Reports from Observers

Measuring Improved Distances to Nearby Galaxies: The Araucaria Project

Wolfgang Gieren1 Grzegorz Pietrzynski1 Fabio Bresolin 2 Rolf-Peter Kudritzki 2 Dante Minniti 3 Miguel Urbaneja 2 Igor Soszynski1 Jesper Storm 4 Pascal Fouqué 5 Giuseppe Bono 6 Alistair Walker 7 José García1

1 Universidad de Concepción, Chile 2 Institute for Astronomy, Hawaii, USA 3 Universidad Católica, Chile 4 Astrophysikalisches Institut Potsdam, Germany 5 Observatoire Midi-Pyrénées, France 6 Osservatorio Astronomico di Roma, Italy 7 Cerro Tololo Inter-American Observatory, Chile

Figure 1: The spiral galaxy NGC 55 in the Sculptor Group, one of the target galaxies of the Araucaria An intense use of ESO telescopes over Project. The presence of an abundant young stellar the past years has allowed us to make population is revealed by the many blue objects con- significant improvements in the charac- centrated towards the disc of this galaxy. terisation of stellar populations, and in the determination of the distances of of nearby galaxies in order to calibrate number of important scientific results nearby galaxies. We report on recent far-reaching secondary methods of dis- have emerged from the abundant data progress on the use of Cepheid vari- tance measurement which could be used obtained with ESO telescopes, some ables and blue supergiant stars for the to determine the distances to galaxies of which we will briefly describe in this accurate measurement of galaxy dis- remote enough to find an unbiased value article. In this progress report, we will tances. This work will finally lead to a of H0. Their very successful work was focus on two types of distance indicators, significant reduction of the uncertainty hampered by the fact that Cepheid vari- the pulsating Cepheid variables, and the on the Hubble constant, which meas- ables are not a perfect instrument for extremely luminous blue supergiant stars. ures the current rate of expansion of the distance measurement. Cepheids, like Universe. other stellar standard candles, are affect- ed to some degree by the environmental Progress on the Cepheid properties of their host galaxies, most period-luminosity relation The measurement of distances to astro- notably abundances of the heavy ele- nomical objects is a fundamental problem ments, and the ages of the stellar popula- The radially pulsating and relatively cool ever since humanity began to look at the tions. Such effects must be taken into ac- Cepheid supergiant stars exhibit a well- stars. Knowledge of precise distances count if truly accurate distances to nearby known relation between their mean in- to galaxies is important for the study of a galaxies are to be measured. With this trinsic luminosity, and their pulsation peri- broad range of astrophysical phenomena, motivation in mind, our group set out, ods – the famous period-luminosity including the true energy outputs of lu- a few years ago, to thoroughly investigate (PL) relation, which is normally written in minous sources; it is also fundamental for the environmental dependences of a the form M = a log P + b, where M is establishing accurate cosmological pa- number of stellar distance indicators, the mean absolute magnitude (in a given rameters which describe the actual state, including Cepheids, blue supergiants, RR photometric band), and P the period and history of the Universe. One par- Lyrae stars, red clump giants, and the (in days). With the PL relation calibrated, ameter of particular interest is the Hubble tip of the red giant branch (TRGB) magni- the mean luminosities of Cepheids, constant H0 which measures the cur- tude in the Araucaria Project (http://ifa. and thus their distances, can be inferred rent acceleration of the expanding Uni- hawaii.edu/~bresolin/Araucaria/). This from their periods. In order to determine verse. Some ten years ago, the HST Key project is a necessary complement to the the dependence of the PL relation on Project on the Extragalactic Distance HST Key Project. Over the past two metallicity, we have been conducting sur- Scale (Freedman et al. 2001) set out to years, the Araucaria Project has obtained veys for Cepheids in a number of spiral measure Cepheid distances to a sample a Large Programme status at ESO, and a and irregular galaxies of widely different

