SALT and RR Lyrae Variables: Our Galaxy, The Magellanic Clouds and the Local Group (2024)

Michael Feast

Abstract

A review is given of the possibilities for the study of the kinematicsand metallicities of the old populations in galaxies of the Local Groupusing SALT/PFIS observations of RR Lyrae variables.

Astronomy Department, University of Cape Town, Rondebosch, 7701,South Africa.
mwf@artemisia.ast.uct.ac.za

1. Introduction

In her summary talk at the recent Carnegie Cosmology conference,Sandra Faber (2003) made two predictions:

1. “The major era of chasing cosmological parameters is now closing”

2. “Understanding galaxy formation…..will continue to occupycosmologists for some time……But there is no fundamentally newphysics 111As distinct, presumably, from new and unusualapplications of known physics. to be discovered there.”

These are bold, perhaps rash, predictions, of the kind that are often falsifiedby observations and SALT could well play a part in such falsification.

As regards galaxy formation, structure, and evolution, one gets some flavourof the present situation by considering the dwarf spheroidal galaxies which areoften considered the simplest of systems. Below are summarized theconclusions of some papers, all on dwarf spheroidals, published in the last fewmonths.
On the one hand:
Dwarf spheroidals are formed by the coalescing of clusters in dark matterhalos.
(astro-ph/0309202)
On the other:
Draco and UMi are weakly unbound systems - no significant dark matter.
(astro-ph/0309207)
But:
Draco is not the remnant of a tidally disrupted satellite but probablyis strongly dark matter dominated.
(ApJ 589, 798, 2003)
Though:
It is impossible to reproduce the tidal tail of UMi if it has a largedark matter content.
(ApJ 586, L123, 2003)
But again:
UMi is one of the most dark matter dominated galaxies known.
(ApJ 588, L21, 2003)
And finally:
The extended outer structure of UMi could be extra-tidal stars either unboundor within a dark matter halo.
(AJ 125, 1352, 2003)

I think it might be fair to summarize the above by saying that, notonly do we not know what dark matter is, we are not certain where it is,and some would even say, if it is. It is worth noticing in passing thatit has recently been suggested(Scarpa et al. 2003) that some globular clusters may contain dark matteror that modified Newtonian dynamics is required.

As for Our Own Galaxy, it has been commonly supposed that its halo was formedby the infall of dwarf-spheroidal-like objects. It was also arather general belief that the halo of Our Galaxy is typical of thehalos of spirals. However, we now know that the abundance of the α𝛼\alphaelements (i.e. [α𝛼\alpha/Fe]) in dwarf spheroids is not similar to theirabundance in our halo(e.g. Shetrone et al. 2003, Tolstoy et al. 2003). Evidently infallingdwarf spheroidals are notthe major source of at least the inner halo of Our Galaxy. Furthermore,recent work (Brown et al. 2003) suggests that the halo of M31 is distinctlydifferentfrom that of Our Galaxy. Thus bringing into question the notion of a“typical” halo.

It is clear that the current need is for detailed studies of theinternal structureand kinematics of all types of galaxies as a function ofthe chemical compositionsand ages of their component populations.

2. Variable stars and SALT

In carrying out programmes of this type we need in each galaxyto isolate hom*ogeneous groups of objects. Variable stars are particularlyuseful in this respect and amongst the variables there are three particularlyuseful types. These are the Cepheids which trace young, metal rich populations(the disc in Our Galaxy); RR Lyraes, old metal-poor objects,which trace halo-type populations; and, the Miras which, as a function ofperiod, trace much of the intermediate age populations. These three typesof variable star can be used to study the distances, kinematics anddistributions of different populations in Our Galaxy, in the Local Groupand, in some cases in more distant galaxies.

SALT is coming into operation in the era of massive variable star surveys.This offers great opportunities and will doubtless have a major effecton SALT programmes.MACHO, OGLE, MOA, SLOAN, QUEST are just some of the moreobvious current surveys. Such surveys will undoubtedly be extended in thefuture either with the direct aim of finding variables or elsefinding them as a by-product. For instance, planned surveys of thesky for near-earth objects should also find variables in large numbers.Not only are surveys finding very large numbers of variables tofaint limits, but they are being found in systematic ways which is of greatimportance in their use. In addition to this, surveys such as 2MASS allowpotential variables to be selected by their colours.

