CO(1-0) in High-Redshift Radio Galaxies using the ATCA

Transcription

1 The Interstellar Medium in High Redshift Galaxies Comes of Age NRAO Conference Series, Vol. 28 J. G. Mangum c 2012 National Radio Astronomy Observatory CO(1-0) in High-Redshift Radio Galaxies using the ATCA Minnie Y. Mao 1, Bjorn Emonts 2 and Ray P. Norris 2 1 National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 87801, USA 2 CSIRO Astronomy and Space Science, Australia Telescope National Facility, PO Box 76, Epping, NSW, 1710, Australia Abstract. In 2009, the Compact Array Broadband Backend (CABB) was installed on the Australia Telescope Compact Array (ATCA). ATCA/CABB offers 2 2 GHz instantaneous bandwidth, mm-capabilities from GHz and versatile array configurations, making it an ideal instrument to detect and spatially resolve high-redshift molecular gas. Our team have observed a representative sample of high-redshift radio galaxies using CABB with the intention of detecting CO(1-0) (Emonts et al. 2011a,b). Here I summarise ATCA s upgraded spectral line capabilities and present and discuss our work on molecular gas in high-redshift radio galaxies, including the detection of CO(1-0) in four z 2 radio galaxies. 1. Introduction High-redshift radio galaxies (HzRGs) are among the most luminous and massive galaxies in the early Universe (e.g Seymour et al. 2007; Miley & De Breuck 2008). They display clumpy optical morphologies (Pentericci et al. 2001) and are typically surrounded by rich proto-cluster environments (Venemans et al. 2007). HzRGs are believed to be the ancestors of bright cluster galaxies found in the centres of clusters of galaxies in the local Universe. The interaction between HzRGs and their surrounding protocluster make HzRGs the ideal laboratories for understanding the formation and evolution of clusters of galaxies, as well as the relationship between star-formation and AGN activity. Moreover, studying how HzRGs evolve can provide valuable insight towards star-formation in clusters of galaxies in the local Universe. The main ingredient for star-formation is cold molecular gas. Cold molecular gas (mostly in the form of H 2 ) is a key ingredient for the formation of stars. However, unless shocked or heated to very high temperatures, H 2 is very difficult to see due to its strongly forbidden rotational transitions. The presence of H 2 may be inferred by the presence of carbon monoxide (CO), which emits strong rotational transition lines that occur primarily through collisions with H 2 (Solomon & Vanden Bout 2005). Although many systematic searches for CO in HzRGs have been attempted (e.g Evans et al. 1996; van Ojik et al. 1997), the results have traditionally been hampered by narrow bandwidths and unsuitable frequency converage. With the advent of ALMA, and recent upgrades on the Karl G. Janksy Very Large Array (VLA) and the ATCA, these challenges may be overcome. Using CABB on the ATCA, we have searched fourteen HzRGs for CO(1-0) in order to study the molecular gas content of HzRGs and have detected CO(1-0) in three (Emonts et al. 2011a,b, 2012), with a likely fourth detection currently under investigation. 1

2 2 Mao, Emonts and Norris 2. The Compact Array Broadband Backend on the Australia Telescope Compact Array The ATCA is located in Narrabri, New South Wales, approximately a six hour drive from Sydney, and is operated by the CSIRO Astronomy and Space Science division as part of the Australia Telescope National Facility (ATNF). The ATCA comprises six 22 m antennas with a maximum baseline of 6 km. Five of the six antennas are on a 3 km East-West track with a shorter North-South spur and may be moved into different configurations to maximise uv coverage. The ATCA operates from 1.1 GHz to 106 GHz in five discrete bands. In 2009, the ATCA was upgraded with the Compact Array Broadband Backend (CABB). Previous to the CABB installation, the ATCA provided an instantaneous bandwidth of MHz. CABB has an instantaneous bandwidth of 2 2 GHz, which is a 16-fold increase in bandwidth, corresponding to a 4-fold increase in continuum sensitivity. A description of CABB and some early science highlights are presented in Wilson et al. (2011). The ATCA has three millimetre receivers. The 15 mm, 7 mm and 3 mm receivers, which cover the frequency ranges GHz, GHz and GHz respectively. The broad frequency coverage coupled with the compact, hybrid configurations (with baselines as short as 31 m) make the ATCA an excellent instrument to detect molecular gas at high-redshifts. Figure 1 shows the redshift ranges at which the ATCA could observe the CO(J,J-1) lines. 3. CO(1-0) in HzRGs using the ATCA We are in the final stages of performing a systematic survey of CO(1-0) in a representative sample of HzRGs. The ATCA s ability to observe at <50 GHz enable us to observe the ground transition CO(1-0) (ν rest = 115 GHz) at z > 1.3. Despite higher order CO lines being more likely to have a higher flux density than the lower ones, the higher transitions are tracing the dense and thermally excited gas, mostly in the central regions of the galaxy. Papadopoulos et al. (2001) suggest that widely distributed large resovoirs of less dense, subthermally excited gas may be missed. CO(1-0) is least affected by the excitation conditions of the gas and hence observations of the ground transition are crucial for deriving the most accurate estimates of the overall molecular gas content in HzRGs. The sample of HzRGs was selected using the following criteria: Each source has a declination less than -10 degrees, so as to be observable by the ATCA, The source is at a redshift between 1.3 and 2.9 so that the CO(1-0) transition would be observable with the ATCA s 7 mm band, and Ancillary Hubble Space Telescope and Spitzer data (Pentericci et al. 2001; De Breuck et al. 2010) be available so as to maximise scientific output. In total fourteen sources satisfied the selection criteria. Observations were performed using the two most compact hybrid configurations of the ATCA, H75 and H168, to mitigate the effect of atmospheric phase fluctuations, which worsen with baseline length. The average on-source integration time is 15 hours. Observations for this project began in 2009 when CABB was commissioned. To date, we have detected CO(1-0) in four HzRGs, three of which are shown in Figure 2. These are MRC , MRC (Emonts et al. 2011b) and MRC (the Spiderweb Galaxy, Emonts et al. 2012) with molecular gas masses of M H2 of 3, 6 and M respectively. Furthermore, one galaxy in our sample has a tentative CO(1-0) detections. MRC , was observed at the extreme edge of the CABB band and is presented in Emonts et al. (2011a). Details about the observations and data reduction are described in Emonts et al. (2011a). Assuming X CO = M H2 /L CO = 0.8.

