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
APPLICATION FOR OBSERVING TIME
from
1.
X
MPIA
MPG institute
Telescope:
2.2-m
X
Benjamin P. M.
2.1 Applicant
other
Laevens
Max-Planck Institut für Astronomie
Name
Institute
Königstuhl 17
D-69117 Heidelberg
street
ZIP code - city
blaevens
[email protected]
ESO User Portal username
e-mail
Dr. N. F. Martin
Max-Planck Institut für Astronomie
name(s)
institute(s)
Prof. Dr. H-W Rix, Dr. Branimir Sesar
Max-Planck Institut für Astronomie
name(s)
institute(s)
Benjamin P. M. Laevens
Dr. Nicolas F. Martin
name
name
2.2 Collaborators
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: A
Observing programme:
Title
:
Abstract :
Follow-up for potential Milky Way Dwarf Galaxy Candidates
We propose to follow-up our 15 most significant Milky Way (MW) dwarf galaxy (DG) candidates found in the Pan-STARRS1 3π (PS1) survey to confirm their nature. The proposed
observations in the B- and V-band with the 2p2 Wide Field Imager will produce deep CMDs
of our candidates, reaching ∼ 1.5 mags deeper than the survey data, and unambiguously
confirm the presence/absence of DGs among our targets. Despite spanning twice the coverage of the SDSS that enabled the discovery of a dozen new MW DGs, we have discovered
just one new globular cluster (GC) and no new DGs, hinting at a strong anisotropy in their
distribution, at odds with predictions from ΛCDM. The proposed observations will bolster
this conclusion by confirming no faint DG lies just below the PS1 detection limit.
X
4.
Instrument:
WFI
FEROS
GROND
5.
Brightness range of objects to be observed:
6.
Number of hours:
15
from
applied for
20
no restriction
7.
grey
to
24.5
V-band
already awarded
still needed
none
none
dark
Optimum date range for the observations: ............................ 01.09.2014 – 01.03.2015
Usable range in local sideral time LST: ................................................ h – h
8a.
Description of the observing programme
Astrophysical context
Previous work
Over the course of the last forty years, Local Group
DGs have become a source of increasing interest for a
variety of reasons. Such galaxies are the faintest and
smallest galaxies known, with large mass to light ratios
and are probes of both the dark matter (DM) on the
galactic scale, and of the faint end of galaxy formation. In the currently favoured ΛCDM Cosmological
Model, there appear to be discrepancies between the
distribution of DGs around their host galaxy, with simulations favouring an isotropic distribution [1]. However, current observations show most DGs to be distributed anisotropically in a plane around their host,
e.g. the Vast Polar Structure of MW Satellites (VPOS)
[2]. Similarly, 50% of DGs around M31 lie in a plane [3].
Until recently, only regions focussing on the plane of
DGs were probed around the MW. Therefore, searching for DGs away from the plane is essential to contradict or reinforce this tension between models and
observations.
The Sloan Digital Sky Survey (SDSS) revolutioned
MW science relatively recently, discovering numerous
faint DGs, trebling the number of known MW DGs and
bringing the current census to 24. With new coverage
equalling that of the SDSS again and attaining similar
depths in optical bands, the Panoramic Survey Telescope And Rapid Response System 1 (Pan-STARRS 1,
PS1) gives us an unprecedented and unique opportunity to search for new MW satellite DGs away from the
proposed plane of satellites. The promising prospects
of the PS1 survey were fulfilled with the discovery of
three new M31 satellites: Lacerta I, Cassiopeia III [4]
and Perseus I [5]; however, our current analysis produces no new, unmistakable MW DGs, possibly emphasising the strong anisotropy of the MW satellite
distribution. Therefore, we wish to observe our most
significant candidate-DG detections to either confirm
they are simply noise fluctuations in the PS1 data, or,
alternatively, discover new, faint DGs that have so far
gone unnoticed.
