The IRAS colors of galaxies

@ Pergamon Press Ltd Printed in Great Britain 0275-1062,‘92$10.00+.00

Chin. Astron. Astrophys. (1992)16/1,8-16 A translation of Acta Astrophys.Sin.(l991)11/4,311-320

The IRAS

colors of galaxies* WANG Gang

Beijing Astronomical

Observatory, Chinese Academy of Sciences, Beijing 100080

Abstract Based on a sample of 369 galaxies which has been detected by IRAS at all of the four bands, the colors of IRAS galaxies are discussed in the paper. By their colors, they could be classified into four groups, i.e. normal, starburst, AGN and mixture. The IRAS emission of most of these galaxies could be fitted well using a simple model of two black bodies. Key

words:

Galaxies-IRAS

color-active

galaxies

1. INTRODUCTION

Helou ~1 found that IRAS galaxies, in the color-color diagram of logFl2/F25 versus logFso/F~oo, are distributed in a band such that as systems get cooler at 12/25 pm, they get warmer at 60/100 pm. It is due to the different ratio of cirrus to star formation components in these galaxies. Soifer et al L21d’lscussed the IRAS colors of galaxies, based on their IRAS Birght Galaxy Sample i31 which is selected by flux greater than 5.24 Jy at 60 pm. The galaxies are separated into two groups at F25/Fso = 0.17. They found that for both cool and warm galaxies, there are correlations among their IRAS colors, IRAS luminosity and the ratio between IRAS and optical luminosities. The reemission of dust absorbing UV and optical emission is the most important mechanism of galaxy IRAS emission. Based on the preliminary IRAS results, de Jong et al 141found that IRAS emission of galaxies comes from cool dust which is distributed in whole galactic disk and from warm dust which is associated with star formation regions. A two component model of IRAS emission of galaxies was proposed by de Jong and Brink 151. Rowan-Robinson & Crawford ~1 used a three component model to fit the IRAS emission of a few hundred bright galaxies. The first one is dust distributed along *Original version received

1990 June

11; revised

version

received

November

10

IRAS Galaxies

galactic disk, which could One is at 30K, and another and hot one presents small interpreted as being due to third one is active galactic dust.

9

be contributed by a combination of two black bodies. one is at 200K. The cool one presents large dust grain, dust grain. The second one is starburst, which could be stars with temprature 1OOOOKsurrounded by dust. The nucleus, which has a power law spectrum absorbed by

2. THE

SAMPLE

Using the following criteria, a sample of 369 IRAS galaxies is selected from IRAS Point Source Catalog. 1. Flux quality Q > 1 at all of the four bands. It means the fluxes are better than upper limit only, however, Q = 2 indicates a moderate quality. It is 2. lb1 > 3o” . At low Galactic latitude, the sky is confused in infrared. difficult to detect galaxies and separate them from the objects in the Galaxy. For the same reason, the regions of Large and Small Megellanic Clouds are excluded as well. 3. Fss > F25. Lonsdale 171indicated that at high Galactic latitude, the sources This criterion is widely accepted. Our criterion is with F60 > SF25 are .gdaxieS. more relaxed than it slightly. There are 396 sources matching the above criteria in IRAS Point Source Catalog version 2. Excluding 18 stars, 6 Galactic nebulae and 3 HI1 regions in large galaxies, our sample contains 369 IRAS galaxies. Some of them are included in IRAS Small Structure Catalog. The ISSC fluxes are used instead of IPSC fluxes. 366 of the 369 sources are associated with optical or radio galaxies. Only 3 sources are observed by IRAS only. We take the morphological types of these galaxies from optical catalogs. There are 3 elliptical, 18 lenticular, 216 spiral, 6 irregular, 42 barred spiral, 13 doubled and 14 peculiar galaxies. The morphological types of other 57 galaxies are unknown. & Veron’s catalog [lo1 and Markarians catalog Arp’s catalogs [81~[gl,Veron-Cetty 111have been checked for the peculiar galaxies in our sample.

3. IRAS The distributions of 3 IRAS colors are two peaks at log F25/Fl2 = 0.2 et al’s sample too. It implies that The distribution of logFso/Fzs ( galaxies are the most.

COLORS

for these galaxies are shown in Figure 1. There and 0.45 (Fig. 1 (a)). These can be seen in Soifer there are two kinds of IRAS galaxies probably. Fig. 1 (b)) h as a tail below 0.5, where Seyfert

3.1. Morphology The distributions of IRAS colors of galaxies according are plotted in Figure 2 and listed in Table 1.

to their morphological

types

norm. 64

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of colors by morphological

2 The distributions

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TABLE

30

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TABLE

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IRAS Galaxies

Fig. 1 Distributions Galaxies

of the Colors of IRAS

11

Fig. 2 Distributions of colors of galaxies for each morphological type

(a). 70% of the galaxies with log Fzs/Flz < 0.35 are spiral ones, but to the other side only 40% are spiral galaxies. (b). There is no obvious difference among the distributions of log F~-J/Fz~ of each morphological type. (c). The proportion of spiral galaxies increases with Fim /Fso from 20% to 90%. Other galaxies have a reversed distribution. Peculiar galaxies and faint galaxies with The proportion of them decreases unknown morphological type are the “bluest”.

WANG

12

Gang

from 30% to a few percent. 3.2.