3. The RR Lyrae Variables

Of the three main types of variable star mentioned above, this paper willconcentrate on the RR Lyrae variables. It is in some ways quite appropriateto discuss here the potential of SALT for studying these stars in ourown and other galaxies. Just over 50 years ago when the 1.9m telescope, nowat Sutherland and then in Pretoria, was the largest telescope in thesouthern hemisphere, David Thackeray and Adriaan Wesselink used it todiscover the first RR Lyrae variables in the Magellanic Clouds and Thackerayused it to prove that the Hubble-Baade variables in the Sculptordwarf spheroidal were indeed RR Lyraes. These two discoveries were of majorimportance. The first, increased the distance modulus of the Clouds by1.5mag and was the most decisive evidence for a major increase in theextragalactic distance scale. The second, was particularly important inthe context of the population scheme that Baade was then proposing.The historical background to this work is given in the Baade-Thackeraycorrespondence (Feast 2000).

RR Lyrae variables are found in globular clusters as well as the general field.They are indicators of old, metal-poor populations, with [Fe/H] rangingfrom about 0.50.5-0.5 to 2.52.5-2.5. Their pulsation periods range fromabout 0.4 to 1.0 days if they are fundamental pulsators (the “ab” type)or about 0.2 to 0.5 days if they are overtone pulsators (“c” type).Table 1 lists some estimates of their absolute magnitudes.

MethodMVsubscript𝑀𝑉M_{V}reference
Parallax (HST)+0.62 ±plus-or-minus\pm 0.16Benedict, et al. 20021superscript200212002^{1}
Parallax (Hipparcos)+0.40 ±plus-or-minus\pm 0.22Koen & Laney 1998
Horizontal Branch+0.63 ±plus-or-minus\pm 0.12Gratton 1998
Globular Clusters+0.47 ±plus-or-minus\pm 0.12Carretta et al. 2000
δ𝛿\delta Scuti variables+0.49 ±plus-or-minus\pm 0.10McNamara 1997
Statistical parallaxes+0.79 ±plus-or-minus\pm 0.13Gould & Popowski 1998
Adopted Mean+0.58

1 see Feast (2002)

The adopted mean gives half weight to the result from Hipparcosparallaxes because this has a large standard error. Some of theother values may not be as well determined as their standard errorsmight imply. For instance the statistical parallax solution depends ona simple model of the galactic halo. Using the photometry ofRR Lyraes in the LMC (e.g. Clementini et al. 2003) with the adopted value ofMVsubscript𝑀𝑉M_{V} leads to anLMC distance modulus of 18.53 in good agreement with other values (seee.g. Feast 2003).

It has been known for some long while that the absolute magnitude ofRR Lyrae variables depends on their metallicity. In its simplestform this relationship can be written as:

MV=α[Fe/H]+γsubscript𝑀𝑉𝛼delimited-[]𝐹𝑒𝐻𝛾M_{V}=\alpha[Fe/H]+\gamma(1)

The MVsubscript𝑀𝑉M_{V} values given in Table 1 are for [Fe/H] = –1.5 assumingα=0.18𝛼0.18\alpha=0.18, a value often used. There has been considerable discussionof the true value of α𝛼\alpha. One reason that this is important isthat it affects the relative distances of globular clusters ofdifferent metallicities when using RR Lyraes as the distance indicator.These distances determine the absolute magnitudes at the main sequenceturn-off and thus the cluster ages. If α=0.18𝛼0.18\alpha=0.18, metal-poor clusters([Fe/H]2.2similar-todelimited-[]FeH2.2\rm[Fe/H]\sim-2.2) are older than metal-rich([Fe/H]0.7similar-todelimited-[]FeH0.7\rm[Fe/H]\sim-0.7) clustersby about 3Gyr. If however, α=0.39𝛼0.39\alpha=0.39 (Sandage 1993), then allglobular cluster would be about the same age, independent of metallicity(see e.g., Sandage & Cacciari 1990).Thus, establishing the dependence of RR Lyrae absolute magnitudes onmetallicity is of important for understanding the early history andevolution of Our Galaxy.

