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STE+AzTEC ★

SM G S U RVEY

Serendipitous detection of ultra-bright submillimeter galaxies beyond the Lupus-I molecular cloud

The most luminous submillimeter galaxies (SMGs) are unique probes of star-formation history in the early universe, but are very rare. We report detections of two candidates for distant SMGs, MM J15451‒3443 and MM J15415‒3503, which are discovered by our AzTEC/ASTE 1.1-mm survey toward the Lupus-I star-forming region. The two objects have 1.1-mm flux densities of 43.9 and 27.1 mJy, making them the brightest SMGs found in the AzTEC/ASTE survey. Photometric redshift estimates using any available data from the mid-infrared to the radio suggest that the redshifts of the two SMGs are zphoto ~ 4‒5 and 3, respectively. Near-infrared objects are found very close to the SMGs and they are consistent with low-z ellipticals, suggesting that the high apparent luminosities are attributed to gravitational magnification. The inferred number counts at S1.1mm > 25 mJy constrains the brightest end of the number counts and is consistent with the estimates from the counts obtained by the South Pole Telescope 1.4-mm survey, supporting the gravitationally lens scenario.

Discovery of MM J1545 and MM J1541

Is MMJ1545 a Galactic First Hydrostatic Core?

ABSTRACT

Yoichi Tamura (U. Tokyo), R. Kawabe (NAOJ), Y. Shimajiri (NRO/CEA Sacley), T. Tsukagoshi (Ibaraki U.), K. Saigo, H. Ezawa (NAOJ), D. J. Wilner (Harvard-Smithsonian CfA), Y. Oasa (Saitama U.), Y. Nakajima (NAOJ), R. Dickman, C. Chandler, M. Goss (NRAO), K. Kohno, Y. Otomo (U. Tokyo), M. S. Yun, G. W. Wilson (UMass), D. H. Hughes (INAOE),

and the AzTEC/ASTE collaboration

MMJ1545 is a very unusual source if this would be associated with the Lupus-I cloud because:

- No MIPS 24 micron emission is detected down to the 0.3 mJy level.- We did not detect C18O (1‒0) and (2‒1) that are universally observed in starless cores.- very compact, extremely cold (6.3 ± 0.2 K), low Av (< 1).

A possible Galactic interpretation might be a first hydrostatic core (Larson 1969) in its very early phase. As shown in Fig. 6, the unresolved cusp-like structure appeared at u > 100 kλ and the extended envelope predicted from the first core model (Tomida et al. 2010) are consistent with the actual SMA visibility amplitudes.

Note that the adjacent compact NIR source, J1545B, and the observed JVLA 6 cm flux at the SMA position cannot be explained even in the first hydrostatic core models.

Figure 6 | The visibility amplitude versus projected baseline of MMJ1545.

MMJ1545 and MMJ1541 are detected at 1.1 mm at signal-to-noise ratios (SNR) of 7.8 and 5.4, respectively. The flux densities of MMJ1545 and MMJ1541 are 43.9 ± 5.6 mJy and 27.1±5.0 mJy.

These two are amongst the four brightest ever discovered in the AzTEC/ASTE campaign, one of which is “Orochi”, a S1.1mm = 37 mJy SMG at z = 3.39 (Ikarashi et al. 2011) and the other is a 43-mJy source toward the peripheral field of the Small Magellanic Cloud (Takekoshi et al., submitted).

Both of the sources toward the Lupus-I are also detected in the Herschel/SPIRE 250, 350, 500μm bands, most of which have flux densities of > 100 mJy. In this respect both are similar to the brightest SMGs detected in the Herschel and SPT surveys.

High-redshift Dusty Starbursts Magnified by Gravitational Lenses

We find that the SEDs (Fig. 3) require hyper-luminous (LFIR ~ 10¹⁴ Lsun) dusty systems with high star-formation rate (SFR ~ 104 Msun/yr) redshifted at z ~ 4‒5 (MMJ1545) and z ~ 3 (MMJ1541).

The rest-frame UV to optical SEDs clearly exceed to the template, suggesting that a foreground galaxy that lenses a background SMG.

The derived properties are summarized in Table 1.

