In the search for CO emission in young, low-metallicity spiral disks and dwarf galaxies: Prospects for ALMA

Armando Gil de Paz (UCM), Kartik Sheth (Caltech/SSC), Carmen Sánchez Contreras (DAMIR, IEM, CSIC), Samuel Boissier (LAM), Barry F. Madore (OCIW), Jaime Zamorano (UCM)

12CO(1-0) & 12CO(2-1) main beams at IRAM 30m

I Zw 18 (HST) I Zw 18

21

3 4

5

Center NW XUV complexes with ionized-gas line-emission

SECenter SWCenter

Motivation: Many high-redshift galaxies have been detected in CO emission to date. However, these are very massive systems that only represent the tip of the iceberg of the galaxy population at these redshifts. At even higher redshifts (z>6) some of these objects might not even exist (e.g. Labbe et al. 2006). Indeed, it is now thought that dwarf galaxies were the main contributors to the reionization of the Universe.

In this poster we analyze the CO emission from two nearby galaxies whose properties resemble those of galaxies expected to be dominating (at least in number) the high-z galaxy population: spiral galaxies in formation (NGC 4625; see 1, 2) and dwarf galaxies (I Zw 18; 3, 4).

Conclusion: CO emission from young spiral disks could be detected up to z=1 and their (proto) bulges up to z=2 in a Legacy-like (several hundred hours long) program. However, the detectability of low-metallicity dwarf galaxies like I Zw 18 even at z>0.2 is unlikely.

NGC 4625: The properties of NGC 4625 and its Extended UV (XUV) disk resemble those of a young, forming spiral disk (Boissier & Prantzos 2000): faint total optical luminosity and surface brightness, high neutral gas and dark matter content, and low metallicity (Bush & Wilcots 2004, Gil de Paz et al. 2005, 2006).

Panel 2 shows the optical (left) and GALEX UV (right) images of NGC 4625 along with the 12CO(1-0) spectra (bottom) obtained with the IRAM 30m MRT. Emission is clearly detected in the nuclear regions (proto-bulge?) and marginally in positions #9 and #10 of the disk.

I Zw 18: I Zw 18 (the most metal poor galaxy known) is the best local example of a young galaxy in formation. Its properties are similar to those of the first galaxies, including those responsible for the reionization of the Universe at z>6. In Panel 3 we show a HST image of I Zw 18 (left) along with new deep 12CO(1-0) and (2-1) (right) spectra obtained with the IRAM 30m. Despite the high sensitivity of these observations no CO emission is detected (rms=4 mJy for the 1-0 line).

Optical (R-band)

Far-UV (GALEX)

LCO vs. [O/H]: The faint CO

luminosity at Z<0.1Zsun (Taylor et al. 1998) might preclude the detection of CO from low-mass high-z young galaxies (see 5).

Right: CO detections and upper limits at low-metallicities. The predicted change in LCO with metallicity (normalized to LIZw36~LMrk86) is shown: Wilson (1995; solid line), Arimoto et al. (1996; dotted line).

Prospects for ALMA: On the detectability of young spiral disks and dwarfs at high redshift

Higher CO transitions: Despite the limitations of current instrumentation for detecting high CO transitions in low-metallicity dwarf galaxies, the few objects detected to date indicate that their molecular gas is warm (Tk>20K), and moderately dense (nH2

~103-104 cm-3;

Meier et al. 2001).

These properties are similar to the “warm cloud” component of the models of high-redshift CO emission by Combes et al. (1999). Assuming that this model is valid for both high-redshift young forming dwarfs and spiral disks we expect an increase of ~1.3 dex (x20) in brightness of the CO lines between CO(1-0) and the optimal (6-5), (5-4), and (4-3) transitions.

Future: A complete mapping of the Extended UV disk of NGC 4625 to fainter levels with ALMA and its comparison with our GALEX UV data will allow us studying the photo-dissociation efficiency of the UV radiation field in this young disk and in its high-redshift analogous.

A deeper study of I Zw 18 using ALMA should provide excellent constraints on the dependence of the CO emission with [O/H] and on the reasons behind it (e.g. dependence of XCO on [O/H] and spatial scale, lack of H2).

NGC 4625

Bibliography: Arimoto et al., 1996, PASJ 48, 275

Boissier & Prantzos, 2000, MNRAS 312, 398

Bush & Wilcots, 2004, AJ 128, 2789

Combes et al., 1999, A&A 345, 369

Gil de Paz et al., 2005, ApJ, 627, L29

Gil de Paz et al., 2006, ApJ, submitted.

Labbe et al., 2006, ApJ 649, L67

Meier et al., 2001, AJ 121, 740

Taylor et al., 1998, AJ 116, 2746

Wilson, 1995, ApJ 448, L97

Observed line fluxes: The peak temperatures for the center (proto-bulge?) and outer young disk of NGC 4625 yield fluxes of 0.5 Jy (regions #1 & #2) and 0.1 Jy (region #9), respectively (see 2). In the case of I Zw 18 we estimate a r.m.s. of 4 mJy (see 3). These are all values measured in the 12CO(1-0) transition. We then derived the expected line fluxes that objects like NGC 4625 and I Zw 18 would have if seen at high redshift in the 12CO(6-5), (5-4) and (4-3) transitions (right, solid grey lines). The same brightening factor (x20 relative to the 1-0) has been applied to all three transitions. The distances to NGC 4625 and I Zw 18 are 9.5 Mpc and 12.6 Mpc, respectively. We adopted a cosmology with (h,M,)=(0.7,0.27,0.73).

Young disk (NGC 4625 #9)

Proto-bulge? (NGC 4625 #1)

Young dwarf (I Zw 18)

Small program = 1x8h

Large program = 10x8h

Legacy program = 100x8h12CO(6-5) ALMA rms12CO(5-4) ALMA rms12CO(4-3) ALMA rms

Above: Large (Legacy) ALMA programs of 10x8h (100x8h) could detect sources like the center of NGC 4625 (proto-bulges?) up to z=1 (z=2). Young disks could be seen up to z=0.5 (z=1). Young dwarfs like I Zw 18 would be missed already at z=0.2 or even closer.

(4 mJy)

Main Beam at the observed positions

5 arcsec