RR Lyrae variables are ideal spectroscopic targets for SALT. Photometry,fromlarge scale surveys is, or will be, available;the short periods of the variables limit thepossible exposure time to less than 30min (or perhaps 60min for thelonger period stars), and their radial velocities and metallicitiescan be adequately determined, for many purposes, at modest resolution(say R similar-to\sim 1000)well within the range of PFIS. Thus they will be good targets forthe first generation of SALT instrumentation. RR Lyrae metallicitiesare generally estimated using the method devised by Preston (1959).This method compares the strengths of the Balmer lines with that ofthe CaII(K) line, leading to a quantity ΔSΔ𝑆\Delta S which can be calibratedas a function of [Fe/H] from high resolution observations of nearbyRR Lyraes. (For discussions of the calibration and accuracy of the methodsee for instance: Suntzeff et al. 1991, Clementini et al. 1995,Lambert et al. 1996, Solano et al. 1997, Fernley and Barnes 1997.)

4. RR Lyraes and the LMC

Until recently the only spectroscopic metallicities knownfor LMC RR Lyraes were of six stars observed with the ESO 3.6m telescopeat R450similar-toabsent450\sim 450 and using the ΔSΔ𝑆\Delta S method(Bragaglia et al. 2001). However, a short paper and aconference proceedings have now appeared giving some results from theVLT. It is useful to look at these preliminary data in some detail as itgives some idea of what may be achieved with SALT both in the MagellanicClouds and elsewhere.

Both these studies used the VLT with the FORS1 instrumentation. Thishas a field of view of 7×7777\times 7 arcmin, somewhat smaller thanSALT/PFIS, and 19 slitlets. Note that FORS2 has the same size field but,like PFIS, can accommodate more slits. Minniti et al. (2003) obtainedtwo exposures of 20 min on each of six fields in the LMC bar. There werefive to ten RR Lyraes per fieldand the resolution was about 1000. Photometry(e.g. Clementini et al. 2003, Soszyński et al. 2003b) indicatesthat these stars are at V19.3similar-to𝑉19.3V\sim 19.3 and B19.7similar-to𝐵19.7B\sim 19.7.From these spectra Minniti et al. made an estimate of the velocitydispersion. To do this they needed to correct the observed velocitiesfor pulsational effects (since they did not have full velocity curves).This correction was made using a standard template with the phaseof the programme starknown from published photometry222It is worth noting that in this type of work the templateused needs to be carefully chosen since strong and weak lines givedifferent velocity amplitudes (e.g. Oke et al. 1962).The velocity dispersion derivedthen needed to be corrected for the scatter introducedby the template method and also for the estimated uncertainty in theradial velocity measurements. Table 2 shows their results.

Measured61 ±plus-or-minus\pm 7
Phase correction20
Measuring uncertainty22
“True” dispersion53 ±plus-or-minus\pm 10
kms1kmsuperscripts1\rm km\,s^{-1}

This result is of interest because the estimated dispersion islarger than that of other objects in the LMC: i.e.young population, 9kms1similar-toabsent9kmsuperscripts1\sim 9\rm km\,s^{-1}; Planetary nebulae,20kms1similar-toabsent20kmsuperscripts1\sim 20\rm km\,s^{-1}; Miras, 33kms1similar-toabsent33kmsuperscripts1\sim 33\rm km\,s^{-1}. However, it isclearlyonly a fore-taste of what might be done.

The other VLT/FORS1 study (Clementini 2003) summarizes the[Fe/H] results from Preston’s ΔSΔ𝑆\Delta S(resolution about 800)for about 100 RR Lyraes in theLMC Bar. From a plot of Vosubscript𝑉𝑜V_{o} against [Fe/H]Clementini and her co-workers findthat α𝛼\alpha in equation 1 above is 0.21. However, their figure 2 shows thatthis value is still rather uncertain, due mainly to the few points athigh and low metallicitiesConsiderably higher or lower values cannot at present be ruled out.Most of the points lie in the range,[Fe/H] –1.2 to –1.8. An estimate of the distribution of these pointstogether with the quoted uncertainty of a single value(σ[Fe/H]subscript𝜎delimited-[]𝐹𝑒𝐻\sigma_{[Fe/H]} = 0.2) suggests that much of the scatter in [Fe/H]is observational. Evidence from galactic work (see e.g., Suntzeff et al. 1991)suggests thatit should be possible to derive relative [Fe/H] values by the ΔSΔ𝑆\Delta Smethod with an uncertainty of about 0.1 and this will be necessary tostudy the distribution of RR Lyrae metallicities in the LMC in detail.

