STUDYING THE MORPHOLOGY AND STAR FORMATION OF GALAXIES AS A PROBE OF GALAXY EVOLUTION

Publications of the Korean Astronomical Society pISSN: 1225-1534

30: 511 ∼ 512, 2015 September eISSN: 2287-6936

2015. The Korean Astronomical Society. All rights reserved. c http://dx.doi.org/10.5303/PKAS.2015.30.2.511

STUDYING THE MORPHOLOGY AND STAR FORMATION OF GALAXIES AS A PROBE OF GALAXY EVOLUTION

Hsuan-Ju Chen & Chorng-Yuan Hwang Institute of Astronomy, National Central University, Taiwan

E-mail: [email protected]

(Received November 30, 2014; Reviced May 31, 2015; Aaccepted June 30, 2015)

ABSTRACT

Star formation activities dominate the evolution of galaxies. Elliptical galaxies are believed to be old galaxies in the Hubble sequence, and elliptical galaxies at different evolution epochs might have different star formation activities and/or morphologies. We investigate the connection between star formation rates and the morphology of elliptical galaxies. With the Sloan Digital Sky Survey (SDSS) and the Galaxy Zoo, we select a sample of elliptical galaxies by morphology and consider their infrared emission as an index of star formation rate to study the relation between the star formation rates and their morphological properties, such as ellipticities. In addition, we select some nearby spiral galaxies with very low MIR emission to probe the mechanisms of these red spiral galaxies. We display our preliminary results and discuss their implication on the evolution of galaxies in this poster.

Key words: galaxies: star formation – galaxies: evolution

1. STAR FORMATION RATE

Star formation rates (SFRs) are the key probe of the evolution of galaxies. There are many SFR tracers such as Hα, and radio and infrared emission (Kenni- cutt, 1998). The Wide-field-Infrared-Survey Explorer (WISE) is an all-sky survey with high resolution at 3.4 µm, 4.6 µm, 12 µm, 22 µm. Elliptical and spiral galaxies were found to be located at different places in the NIR color-color diagram (Wright et al., 2010). In this report, we consider 22 µm flux as a SFR tracer to select elliptical galaxies with high SFRs. In addition, we also select some spiral galaxies with poor emission at 22 µm.

We refer to Jarrett’s relation of luminosity and SFR (Jarrett et al., 2013):

SF R(M /yr) = 7.5 × 10

νL

(L ) (1) 2. SAMPLE SELECTION

• From SDSS DR10, we selected 20,000 elliptical galaxies and 20,000 spiral galaxies with redshifts less than 0.1. Both elliptical and spiral galaxies were voted by more than 50% people in the Galaxy Zoo.

• For elliptical galaxies, we constrained their mor- phologies with the parameter fracDev =1, which in- dicates that the optical emission is dominated by the bulge.

http://pkas.kas.org

• From WISE, we downloaded the W4(22 µm) data for these galaxies as the star formation rate indices.

• There are 1,295/20,000 ellipticals and 7,218/20,000 spirals which have 22 µm data.

3. RESULTS

In Fig.1, we show the 1,295 ellipticals (red line) and 7,218 spirals (blue line) which have 22 µm emission. For ellipticals, there are only 221 sources classified as normal galaxies from the SDSS spectra. All of them have SFRs smaller than 17.2 M

/yr; others are classified as AGNs, starbursts, or starforming galaxies. We also note that there are spiral galaxies with very low SFRs (Cortese, 2012; Masters et al., 2010; Tojeiro et al., 2013). We esti- mated the correlation of ellipticity and SFR for the 221 normal galaxies but did not find obvious correlation be- tween them. We show some images and spectra of these galaxies from the SDSS in Fig.2. There are 4 ellipticals with high SFRs and 4 spirals with non-detected 22 µm emission. Table 1 lists information for the 4 ellipticals with high SFRs, and Table 2 for the 4 red spirals. In the left half of Fig.2, the images show 4 normal ellip- ticals with strong 22 µm emission. Ellipticals ‘b’, ‘c’

and ‘d’ show strong Hα and/or [O III] emission line, so they truly have significant star formation. The spec- trum of elliptical ‘a’ is corrupted at the Hα line. In Fig.2, the 4 spirals are close to us and have absolute magnitudes brighter than -19. However, these spirals are not detected at 22 µm. In addition, all of them have non-detected emission lines but have an obvious 4000 ˚ Abreak.

511

512 CHEN & HWANG

Figure 1. Elliptical and spiral galaxies of different SFRs. The lower diagram is the zoom in of the upper one. The red line is the number distribution of the ellipticals, and the blue line is that of the spirals.

4. SUMMARY AND FUTURE WORK

We find some ellipticals with high SFRs and some spi- rals with undetected 22 µm emission. There is no cor- relation between the SFRs and the ellipticities of the normal ellipticals. It is still unclear what is the origin for the high SFRs in these normal ellipticals. On the other hand, the spirals with undetected 22 µm emission have very low SFRs; this is consistent with the spectra and the red images of SDSS for these sources. Although the SDSS spectra were mainly obtained from a central 3” aperture, the bulges and disks of the spirals have similar colors, indicating that the disks also have very low SFRs, consistent with non-detection of the 22 µm emission. These results indicate that the pattern form- ing mechanism might be different from that of normal spirals. We will investigate morphological (e.g., Sersic index, compactness etc) and environmental (e.g., clus- ters, groups etc) effects on the SFR of these galaxies in the future.

ACKNOWLEDGMENTS

This work is partially supported by the Ministry of Science and Technology (MOST) of Taiwan through the grant MOST 103-2119-M-008-017-MY3.

REFERENCES

Cortese, L., 2012, Are Passive Red Spirals Truly Passive?

The Current Star Formation Activity of Optically Red Disc Galaxies, A&A, 543, AA132

Jarrett, T. H., Masci, F., & Tsai, C. W., et al., 2013, Extend-

Figure 2. Images ‘a’ to ‘d’ are 4 normal elliptical galaxies with strong 22 µm emission. Images ‘e’ to ‘h’ are 4 nearby spiral galaxies with non-detected 22 µm emission.

Table 1

4 elliptical galaxies with high SFRs High SFR w4 luminosity SFR redshift

ellipticals (L

) (M

/yr)

a 1.45 × 10

10.87 0.082

b 2.29 × 10

17.16 0.078

c 8.07 × 10

6.05 0.090

d 6.97 × 10

5.22 0.095

Table 2

4 spiral galaxies with non-detected 22 µm emission Red Upper limit w4 r-band absolute redshift spirals luminosity(L

) magnitude

e 7.69 × 10

-21.102 0.016

f 3.35 × 10

-20.683 0.011

g 1.59 × 10

-20.312 0.015

h 2.28 × 10

-19.732 0.016

ing the Nearby Galaxy Heritage with WISE: First Results from the WISE Enhanced Resolution Galaxy Atlas, AJ, 145, 6

Kennicutt, R. C., Jr., 1998, Star Formation in Galaxies Along the Hubble Sequence, ARA&A, 36, 189

Masters, K. L., Mosleh, M., & Romer, A. K., et al., 2010, Galaxy Zoo: Passive Red Spirals, MNRAS, 405, 783 Tojeiro, R., Masters, K. L., & Richards, J., et al., 2013, The

Different Star Formation Histories of Blue and Red Spiral and Elliptical Galaxies, MNRAS, 432, 359

Wright, E. L., Eisenhardt, P. R. M., & Mainzer, A. K., et al., 2010, The Wide-field Infrared Survey Explorer (WISE):

Mission Description and Initial On-orbit Performance, AJ,

140, 1868