REVEALING THE YOUNGEST STAR CLUSTERS IN THE STARBURST GALAXY NGC 4449

Transcription

1 REVEALING THE YOUNGEST STAR CLUSTERS IN THE STARBURST GALAXY NGC 4449 Amy E. Reines Advisor: Kelsey E. Johnson Department of Astronomy, University of Virginia, Charlottesville, VA, ABSTRACT Star formation is arguably the most important process in the universe, with implications from galaxy evolution to planet formation and the origin of life. Super star clusters (SSCs) are the most massive and dense of all stellar clusters and are consistent with being young analogues of the ancient globular clusters we see today around massive galaxies like our own Milky Way. I present a study of the irregular starburst galaxy NGC 4449 in which I find 37 newborn stellar clusters. Using a combination of data at radio and optical wavelengths from the Very Large Array and the Hubble Space Telescope, I determine the physical properties such as ages, masses and extinctions of the young clusters. Studying the formation and evolution of local super star clusters as they emerge from their dusty birth cocoons will shed light on an extreme mode of star formation that was prevalent in the early universe and continues to have a major impact on the evolution of galaxies. Introduction The most intense episodes of star formation in the local universe result in the formation of super-star clusters. Super-star clusters (SSCs) are the most massive and dense of all stellar clusters and are consistent with being young analogues of classical globular clusters. Interest in SSCs was sparked by the discovery of a swarm of young massive star clusters in the galaxy NGC 1275 by Holtzmann et al. (1991) using the Hubble Space Telescope (HST). Since then, SSCs have been found in a large number of galaxy systems, primarily at optical wavelengths with HST. During the last few years, however, ultra-young SSCs still deeply embedded in their birth material have been discovered using infrared and radio imaging (Johnson et al. 2004; Johnson, Indebetouw, & Pisano 2003). The current physical model for a super star cluster is as follows (Johnson, Indebetouw, & Pisano 2003). While in the earliest stages of evolution, the embedded cluster contains a compact stellar core dominated by very hot and luminous O and B stars. These infant stars ionize the surrounding gas, producing a dense HII region observable in the radio regime via thermal free-free emission. This dense HII region is in turn surrounded by a warm dust cocoon, detectable by its infrared emission of re-radiated starlight. Shortly after the cluster is born, its HII region expands in order to achieve pressure equilibrium, and quickly becomes unobservable in the radio regime since free-free emission is critically dependent on electron density. Simultaneously, intense radiation pressure and stellar winds, emanating from massive stars within the cluster, disperse the surrounding dust and the cluster stars become visible at optical wavelengths. We present our search for ultra-young embedded star clusters in the galaxy NGC 4449 as part of a program to study SSCs as they emerge from their birth cocoons and transition from being visible in the radio regime to the optical regime. Optical and ultraviolet HST/WFPC2 broadband images have already revealed 