The

Peculiar

Galaxies

The distributions of IRAS colors are plotted in Figure 3 and listed in Table 2 according to their types. There are 100 Arp’s galaxies, 38 Markarian galaxies, 13 of them are called starburst galaxies Ill], and 93 Veron-Cetty & Verons’ galaxies included in our examples. Other galaxies which are not included in these catalogs are called normal galaxies hereafter. (a). To the left side of logF25/Fr2 = 0.35, there are more normal galaxies. Arp’s galaxies take 20% to 30% over the whole distribution. The proportion of AGNs and Markarian galaxies increases with FZS/FIZ. (b). The percentage of normal galaxies increases with F~cJ/F~~. The percentage of Arp’s galaxies has a flat distribution, but it is more at log F~o/Fzs < 0.5. Markarian galaxies and AGNs have “bluer” colors. They take a large proportion at the tail of log F~o/Fzs < 0.5, where the nercentage of AGNs is 67%. (c). Beyond log Fim /FM = 0.2, normal galaxies take a large part, but to the other side, peculiar galaxies are more. From these color distributions, the following conclusion could be drawn. IRAS galaxies could be separated into two groups. For log F25/F12 < 0.35, normal galaxies take a large part. To the other side peculiars are more. log Feo/Fzs could be used to select AGNs. Most of the galaxies with log FG,-J/F~~< 0.5 are AGNs. Most of the galaxies with log Fso/F~~ > 0.9 are normal galaxies. The relation of the color and the morphological type is obvious at 25/12pm and 100/60pm, but not at 60/25pm. The differences of each type of peculiar galaxies are clear at all the three colors. Normal galaxies have smaller Fzs/F~z and larger Fso/Fzs and F~oo/F~o. AGNs and Markarian galaxies are in the contrary way. Arp’s galaxies are in the middle between the two.

4. THE

COLOR-COLOR

DIAGRAM

The color-color diagrams of IRAS galaxies are showed in Figure 4, 5 and 6. Each type of galaxies are marked differently. In Figure 4, most of galaxies are concentrated in two regions. Around log Fzs/Flz = 0.2, log FI~~/F60 = 0.3, the galaxies are the most concentrated. Most of them are normal galaxies. Around log Fzs/Flz = 0.5, log F~oo/F~o = 0.2, there are more AGNs and starburst galaxies. In Figure 5, normal galaxies are concentrated near log F,,,lF25 = 1.0. Peculiar galaxies extend to down-left from here. In Figure 6, the galaxies could be separated into four groups. They are normal galaxies, starburst galaxies, AGNs and the mixture. The spectra of the galaxies are plotted in 5 groups in Figure 7. In the first group, there are 153 galaxies. 101 of

13

IRAS Galaxies

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Fig. 3 The distribution culiar galaxies

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.

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hzFzs /FIZ

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of the color of pe-

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Fig. 4 Color-color

diagram of lg F&F60

vs.lg WFIZ

them are not in the catalogs which we used to identify peculiar galaxies. Therefore it is called normal group. 35 of 52 peculiar galaxies are included in Arp’s catalog only. But, Arp’s galaxies are selected by the morphology, some of them have no obvious interaction. In the second group, there are 107 galaxies. In our sample, there are 22 starburst galaxies and H2 type AGNs. 19 of them are in this group. It is called starburst group. 30 of 39 galaxies in the third group are in Veron-Cetty & Veron’s catalog, therefore it is called active galaxy group. In the fourth group there are 53 galaxies. It is mixed with normal and peculiar galaxies. Other 17 galaxies with unusual colors are plotted in the fifth group. The differences of the spectra among the 5 groups are obvious.

WANG Gang

14

.

--L-.-J

0

0.5

1

1.5

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-0.5

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-

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NcFm IFIS

Fig. 5 18FlOO/F60

0

Fig. 6

~‘6 lg F60/&5

lg&O/F25

~‘6 lgF25/Fl2

5. The Model IRAS observed the galaxies at four bands. A three parameter model could be used to fit the IRAS emission of galaxies. In Rowan-Robinson & Crawford’s model, the three parameters are the ratios among warm dust, cool dust, starburst and Seyfert components. But the model with fixed temperature, density and space distribution of dust is not rational for the galaxies with different dust component and radiation field. Without concerning of the radia.tive transfer, two black body model is the simplest one. Assuming the IRAS emission of galaxies is avB,(Tl) + ,DvB,(T~), we calculated all of the galaxies in our sample and got the temperatures and ratio of two components. The histograms of 2’1 and 2’2 are showed in Figure 8. 2’1 reflects the behavior of Fzs/F,z. The two peaks are clear as well. T2 reflects the behavior of Free /Pso . The ratio of the two components reflects the behavior of Fse/Fzs. We will not discuss them more. Only for IRAS00450-2533 and IRAS02509+1248, we do not have a reasonable result. We got Tl > lOOOK for 00450-2533. It is too hot. This IRAS source is confused with a nearby red star probably. IRAS02509+1248 has a extremely high flux at 6016rn. It has Cl = 6, which means that the area is influenced by “Cirrus”.

15

IFUS Galaxies

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Fig. 7 Spectra

of gahxies

16

WANG

Number

Gang

-

40,

1 loo

loo

lb)

(I)

Fig. 8 (a) Histogram of the temperature of hot component (b) Histogram of the temperature of cool component

6. CONCLUSION We conclude the paper as the following. 1. IRAS colors of galaxies are correlative with their morphological type. Spiral galaxies have smaller Fss/Fiz and larger FIOO /Fse. 2. IRAS galaxies can be classified into four groups , i.e. normal, starburst, AGN and mixed, in the color-color diagram of Fzs/Fiz and Fse/Fzs. 3. The simple model of two black body could fit most of their infrared emission. Acknowledgement The idea of this paper is from my Ph.D. thesis. I would like to thank Professor Hu Jingyao for kind advice, and Professor Deng Zugan and Professor Xia Xiaoyang for discussions in preparing this paper. References PI

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G., ApJ.,

PI

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