3 CO(1-0) in HzRGs using the ATCA 3 Figure 1: The ATCA s mm-bands and corresponding ability to detect CO(J,J-1) to J=6. The ATCA can detect CO(1-0) from 1.3 < z < 2.9 with its 7mm receiver.

4 4 Mao, Emonts and Norris Figure 2: The three CO(1-0) detections in HzRGs observed using the ATCA. Figure from ATNF Newsletter (Issue 72, April 2012) MRC MRC (z = 1.92) contains the strongest CO(1-0) signal detected in a HzRG to date (Emonts et al. 2011b). The galaxy is an infrared-bright starburst merger-system. The strong CO(1-0) suggests the system contains large amounts of molecular gas that has not yet been depleted by star-formation or AGN-feedback. High-resolution follow-up observations of the CO(1-0) in this galaxy using the 750 m and 1.5 km ATCA configurations suggest that the CO(1-0) is resolved. The widespread CO(1-0) is consistent with a turbulent merger system MRC (the Spiderweb Galaxy ) The Spiderweb proto-cluster is a massive proto-cluster system at z = It is known to host extreme amounts of star-formation (Seymour et al. 2012). This massive proto-cluster is likely a high-redshift analogue to rich clusters of galaxies in the local Universe. The CO(1-0) detection is described in Emonts et al. (2012). 4. Conclusions The CABB on the ATCA is an excellent instrument for observations of redshifted CO(1-0) and, being situation in the Southern hemisphere, is a good compliment to ALMA for frequencies below 50 GHz. Observations of CO(1-0) are imperative as the ground transition is the least affected by the excitation conditions of the gas and hence the most robust tracer of the overall molecular gas content. We have observed a sample of 14 HzRGs and have detected CO(1-0) in four, of which three have been published. Results for the fourth detection will be published in an upcoming paper. References De Breuck C., et al., 2010, ApJ, 725, 36 Emonts B. H. C., et al., 2011, MNRAS, 415, 655 Emonts B. H. C., et al., 2011, ApJ, 734, L25 Emonts B. H. C., et al., 2012, MNRAS, Submitted Evans A. S., Sanders D. B., Mazzarella J. M., Solomon P. M., Downes D., Kramer C., Radford S. J. E., 1996, ApJ, 457, 658 Miley G., De Breuck C., 2008, A&ARv, 15, 67 Papadopoulos P., Ivison R., Carilli C., Lewis G., 2001, Natur, 409, 58 Pentericci L., McCarthy P. J., Röttgering H. J. A., Miley G. K., van Breugel W. J. M., Fosbury R., 2001, ApJS, 135, 63

5 Seymour N., et al., 2007, ApJS, 171, 353 Seymour N., et al., 2012, ApJ, 755, 146 Solomon P. M., Vanden Bout P. A., 2005, ARA&A, 43, 677 van Ojik R., et al., 1997, A&A, 321, 389 Venemans B. P., et al., 2007, A&A, 461, 823 Wilson W. E., et al., 2011, MNRAS, 416, 832 CO(1-0) in HzRGs using the ATCA 5