61 candidate compact stellar clusters in NGC 4449 (Gelatt, Hunter, & Gallagher 2001), including a bright central cluster with an age of 6-15 Myr (Gelatt, Hunter, & Gallagher 2001; Böker et al. 2001). NGC 4449 is a barred Magellanic-type irregular starburst galaxy (de Vaucouleurs et al. 1991) and is part of the Canes Venaticorum group of galaxies with a right ascension of 12h28m11.900s and a declination of +44d05m39.60s (J2000). NGC 4449 has unusually extended streamers of neutral hydrogen wrapping around it and the galaxy s central and outer HI gas systems are counterrotating (van Woerden, Bosma, & Mebold 1975; Bajaja, Huchtmeier, & Klein 1994; Hunter et al. 1998), both suggesting that NGC 4449 may have in- Reines 1

2 teracted with a neighboring galaxy in its past (Bajaja, Huchtmeier, & Klein 1994; Hunter, van Woerden, & Gallagher 1999). Using results from Bajaja, Huchtmeier, & Klein (1994) and a Hubble constant of 72 km s 1 Mpc 1 (Spergel et al. 2003), we calculate for NGC 4449 a distance of 3.9 Mpc, a total HI mass of M, and a total mass of M within a 33 kpc radius. The galaxy has an angular size of 6.2 x 4.4 corresponding to a linear size of 7 x 5 kpc at its distance. NGC 4449 has a star formation rate of 0.01M yr 1 kpc 2, twice that of the Large Magellanic Cloud (Hunter, van Woerden, & Gallagher 1999). Observations and Data Reduction We have obtained observations of NGC 4449 spanning the radio and optical wavelength regimes. Our Very Large Array (VLA) data at centimeter radio wavelengths reveal the HII regions surrounding the youngest clusters in the galaxy, via their thermal freefree emission. Optical observations from the HST archive allow us to study the stellar light emanating from the infant clusters, as well as hydrogen recombination lines from the surrounding ionized gas. VLA Data Radio observations of NGC 4449 were take with the VLA in the C configuration at 6.0 cm (C band), 3.6 cm (X band), and 1.3 cm (K band). The data were imaged to have matched beams of 1 using the Astronomical Image Processing System (AIPS). Our highest sensitivity image of NGC 4449 at 3.6 cm is shown in Figure 1. HST/ACS Data Optical images of NGC 4449 have been taken with the Advanced Camera for Surveys (ACS) aboard HST and I have obtained these calibrated, drizzled data from the archive. I analyze images taken through the following filters: F550M (Narrow V), F814W (Wide I), F658N (Hα), F660N([NII]), and F502N([0III]). A composite image is shown in Figure 2. Data Analysis and Results All of the data analysis was performed using code I wrote in the Interactive Data Langauage (IDL). Radio Selected Ultra Dense HII Regions I have identified 39 radio sources in our VLA data of NGC Their coordinates, flux densities and spectral indexes (α) are listed in Table 1 in order of increasing right ascension. A flat or positive spectral index, α > 0 (S ν ν α ), is indicative of a thermal radio source producing free- Fig. 1. A VLA X-band image of NGC 4449 revealing radio knots as current sites of star formation. Fig. 2. An HST/ACS optical image of NGC Hα emission is shown in red, which highlights regions of hot gas being ionized by young massive star clusters. Stellar continuum is shown in blue, highlighting the older stellar populations within this galaxy. free emission. A negative spectral index is caused by the steep decline of non-thermal synchrotron emission. Non-thermal sources are not of interest here since they are not associated with the youngest natal stellar clusters. Reines 2