The Messenger 121 – September 2005 23 Reports from Observers Gieren W. et al., Measuring Improved Distances to Nearby Galaxies

Figure 2: The Cepheid period-luminosity relation de- fined by Cepheid variables in NGC 55, in the I-band. The period P is in days. From wide-field images taken on about 50 different nights, we discovered 81 Cepheids with periods longer than 10 days – these are the first Cepheid variables ever discovered in this galaxy. Differential reddening in this rather inclined galaxy is likely to contribute significantly to the observed scatter. This effect should be greatly reduced in the near-infrared PL relations we are cur- rently measuring from VLT/ISAAC images. metallicities in the Local Group, and in the Sculptor Group. From a comparative NGC 55 study of the PL relations exhibited by the Cepheids in the different galaxies, in a 19 variety of optical and near-infrared photo- metric bands, we can expect to deter- mine both, the effect of metallicity on the slope, and on the zero point of the PL 20 I relation, and to filter out the most appro- [ m a

priate photometric band for distance g work in which the effect is minimised. ] 21 So far, we have completed optical (BVI) surveys for Cepheids over the whole spatial extents for the Local Group galax- ies NGC 6822 (Pietrzynski et al. 2004), 1 1.2 1.4 1.6 1.8 2 NGC 3109 and WLM; these data comple- logP ment previous surveys for Cepheids in other Local Group irregular galaxies (LMC, SMC, and IC 1613). In Sculptor, Cepheid NGC 55 Cep 7 P = 67.13 d surveys have been completed for the spi- ral galaxies NGC 300 (Gieren et al. 2004),

NGC 55, NGC 247, and NGC 7793. 19 21 The wide-field imaging data used in these surveys were obtained at the ESO 2.2-m telescope and WFI instrument, with a strong complement from the Polish 1.3-m 21.5 telescope and mosaic camera at 0.5 1 1.5 2 0 0.5 1 1.5 2 Las Campanas Observatory, and the 4-m phase phase Blanco telescope and mosaic camera at CTIO. In all Araucaria target galaxies, NGC 55 Cep 14 P = 45.99 d including those in the Local Group, we 19 20.5 were able to very substantially enlarge the number of known Cepheids, and in par- 21

ticular find long-period ones which carry 19,5

I

V [

the strongest weight in the distance m 21.5

[

a

m

g

a ] determinations. In three of the four Sculp- g 20 ] tor Group spiral galaxies, we have dis- 22 covered the first Cepheid variables ever. 0 0.5 1 1.5 2 0 0.5 1 1.5 2 phase phase An example is NGC 55 (Figure 1); in this galaxy, we have detected 81 Cepheids with periods in the range 10–100 days Figure 3: Phased light curves in V- and I-bands for which define a tight PL relation (Figure 2). two of the Cepheids we discovered in NGC 55. The periods are indicated on the plots. Such data lead In Figure 3, we show the light curves to a very precise determination of the mean magni- of two of these variables in the V- and I- tudes of the variables.

bands, obtained from our mosaic images I

V

[

m

[

m

a g

taken on about 50 different nights. As an One first conclusion from these data is witha the direct Baade-Wesselink-type

] g example of the improvement on the that in the optical V and I bands, the infrared] surface brightness technique Cepheid census in the Local Group, we slopes of the PL relations observed in all (Gieren et al. 2005a) has furthermore pro- show in Figure 4 (see next page) the these different galaxies are consis- vided strong evidence that the slope of PL relation defined by some 100 Cephe- tent with the slopes defined by the LMC the PL relation keeps being independent ids in NGC 3109, most of them discov- Cepheids observed in the OGLE-II Project of metallicity up to the solar abundance ered in the Araucaria Project from data (Udalski 2000), arguing for a very small shown by the Milky Way Cepheids. This is taken during 80 nights at the Polish 1.3-m metallicity effect (consistent with a null ef- an especially important result since many telescope on Las Campanas. In contrast fect) on the PL relation slope in the [Fe/H] of the massive spiral galaxies in the to the more massive spiral NGC 55, range from about – 1.0 dex to – 0.3 dex, HST Key Project have near-solar heavy- NGC 3109 does not contain a population spanned by the young populations of our element abundances. Application of of very long-period Cepheids. target galaxies. Recently, our distance the LMC Cepheid slopes in V and I to the work on LMC and Milky Way Cepheids observed Cepheid PL relations in such

