
grey
to
24
r-mag
already awarded
still needed
none
none
dark
Optimum date range for the observations: ............................ 01.08.2017 – 30.09.2017
Usable range in local sideral time LST: ...................................... 14:00h – 23:20h
8a.
Description of the observing programme
Astrophysical context
deep photometry, searching for other CMD signatures
of small galaxies (RGB, MS-turn off etc..): a promising
The condensation and concentration of baryons in low
avenue to find ultrafaint galaxies at greater distances.
mass halos, to densities high enough that it leads to
star formation, appears to be increasingly difficult in
dark matter halos of lower and lower mass: conse- Immediate aim
quently galaxy formation as a whole must have a lower Indeed the new RRL sample contains about a dozen
(halo) mass limit, as star-formation becomes impos- pairs of RRL, close on the sky and in distance modulus;
sible. In ultra-faint galaxies, where only a few hun- the distance moduli of the best candidates range from
dred stars managed to form over 10 billion years in- 18. to 19.5, i.e. from 50-80 kpc. These pairs of RRL
side a small dark matter halo, Nature seems to have have projected separations of ∼ 250 pc, comparable
nearly reached that limit. There are many reasons to to the expected diameters of ultrafaint satellite galaxunderstand the lower limit of galaxy formation well. ies. For the four directions accessible in the upcoming
The only systems we know about and can study are semester with the [email protected], we propose to obtain
in our immediate vicinity (≤ 40 kpc), because current deep V & I WFI photometry in the 300 × 300 FOV.
searches have been unable to discover them beyond. This photometry should reach V=24 and I=23.4 mag
Yet, the properties of the faintest galaxies that are not (at 7σ) respectively, which is 1-1.5 mag below the MS
deep inside the halo of a big galaxies (with the ensuing turn-off if there is a satellite galaxy at the (known) disdynamical tides and the gas stripping) could be quite tance modulus of the RRL. This photometry will prodifferent. Therefore, finding the faintest galaxies, and vide a definitive test whether the pairs of RRL were
among them those furthest away from the center of the signposts of a faint dwarf galaxy or not. In this way,
Milky Way, remains a crucial goal.
the proposed observations offer the best current opOver the last decade or so, the SDSS survey, then portunity to find very faint satellite galaxies at greater
PS1 and DES, have been sky surveys that have revolu- distances from the MW, where their formation may
tionized this field, by discovering “ultra-faint” Milky have been less affected by the central galaxy.
Way satellite galaxies, whose formation and enrichment histories could then be studied in detail. But
the current paradigm predicts that there should be nu- Previous work
merous small galaxies at larger distances.
This proposal is on the one hand based on the longWhy may they not have been discovered, yet? Ul- term MPIA effort to identify a clean sample of faint
trafaint galaxies are best discovered as clumps of main- RRL across the sky; this sample is an unmatched resequence-turn-off (MSTO) colored stars in magnitude, source at this point. The proposers have a long track
color position space. Current all-sky surveys go to record of discovering new satellite galaxies, as well as
∼ 21 mag, limiting this approach to distance moduli obtaining, analysing and publishing multi-band phoof ≤ 17 (given an absolute magnitude of the MSTO tometry.
of 4). At greater distances, such galaxies can be (and
have been) found as clusters of RGB stars; but as these
Layout of observations
stars are hard to distinguish from foreground K-dwarfs,
many RGB stars are needed for detection.
The goal is to construct a V-I color magnitude diaIdeally, one would like to have a luminous but high- gram that extends about a magnitude below the MSTO
contrast tracer of the old metal-poor populations ex- (presuming there is a galaxy with an old, metal poor
pected for ultrafaint galaxies. RR Lyrae stars (RRL) population) at the distance and direction indicated by
are such a tracer. With absolute magnitude of ∼ 0.5, the RRL. The pairs of RRL are separated by 15-20 arthey can be seen to D ∼ 100 kpc; and they have little cmin on the sky, suggesting a single deep pointing with
foreground contaminants (e.g. Sesar et al 2015). In a WIFI in each case. We have tools for data reduction
sequence of papers (Hernitschek et al 2016, Sesar et al and CMD analysis in place, and can therefore ensure
2016) we have managed to identify a high-quality sam- rapid turnaround. Any population that produces two
ple of RRL (90% purity, 80% completeness) extending RRL, will have enough stars to show a very marked
to ∼ 120 kpc across 3/4 of the sky, based on PS1 data; main sequence turn-off. The data will decide cleanly
the data also afford a distance precision of 3%. This whether there is an ultrafaint galaxy in any of these
spectacular new sample of RRL is presented in Figure directions or not; alternatively , the two RRL are not