Inspired by previous MPIA searches for DGs in the
Local Group [6], and methods used in the SDSS [7],
we have developed an automated search algorithm
adapted to PS1[8]. We isolate typical potential blue,
metal-poor DG stars, based on their photometric properties and account for varying distances of such DGs
in CMD space. We then convolve these sources with
window filters (Gaussians) of varying sizes to find local
over densities, accounting for background effects, spatial incompleteness of the survey, the complex survey
footprint and MW contamination. Finally, we quantify the detection significance of our convolution maps.
Applying our search algorithm delivers a 9/13 recovery rate of the known SDSS DGs, reinforcing our confidence in the search method. We quantify the significance of our detections (see Fig. 2). Significant detections are investigated by looking at the over density’s
CMD and comparing it to a field region to discern the
presence of a real over density as well as studying the
(non-) presence of CMD features of a DG (Fig. 1).
Besides the discovery of one new GC[9], a lack of new,
definite DGs, leads us to build a list of candidates that
are too faint to be reliably found in PS1.
Layout of observations
DGs have a typical size of 5-20 arcmin; hence one WFI
pointing per candidate will suffice since the field of
view is several half-light radii of a DG. We will acquire photometry in two bands (B and V) allowing us
to construct CMDs of our objects. Past experience
in DG searches has shown that observations down to
V∼24.5 are deep enough to reliably confirm DG detections out to ∼ 200 kpc [10]. Finally, we motivate
the number of candidates we have chosen such that we
have enough potential candidates, so we do not miss
a satellite, whilst at the same time not choosing too
many with low S/N values, eating in to the noise and
therefore being a likely waste of telescope time. We
were awarded 50 hours of time at La Silla (May 2014)
to observe our candidates for half of the sky in the
Immediate aim
Southern hemisphere and 20 hours at Calar Alto for
We propose to follow-up our 15 most likely DG candi- our search in the Northern hemisphere. We submit this
dates by probing approximately 1.5 magnitudes deeper second proposal to complete our search for candidates
than the limiting magnitude of the PS1 survey to estab- for the other half of the Southern sky.
lish the true nature of these over densities. We envisage
this follow-up to reinforce or contradict the anisotropic Strategic importance for MPIA
distribution of DGs. Identifying a penury of DGs in
PS1, would lead to a paper relating the implications The follow-up of MW DG candidates is a continuaof this result to the ΛCDM prediction and therefore tion of the work done at the institute with respect to
unambiguously confirm the observed anisotropy. Dis- untangling the general characteristics of Local Group
covery of new DGs would remain an astrophysically DGs with the aim of testing cosmological predictions
relevant result, providing additional examples of DGs on galactic scales. This follow-up is of key importance
whose properties can be studied in more depth. Fi- in the context of the PS1 Consortium’s work where the
nally, the discovery of new DGs would seriously lower Key Project 5 (MW and Local Group science) is led by
two members of the institute (Dr. N. Martin and Prof.
the significance of any planar alinement of satellites.
Dr. H.-W. Rix).
2
8d.
Figures and tables for backup programme
3
9.
Objects to be observed
(Objects to be observed with high priority should be marked in last column)
Designation
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
α (2000)
δ (2000)
magnitude in
spectral range
to be observed
priority
01h 31m 58.s
07h 59m 32.s
05h 41m 53.s
06h 03m 50.s
08h 45m 10.s
10h 33m 38.s
05h 29m 42.s
08h 48m 49.s
09h 16m 40.s
09h 37m 18.s
03h 05m 10.s
02h 40m 48.s
05h 40m 01.s
08h 16m 28.s
08h 35m 44.s
−13◦ 380 4300
+03◦ 260 2500
−23◦ 550 2600
−17◦ 400 1500
−01◦ 470 5300
−16◦ 030 5600
−21◦ 550 1600
−16◦ 300 1500
−09◦ 240 1400
−27◦ 090 4100
−28◦ 380 5200
−06◦ 030 1400
−21◦ 390 5400
+01◦ 410 5600
−12◦ 090 3700
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
V∼24.5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
4
10. Justification of the amount of observing time requested:
In the interests of making our DG search as efficient as possible, we propose to reach a depth of V∼24.5.
