ΛCDM SATELLITES AND H i COMPANIONS—THE ARECIBO ALFA SURVEY OF NGC 2903

Weak gravitational lensing studies have shown that stellar light traces dark matter on supercluster and cluster scales (Heymans et al. 2008). The issue is much less clear on subgalactic scales, however, as evidenced by the well known "missing satellites" problem around the Milky Way (MW). At issue is the fact that the predicted number of satellites based on cold dark matter (CDM) and Λ cold dark matter (ΛCDM) simulations of galaxy formation is significantly greater than the observed number of dwarf MW companions (Kauffmann et al. 1993; Klypin et al. 1999; Moore et al. 1999; Diemand et al. 2007b).

A number of explanations for this discrepancy have been proposed. These include the suppression of star formation due to the ionization of the gas (Barkana & Loeb 1999; Benson et al. 2002; Shaviv & Dekel 2003; Gnedin et al. 2008) or dissociation of molecular hydrogen (Haiman et al. 1997), gas stripping due to supernovae-driven winds from an early star formation epoch (Hirashita et al. 1998; Klypin et al. 1999), only the most massive halo substructures forming stars (Stoehr et al. 2002), the disruption of satellites by tidal stripping/stirring (Klypin et al. 1999; Mayer et al. 2001a, 2001b; Kravtsov et al. 2004), confusion between dwarf satellites and high-velocity clouds (HVCs, see Klypin et al. 1999 for a summary), the suppression of small-scale power in simulations via contributions from warm dark matter (Avila Reese et al. 2001; Zentner & Bullock 2002), and incompleteness in the census of MW satellites (Mateo 1998).

It is now clear that the latter explanation has played a part, since in the past few years, the known population of dwarf MW satellites has almost doubled based on scrutiny of the Sloan Digital Sky Survey (SDSS; York et al. 2000, see also Koposov et  al. 2008); however, although the detection of the new satellites alleviates the problem, it does not eliminate it entirely since, after making corrections for sky coverage, the discrepancy is still approximately a factor of 4 (see Simon & Geha 2007).

The concept that star formation might, in some way, have been suppressed in systems of low total mass, suggests the possibility that dark-matter substructure could still be traced by atomic hydrogen (H i) even though appreciable stellar content may be missing. However, many searches, with a variety of sensitivity and coverage, have been undertaken for low-mass starless companions, with little success (see Section 2) and some have suggested that small galaxies which retain H i are likely to have developed stars as well (Briggs 2004; Taylor & Webster 2005).

The advent of the seven-beam Arecibo L-band Feed Array (ALFA; see Giovanelli et  al. 2005a)¹⁷ has now provided an opportunity to survey nearby systems for the possible presence of such "dark companions," with unprecedented sensitivity, coverage, and speed. We report here the results of a targeted deep survey of a single, isolated galaxy, NGC 2903 (see Section  3). In this paper, we outline our observational procedure and data reduction, we present global results for NGC 2903 and then concentrate on companions and their implications for primordial dark matter searches. The details of the H i in NGC 2903, itself, will be left for a subsequent paper. Please note that data related to this project, including some software that we have developed, data from related observations, our final cubes and beam maps can be found on our NGC 2903 Web site.¹⁸

In Section 2, we outline previous surveys that have taken place, presenting a comparison with our approach and, in Section 3, we discuss NGC 2903 and its environment. Since this paper introduces new techniques for observing and reducing Arecibo/ALFA data, we discuss these in detail in Sections 4 and 5. Our detection thresholds and data quality are given in Section 6, Section 7 presents the results for NGC 2903 and its environment, and Sections 8 and 9 provide the discussion and conclusions, respectively.

Current observational data suggest that H i clouds tend not to be "intergalactic," but rather associated with galaxies (Briggs 2004). Various searches for faint H i around galaxies, however, have typically been hampered by the need to choose sensitivity at the expense of coverage or vice versa.

For wide coverage, the blind H i Parkes All Sky Survey (HIPASS; Barnes et  al. 2001; Koribalski et  al. 2004; Meyer et  al. 2004) revealed only one definite extragalactic H i cloud in the NGC 2442 group with a high H i mass of about 10⁹ M_☉ (Ryder et  al. 2001). This cloud has been interpreted as a remnant of a tidal interaction (Bekki et  al. 2005) and cannot be considered primordial.