24 The Messenger 121 – September 2005 Figure 5: The near-infrared PL relations in J- and K-bands determined from VLT/ISAAC data for Cepheids in the Sculptor galaxy NGC 300. The variables were previously discovered by Pietrzynski et al. (2002) from wide-field images obtained at the ESO-MPI 2.2-m telescope. Each Cepheid was Figure 4: The Cepheid PL relation in the I-band for the observed at two different epochs and its mean mag- Local Group galaxy NGC 3109. Most of the Cepheids nitude determined with the method of Soszynski in this plot were discovered in the Araucaria Project. et al. (2005). The data fit very well the PL relations in Note that NGC 3109, in contrast to NGC 55, does not J and K as obtained for the LMC Cepheids by contain truly long-period Cepheids – the longest ob- Persson et al. (2004). The slopes of the solid lines served period is 31 days. were taken from this work.

19 NGC 3109 NGC 300

18

20 19 I K [ m [ m a g 21 a ] g 20 ]

21 22

0.6 0.8 1 1.2 1.4 logP

18 galaxies should therefore not cause any important systematic problem.

Very recently (Bresolin et al. 2005a), we 19 have been testing the effect blending J

of Cepheids with nearby projected neigh- [ m a bour stars in the crowded images of dis- g 20 tant galaxies has, by comparing ground- ] based photometry of Cepheids in NGC 300 from ESO-WFI data to BVI images of the same Cepheids obtained with the 21 Hubble Space Telescope and ACS. In the case of NGC 300, with its distance of 1 1.2 1.4 1.6 1.8 2 logP close to 2 Mpc, the systematic effect of blending on the distance derived from the ground-based images is found to be only ~ 2 per cent. The main reason is that in the LMC with the Polish 1.3-m tele- and provide a definitive answer about long-period Cepheids are intrinsically scope on Las Campanas which is expect- the reality of a period break in the relation. bright enough to outshine nearly all of the ed to discover a large number of new close companions on the images, making bright, long-period Cepheids close to the their relative contributions to the Cepheid bar. These data, which cover most of Cepheid work in the near-infrared flux measured on ground-based images the spatial extent of the LMC and which insignificant in most cases. This is good will become available in 2007, will also There are at least three substantial advan- news for ground-based Cepheid distance decide the nagging question of whether tages when Cepheid distance work is car- work on relatively distant galaxies. the LMC Cepheid PL relation shows ried out in near-infrared bands. The first a break near 10 days, as claimed by obvious advantage is the strong reduction Since the LMC Cepheids are currently Sandage et al. (2004). Such a departure of the effect of dust absorption. A second providing the fiducial PL relations for from a uniform slope over the whole advantage is that Cepheid light curves in the distance determination of other galax- period range, if real, would evidently con- the near-infrared, and particularly in the ies, owing to the very large number of stitute a serious problem in the use of K-band, are more symmetrical than their Cepheids discovered by the OGLE-II and the Cepheid PL relation for distance work. optical light curves, and have smaller am- other microlensing projects, it is extremely If the break at 10 days turns out to be plitudes. This makes it possible to mea- important to establish the Cepheid PL re- real, new fiducial LMC PL relations must sure a Cepheid’s mean K-band brightness lations (in different bands) in the LMC with be established in the period range long- with a very good precision from just one the highest possible accuracy. Since wards of 10 days, which is the relevant random phase photometric observation, if the existing microlensing surveys have not range for the measurement of the dis- the star’s optical light curve and period is found many long-period Cepheids in the tances of galaxies beyond about 1 Mpc. known (Soszynski et al. 2005). K-band LMC, due to long integration times which Our current “LMC shallow Cepheid PL relations can therefore be determined overexposed any Cepheids longward survey” is expected to provide a signifi- very economically if the Cepheids of a of periods of ~ 30 days, we are currently cant improvement of the calibration of galaxy have been previously found and undertaking a “shallow” Cepheid survey the LMC Cepheid PL relation in V and I, characterised in the optical spectral range.

