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Abstract

Several properties of the M 31 disk, namely: opacity, extinction law and gas-to-dust ratio are studied by means of optical and near-infrared photometry of ten globular clusters and galaxies seen through the disk. The individual extinctions of these objects were estimated with respect to several sets of theoretical spectral energy distributions for globulars and galaxies. Seven targets are consistent with reddened globulars, two - with starburst galaxies and one - with an elliptical. The extinction estimates agree with semi-transparent disk () in the inter-arm regions. The total-to-selective extinction ratio in those regions 2.750.1 is lower on average than the typical Galactic value of =3.1. We also obtained a gas-to-dust ratio, similar to that in the the Milky way. It shows no correlation with the distance from the M31 center.

galaxies: ISM, photometry, dust, extinction; globular clusters

Opacity of M31 disk] Dust properties of nearby disks: M 31 case P. Nedialkov, A. Valcheva, V. Ivanov & L. Vanzi] Petko L. Nedialkov, Antoniya T. Valcheva,
Valentin D. Ivanov & Leonardo Vanzi 2008 \volume254 \pagerange1–7 \jnameThe Galaxy Disk in Cosmological Context \editorsJ. Andersen, J. Bland-Hawthorn & B. Nordström, eds.

\firstsection

1 Introduction

The dust in galaxies attenuates the light of background extragalactic sources. Recent studies ([Holwerda et al. (2005), Holwerda et al. 2005]) of morphologically representative samples of spirals shows that dust opacity of a disk arises from two distinct components: (i) optically thicker () and radially dependent component, associated with the spiral arms, and (ii) relatively more constant optically thinner disk (), dominating the inter-arm regions and the outskirts of the disk.

The nearby giant spiral galaxy M 31 is well suited for comprehensive studies of the interplay between the stars and the ISM. The radial distribution of the opacity in M 31 spiral arms, based on individual estimates towards OB stars, shows that the opacity exponentially decreases away from the bulge ([Nedilakov & Ivanov (1998), Nedilakov & Ivanov 1998]). However, a study of 41 globular clusters in M 31 indicates the absence of radial extinction gradient with the galactocentric distance ([?, Savcheva & Tassev 2002]).

Measuring the color excesses of objects behind the disks is an alternative method to constrain the disk opacity. It was applied to M 31 by [Cuillandre et al. (2001), Cuillandre et al. (2001)] who used background galaxies. They concluded that the M31 disk is semi-transparent for distances larger than .

Here we complement this work, presenting opacity estimates for galactocentric distance smaller than , derived from the comparison of apparent colors of background globulars and ellipticals with models.

2 Observations and data reduction

Our sample includes 21 background galaxy candidates located well within the standard M 31 radius . They were selected from a number of heterogeneous sources: visual inspection of DSS and the NOAO archive photographic plates, dropouts from M 31 globular cluster searches ([Battistini et al. (1980), Battistini et al. 1980]). Our original intention was to base the study on background ellipticals only but five of our targets were recently identified as globulars, and their [Fe/H] and v became readily available from [Galleti et al. (2006), Galleti et al. (2006)].

We obtained imaging in Dec 1996 and Jan 1997 with ARNICA ([Lisi et al. (1993), Lisi et al. 1993]) at 1.8m Vatican Advanced Technology Telescope on Mt. Graham. The instrument is equipped with a NICMOS 3 (256 256 pixels) detector array, with scale of 0.505 arcsec/pixel. The data reduction includes the typical steps for infrared imaging: “sky” removal, flat-fielding, alignment and combination of individual images, separately for every filter and field. Ten of the targets (Table 1, Fig. 1) were identified on the images from the Local Group Survey ([Massey et al. (2006), Massey et al. 2006]), obtained with the KPNO Mosaic Camera at 4m Mayall telescope.

No. 2MASX/2MASS Other Object v [Fe/H] r N(HI+2H)
name name Type [km s] [arcmin] [10 at. cm]
1. 2MASX J00451437+4157405 Bol 370 1 347 1.80 52.2 1.895
2. 2MASX J00444399+4207298 Bol 250D 1 442 - 84.2 9.028
3. 2MASS J00420658+4118062 Bol 43D 1 344 1.35 30.8 12.836
4. 2MASS J00413428+4101059 Bol 25D 1 479 - 20.8 0.668
5. 2MASS J00413436+4100497 Bol 26D 1 465 1.15 20.8 2.170
6. 2MASS J00413660+4100182 Bol 251 2 - - 20.6 1.350
7. 2MASS J00430737+4127329 Bol 269 2 - - 19.6 12.384
8. 2MASS J00421236+4119008 Bol 80 2 - - 29.3 15.248
9. 2MASX J00410351+4029529 Bol 199 2 - 1.59 79.1 16.120
10. 2MASS J00425875+4108527 Bol 140 3 413 0.88 31.7 6.654

Notes:
Following Galleti et al. (2006): 1 - confirmed globular clusters (GC), 2 - GC candidates, 3 - uncertain objects
Total hydrogen column density based on pencil beam estimates of CO(10) intensity ([Nieten et al. (2006), Nieten et al. 2006]), converted to molecular hydrogen column density using a constant conversion factor ([Strong & Mattox (1996), Strong & Mattox 1996]) and 21 cm emission from the Westerbork map ([Brinks & Shane (1984), Brinks & Shane 1984)].