As for targeted searches, lower mass limits have been achieved. Zwaan (2001) and de Blok (2002) made incomplete samplings of several galaxy groups to limits of a few × 10⁶M_☉. Minchin et  al. (2003) surveyed the Cen A group to a limit of 2 × 10⁶ M_☉, and Pisano et  al. (2004, 2007) observed six Local Group analogs to (2–5) ×10⁶ M_☉. Kovac et  al. (2005) completely surveyed the Canes Venatici group to a limit of 10⁶–10⁷ M_☉. Barnes & de Blok (2004) searched for faint H i companions around NGC 1313 and Sextans A to ∼ 10⁶ M_☉. Pisano & Wilcots (1999, 2003) searched for gas rich companions around six isolated galaxies to an approximate detection limit of only 10⁸M_☉.¹⁹ In these and other targeted surveys (see also Kilborn et  al. 2006), neither starless H i companions nor HVCs, where the sensitivity was sufficient (e.g., Zwaan & Briggs 2000; Pisano et  al. 2007), were detected with the exception of H i that could again be attributed to tidal debris (e.g., see Bekki et  al. 2005).

In contrast to the targeted searches described above, our study of a relatively isolated system (see Section 3) simplifies the interpretation of any H i detections since tidal explanations are much less likely. Moreover, use of the 305 m diameter Arecibo radio telescope has placed these observations among the most sensitive yet achieved (see Section 6.1). While slightly lower H i mass limits have been claimed in deep interferometric observations of NGC 891 (Oosterloo et  al. 2007), M 31, and M 33 (Westmeier et  al. 2005a), our combination of low-mass detection limits, the lowest H i column density limits yet achieved in such studies, the sensitivity to broad scale structure not possible via interferometers, and the complete (and large) sky coverage combine to make this survey unique.

NGC 2903 (Table 1 and Figure 1) has a number of assets that make it a good target for deep H i mapping. It falls within the declination range of the Arecibo telescope, it is bright and massive so there is a reasonable expectation of the presence of ΛCDM (or other) satellites, it is of large angular size so is easily resolved by the Arecibo beam, it is nearby yet lies beyond the Local Group (D = 8.9 Mpc; Drozdovsky & Karachentsev 2000; 1' = 2.6 kpc ²⁰), and some previous H i observations of the galaxy are available for comparison (Begeman 1987; Begeman et  al. 1991; Hewitt et  al. 1983; Haynes et  al. 1998). An important characteristic is that it is noninteracting and isolated, in the sense that no galaxies larger than one quarter of its optical size are present within 20 optical diameters away (No. 0347 in the Catalog of Isolated Galaxies; Karachentseva 1973; ²¹ see also Haynes et  al. 1998).

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NGC 2903 is characterized by its barred, grand-design spiral pattern. It displays a number of "hot spots" in its nuclear region as well as a ring of star formation (e.g., Pérez-Ramírez et  al. 2000). The nuclear dust distribution is chaotic (Martini et  al. 2003). The CO emission is concentrated along the bar (Regan et  al. 1999) and the star formation rate (SFR) per unit area is enhanced by an order of magnitude in the nucleus in comparison to the disk (Jackson et  al. 1991). A soft X-ray halo extending to the west of the nucleus has been interpreted as outflow from a nuclear starburst-driven wind (Tschöke et  al. 2003).

Aside from evidence of nuclear star formation, however, NGC 2903 is a typical massive spiral whose properties are similar to those of the MW. Its global SFR (2.2 M_☉ yr⁻¹; Table 1) is comparable to the MW value (∼ 4 M_☉ yr⁻¹; Diehl et  al. 2006), considering the different methods for estimating this value. More importantly, its rotation curve shows a rise to 210 km s⁻¹ at a galactocentric radius of R ∼ 4 kpc, declining slightly to 180 km s⁻¹ by R ∼ 33 kpc, its outermost measured point (Begeman et  al. 1991). These values agree with the rotation curve of the MW over 4 ⩽ R (kpc) ⩽33 to within error bars (Xue et  al. 2008). Aside from environment, therefore, NGC 2903 appears to be an analog of the MW.