The Messenger 121 – September 2005 25 Reports from Observers Gieren W. et al., Measuring Improved Distances to Nearby Galaxies

In addition to these important advan- 1.4 I I I I I I I I tages, theoretical studies suggest an even I

I 1.6 e e I I I i I smaller dependence of the PL relation N H O S C O H on metallicity in the near-infrared, as com- 1.2 1.4 pared to optical wavelengths. 1.2 For these reasons, we have been under- 1.0

taking near-infrared follow-up imaging x 1.0 u for selected subsamples of long-period l Cepheids in most of the target galaxies F 0.8 e 0.8 v i

of our project. We have been obtaining t

a 3990 4045 4100 4280 4340 4440 6520 6570 6620 these images using VLT/ISAAC, and the l e 1.4 NTT with the SOFI instrument. Superb R I I I I I I I I e e e e I results are being obtained from these very i H S N II + O II H H N H high-quality data, as recently demon- 1.2 strated in the case of NGC 300. Figure 5 shows the PL relations in the J- and K- bands obtained from our VLT images 1.0 for this galaxy, which have led to a very accurate determination of the distance to NGC 300 by combining the near-infared 0.8 and optical data for this galaxy (Gieren et 4470 4510 4550 4590 4630 4670 4710 4850 4900 4950 5000 5050 al. 2005b; see also a recent August 1, Wavelength (Å) 2005 ESO Press Release). The work on NGC 300 has shown how essential in- are, however, considerable uncertainties Figure 6: The technique used to measure the abun- frared images are to achieve an accurate on the gas-phase abundance scale at dance of metals of blue supergiants is shown here. With the stellar gravity fixed by fits of model spectra to determination of the reddening of a high metallicity (around the solar value and the high-order Balmer lines and the effective tem- galaxy, including the contribution coming above), as encountered in the central perature determined from line diagnostics, the abun- from dust absorption inside the galaxy. regions of spiral galaxies (Bresolin et al. dances of different elements are varied in the models Only in this way can it be hoped to 2005b). Stellar abundance studies allow until the best fit (in yellow) to the observed spec- trum (orange line) is obtained. The sensitivity to the achieve the final goal of the Araucaria us to circumvent this difficulty, although abundance is indicated in the lower part of the pan- Project, which is to measure the dis- the chemical analysis in young massive els, where models differing by ± 0.2 dex relative to the tances to nearby galaxies with a precision stars is a complex task, due to strong de- best-fitting model are compared. The effects of strong of at least 5 per cent, or better. partures from conditions of local thermo- stellar winds in this B3 supergiant in NGC 300 are visible in the Hα line, which is in emission (upper right). dynamic equilibrium and to the effects of stellar winds on the atmospheric struc- supergiant stars in NGC 300 (Urbaneja et Blue supergiants ture. It is important to note that both al. 2005). By combining our good-quality the blue supergiants and the H II regions data with modern stellar-atmosphere The spectroscopy of blue supergiants, the are young (< 10–20 million years) objects, analysis techniques for massive stars we brightest young stars visible in galaxies, and therefore the galactic chemical have been able to compare for the first and among the most massive, is an inte- abundances derived from them are rele- time in a galaxy located outside the Local gral part of the Araucaria Project. The vant for the study of young stellar Group the chemical abundance gradient goal of our detailed study of these stars is distance indicators, such as Cepheids. in its disc obtained independently from twofold: to measure chemical abundances the stellar analysis, and from the ionised of heavy elements and to develop and The analysis of the chemical composition gas (Figure 7). apply a new distance determination tech- of blue supergiants complements nique based on a small set of fundamen- the study of H II regions, which is limited The NGC 300 observations were carried tal stellar parameters. mostly to the abundances of oxygen, out utilising the multi-object spectroscop- nitrogen and sulphur, by providing infor- ic capabilities of FORS at the VLT, yield- Gathering information on the metal con- mation not only for these elements, but ing intermediate-resolution spectra of sev- tent of galaxies is essential to obtain also for additional species, such as eral dozens of stars in this galaxy (Bresolin accurate distances, since the techniques magnesium, iron and silicon. Model spec- et al. 2002). This observing strategy, re- used, such as the Cepheid PL relation, tra, calculated accounting for the pres- peated for all the galaxies included in could significantly depend on metallicity. ence of millions of atomic transitions, are the Araucaria Project, has allowed us to The chemical abundances in spiral and compared to the observed supergiant collect several hundred spectra of blue irregular galaxies are commonly obtained spectra to measure the abundances of supergiant candidates (the case of NGC from the spectroscopic analysis of giant these metals. An example of this pro- 247 in Sculptor is shown in Figure 8). H II regions, resulting from the photoioni- cedure is shown in Figure 6, taken from This represents an unprecedented sample sation of gas clouds by hot stars. There our chemical analysis of early B-type of extragalactic massive star spectra, of