We make an estimate of the total time required by using the exposure time calculator. Using a typical
seeing of 1.0”, an airmass of 1.3, a S/N=5, we estimate an exposure time of 19 and 28 minutes for the V
and B-bands respectively. This totals to 47 minutes. We plan to take three dithered sub-exposures per
filter. Accounting for 1 extra minute per sub-exposure, this adds an extra 6 minutes, totaling 53 minutes.
In addition to this, we take into account any additional overheads such as the slewing of the telescope,
rounding up our estimate of time to one hour per candidate. Since we have fifteen candidates, this scales
up to an equal number of observing hours. Finally, we add on an extra 25-30 % to our estimate in buffer
time to account for adverse seeing as well as the disruption induced by GROND, bringing our total to 20
hours observing time.
This telescope is ideally suited for the follow-up of our candidates for two primary reasons. Pan-STARRS
1 extends the SDSS coverage to those parts of the sky situated between δ = 0◦ and δ = −30◦ , leading
to the natural choice of a telescope in the southern hemisphere. We choose candidates spread out over
a range of RA=24h to 12h, to maximise our observing possibilities. We already observed our first set of
candidates during our run in May 2014. We would like to complete our follow-up of candidates for the
other half of the sky. Secondly, the wide field of view of the La Silla 2.2m telescope is ideally suited to
the purposes of this follow-up. With DGs ranging in typical sizes of 5’ to 20’, time can be efficiently and
well spent, with just one pointing per candidate allowing us to obtain photometry of the central regions
of our supposed over densities as well as enough coverage in the vicinity of the object to get reference field
CMDs to compare to the over density’s CMD.
11. Constraints for scheduling observations for this application:
Our targets have been chosen to span a range in RA that is observable all throughout the semester. Owing
to our goal of reasonably deep B-band observations, we request dark time for this programme.
12. Observational experience of observer(s) named under 2.3:
(at least one observer must have sufficient experience)
Laevens has had 9 nights worth of experience with WFI during the current run at La Silla (May 2014).
Martin has ample experience with wide-field imagers and has, in particular, observed numerous times with
the 2p2 in La Silla. We have the pipeline in place for the data reduction, therefore observations and data
reduction will therefore not be an issue for our group.
13. Observing runs at the ESO 2.2m-telscope (preferably during the last 3 years)
and publications resulting from these
Telescope
LBT
2.2m
LBT
2.2m
2.2m
instrument
LBC
WFI
LBC
WFI
WFI
date
Oct-Dec 11
Aug 11
Nov 12
Jan 14
May 14
hours
9h
10n
9h
1h
9n
5
success rate
0%
0%
60%
100%
25%
publications
technical and weather issues
weathered out
in progress
[9]
[8]
14. References for items 8 and 13:
[1] Moore et al. (1999): Dark Matter Substructure within Galactic Halos, ApJ 524, L19
[2] Pawlowski et al. (2012): The VPOS: a vast polar structure of satellite galaxies, globular clusters and
streams around the Milky Way, MNRAS 423, 1109
[3] Ibata et al. (2013): A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy, Nature
493, 62I
[4] Martin et al. (2013): Lacerta I and Cassiopeia III. Two Luminous and Distant Andromeda Satellite Dwarf
Galaxies Found in the 3π Pan-STARRS1 Survey, ApJ 772, 15M
[5] Martin et al. (2013): Perseus I: A distant satellite dwarf galaxy of Andromeda , ArXiv e-prints 1310.4170
[6] Koposov et al. (2008): The Luminosity Function of the Milky Way Satellites, ApJ686, 279K
[7] Walsh et al. (2009): The Invisibles: A Detection Algorithm to Trace the Faintest Milky Way Satellites,
AJ 137, 450W
[8] Laevens et al. (in prep): A search for Milky Way dwarf galaxies in the PS1 survey
[9] Laevens et al. (in prep): Discovery of the most distant Milky Way globular cluster
[10] Belokurov et al. (2007): Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions,
ApJ 654, 897B
6
Tolerance limits for planned observations:
maximum seeing:
photometric conditions:
1.200 minimum transparency:
moon: max. phase /
6
:
% maximum airmass:
/◦ min. / max. lag:
1.5
/ nights