Table 1: Sub-set of our original target list, with photometry.
Figure 1: -band images of our targets from the Local Group Survey ([Massey et al. (2006), Massey et al. 2006]). The field of view is . North is up, and East is to the left. The white circles show the photometric extraction apertures. The numbering corresponds to Table 1.

Clouds were present during most of the observations, forcing us to use the 2MASS Point Source Catalog ([Cutri et al. 2003, Cutri et al. 1993]) stars for the photometric calibration (typically using 4–10 common stars per field). No color dependence was found, and the r.m.s. of the derived zero-points was 0.05 mag for both bands. The typical seeing of the optical images () matches well that of the near-infrared data set (), allowing us to perform simple aperture photometry with radius. We used the standard IRAF1 tasks. The zero points of the optical data are based on stars in common with the catalog of [Massey et al. (2006), Massey et al. (2006)]. The -band magnitudes and the observed colors together with their errors are listed in Table 2. The majority of the infrared colors shows excellent agreement with the available 2MASS colors (see Fig. 2).

No.
1. 16.18 0.08 3.85 0.30 2.23 0.31 1.84 0.32 3.02 0.23 2.53 0.10 3.67 0.30
2. 17.60 0.02 6.19 0.09 3.51 0.06 2.69 0.06 5.11 0.09 4.25 0.08 5.65 0.06
3. 17.15 0.02 6.46 0.05 3.90 0.06 3.08 0.05 5.16 0.07 4.21 0.05 6.13 0.06
4. 18.23 0.03 6.25 0.07 3.49 0.10 2.80 0.05 5.34 0.10 4.19 0.06 5.74 0.07
5. 18.23 0.03 5.72 0.07 3.43 0.10 2.70 0.05 4.85 0.10 3.81 0.07 5.39 0.07
6. 17.77 0.03 5.82 0.07 3.46 0.10 2.79 0.04 4.65 0.09 3.69 0.06 5.35 0.06
7. 18.49 0.03 5.83 0.09 3.51 0.07 2.96 0.06 4.68 0.11 3.72 0.09 5.31 0.09
8. 17.14 0.02 5.10 0.06 3.28 0.06 2.59 0.04 3.98 0.08 3.25 0.05 5.01 0.06
9. 18.26 0.02 6.36 0.07 3.76 0.07 2.95 0.06 5.20 0.08 4.27 0.04 5.90 0.08
10. 17.37 0.02 4.64 0.07 2.38 0.12 2.19 0.04 3.59 0.09 3.08 0.07 4.43 0.07
Table 2: photometry of the objects listed in Table 1. Photometric systems: – Johnson, - Cousins, - [Bessell & Brett (2006), Bessell & Brett (1988)]. The uncertainties include both the zero-point errors and the statistical errors of the individual measurements.
Figure 2: Comparison between the colors derived in this work and the available 2MASS colors. Note that the photometry of Bol 269 (target No. 7 in our list) was flagged as suspect in the 2MASS Point Source Catalog.

3 Dust properties of M 31 disk from -test minimization

The intrinsic near-infrared colors of ellipticals are nearly identical: ()0.22 mag, as demonstrated by [Persson et. al (1979), Persson et al. (1979)]. Assuming all the targets belong to that Hubble type, the opacity of the disk that lay between them and the observer can easily be calculated, taking into account the internal Milky Way extinction, i.e. from the work of [Schlegel et. al (1998), Schlegel et. al (1998)]. However, the evident contamination of the sample by globular clusters requires also to consider for each object the possibility that it is a M 31 globular, with the typical globular cluster colors. Furthermore, a cluster may be located in front of the M 31 disk, adding extra degree of complication to the analysis.

To address these issues we developed a multicolor minimization technique to derive simultaneously the disk opacity and number of other parameters: gas-to-dust ratio, extinction law and last but not least – the nature of the object (elliptical galaxy or globular cluster). It allows also to fix some of these parameters, while still varying the rest of them. The intrinsic colors of globulars were adopted from the model of [Kurth et al. (1999), Kurth et al. (1999)] and for the ellipticals – from [Bicker et al. (2004), Bicker et al. (2004)].