1. Remarkably, basically all known faint satellite galax- part of a bound system.
ies of the Milky Way show up as clumps of equidistant
RRL stars. [So do many globular clusters, though the Strategic importance for MPIA
PS1 photometry is not tuned to the crowded fields.]
With this sample at hand, an obvious approach This may be the last program of photometric follow-up
would be to ask whether there are clumps of distant of PS1 candidate galaxies, which has been a strategiRRL that are not identified with known MW satellites. cally important (and successful) program for the last
Those directions would be ideal directions to obtain five years.
2
8b.
Figures and tables
angular distribution of RR Lyrae candidates
NGC5024 ComaBerenices(CBe)
CVnI
NGC5272
NGC5053
NGC5466
BootesI(Boo)
LeoV
Sagittarius(trailing)
Sagittarius
(trailing)
UrsaMajorI(UMaI)
Latitude (b)[degrees]
LeoII
60
LeoIV
LeoI
NGC5904
UrsaMinor(UMi)
UrsaMajorII(UMaII)
dwarf galaxies
globular cluster
galaxy
E(B-V)=0.5
E(B-V)=1
E(B-V)=1.5
Draco(Dra)
Sagittarius
(leading)
LeoT
NGC4590
Hercules(Her)
Sextans(Sex)
NGC5897
30
NGC5694
135
90
45
0
315
270
225
10
180
0
-10
NGC6934
M31
NGC6864
NGC7078
NGC6981
M33
Segue2
Orion
Pal13
NGC1904
NGC7089
PiscesII
-60
Longitude (l)[degrees]
Figure 1: Sky map of the new, deep and pure sample
of RR Lyrae across 3/4 of the skay, derived from PS1
(Sesar et al 2016, in prep.). Basically all known faint
satellite galaxies in the PS1 footprint show up (perhaps not in the Figure) as tight clumps or pairs of RR
Lyrae stars. The numerous RRL in the Draco dwarf
galaxy confirm a 3% distance precision. This distance
precision makes for a powerful discriminant in isolating
pairs of RRL that are close on the sky and equidistant.
3
-30
9.
Objects to be observed
(Objects to be observed with high priority should be marked in last column)
Designation
PS1-RRL-1
PS1-RRL-2
PS1-RRL-3
PS1-RRL-4
α (2000)
δ (2000)
magnitude in
spectral range
to be observed
priority
13h 58m 10.s 0
21h 43m 53.s 0
22h 59m 07.s 0
23h 10m 13.s 0
−24◦ 290 5900
−06◦ 420 3100
15◦ 380 5700
−7◦ 270 5400
23
23
23
23
1
1
1
1
4
10. Justification of the amount of observing time requested:
To reach 1 magnitude below the turn-off for old pupulations at a DM of 18.5-19.2, we need to reach V=24
and I=23.4 at 7σ. This requires dark time. Based on the ETC, and assuming 1 arcsec seeing, this requires
1 hour per pointing and filter, or 2.5 hours total per candidate object (RRL pair), including overheads.
This makes for a total request of 10 hours of dark time, late in the semester. We would prefer service
mode; visitor mode is feasible.
11. Constraints for scheduling observations for this application:
12. Observational experience of observer(s) named under 2.3:
(at least one observer must have sufficient experience)
N. Martin has often observed at La Silla
13. Observing runs at the ESO 2.2m-telscope (preferably during the last 3 years)
and publications resulting from these
Telescope
instrument
date
hours
success rate
publications
PI of this proposal has not submitted PI proposals to the 2.2m in the last 3 years.
5
14. References for items 8 and 13:
6
Tolerance limits for planned observations:
maximum seeing:
photometric conditions:
1.200 minimum transparency:
yes moon: max. phase /
6
:
70% maximum airmass:
0.2/45◦ min. / max. lag:
1.5
/ nights