As indicated above, NGC 2903 is considered to be an isolated galaxy, given the dearth of nearby companions sufficiently massive to perturb it. However, two small companions, UGC 5086 and D565-06, are known to be associated (Drozdovsky & Karachentsev 2000) and, from an optical search for additional possible companions within similar radii, we have now identified a third companion, D565-10. D565-10 was found from a search over the spatial region and velocity range within which UGC 5086 and D565-06 have previously been found. Since its separation from NGC 2903 in both position and velocity space is less than that of D565-06, we include it as a newly identified companion here. These three galaxies and their known properties are listed in Table 2. Of the three, only UGC 5086 is within our surveyed field of view and is labeled in Figure 1.

Aside from the H i observations listed above, more recent H i data from the HIPASS (Wong et  al. 2006) and the Westerbork SINGS survey (Braun et  al. 2007) are now also available. NGC 2903 is also in The H i Nearby Galaxy Survey (THINGS, Walter et  al. 2008) which makes use of Very Large Array (VLA) data. At the time of our observations, five archival VLA unpublished H i data sets were available, all of which we have reduced. Of these, two sets produced good data. These are: (1) observing run AO125, taken 1996 September 29 constituting 2.03 hr on source in D configuration, and (2) run AW536, taken 2000 April 22, constituting 2.68 hr on source in C configuration. We do not reproduce the VLA cubes here, but make them available on our NGC 2903 Web site,¹⁸ and refer to them, as needed, only for comparison purposes. The VLA data sets are of higher spatial resolution than the Arecibo/ALFA data, but are much less sensitive (see Section 7.1.2, for example). These reference VLA data sets predate those of THINGS.²²

Observations were carried out with the 305 m telescope of the Arecibo Observatory²³ using the seven-beam ALFA receiver system (see Figure 2 of Giovanelli et  al. 2005a, for the ALFA beam geometry) with the Wideband Arecibo Pulsar Processor (WAPP) back-end spectrometer system. The total observing time allocated for this project was 97 hours, divided into 37 observing blocks (2004 November 28–30, December 1–6, 14–23, 26; 2005 February 10–13, 28, March 1–6, 21–26) carried out during the commissioning phase of ALFA. The observing setup is summarized in Table 3.

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Because the ALFA beams can have coma lobes as high as 20% (7 dB), high-sensitivity observations of extended objects with this instrument must account for contributions from stray/unwanted radiation into these lobes; that is, we obtain a "dirty map" which must be "cleaned." Our basic approach is therefore to map the field in Figure 1 as well as the seven ALFA beams in a fixed number of telescope configurations. The beam maps are used to deconvolve the sidelobe contribution to the galaxy map in each configuration after which these clean maps are combined to form our final datasets. In Sections 4.1 and 4.2, we describe our observing strategy for mapping the galaxy and the beams, respectively.

4.1. Observations of NGC 2903

4.2. Beam Mapping

The drift scans were bandpass subtracted and baseline flattened using IDL procedures written by P. Perillat for general use at Arecibo Observatory, and by R. Giovanelli and B. R. Kent for the ALFALFA precursor data (Giovanelli et  al. 2005b). Preliminary calibration was accomplished using the system's equivalent flux density for each beam as a function of zenith angle, provided by Arecibo Observatory staff. Data reduction specific to the galaxy and beams is described below.

5.1. Galaxy Data Reduction

5.2. Beam Data Reduction

5.3. Image Deconvolution and Final Cubes

A selection of R.A.–V plots and R.A.–decl. plots of the final cubes are shown in Figures 3 and 4, respectively. The grayscale in the plots emphasizes low-intensity emission to illustrate the data sensitivity in low dynamic range regions of the cube as well as residual map errors near NGC 2903. We discuss these map properties in turn below.

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6.1. Data Sensitivity

The mean, $\bar{S}$, and rms noise, σ, of all regions beyond the extended envelope of NGC 2903 (see Section 7.1) in each cube is listed in Table 4. As expected in these low dynamic range regions, the final baseline is consistent with zero ($\bar{S}\ll \sigma$) and a histogram of σ over all line-free channels is Gaussian.

The variation in σ as a function of right ascension, declination, and V was examined. While we find that σ is independent of R.A. and V, it does vary with declination, a result that is illustrated in Figure 5(a). This is primarily caused by the higher gain of the central beam, Beam 0 (∼11 K Jy⁻¹) relative to the outer ones (∼8.5 K Jy⁻¹). The result is that declinations surveyed with the former have lower σ. The correspondence between the declination of Beam 0 for each drift (location of points along x-axis of Figure 5(d)) and the minima in Figure 5(a) illustrates this effect. Different numbers of drift scans at some declinations (Figure 5(d)), uncertainties in beam calibration and variations in beam spacing (see Section 4.1) also contribute to changes in σ with declination.

