The Astronomical Journal, 135:1697–1707, 2008 May c 2008. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
doi:10.1088/0004-6256/135/5/1697
THE RADIO CONTENT OF GLIMPSE 1
U. Giveon1 , R. H. Becker2 , and R. L. White3
Department of Physics, University of California, Davis, CA, USA;
[email protected] 2 University of California, Davis & Lawrence Livermore National Laboratory, Livermore, CA, USA;
[email protected] 3 Space Telescope Science Institute, Baltimore, MD, USA;
[email protected] Received 2007 August 22; accepted 2008 February 13; published 2008 March 31
ABSTRACT We present results from matching a Very Large Array Galactic plane catalog at 6 cm covering the first Galactic quadrant (350◦ l 42◦ , |b| 0.4◦ ), the MSX6C Galactic plane catalog, and the Galactic Legacy Infrared MidPlane Survey Extraordinaire v1.0 (GLIMPSE i) catalog. The much higher angular resolution and better sensitivity provided by GLIMPSE should improve the identification of embedded Galactic star-forming regions, and enable the study of the relationships between the central stellar sources, the ionized gas, and the surrounding dust. The GLIMPSE catalog is so crowded compared to our 6 cm catalog that it actually leads to confusion in identifying chance alignments, but this is resolved when separating GLIMPSE into red (M3.6 µm − M8 µm > 2.5) and blue (M3.6 µm −M8 µm < 2.5) sub-catalogs. In spite of the improved properties of GLIMPSE, we find a very small number of GLIMPSE–6cm high-reliability matches in the overlapping area of the two samples (10◦ l 42◦ , |b| 0.4◦ )— 132, only 55 of them have a Midcourse Space Experiment (MSX) counterpart. Even though the matching results are less successful than expected, there are still some general outcomes to them. First, we discover an obscuration effect around our candidates: the average local source density of blue sources, dominated by stars, is decreasing toward the radio positions; their average brightness increases, and their color reddens significantly, supporting the picture in which background sources disappear behind the opaque nebulae associated with the radio source. Second, the selected sources define near and mid-infrared color criteria, which are used to detect a total of 849 GLIMPSE sources in the entire GLIMPSE survey that have MSX matches and that show the same collective behavior. Only 15% of these sources are previously classified, mainly as H ii regions, masers, young stellar objects, and molecular clouds. Key words: catalogs – Galaxy: disk – H ii regions – infrared: general – radio continuum: general
source morphologies. With higher IR resolution, it may be feasible to study the relationships between the central stellar sources, the ionized gas, and the surrounding dust. The Spitzer program, Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE i; hereafter, GLIMPSE) provides maps of large parts of the Galactic plane (10◦ |l| 65◦ , |b| 1◦ ; Benjamin et al. 2003) at wavelengths 3.6, 4.5, 5.8, and 8 µm, and its v1.0 catalog comprises 47 million entries. It has higher angular resolution (2 ), and much higher sensitivity— 0.4 mJy compared to the MSX with 0.1 Jy at 8 µm. GLIMPSE provides a comprehensive view of the stellar content in the inner Galaxy that is mostly obscured in the optical. It is ideal for studying the Galactic structure: the disk, molecular ring, spiral arms, and central bar; for studying the Galactic stellar population in terms of its mass function and evolution; and for accurate comparison with other Galactic plane surveys. However, deeply embedded stellar nurseries may be too cool for GLIMPSE to detect, requiring observations at longer wavelengths. The present work is a step toward identifying these highly-embedded starforming regions in the area covered by GLIMPSE, in conjunction with the MSX and radio data in a limited part of the Galaxy. We use a preselected sample of 740 ultra-compact (UC) H ii regions candidates—a result of matching a VLA Galactic plane catalog at 6 cm (White et al. 2005) and the MSX catalog (Giveon et al. 2005), with the aim of characterizing the GLIMPSE colors of these objects, and identifying them across the GLIMPSE survey area. In Section 2, we describe the selection of the sample of UC H ii regions candidates using the MSX and VLA observations and its matching with the GLIMPSE catalog. In Section 3, we study the color properties of the resulting subset of sources,
1. INTRODUCTION Infrared (IR) surveys of the Galactic plane have played a crucial role in revealing the early phases of star formation which take place within the still existing dusty natal clouds. The progressively lower extinctions at near-IR, mid-IR (MIR), and far-IR (FIR) wavelengths have allowed a better estimation of the Galactic star-formation rate and the understanding of the connection between star formation and the Galactic structure. The IRAS mission produced the first MIR and FIR (12, 25, 60, and 100 µm) all-sky survey that transformed our understanding of the IR sky with 250,000 point sources. The dominant Galactic sources of IR emission were found to be diffuse Galactic emission, and star-forming regions where protostars and young stars are still embedded in cocoons of gas and dust. Many studies made use of IRAS colors to select and classify objects of a variety of types (e.g., Beichman 1987; Wood & Churchwell 1989). However, the small scale-height of the distribution of massive stars (FWHM ≈ 15 ; White et al. 1991; Becker et al. 1994) meant that there is considerable source confusion in the IRAS survey as a result of its poor angular resolution (∼1 ). In the following decade, the Midcourse Space Experiment (MSX) produced a higher spatial resolution survey (∼18 ) of the Galactic plane, covering the entire Galaxy for latitudes 90% complete in detecting all embedded O stars across the Galaxy in the survey’s area, assuming ionization-bounded nebulae. The comparison of the 6 cm catalog and the MSX Galactic plane catalog resulted in a sample of 687 matches. The number of matches is currently 740—slightly higher due to improved image processing of the radio maps. Most of the matches show red MSX colors and a thermal radio spectrum between 6 and 20 cm. The MIR colors of the matching subset are approximately M8 µm − M21 µm 1.2, M15 µm − M21 µm 0.4, if M8 µm − M21 µm < 1.2, then M12 µm − M15 µm < −0.4,