Object Photometric redshift *1 log μ LFIR (Lsun) *2 μSFR (Mo/yr) *²

MM J1545 4.7 (+0.9/‒0.8) 14.3 (+0.1/-0.2) 40000

MM J1541 3.0 (+0.3/‒0.4) 14.0 (+0.1/-0.2) 20000

Table 1 | Properties of MM J1545 and MM J1541.

*1 The value taken from the best-fit SED template (Arp 220).*2 The apparent value which is not corrected for lens. μ is a magnification factor.

Figure 3 | The spectral energy distribution (SED) of MMJ1545 (left) and MMJ1541 (right). The best-fit SEDs and photometric redshifts are shown as well.

Figure 5 | The Euclidean-normalized differential number counts at 1.1 mm (from Scott et al. 2012).

The 1.1-mm Counts at the Brightest End

The 1.1-mm number counts at S1.1mm = 25‒50 mJy is (1.8±1.3) ×102 mJy1.5 deg－2 (Fig. 5). This brightest end of the 1.1-mm counts is investigated for the first time.

The individual analysis of MMJ1545 and MMJ1541 has shown the presence of strong-lensing effects on both sources, which supports the enhancement in the 1.1-mm counts at the brightest part that is assisted by gravitational amplification.

Herschel

SubaruSpitzer

VLA

Nobeyama Radio Observatory User’s Meeting 2013

Figure 1 | The large-scale view of the Lupus-I molecular cloud (Left) and the close-up cutouts of MM J1545 and MM J1541, distant starburst galaxies. The background of each panel shows Herschel/SPIRE 250‒500 micron image, while contours show the AzTEC/ASTE 1.1-mm images (2, 4, 6σ).

Lupus-I molecular cloud

MM J1545 MM J1541

This work

Figure 2 | The 15’’ x 15’’ images of MMJ1545 (left) and MMJ1541 (right) at various wavelengths. The red contours shows the images taken by SMA 890 micron (MMJ1545) and MIPS 24 micron (MMJ1541).

Photometric redshift estimates

Gravitational lensing effect

(c) NASA/JPL, caltech

Observed properties of MM J1545 and MMJ1541

Background: Herschel/SPIREContours: AzTEC/ASTE coverage

UKIRT

Spitzer SMA

ASTE

NMA

JVLA

DSS/2MASS

Herschel

ASTE

Figure 4 | The 6’’ x 6’’ SMA image (contours) of MMJ1545 overlaid on the MOIRCS Ks image (background). The ellipses indicate 1σ positional uncertainties in the near-IR (MOIRCS and IRAC) and the submm to centimeter (SMA, VLA). The near-IR image (J1545B) clearly offset (0.9’’) from the SMA position. We also show a schematic picture of gravitational lens effect.

MM J1545

MM J1541

MM J1545J1545BThe near-IR object closely (0.9’’) associated with MMJ1545, referred to as J1545B, is likely a lensing galaxy (Fig. 4). A single, but elongated (~1’’) structure of MMJ1545 in the SMA image (see also Fig. 6) indicate the Einstein radius of < 0.9’’, which requires the lens redshift of zlens < 0.6. The magnification factor may be as high as μ~10.

The near-IR / optical object closely (3’’) associated with MMJ1541, referred to as J1541B, is likely a lensing galaxy. Gravitational lens modeling with a singular isothermal sphere (SIS) and photometric redshift estimates (zlens = 0.27+0.34‒0.27) for J1541B suggests the Einstein radius of 0.7’’‒1.3’’, suggesting a moderate magnification (μ~1.5).

JVLA 6 cm NMA 2.7 mm SMA 1.3 mm SMA 890μm

15 arcsec

1 arcmin 1 arcmin

0.5 deg

MIPS 24μm IRAC 5.8μm 3.6μm

Contours: SMA 890μm

MOIRCS Ks WFCAM H JMMJ1545

LensLensLens Lens Lens

MMJ1541

Lens Lens LensLens

MIPS 24μm WISE 4.6μm 3.2μm 2MASS Ks H J DSS I R B

Contours: MIPS 24μm

Herschel

ASTE