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From the noise values and some assumptions, we can compute detection limits for each cube in the low dynamic range regime. We consider the limiting flux integral, ${\cal S}_{\rm lim}$, to be from an unresolved signal that is at a 3σ level in two independent channels,

$$\begin{equation} {\cal S}_{\rm lim}=3\sigma \,2\delta V\; {\rm {Jy\;km\;s}^{-1}}, \end{equation} \tag{ 1 }$$

where δV (km s⁻¹) is the velocity resolution (see Table 4) and σ is in Jy beam⁻¹. The minimum detectable H i mass is then,

$$\begin{equation} {M}_{\rm HI\,lim}=2.356 \times 10^5\,D^2\,{\cal S}_{\rm lim} \; {M}_\odot \end{equation} \tag{ 2 }$$

where D is the distance (Mpc), and the minimum detectable column density for a signal that uniformly fills the beam is

$$\begin{equation} {N}_{\rm HI\,lim}=\frac{2.228 \times 10^{24}}{(\theta \,\nu _c)^2}\,{\cal S}_{\rm lim}\; {\rm cm}^{-2}, \end{equation} \tag{ 3 }$$

where θ is the spatial resolution (arcsec, Table 4) and ν_c is the central frequency (GHz, Table 3). If the signal does not uniformly fill the beam, then the right-hand side of Equation (3) must be divided by an areal filling factor.

Plots of M_HI lim and N_HI lim as a function of decl. are shown in Figures 5(b) and (c), respectively, and the mean values for each cube are given in Table 4. Note that the cubes smoothed in velocity have higher M_HI lim and N_HI lim than their full-resolution counterparts because of the larger δ V of the former. Note also that these results are simply detection limits for the data, without imposing assumptions about the properties of any companions that might be present. The limits shown in Table 4 are very low; for example, the column density limits are lower than those of THINGS by two orders of magnitude.²²

6.2. Residual Map Errors near NGC 2903

7.1. Basic Properties of NGC 2903

While a detailed analysis of the H i morphology and kinematics of NGC 2903 is beyond the scope of this paper, we present some of its basic properties here to illustrate the content and quality of our data cubes.

7.1.1. Global Parameters

Figure 6 shows the global profile of NGC 2903, and corresponding global parameters are given in Table 5. The profile shape agrees well with previously published plots (Wong et  al. 2006; Hewitt et  al. 1983) and our integrated flux density (Table 5) agrees with the result of Braun et  al. (2007) to within errors. There is an obvious asymmetry in the galaxy, such that the low-velocity peak (northeast side of galaxy) is higher than the high-velocity peak (southwest side). The integrated flux on the low-velocity side of the galaxy is 13% higher than on the high-velocity side, denoting an intrinsic asymmetry in the H i  distribution of that order.

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Table 5. H i Properties of NGC 2903

Parameter	Value
Δ  V50 (km s−1)a	370 ± 2
Δ  V20 (km s−1)b	383 ± 2
Vsys (km s−1)c	555 ± 2
∫ SV dV (Jy km s−1)d	255 ± 15
MHI (M☉)e	(4.8 ± 0.3) × 109
dHI/doptf	3.2

Notes. ^aFull width at 50% of the average of the two profile peaks. ^bAs in a but at 20%. ^cV at the midpoint of ΔV₂₀. ^dIntegral of flux density associated with NGC 2903. The dominant uncertainty is from the baseline flattening; excluding this effect, the uncertainties are of order 1%. ^eH i mass, from Equation (2) substituting ∫S_VdV for ${\cal S}_{\rm lim}$. ^fRatio of H i to optical major axis diameter. d_HI is measured at 10¹⁸ cm⁻² and d_opt is from Table 2.

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7.1.2. Morphology and Kinematics

We present the integrated intensity and intensity-weighted mean velocity fields, from the RDV-smoothed cubes, in Figure  7. A small, previously unknown H i companion can be seen 248 (64.3 kpc in projection) to the northwest. The eastern companion, UGC 5086, is enveloped in the H i emission from NGC 2903. H i companions will be discussed further in Section  7.2.