3 TABLE 1 RADIO FLUX DENSITIES AND SPECTRAL INDICES Source ID R.A. Decl. F 6.0cm F 3.6cm F 1.3cm α3.6cm 6.0cm (J2000.0) (J2000.0) (µjy) (µjy) (µjy) α 3.6cm 1.3cm < 92 76(19) < 213 > < < (22) < 155 > 0.19 < (37) 281(25) 280(51) -0.27(0.26) -0.00(0.21) (38) 272(34) 334(56) 0.27(0.37) 0.21(0.21) < (25) < 143 > 0.33 < (34) 249(14) 144(32) -0.91(0.18) -0.56(0.24) < (20) 165(44) > (0.31) < (21) < 122 > 0.17 < < (20) < 111 > < < (20) < 128 > 0.30 < (44) 230(27) 221(38) 0.63(0.53) -0.04(0.21) (47) 134(21) 150(39) -0.80(0.49) 0.11(0.31) (49) 115(19) < (0.38) < ,915(99) 2,123(29) 900(46) -1.85(0.04) -0.88(0.05) ,271(62) 2,253(61) 1,506(38) -0.01(0.07) -0.41(0.04) (47) 113(20) 102(32) -0.72(0.60) -0.10(0.36) < (19) < 83 > < < (14) < 157 > 0.78 < < (12) < 67 > < < 94 44(13) < 151 > < < 97 80(15) < 180 > < (41) 164(15) 177(52) 0.13(0.51) 0.07(0.31) (56) 475(23) 485(52) -0.11(0.22) 0.02(0.12) < (13) < 65 > < < (13) < 66 > < ,967(44) 1,802(38) 1,358(94) -0.16(0.06) -0.29(0.07) (40) 297(20) 340(87) 0.06(0.28) 0.14(0.27) < (12) < 74 > < < (26) < 240 > 0.65 < (79) 190(16) 157(51) 0.66(1.09) -0.19(0.34) < (16) < 122 > < < (14) < 123 > < < (19) < 127 > 0.34 < < (13) < 122 > < < (13) < 128 > < < (14) < 134 > < < (19) < 152 > 1.56 < < (17) < 176 > < (35) 138(20) < (0.39) < 0.87 NOTE. Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. Reines 3

4 Two of the radio sources (6 and 14) have steep negative spectral indices, indicating that they are nonthermal. Source 6 is likely a background AGN and source 14 is a known supernova remnant. The remaining 37 sources are consistent with being thermal in nature or a mixed population. Image Registration and the Starburst Morphology In order to study the optical properties of the the radio selected sources in NGC 4449, we must first register the HST images to the VLA images, which are used to define an absolute astrometric reference frame. We register the HST images by first matching sources in both Hα and the radio since both detect HII regions. We find the average x and y pixel offsets between sources in the two images and modify the keywords CRPIX1 and CRPIX2 in the header of the H α FITS file. CRPIX1 and CRPIX2 give the x and y coordinates of the reference pixel corresponding to the right ascension and declination of the reference pixel. The other ACS images are then registered to the Hα image using coincident sources. HST/ACS Photometry of Radio Selected Sources We wish to study areas of NGC 4449 defined by the radio selected ultra dense HII regions at optical wavelengths in an effort to reveal the nature of the natal star clusters. A description of the algorithm developed to do photometry of the radio selected sources in the HST data is given here. The results are listed in Table 2. A contour level of 3σ (x mjy beam 1 ) in the 1.3 cm VLA data is used to define a region of interest. The x,y coordinates of the contour vertexes are transformed into pairs of right ascension and declination using information in the FITS header. These WCS coordinates are in turn converted into