The results from the test are presented in Table 3. The free parameters in the case of globular clusters (left side of the Table 3) are: age, abundance , , and in the case of the elliptical galaxy models (right side of the Table 3) they are: redshift , , and . We created a multi-dimensional grid, with steps of 0.01 along all axes and calculated the for every grid node.


No.
       Globular Cluster Model Fit     Elliptical Galaxy Model Fit Derived
              Parameters               Parameters Type

Age Abun- dust Red- dust
[yr] dance [mag] vs. shift [mag] vs.
        gas? gas?

1.
0.910 0.0001 3.10* 1.81 1.288 no 0.000 3.10* 0.00 39.950 no Globular
0.910 0.0003* 3.10* 1.46 2.510 no 0.000 6.00 0.00 39.950 no
0.610 0.0003* 2.43 1.45 1.534 no

2.
0.410 0.0500 3.10* 2.41 2.400 no 0.075 3.10* 0.98 7.562 yes Globular
0.069 3.42 1.05 6.900 yes

3.
0.810 0.0500 3.10* 0.50 3.406 yes 0.026 3.10* 1.41 45.111 yes Globular
3.010 0.0009* 3.10* 2.60 19.558 no 0.021 2.15 1.18 10.404 yes
2.010 0.0009* 2.77 2.54 8.552 no

4.
3.010 0.0500 3.10* 0.86 2.152 no 0.072 3.10* 1.06 7.180 no Globular
0.080 2.75 0.97 5.692 no

5.
8.010 0.0400 3.10* 0.21 0.870 yes 0.046 3.10* 0.86 24.117 no Globular
1.110 0.0014* 3.10* 1.73 7.522 no 0.045 1.77 0.62 3.649 no
2.010 0.0014* 2.75 2.00 2.786 no

6.
1.410 0.0400 3.10* 0.01 1.707 no 0.061 3.10* 0.83 69.838 no Globular
0.072 1.08 0.40 8.572 no

7.
1.410 0.0450 3.10* 0.00 5.913 no 0.064 3.10* 0.84 61.119 yes Globular
0.068 1.00 0.40 13.223 yes

8.
1.310 0.0200 3.10* 0.00 34.012 no 0.024 3.10* 0.56 137.883 yes Uncertain
0.000 1.00 0.33 27.456 yes

9.
3.010 0.0500 3.10* 0.97 4.321 yes 0.042 3.10* 1.26 12.802 yes Uncertain
2.010 0.0005* 3.10* 2.67 7.154 no 0.061 2.63 1.11 9.039 yes
2.010 0.0005* 3.10 2.67 7.154 no

10.
1.010 0.0500 3.10* 0.50 13.716 yes 0.042 3.10* 0.15 54.800 yes Uncertain
0.910 0.0025* 3.10* 2.25 26.747 no 0.000 1.00 0.15 30.674 yes
0.910 0.0025* 2.69 2.06 17.238 no

Table 3: Summary of the minimization. The matches of the apparent colors to the globular cluster models of [Kurth et al. (1999), Kurth et al. (1999)] is given in the left and to the intrinsic colors of ellipticals predicted by the GALEV models of [Bicker et al. (2004), Bicker et al. (2004)] is given on the right. The numbers of the targets as the same as in Table 1. The table also reports if the corresponding to the minimum agrees (within the errors) with the total gas density derived from the combined map of [Brinks & Shane (1984), Brinks & Shane (1984)] and the CO(10) map of [Nieten et al. (2006), Nieten et al. (2006)]. The asterisk indicates a fixed parameter.

The preliminary tests reveal that in all cases -bands dominate the values of . These bands have the largest systematics with respect to external photometry ([Massey et al. (2006), Massey et al. 2006]). To account for that and to minimize their impact we tentatively added 0.20 mag to the -band and 0.12 mag to the -band magnitude errors. The errors listed in Table 2 do not reflect this modification. As a result, we have relatively more equal contribution of the different colors to the .

The globular cluster model fits much better the colors of most targets than the elliptical model. The typical opacity across the M 31 disk is 1 mag. There are two exceptions (objects No. 8 and No. 10) for which neither model yields a reasonable match.

Interestingly, in M31 is lower than the typical Galactic value of 3.1, and it is similar to the one obtained by [?, Savcheva & Tassev (2002)]. This may indicate a smaller mean size of the dust grains in the diffuse component of M 31 ISM, in comparison with the Milky way. Although the targets are located well within standard radius , where the active star formation still takes place, all of them are projected in the inter-arm regions where the opacity of the disk stays relatively law, as seen from the column density values in Table 1. Here we assumed the Galactic gas-to-dust ratio ([Bohlin et al. (1978), Bohlin et al., 1978]).