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We find a very large H i envelope around NGC 2903, even accounting for the Arecibo beam and residual map errors. For comparison, the gray contour in Figure 7(a) shows the outermost significant integrated intensity level (1 × 10¹⁹ cm⁻²) in the archived D-configuration VLA observations (see Section 3), smoothed to the same resolution as the Arecibo data in the figure. The major axis diameter at 10¹⁸ cm⁻² in our Arecibo data—which should be immune to residual map errors (see Section 6.2) is  d_HI = 407 (105 kpc) after correcting for the beam, nearly twice the value measured from the VLA data. Thus, the H i extent of NGC 2903 is at least 3.2 times its optical diameter (Table 1) and ranks among the largest known (Matthews et  al. 2001; del Rio et  al. 2004; Spekkens & Giovanelli 2006; Oosterloo et  al. 2007; Curran et  al. 2008).

The velocity field of NGC 2903 (Figure 7(b)) shows regular rotation, with the northeast side advancing with respect to the center. The contours indicate that the outer H i disk of the galaxy is warped in spite of its apparent isolation. A position–velocity plot along a 270'' wide strip of the major axis is shown in Figure  8. The inner rotation curve of NGC 2903 appears to be regular, but it is strongly biased by beam smearing and not a good indicator of the gravitational potential in these regions. We find no evidence for gas at anomalous velocities at the sensitivity and resolution of our data.

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7.2. H i Companions of NGC 2903

7.2.1. The Known Companion, UGC 5086

UGC 5086 is the only previously known companion of NGC 2903 that lies within the surveyed region (see Section 3). This galaxy overlaps the large H i envelope of NGC 2903 in both position and velocity space. Figure 9, which shows a single channel at the systemic velocity of UGC 5086, illustrates this blending. A global profile from a 3' × 3' box centered on UGC 5086 in the RDV-smoothed cubes yields a spectrum (not shown) with a peak of 4.4 mJy at V = 497 km s⁻¹, resulting in a measured H i mass of M_HI =  9.0 × 10⁶ M_☉ (Equation (2) substituting the integrated flux of the profile for ${\cal S}_{\rm lim}$). While the velocity of the spectral peak agrees with the systemic velocity of UGC 5086 (Table 2), the detected signal could also arise from the envelope of NGC 2903 itself. To help distinguish between signal from UGC 5086 and NGC 2903, we have searched our reduced VLA D-configuration (Section 3) data, which clearly separate the two galaxies spatially. We find no emission from UGC 5086, and place an upper limit on the H i mass in a single 54'' beam of M_HI lim = 5.6 × 10⁵ M_☉ from the VLA data. Thus, our Arecibo detection is primarily from NGC 2903 iself.

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UGC 5086 is well resolved in the SDSS Data Release 6 (DR6). Its image appears almost perfectly circular and it has a red color (B₀ − V₀ = 0.79, using SDSS magnitudes and applying transformations referenced in Section 7.2.3). Given the absence of H i and known optical parameters, it is likely that UGC 5086 is a dwarf spheroidal galaxy without H i content. We note that near-UV emission can be seen in UGC 5086 also,³¹ suggesting the presence of a young stellar population. This is similar to the dwarf spheroidal galaxy, Fornax, which contains both old, intermediate, and young stars (Battaglia et  al. 2006).

7.2.2. Search for New Companions

A visual search of the data cubes has revealed a new H i-rich companion to NGC 2903, visible in Figure 7. This companion, which we designate N2903-HI-1, will be discussed further in Section 7.2.3. In order to detect companions in a more quantitative fashion, it is important to account for the variation in map noise, σ, with declination, illustrated in Figure 5(a). To this end, we formed signal-to-noise ratio (S/N) cubes³² by dividing each datapoint by the σ corresponding to its decl., and searched for emission exceeding ${\cal S}_{\rm lim}$ from Equation (1). Specifically, we integrated each S/N cube over all V including only those points that exceed 3σ over at least two adjacent independent velocity resolution elements.³³ The resulting summed maps, which emphasize faint emission, are shown in Figure 10.

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Figure 10 confirms that N2903-HI-1 is the only bona fide H i-rich companion to NGC 2903 in our data. While other isolated nonzero pixels are also seen in the various maps, they are clearly random noise peaks. This is corroborated by the lack of correlation between the locations of these pixels in the different maps.

There is a possibility that companions may lie within the spatial region over which NGC 2903 is found, but at anomalous velocities in comparison to NGC 2903. We have searched through this parameter space and find no evidence for such H i clouds (see also the major axis slice of Figure 8).