x,y coordinate pairs in an HST image using its FITS header. This process requires the images to be accurately registered ( 3.2). The pixel counts (e ) are then summed up within the contour boundary. The background level is determined in an annulus bounded by two larger expanded source contours. These larger contours are found by first converting the x,y coordinates of the source contour into polar coordinates. The radial distance from the center of the source to each vertex is then multiplied by a set of two appropriate factors for the inner and outer annulus boundaries. The total background subtracted source counts are converted into a HST system magnitude and flux densities are obtained by using the header keywords PHOT- FLAM for F λ (erg s 1 cm 2 Å 1 ). The HST photometry is summarized in Table 2. Physical Properties of Star Clusters: Ages, Extinctions, and Masses In order to estimate the ages, extinctions and masses of the stellar clusters, we compare the observed flux densities to the Starburst99 population synthesis models of Leitherer et al. (1999). We adopt an instantaneous star formation law, a Salpeter IMF (α = 2.35), a total stellar mass of 10 6 M with limits of 1 M and 100 M, and a metallicity of Z= We obtain an upper limit on the number of ionizing photons from the X-band flux densities following Condon (1992). For optically thin thermal free-free radiation the number of Lyman continuum ionizing photons is given by Q Lyc s 1 ( Te 10 4 K ( ) 0.45 ( ν ) 0.1 GHz L thermal ergs s 1 Hz 1 ). Age estimates are obtained from the measured Hα equivalent widths and extinctions are calculated by comparing the measured Hα flux with the expected value (also given in Condon 1992). Masses are estimated by comparing the inferred number of ionizing photons to that predicted by the model at the age found from the Hα equivalent widths. A consistency check is also done using the V and I band photometry. The physical properties of the young stellar clusters are listed in Table 3. Summary I have identified 37 radio-selected natal star clusters in the irregular starburst galaxy NGC Radio observations taken by Kelsey Johnson using the Very Large Array (VLA) reveal dense regions of ionized hydrogen gas engulfing the newborn stellar clusters in the galaxy and I have obtained optical HST/ACS images for comparison. By selecting clusters based on the radio imaging, I have found the youngest clusters in the galaxy that have just recently been born since the dense, ionized gas surrounding a young cluster will not be visible in the radio after 10 Myr, when the hottest and most massive stars in the cluster have died. Synthesizing images at radio and optical wavelengths has yielded the physical properties of these clusters. I have estimated ages, extinctions, and masses of the clusters and find that all of the clusters are younger than 10 Myr with a mean age of 4 Myr. Most of the clusters have masses in range of M, however there is a whopping nu- Reines 4