26 The Messenger 121 – September 2005 Figure 7 (left): A comparison of the radial oxygen Log (O/H) + 12 abundance gradient in NGC 300 obtained from the 8.9 NGC 300 B supergiants (green star symbols) and the H II regions (orange disks). The oxygen abundance is in the 12 + log(O/H) scale, commonly adopted in neb- ular work (the solar value is ~ 8.7 on this scale), 8.7 while the horizontal coordinate is the deprojected gal- actocentric distance in units of the isophotal radius. The nebular abundances have been obtained from the emission line fluxes available in the literature and 8.5 adopting the Pettini & Pagel (2004) calibration of the R 23 abundance indicator.

8.3

Figure 8 (below): Over 60 blue supergiant candi- 8.1 dates have been observed spectroscopically in the Sculptor Group galaxy NGC 247, as indicated here on a mosaic derived from FORS images. The arrow points to the emission line star whose spectrum 7.9 R/R is shown in Figure 9. Candidates for the FORS/MXU 0 spectroscopic follow-up were selected from colours and magnitudes measured from 2.2-m telescope WFI 0.0 0.2 0.4 0.6 0.8 1.0 1.2 images.

great value for the immediate needs of the in V between –7 and –10), blue super- The Flux-weighted Gravity-Luminosity project (abundances and luminosities), giants are among the brightest stellar Relationship (FGLR) determined from our and for future research on normal B- and objects observed in galaxies, second only analysis of blue supergiants (spectral A-type supergiants, as well as on more to supernovae. The investigation of their types from early-B to mid-A) in galaxies of exotic, rare objects, like the extremely usefulness as extragalactic distance indi- the Local Group and in NGC 300 is shown luminous emission-line star we discovered cators, therefore, is a natural component in Figure 10. The calibration we have ob- in NGC 247 (Figure 9). A dedicated ana- of the Araucaria Project. A simple but tained can be used to measure distances lysis technique is being developed by our powerful technique was developed by to galaxies where spectra and apparent group in order to cope with the large Kudritzki et al. (2003), who found that the magnitudes of blue supergiants are avail- 4 amount of stars for which we are deriving flux-weighted gravity g/Teff (the gravity g able. This independent spectroscopic the basic parameters (gravities, tempera- and the effective temperature Teff are both method, despite the complexities involved tures and metallicities). determined from the spectra) is strongly in the data analysis, provides simulta- correlated with the intrinsic luminosity neously the luminosities, as well as the Thanks to their extreme luminosity in the of blue supergiants, and appears to be chemical abundances of the target optical range (with absolute magnitudes quite insensitive to metallicity variations. stars. In this respect, the FGLR has an