It is also plausible that our search has missed companions which overlap NGC 2903 in both position and velocity space, a possibility raised by the location of UGC 5086 (Section 7.2.1). The H i disk of NGC 2903 occupies an exceedingly small percentage of the total volume surveyed, however, making such a coincidence highly unlikely. Moreover, if a starless H i emission feature exists within the H i position–velocity envelope of NGC 2903, then it becomes moot as to whether such a feature should simply be considered part of NGC 2903 itself.

7.2.3. The New Companion, N2903-HI-1

N2903-HI-1 is separated from NGC 2903 by 248 spatially (64.3 kpc in projection) and by +26 km s⁻¹ in velocity (see Tables 1 and 6). Its global profile is shown in Figure 11, and related parameters are given in Table 6. We also show the total intensity map, a position–velocity slice, and the 1st and 2nd moments of the H i distribution in Figure 12. The full velocity resolution cube has been used for the latter two maps since the profile is narrow.

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Table 6. Properties of N2903-HI-1

Parametera	Value
R.A. (h m s)b	09 30 38
decl. (° ' '')b	21 43 08
Δ  V50 (km s−1)c	23.6 ± 5.2
Δ  V20 (km s−1)d	40.2 ± 5.2
Vsys (km s−1)e	582 ± 4
∫ SV dV (Jy km s−1)f	0.14 ± 0.02
MHI (M☉)g	(2.6 ± 0.3) × 106
R (arcsec)h	90± 40
(kpc)	3.9± 1.7
Mdyn sin2(i) (M☉)i	(1.3 ±  1.1) × 108
Mvir (M☉)i	(4.5 ±  4.0 ) × 108

Notes. ^aTable 5 provides definitions when not indicated here. ^bPosition of peak of the total intensity map (Figure 12(a)). The uncertainty is ∼15'' (1/2 the cellsize) in RA and ∼1'' (1/4 of the beam) in decl. ^cFull width at 50% of the profile peak. The error bar encompasses variations between the cubes. ^dAs in b but at 20%. ^eAverage of V at the midpoint of ΔV₂₀ and ΔV₅₀. ^fUncertainty is dominated by variations between the different cubes and choice of velocity window. ^gAssuming the distance is the same as NGC 2903. ^hRadius of N2903-HI-1, from 1/2 of the FWHM found from deconvolved Gaussian fits to the total intensity map. ⁱDynamical (M_dyn), and virial (M_vir) masses, as given by Equations (4) and (5), respectively.

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Figure 12(a) illustrates that N2903-HI-1 is elongated north-east to south-west and has a "cometary" or "head-tail" morphology. In spite of the large beam size, the companion is spatially resolved in all cubes, hence its radius, R, can be measured. This has been done via a Gaussian fit to the highest spatial resolution data and deconvolving the beam (Table 6).

We do not find convincing evidence of systematic motion in our data, either from the major axis slice (Figure 12(b)), the first moment map (Figure 12(c)), or channel maps (not shown). A rotating disk with an inclination i > 15° would exhibit a gradient across the disk that is greater than the typical velocity dispersion of 6 km s⁻¹ in Figure 12(d). Thus, if the H i represents a disk in rotation, we might have expected, given our fine velocity resolution, to have seen some evidence for this. Given the elongated morphology of the H i, it seems more likely that it is being either tidally perturbed or ram pressure stripped via passage through a gaseous medium. Head-tail morphologies are typically seen in the latter case, but because of low spatial resolution, the former cannot be ruled out.

To determine a total dynamical mass, ideally we would want to associate the radius of N2903-HI-1 with some rotational velocity. As we do not know the precise geometry of the system and do not see rotation, this association cannot be made. However, the measured line width and radius should provide us with some measure of the total mass which we now estimate under two assumptions that should encompass the extrema of possibilities (see Westmeier et  al. 2005a for a similar approach).