5 TABLE 2 HST/ACS PHOTOMETRY OF RADIO SELECTED SOURCES IN NGC 4449 Source ID F502N F550M F658N F660N F814W [OIII]λ5007 V continuum Hα+[NII]λ6584 [NII]λ6584 I continuum Flux Density, F λ (10 16 erg s 1 cm 2 Å 1 ) (0.05) 0.10(0.01) 3.64(0.06) 0.96(0.06) 0.12(0.01) 2 <0.10 < (0.08) <0.06 < (1.44) 2.72(0.30) 33.66(0.93) 9.57(0.38) 1.96(0.08) (1.08) 7.34(0.23) 38.61(1.40) 13.56(0.50) 2.96(0.10) (0.88) 0.96(0.05) 7.00(0.62) 3.19(0.24) 0.45(0.02) (0.02) 0.38(0.04) 0.15(0.04) 0.20(0.02) 0.30(0.03) (0.15) 0.54(0.01) 8.30(0.36) 2.10(0.10) 0.34(0.01) (1.55) 0.77(0.05) 15.55(0.95) 3.26(0.23) 0.48(0.02) (0.89) 1.04(0.09) 2.14(0.26) 1.33(0.13) 0.69(0.05) (0.49) 1.01(0.04) 8.62(0.65) 2.22(0.16) 0.56(0.02) (0.89) 2.20(0.56) 12.58(0.89) 4.70(0.75) 1.26(0.35) (1.22) 2.59(0.57) 9.48(1.27) 3.95(0.83) 2.03(0.19) (0.40) 0.37(0.07) 6.80(0.47) 1.71(0.11) 0.26(0.04) (0.78) 1.18(0.17) 4.15(0.77) 1.92(0.20) 0.80(0.04) (5.92) 66.01(3.29) (6.43) 86.59(3.46) 44.43(1.99) (0.06) 0.66(0.03) 3.12(0.09) 1.35(0.22) 0.60(0.02) (1.01) 1.63(0.12) 12.18(0.46) 3.50(0.19) 1.17(0.08) (0.34) 0.93(0.01) 19.18(0.34) 3.78(0.07) 0.56(0.01) (0.17) 0.24(0.05) 2.33(0.40) 0.77(0.11) 0.17(0.02) 20 <0.09 < (0.05) 0.42(0.11) 0.09(0.02) (0.34) 0.95(0.02) 14.40(0.49) 3.22(0.09) 0.48(0.02) (0.25) 2.40(0.01) 14.85(0.29) 4.16(0.03) 1.05(0.01) (0.30) 1.16(0.04) 29.01(1.21) 10.11(0.36) 1.10(0.03) (0.73) 0.62(0.10) 1.20(0.16) 1.28(0.12) 0.46(0.07) (0.39) 0.86(0.08) 8.85(0.47) 3.00(0.15) 0.53(0.05) (1.95) 9.03(0.15) (2.23) 32.98(0.59) 5.21(0.08) (0.30) 1.44(0.03) 23.33(0.66) 5.72(0.22) 0.83(0.02) (0.02) 0.12(0.01) 0.08(0.01) < (0.01) (1.02) 0.54(0.13) 10.38(1.50) 2.59(0.42) 0.35(0.07) (0.20) 0.27(0.01) 10.18(0.22) 1.95(0.20) 0.30(0.01) (0.19) 1.23(0.02) 11.91(0.23) 3.11(0.04) 0.59(0.00) (0.31) 0.20(0.04) 4.18(0.48) 1.18(0.08) 0.12(0.03) (0.77) 0.28(0.07) 6.99(0.73) 1.66(0.25) 0.22(0.04) (0.05) 0.06(0.01) 0.84(0.08) 0.35(0.07) 0.06(0.01) (0.35) 0.14(0.02) 2.64(0.49) 0.75(0.08) 0.12(0.02) (0.05) 0.04(0.01) 0.33(0.05) 0.17(0.02) 0.04(0.01) (0.43) 0.94(0.02) 14.98(0.34) 3.16(0.12) 0.57(0.02) (0.16) 0.79(0.06) 6.83(0.15) 1.97(0.05) 0.39(0.04) (0.27) 0.42(0.02) 7.94(0.22) 1.38(0.06) 0.24(0.01) Reines 5

6 TABLE 3 PROPERTIES OF RADIO SELECTED SOURCES IN NGC 4449 Source ID Q Lyc F Hα log W(Hα) A Hα Age Mass (10 49 s 1 ) (10 14 erg s 1 cm 2 ) (Å) (mag) (Myr) (10 3 M ) 1 11(3) 2.42(0.06) 3.37(0.17) 1.2(0.3) (2.5)2.5(0.4) 1.8(0.5) 2 17(3) 0.37(0.06) > (0.3) <3.4 < (4) 21.52(1.25) 2.96(0.21) 0.3(0.1) (1.3)4.5(0.4) 17.7(1.6) 4 39(5) 22.75(1.37) 2.65(0.08) 0.2(0.1) (0.1)5.0(0.1) 32.5(4.0) 5 19(4) 3.92(0.52) 2.74(0.12) 1.3(0.3) (0.2)4.9(0.1) 11.5(2.2) (3) 5.47(0.29) 3.09(0.09) 0.9(0.2) (1.0)3.9(0.4) 5.6(0.9) 8 22(3) 10.58(0.77) 3.22(0.15) 0.4(0.2) (0.4)2.9(0.2) 4.7(0.6) 9 10(3) 0.80(0.39) 1.96(0.28) 2.4(0.6) (1.0)7.3(1.9) 37.4(10.5) 10 23(3) 5.53(0.54) 2.84(0.11) 1.2(0.2) (0.3)4.8(0.1) 12.8(1.6) 11 33(4) 7.26(2.33) 2.62(0.64) 1.2(0.4) (2.2)5.0(2.3) 27.6(3.2) 12 19(3) 4.92(2.27) 2.32(0.47) 1.1(0.5) (1.2)6.0(2.2) 29.6(4.6) 13 16(3) 4.50(0.46) 3.15(0.42) 1.0(0.2) (3.2)3.2(1.7) 3.1(0.5) (0.82) 2.31(0.32) (9) 58.78(13.86) 2.02(0.15) 1.4(0.3) (0.7)7.2(1.0) 1,042.8(28.3) 16 16(3) 1.66(0.17) 2.42(0.11) 2.1(0.2) (0.5)5.6(0.4) 18.7(3.3) 17 16(3) 7.55(0.60) 2.72(0.17) 0.4(0.2) (0.2)4.9(0.2) 10.0(1.7) 18 21(2) 13.16(0.27) 3.24(0.02) 0.1(0.1) (0.1)2.8(0.1) 4.2(0.4) 19 9(2) 1.43(0.37) 2.83(0.46) 1.6(0.3) (2.1)4.8(1.0) 4.9(0.9) 20 6(2) 0.33(0.06) > (0.4) <4.4 < (2) 9.67(0.39) 3.13(0.06) 0.0(0.2) (0.5)3.4(0.3) 2.6(0.5) 22 23(2) 9.26(0.23) 2.73(0.02) 0.6(0.1) (0.1)4.9(0.1) 14.4(1.3) 23 68(3) 18.15(0.97) 3.21(0.08) 