The relation between total gas column densities and the derived extinctions, corresponding to =3.1 and , is plotted in Fig. 3. Surprisingly, the derived extinctions of the candidate globulars (targets No. 6 and 9) correlate better with the gas density if we use intrinsic colors derived from the elliptical models. We contribute this to the spatial variations of the reddening law.

Figure 3: The agreement between the total hydrogen column density N(H) and the color excess E with respect to [Kurth et al. (1999), Kurth et al. (1999)] model colors of globulars (left) and with respect to colors of ellipticals (right) as predicted by the GALEV models ([Bicker et al. (2004), Bicker et al. 2004]). The extinction values are listed in Table 2 and correspond to =3.1 and . Dashed lines represent the expected range and the thick line is the mean Galactic gas-to-dust ratio ([Bohlin et al. (1978), Bohlin et al. 1978]).

Our analysis also considers the K-correction. We used the HyperZ code of [Bolzonella et al. (2000), Bolzonella et al. (2000)], that includes a variety of spectral energy distributions for different morphological types of galaxies. The results are presented in Table 4. Both cases – fixed to the elliptical type and free morphological types yield reasonable values. The extinction estimates are lower and the redshifts are higher than those derived earlier, indicating a degeneracy between these two quantities. The metallicity-opacity degeneracy is apparent in Table 3 as well: the higher is the abundance , the lower is the derived extinction and vice versa. The HyperZ tends to classify our targets as starburst galaxies, explaining the larger extinction values in comparison with the case of fixed elliptical morphological type. Note that a large fraction of the extinction may be internal to a starburst galaxy and not related to the M 31 disk. This might be the case for targets No. 8 and 10 which, together with No. 9 are our best candidates for galaxies, laying behind the M 31 disk.

No. Red- dust Galaxy Red- dust Galaxy
shift [mag] vs. Type shift [mag] vs. Type
gas? gas?

1.
0.130 0.05 4.562 yes elliptical* 0.135 0.05 0.673 yes starburst

2.
0.200 0.55 1.743 yes elliptical* 0.200 0.55 1.743 yes elliptical

3.
0.145 0.70 1.109 yes elliptical* 0.140 1.00 1.025 yes starburst

4.
0.200 0.35 1.482 no: elliptical* 0.150 1.45 1.365 no starburst

5.
0.150 0.25 0.546 yes elliptical* 0.150 0.75 0.464 no starburst

6.
0.145 0.15 2.700 yes elliptical* 0.155 0.05 1.807 yes starburst

7.
0.145 0.15 3.926 no elliptical* 0.350 0.10 1.051 no starburst

8.
0.110 0.10 2.417 no elliptical* 0.105 0.95 2.230 yes starburst

9.
0.155 0.75 1.246 yes elliptical* 0.155 0.75 1.246 yes elliptical

10.
0.050 0.00 4.659 no elliptical* 0.150 0.50 2.571 yes starburst
Table 4: Summary of the minimization considering the K-corrections, for two different redshifts. The “intrinsic” redshifted colors of the galaxies were determined with the HyperZ ([Bolzonella et al. (2000), Bolzonella et al. 2000]). The numbers of the targets as the same as in Table 1. The grid resolution is 0.05 along all axes and the Galactic extinction law of [Allen (1976), Allen (1976)] is assumed. The rest of the columns are identical with those in Table 3.

4 Conclusions

We measure the opacity of the M 31 disk from the color excesses of 21 objects – a mixture of galaxies behind the disk and globular clusters. Seven of them are consistent with globulars, two - with starburst galaxies and one - with an elliptical galaxy. Their extinction estimates are consistent with a semi-transparent disk () in the inter-arm regions. We confirm the conclusion of [Savcheva & Tassev (2002), Savcheva & Tassev (2002)] that the total-to-selective extinction value in the diffuse ISM of M 31 is on average lower than the typical Galactic value of =3.1. The gas-to-dust ratio appears similar to that in the Milky way and it is independent from the galactocentric distance, which might indicate Solar abundances along the line of sight studied here (within 2085 from the M 31 center).

Acknowledgments

This work was partially supported by the following grants: VU-NZ-01/06, VU-F-201/06 & VU-F-205/06 of the Bulgarian Science Foundation. One of the authors (P.N.) thanks to organizing committee of IAU Symposium No.254 for the grant which allowed him to participate.

Footnotes

  1. IRAF is the Image Analysis and Reduction Facility made available to the astronomical community by the National Optical Astronomy Observatories, which are operated by AURA, Inc., under contract with the U.S. National Science Foundation.

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