(1)
where colors are expressed as magnitude differences. The second criterion is required to separate the matching subset from stars. The matching subset is tightly confined to the Galactic plane, with FWHM of 16 or ∼40 pc, assuming a distance of 8.5 kpc to the Galactic center. This suggests that the sample is dominated by Population I objects, most of them seen as radio point sources, therefore comprising young UC H ii regions, most of which are previously uncataloged. We matched the 6 cm catalog and the MSX catalog based on source position. We estimated the number of chance alignments by comparing the MSX catalog to a few mock radio catalogs with the same spatial distribution by shifting the original radio catalog by ±10 and ±20 in Galactic longitude. The matches from the mock catalogs were then purely coincidental. Typically, about 15% of the matches with the real radio catalog are thus chance alignments. However, it was possible to select sub-groups in the MSX catalog for which this fraction was lower, based on detection in the IR band. For example, sources that were detected at 12, 15, and 21 µm are more likely to be dust envelopes around UC H ii regions than sources detected only at 8 µm, which are mostly stars. For the former, the chance coincidence level is as low as 4%, while for the latter it is 98%.
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Figure 1. Average GLIMPSE local source density as a function of distance from the real radio positions of the MSX–6cm matches (filled circles), and from the positions of a mock catalog (open circles). The distribution of the real matches has a peak of 0.0273 (# arcsec−2 ) at a zero-matching radius. The plot does not include it to allow a more detailed view of the distributions. Error bars represent Poissonian 1σ uncertainties. The depression in the source density around the real sources toward zero separation is a result of obscuration by extended sources.
The still relatively poor resolution of MSX can lead to confusion regarding IR-radio associations and in many cases one MSX source was matched by a few radio sources. With the higher resolution of GLIMPSE (2 ), which is comparable to that of our radio surveys, sources that appeared to be point sources in the MSX images are broken down into smaller IR sources that better correlate with the radio sources. 2.2. The Matching Process All the results described in this section are for the complete GLIMPSE i v1.0 archive comprising 47 million entries. Similar results are obtained for the v1.0 high-reliability archive (30 million entries), but the matching signal (i.e., the number of real matches) is stronger for the general archive. This is probably due to the exclusion of real objects by the high-reliability criteria. Out of the 740 MSX–6cm matches described in the previous section, only 494 fall within the GLIMPSE i area (10◦ |l| 65◦ , |b| 1◦ ). Applying the same matching procedure to these 494 sources and GLIMPSE resulted in abnormally large number of chance coincidences for all matching radii (0– 120 ): matching GLIMPSE with the mock MSX–6cm catalog (shifted in Galactic longitude by an arbitrary amount) gave more matches than for the real radio catalog. This is in spite of the fact that the mock catalogs should have produced only the fraction of the total matches that were chance coincidences. Figure 1 demonstrates that effect by showing the GLIMPSE average local source density as a function of distance from the radio position. Filled symbols represent GLIMPSE matches around the real MSX–6cm sources, while the open symbols represent GLIMPSE matches around a mock catalog generated by shifting the real catalog by 10 . The density of the mock catalog remains flat, while for the real catalog it falls down by a factor of ∼2 toward the radio positions. However, there is still a peak of 0.0273 (# arcsec−2 ) close to 0 . For clarity reasons, the vertical scale of Figure 1 is limited and does not include this peak. The drop in GLIMPSE matches near MSX–6cm sources by that factor must suggest that at least half of the GLIMPSE sources are background to the MSX–6cm sources if produced by extinction. We realized that this effect might be created by the very crowded maps of GLIMPSE combined with the extended nature
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RADIO CONTENT OF GLIMPSE
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Figure 2. Small patch of the Galactic plane which demonstrates the obscuration effect. GLIMPSE sources are marked by small dots and MSX–6cm matches are marked by filled circles. The six MSX–6cm sources are located in “voids,” producing only one match with GLIMPSE (open circle). When shifted by 10 , five out of the six sources find a GLIMPSE match within 3 —a significantly higher match rate (open squares).