1.0(0.1) (0.2)2.9(0.5) 14.6(0.7) 24 15(2) <0.38 > (2) 5.43(0.48) 2.89(0.21) 0.5(0.2) (0.9)4.7(0.3) 6.2(0.9) (5) (1.84) 3.16(0.04) 0.6(0.0) (0.3)3.2(0.4) 48.8(1.0) 27 42(3) 15.47(0.54) 3.13(0.06) 0.7(0.1) (0.5)3.4(0.5) 9.5(0.6) 28 11(2) <0.06 > (4) 6.88(1.28) 3.18(0.53) 0.9(0.3) (2.9)2.9(2.1) 4.9(0.8) 30 27(2) 7.05(0.19) 3.40(0.10) 1.1(0.1) (2.4)2.4(0.3) 4.1(0.3) 31 9(2) 7.70(0.19) 2.93(0.03) 0.0(0.3) (0.1)4.6(0.1) 4.3(1.0) 32 8(2) 2.73(0.41) 3.23(0.53) 0.7(0.3) (2.8)2.8(2.1) 1.5(0.4) 33 18(3) 4.69(0.64) 3.27(0.54) 1.0(0.2) (2.8)2.8(2.1) 3.6(0.5) 34 8(2) 0.50(0.08) 2.91(0.36) 2.6(0.3) (1.8)4.6(0.5) 3.5(0.9) 35 7(2) 1.71(0.39) 3.12(0.37) 1.2(0.4) (3.5)3.5(1.4) 1.7(0.4) 36 6(2) 0.17(0.06) 2.62(0.56) 3.5(0.5) (2.1)5.0(2.0) 5.3(1.7) 37 35(3) 10.13(0.29) 3.12(0.07) 0.9(0.1) (0.6)3.5(0.4) 8.2(0.6) 38 12(2) 4.31(0.26) 2.86(0.19) 0.8(0.2) (1.6)4.7(0.3) 6.0(1.1) 39 20(3) 5.52(0.19) 3.22(0.11) 1.0(0.2) (0.3)2.9(0.1) 4.3(0.6) Reines 6

7 clear super star cluster with a mass of 10 6 M. Most of the clusters suffer from at least one magnitude of extinction and we find 2 sources completely obscured in optical light. NGC 4449 has been a fruitful target for studying extragalactic star formation. In the near future, I plan to incorporate Spitzer Space Telescope infrared data into this study to gain an even better understanding of the star formation in this exciting galaxy. Acknowledgments I would like to thank my advisor Kelsey Johnson for the radio observations used here and for advising me throughout my time at the University of Virginia. I gratefully acknowledge support for this paper provided by the Virginia Space Grant Constoritum through a Graduate Research Fellowship and the Univeristy of Virginia through a Governor s Fellowship. Kennicut, R. C., Jr. 1998, ARA&A, 36, 189 Misselt, K. A., Clayton, G. C., & Gordon, K. D. ApJ, 515, 128 Spergel, D. N., Verde, L., Peiris, H. V., Komatsu, E., Nolta, M. R., Bennett, C. L., Halpern, M., Hinshaw, G., Jarosik, N., Kogut, A., Limon, M., Meyer, S. S., Page, L., Tucker, G. S., Weiland, J. L., Wollack, E., & Wright, E. L. 2003, ApJS, 148, 175S van Dokkum, P. G. 2001, PASP, 113, 1420 van Woerden, H., Bosma, A., & Mebold, U. 1975, in La dynamique des Galaxies Spirales, ed. L. Weliachew (Paris: Editions CNRS), 483 REFERENCES Bajaja, E., Huchtmeier, W. K., & Klein, U. 1994, å, 285, 385 Böker, T., van der Marel, R. P., Mazzuca, L., Rix, H.- W., Rudnick, G., Ho, L. C., & Shields, J. C. 2001, AJ, 121, 1473 Condon, J. J. 1992, ARA&A, 30, 575 de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H., Buta, R., Paturel, G., & Fouqu, P. 1991, Third Reference Catalogue of Bright Galaxies (New York: Springer-Verlag) Fitzpatrick, E. L. 1985, ApJ, 299, 219 Gelatt, A. E., Hunter, D. A., & Gallagher, J. S. 2001, PASP, 113, 142 Holtzmann, J. A., Faber, S. M., Shaya, E. J., Lauer, T. R., Groth, E. J., & Hunter, D. A. 1992, BAAS, 23, 1473 Hunter, D. A., van Woerden, H., & Gallagher, J. S. 1999, AJ, 118, 2184 Hunter, D. A., Wilcots, E., van Woerden, H., Gallagher, J.S., & Kohle, S. 1998, ApJ, 495, L47 Johnson, K. E., Indebetouw, R., & Pisano, D. J. 2003, AJ, 126, 101 Johnson, K. E., Indebetouw, R., Watson, C., & Kobulnicky, H. A. 2004, AJ, 128, 610 This 2-column preprint was prepared with the AAS L A TEX macros v5.2. Reines 7