First, in the event that a rotation might have remained undetected (for example, if the companion represents a rotating galaxy that is near face-on), the dynamical mass, M_dyn, can be estimated via,

$$\begin{equation} {M}_{\rm dyn}\,\sin ^2 i=\frac{R}{G}\,\left(\frac{\Delta V_{50}}{2}\right)^2,\vspace*{-2pt} \end{equation} \tag{ 4 }$$

where ΔV₅₀ is the width of the profile in Figure 11 at 50% of the peak (Table 6). ΔV₅₀ has been adopted, rather than ΔV₂₀, since the latter is a measurement at a level within the noise. With the assumption of rotation, R may be overestimated if there is a cometary tail that does not take part in the rotation; however, the velocity width, which has been minimized by the choice of ΔV₅₀, has a greater effect. Together with what would be a substantial correction for the unknown inclination, the result for M_dyn (Table 6) will be a minimum.

Secondly, for comparative purposes, we assume virial equilibrium for which,

$$\begin{equation} {M}_{\rm vir}=\frac{5\,R\,\left(\Delta V_{50}\right)^2}{8\,G\,\ln {2}}. \end{equation} \tag{ 5 }$$

Here, we have related the mean-square velocity of the particles, 〈v²〉, to the observed FWHM velocity width, ΔV, according to 〈v²〉 = 3(ΔV)²/(8  ln 2) (Westmeier et  al. 2005b).

The results (Table 6) indicate a total mass for N2903-HI-1 which exceeds 10⁸ M_☉ by either estimate (Table 6), and we adopt the mean, 3 × 10⁸ M_☉, as a "characteristic" dynamical mass. Although the error bars are substantial, it is nevertheless clear that the H i mass of N2903-HI-1 is only a small fraction of its total mass.

Does N2903-HI-1 have an optical counterpart? Because of the elongated shape of the H i emission, it is possible that an optical galaxy could be displaced with respect to the H i central peak. We have therefore searched the SDSS DR6 database over the original full beam width, (39), centered on the peak position of N2903-HI-1 (Table 6). There are 207 cataloged sources in this area, each with photometric, rather than spectroscopic redshifts. Given that the dispersion between spectroscopic and photometric redshifts is of order ≈ 0.06 (Csabai et  al. 2003) at low redshift, we have identified all galaxies with redshifts less than 0.06 over this spatial region. The result gives 34 galaxies, all of which are plotted in Figure 12(a). Stellar masses have been calculated for each of these galaxies according to the formalism of Bell et  al. (2005), assuming a Kroupa (2001) Initial Mass Function (IMF), and assuming that they are at the distance of NGC 2903. If a modified Salpeter IMF is used instead (Bell et  al. 2003), the stellar mass increases by only a factor of 1.4.

The galaxy, SDSS J093039.96+214324.7 (see Table 7), which is marked in red in Figure 12(a), is the most likely optical counterpart for several reasons. First, the position of this galaxy agrees with the peak of N2903-HI-1 within errors (Tables 6 and 7). It is also significantly brighter (by at least 2.9 mag in g), larger and has more stellar mass³⁴ than other candidates in the field. A 2nd Digital Sky Survey (DSS2) image of this galaxy is shown in Figure 13, and reveals a galaxy that is elongated roughly northen–south, similar to the H i morphology of N2903-HI-1 (see Figure 12) but at a different position angle. Assuming that J093039.96+214324.7 is at the distance of NGC 2903, we list its properties Table 7.

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Table 7. Properties of J093039.96+214324.7

Parameter	Value
2a × 2b ('' × '')a	22 × 12
(pc × pc)a	950 × 518
cz (km s−1)b	200
u, g, r (mag)c	18.71, 18.14, 18.00
B0, V0 (mag)d	18.28, 17.98
MB, MV (mag)e	−11.47, −11.77
M⋆ (M☉)f	1.8 × 106
MHI/LB (M☉/L☉)g	0.43

Notes. ^aMajor× minor axis dimensions from the 2 σ contour of Figure 13. ^bPhotometric redshift from the SDSS DR6. ^cMagnitudes in SDSS u, g, and r bands. ^dB and V magnitudes from the transformation of Lupton (2005), corrected for Galactic extinction (Schlegel et  al. 1998). ^eAbsolute B and V magnitudes. ^fStellar mass, derived from B₀ − V₀ and M_V assuming a Kroupa IMF (Bell et  al. 2005). ^gRatio of H i mass (Table 6) to blue luminosity, the latter from M_B, with a Solar B-band magnitude of 5.48.