of our sources. The GLIMPSE i v1.0 catalog has about 9 million sources within the area covered by the 6 cm survey (excluding the 20 deg2 hole around the Galactic center), while MSX had only ∼30,000 within the same area, making GLIMPSE ∼300 times denser on average. This increase in density is dominated by stars, which are less likely to be real radio sources. On the other hand, the real IR counterparts of the radio sources are usually extended, so the GLIMPSE catalog often has one entry (if at all) with a spatial “void” around it. The original MSX–6cm catalog will at the most match that one entry, but the shifted MSX–6cm catalog, which is matched merely against a random crowded field of stars, will produce significantly more matches. A clarifying example can be seen in Figure 2, where the shifted MSX–6cm catalog produces a much higher match rate with GLIMPSE compared with the real catalog (five squares compared to one circle). The real MSX–6cm positions correlate with “voids” in the GLIMPSE patch of the plane. These voids may be the result of higher extinction at NIR wavelengths of the MSX source. Thus, the average decrease in the number of matches toward the radio positions is simply explained by an obscuration by the extended IR source. The highest optical depth is in the center, so the density of the matches there will be the lowest, and it will gradually reach the “field” value when going away from the center. In Section 2.3, we show results that support this explanation. Alternatively, the poorer resolution of MSX may have produced a match between the IR source and the radio source, but the higher GLIMPSE resolution broke down the IR source in a way that excluded that match (see the upper left corner of Figure 2). This excluded match might still find a match more easily when the catalog is shifted, that is, when it is compared with the crowded stellar field. Examining the GLIMPSE images revealed that there truly is a paucity of GLIMPSE sources near the MSX–6cm objects. This can result from two effects: either the IR source may be optically thick and obscures background GLIMPSE sources (a dark hole in the GLIMPSE image), or the GLIMPSE image may have bright extended emission that makes it difficult to detect other sources that would otherwise be in the GLIMPSE
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Figure 3. Plot used for estimating the fraction of GLIMPSE sources lost around the radio sources due to varying background levels. Top panel: GLIMPSE’s 3.6 µm sky values as a function of the minimal 3.6 µm flux levels detected at these values for all GLIMPSE sources. Bottom panel: the accumulated fraction of all GLIMPSE sources as a function of their 3.6 µm flux. Vertical dashed lines indicate the GLIMPSE 5σ sensitivity and three background values with the corresponding fractions of lost sources.
catalog (a bright region in the GLIMPSE image). To quantify the contribution of the latter effect, we measured the background around our objects to estimate the fraction of lost GLIMPSE sources. We measured background values at 3.6 µm since at this wavelength the nebulae around our objects are not glowing as much as they do at 8 µm, and so the artificial exclusion of stars by the emission nebulae is minimal. In the top panel of Figure 3, we show GLIMPSE’s 3.6 µm sky values as a function of the minimal 3.6 µm flux levels detected at these values (lower sky values allow detecting fainter objects). The sky values were retrieved from the listed entries in the GLIMPSE catalog. The bottom panel shows the fraction of the GLIMPSE catalog as a function of the upper flux cut-off. We can use this plot to determine what fraction of sources is lost due to the background for increasing background levels. Three background levels are indicated in the plot—3, 7, and 10 MJy sr−1 —with the corresponding percentage of lost sources. We have measured the 3.6 µm background around the positions of the 494 MSX–6cm matches within GLIMPSE using the IRAF package DAOPHOT4 . The sky values of the GLIMPSE objects that match the radio objects were not as accurate because the nearest GLIMPSE positions to the 6 cm position may be off by as much as 30 . DAOPHOT measured the median of all pixels within an annulus of a radius of 15 pixels (∼25 ) and a width of 5 pixels (∼8 ). In Figure 4, we show that the background around the GLIMPSE–MSX–6cm sources (filled bars) is typically higher compared to the entire GLIMPSE survey (open bars). For clarity, the histograms in this plot are normalized to have a sum of 1. The plot shows that the typical IR counterpart of a radio source has a higher background. 4 IRAF (Image Reduction and Analysis Facility) is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.
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Figure 4. Histograms of background values of the entire GLIMPSE survey (open bars), taken from the point source catalog, and of the GLIMPSE–MSX– 6cm sources (filled bars), measured using IRAF. For clarity, the histograms in this plot are normalized to have a sum of 1. The typical IR counterpart of a radio source has a higher background. 0.022 0.02
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Figure 6. (a) Average GLIMPSE local source density as a function of distance from the radio positions of the 494 MSX–6cm matches. Filled symbols indicate all GLIMPSE sources. Open symbols indicate red GLIMPSE sources, and open triangles symbols indicate blue GLIMPSE sources. The depression in the local source density is apparent only for the blue sources, dominated by stars, which are obscured by the extended IR sources. (b) Zooming in on close separations: red sources show a very dominant central peak, while the source density of blue sources keeps dropping.
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All 494 matches 322 matches, sky