The two investigations just described suggest that with sufficient starsand careful (and probably repeated) observations, it will be possibleto study the kinematics and metallicities of RR Lyraes (and their relationship)as a function of position in the LMC. With care it may even be possibleto study the kinematics and metallicities as a function of depth in the LMC.ΔVo=0.5magΔsubscript𝑉𝑜0.5mag\Delta V_{o}=0.5\rm mag corresponds to a depth in the line ofsight of similar-to\sim 12 kpc at the distance of the LMC and for a sphericalsubsystem this would be equivalent to a diameter of 14similar-toabsentsuperscript14\sim 14^{\circ}on the sky; a not unreasonable size for such a subsystem.

The current position regardingthe discovery of RR Lyraes in the general field of the LMCis as follows. The OGLE II survey (Soszyński et al. 2003b) whichcovered 4.5 sq.deg. overthe Bar region, has given data for 7600 RR Lyraes variables. This is anaverage of about 26 variables per PFIS field. Numbers drop away from theBar. The MACHO survey covering about 10 sq.deg. gives data for 7900RR Lyraes (Alco*ck et al. 1996). OGLE III now in progress covers about40 sq.deg. of theLMC. In the outer field there will often only be one RR Lyrae inan 8×8888\times 8 arcmin SALT/PFIS field. It is clear that with the numbers ofLMC RR Lyraes known, and surveys still in progress, the only limitations todoing a really good job on the kinematics and metallicities of this oldpopulation will be the care taken in the work and theamount of SALT/PFIS time available.

5. RR Lyraes and the SMC

So far as I am aware no spectroscopic studies have yet been published onRR Lyraes in the SMC where these stars are about 0.5mag fainter thanin the LMC (i.e V19.7magsimilar-to𝑉19.7magV\sim 19.7\rm mag, B20.0magsimilar-to𝐵20.0magB\sim 20.0\rm mag). OGLE II gavedataon 571 RR Lyraes in a 2.4 sq.deg. field, or about 3 per SALT/PFIS field(Soszyński et al. 2003a).OGLE III covers an SMC area of about 15 sq. deg. It would be particularlyinteresting to study the kinematics and metallicities of SMC RR Lyraes.Young objects (e.g. Cepheids, Caldwell & Coulson 1986) show theSMC to be very extended in the line of sight, a depth to width ratioof about 5 to 1. The estimated depth is between 15 and 20 kpc. Sinceat the distance of the SMC, 17kpc corresponds toΔVo0.6magsimilar-toΔsubscript𝑉𝑜0.6mag\Delta V_{o}\sim 0.6\rm mag,it may be possible to resolve the depth structure in the RR Lyraes.

6. RR Lyraes and the Dwarf Spheroidals

Table 3 lists the dwarf spheroidal galaxies of the Milky Way subgroupin order of their total visual absolute magnitude (taken mostly fromvan den Bergh 2000). Also listed are estimates of their distance moduli,the numbers of RR Lyraes currently known in each system (with references),the estimated approximate B𝐵B magnitude of these stars and the numbersexpected per SALT/PFIS field. Draco and UMi are of course too far northfor SALT (but could usefully be tackled by HET). Detailed studies ofclearly old populations in the dwarf spheroidals which are generallysupposed to be relatively simple systems would be very valuable.It would of course be in parallel with thehigh resolution work, some already published(e.g. Shetrone et al. 2003, Tolstoy et al. 2003), on the metallicities ofRGB stars in these systems.Whilst the RGB stars are brighterthan the RR Lyraes, it seems difficult, at leastat present to be certain of the age of single stars of that type.How much will be possible on the dwarf spheroids with SALT willdepend on how PFIS actually performs. We might well hope to be able tostudy the kinematics and metallicities of RR Lyraes in Sextans, Sculptorand Carina, and one might perhaps also hope to observe those in Fornax.It is interesting to note that, so far as I am aware, there has been nopublished spectroscopic study of the RR Lyraes in the core of the Sgr Dwarfgalaxy although these are relatively bright.