The Messenger 121 – September 2005 27 Reports from Observers Gieren W. et al., Measuring Improved Distances to Nearby Galaxies β H x u L

F Figure 9: The FORS2 spectrum of an extremely lumi-

nous (MV ~ –9.3) blue supergiant star in NGC 247. d A strong stellar wind is responsible for the emission e 3 lines visible throughout the spectrum (a nebular com-

s ponent is also present for the Balmer lines), which is i

l reminiscent of the spectra of Luminous Blue Variables. Most of these features are due to Fe II, as identified a γ by the vertical bars. H

m 2 r I I e I I a ε δ o F C H H N 1

4000 4500 5000 Wavelength (Å)

Figure 10: The Flux-weighted Gravity-Luminosity re- lationship is plotted here for blue supergiants analysed –10 in the Local Group and in NGC 300. The legend Milky Way shows the meaning of the different symbols used. The LMC data are taken from Kudritzki et al. (2003), except SMC for the early B-type supergiants in NGC 300, which M31 are taken from Urbaneja et al. (2005). M33 – A Ia –9 NGC 3621 NGC 6822 NGC 300 – early B

l NGC 300 – B8–A4 o b

M –8

–7

1.8 1.6 1.4 1.2 1.0 4 Log (g/Teff,4) (cgs)

advantage over the photometric meth- research. Last not least, we want to thank the differ- Gieren, W., Pietrzynski, G., Soszynski, I. et al. 2005b, ods of distance determination (Cepheids, ent members of the OGLE Project for helping out with ApJ 628, 695 the observations for this programme at the Polish Kudritzki, R. P., Bresolin, F., Przybilla, N. 2003, TRGB, etc.), in that it allows us to ac- telescope on Las Campanas. ApJ 582, L83 count directly for the effects that metallici- Persson, S. E., Madore, B. F., Krzeminski, W. et al. ty has on the distances derived. 2004, AJ 128, 2239 References Pettini, M., Pagel, B. E. J. 2004, MNRAS 348, L59 Pietrzynski, G., Gieren, W., Fouqué, P., Pont, F. 2002, Bresolin, F., Gieren, W., Kudritzki, R. P. et al. 2002, AJ 123, 789 Acknowledgements ApJ 567, 277 Pietrzynski, G., Gieren, W., Udalski, A. et al. 2004, Bresolin, F., Pietrzynski, G., Gieren, W., Kudritzki, R. P. AJ 128, 2815 We thank the ESO Observing Programmes Commit- 2005a, ApJ, in press Sandage, A., Tammann, G. A., Reindl, B. 2004, tee for the generous allocation of observing time Bresolin, F., Schaerer, D., González-Delgado, R. M., A&A 424, 43 to our project. It is a pleasure to thank ESO staff on Stasinska, G. 2005b, A&A, in press Soszynski, I., Gieren, W., Pietrzynski, G. 2005, Paranal and La Silla for their expert collaboration in Freedman, W. L. et al. 2001, ApJ 553, 47 PASP 117, 823 obtaining those observations which were scheduled in Gieren, W., Pietrzynski, G., Walker, A. et al. 2004, Udalski, A. 2000, AcA 50, 279 service mode. Wolfgang Gieren, Grzegorz Pietrzynski, AJ 128, 1167 Urbaneja, M. A., Herrero, A., Bresolin, F. et al. 2005, and Dante Minniti acknowledge support from Gieren, W., Storm, J., Barnes III, T.G. et al. 2005a, ApJ 622, 862 the Chilean FONDAP Center for Astrophysics for this ApJ 627, 224

28 The Messenger 121 – September 2005