GalaxyMVsubscript𝑀𝑉M_{V}ModN(RR)Reference(RR)similar-to\sim B(RR)PFIS
Sgr Dwarf–13.8:17.02370118.21
Fornax–13.120.7515221.815
LeoI–11.922.054323.23
LeoII–10.121.6148422.640
Sculptor–9.819.7226521.150
Sextans–9.519.736620.96
Carina–9.420.075721.24
Draco–8.419.52638
UMi–8.419.0569

References
1. Cseresnjes 2001
2. Bessier & Wood 2002
3. Held et al. 2001
4. Siegel & Majewski 2000
5. Kaluzny et al. 1995
6. Mateo, Fischer & Krzeminski 1995
7. Dall’Ora et al. 2003
8. see Dall’Ora et al. 2003
9. Nemec et al. 1988

7. RR Lyraes in Our Galaxy and the Sgr Dwarf Stream

As regards Our Own Galaxy, some of the outstanding questions are as follows:
Was the Halo formed from infalling satellites?
If so, should we see more than the remnants of Sgr Dwarf?
Can other remnants be found kinematically?
Is the recently found ring at similar-to\sim 20kpc from the centre(e.g. Ibata et al. 2003, Crane et al. 2003, Sikivie 2003, Helmi et al. 2003,Rocha-Pinto et al. 2003, Martin et al. 2003)a satelliteremnant or a structural feature of the galactic disc?
Was the inner halo formed by monolithic collapse or by mixing ofinfalling debris?
Are the RR Lyraes in the galactic Bulge different from those in thesolar neighbourhood or is the Bulge reddening law anomalous? (see, e.g.Stuz, Popowski & Gould 1999, Udalski 2003 )
What is the structure of the galactic bar and its relation to the Bulge?

The RR Lyraes in Our Galaxy are relatively thinly distributed over the sky.Even in the main body of Sgr Dwarf there is unlikely to be more than oneper SALT/PFIS field. They become somewhat more concentrated in the Bulgewhere one expects similar-to\sim 5 per SALT/PFIS field in the Baade windowaround NGC6522 (Oort & Plaut 1975). However, there is no lack ofgalactic RR Lyraes.The SLOAN survey for instance has found 3000 in just 1000 sq.deg.(Ivezic̆ 2003). The SLOAN RR Lyraes extend out to a distance ofsimilar-to\sim100 kpc (i.e. to V magnitudes of 20-21), with clumpingprobably connectedwith the Sgr dwarf stream.

The QUEST survey(Vivas et al. 2001, Vivas, Zinn & Gallart 2003, Zinn et al. 2003) isfinding large numbers of RR Lyraes (probablywith considerable overlap with SLOAN). They report 498 in a 380 sq.deg.region. They also report a clump at a distance of about 50kpc(RR Lyraes with V19.2magsimilar-to𝑉19.2magV\sim 19.2\rm mag). For 16 of these they have twospectra each(R = 800) from VLT/FORS1. The radial velocity error for a single measurementis estimated as 20kms1similar-toabsent20kmsuperscripts1\sim 20\rm km\,s^{-1} and the velocity dispersion of their16 stars is 25kms1similar-toabsent25kmsuperscripts1\sim 25\rm km\,s^{-1}. This is much lower than that expected forhalo objects and is consistent with the suggestion that these starsare part of the Sgr dwarf stream. The mean [Fe/H] of these RR Lyraesis –1.7.

There is a great deal of work to do here which will clarifyour understanding of both the Sgr dwarf (and other) stream(s) andthe structure of the halo, including its very outer parts.Currently there is much interest in whether the great-circle structureof the Sgr dwarf stream implies a spherical halo potential, contraryto the predictions of CDM(e.g. Ibata et al. 2001, Majewski et al. 2003, Helmi 2003 and a largenumber of papers on the Sgr dwarf stream generally).Evidently detailed work on the radialvelocities and metallicities of RR Lyraes in the Sgr dwarf stream (andalso its core) is of importance both for studies of the structureand evolution of Our Galaxy and also, more generally, for the natureand distribution of dark matter.

8. Conclusion

It is clear that SALT/PFIS will make possible the detailed study of thekinematics and metallicities of RR Lyraes in our own galaxy, in theMagellanic Clouds and in Local Group galaxies. This should lead to amajor advance in our understanding of the structure, dynamics, evolutionand origin of galaxies. To do this properly will require a majoreffort but the likely rewards are very considerable. And, if in the processnew physics is revealed, so much the better.

9. Acknowledgments

I am grateful to Dr T.D. Kinman for his comments on an initial draftof this paper and for drawing my attention to some important references.

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SALT and RR Lyrae Variables: Our Galaxy, The Magellanic Clouds and the Local Group (2024)
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