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At the distance of NGC 2903, the properties of J093039.96+214324.7 indicate that it is a low luminosity dwarf galaxy with a linear diameter of approximately 1 kpc. If this galaxy is the optical counterpart of N2903-HI-1, then its stellar mass rivals its H i mass but is at least two orders of magnitude less than its total mass, leading to the conclusion that the system is dark matter dominated. (If J093039.96+214324.7 is not the optical counterpart, the same conclusion is reached.) The H i envelope of N2903-HI-1 extends to ∼8 optical radii. Comparing J093039.96+214324.7 to known Local Group dwarfs (Mateo 1998) or faint irregular galaxies (Begum et  al. 2008), we find that its color, (B₀ − V₀ = 0.31), radius, line width, total mass, and H i mass to blue light ratio (M_HI/L_B) fall within observed ranges for these other known systems. Although its recessional velocity differs from that of N2903-HI-1, the difference falls within typical errors for photometric redshifts of nearby systems (Csabai et  al. 2003). Thus, the position and properties of J093039.96+214324.7, in comparison to the other galaxies in the region, all suggest that this galaxy is the likely optical counterpart of N2903-HI-1. A spectroscopic redshift for this system would decide the matter.

We note that the H i mass of N2903-HI-1 also falls within the range of Galactic HVCs for which there are distance constraints (Putman et  al. 2002, Thom et  al. 2008; Wakker et  al. 2007; Wakker et  al. 2008) and at the upper end of the H i mass distribution found for the HVCs around M 31 and M 33 (Thilker et  al. 2004; Westmeier et  al. 2005a, Westmeier et  al. 2007). However, its separation from NGC 2903 (64 kpc in projection) is larger than the population of HVCs around the MW (less than 10–15 kpc typically; Thom et  al. 2008) or M 31 (within 50  kpc; Westmeier et  al. 2007). In combination with the evidence for an optical counterpart, we consider this interpretation for N2903-HI-1 to be less likely.

In our targeted sensitive H i survey of the isolated MW analog, NGC 2903, we have discovered one new companion, the H i object, N2903-HI-1, which is dark matter dominated (Table 6) and is likely associated with the optical dwarf galaxy, SDSS J093039.96+214324.7. New discoveries of dwarf satellites of the MW from the SDSS place their typical total masses in the 10⁶⁻⁷M_☉ range (Simon & Geha 2007) which is lower than the total mass of > 10⁸ M_☉ (Table 6) found for N2903- HI-1. Combining data from previously known MW companions, the new SDSS companions, and those of M 31, most satellites which still contain detectable H i lie beyond 300 kpc radius, the implication being that H i has been stripped in closer systems (Putman et  al. 2008; Grcevich & Putman 2009), although notable exceptions also occur (e.g., the Large and Small Magellanic Clouds). N2903-HI-1, at a projected distance of 64 kpc from NGC 2903, would have to lie at least 293 kpc in front of or behind NGC 2903 to be at a true separation > 300 kpc. Although this is possible, it is more likely that the companion is closer to NGC 2903, yet has retained its H i–a result that may be related to its high dynamical mass in comparison to most MW systems within the same radius. Thus, if ram pressure stripping is occurring, as suggested by the head-tail morphology of N2903-HI-1, the process may be slower because of its high dynamical mass.

Although the true separation and space velocity of N2903- HI-1 are not known, we can at least adopt the projected separation and radial velocity offset from NGC 2903 to determine whether the above speculation is feasible. Approximating N2903-HI-1 as a sphere, its average ISM H i density, from the values of Table  6, is n_ISM ≈ 4 × 10⁻⁴ cm⁻³. A simple condition for stripping is,

$$\begin{equation} n_{\rm halo}\,V_{\rm rel}^2\,>\,n_{\rm ISM}\,G\,{\rm M}_{\rm tot}/R, \end{equation} \tag{ 6 }$$

where M_tot is the total mass of N2903-HI-1 and R is its radius. Using M_tot = 3 × 10⁸ M_☉, V_rel = 26 km s⁻¹ (Section 7.2.3) and R from Table 6, and solving for halo density, we find n_halo = 2 × 10⁻⁴ cm⁻³. This value is within the expected range of halo densities for the MW at the projected distance of N2903-HI-1 (64 kpc; Grcevich & Putman 2009). If the relative velocity is larger, stripping will be more effective, and if the true separation is larger, stripping will be less effective since the halo density will be lower. Nevertheless, this estimate indicates that N2903-HI-1 could indeed be undergoing ram pressure stripping under the assumption that the halo of NGC 2903 resembles the MW. The stripping timescale depends on the difference between the two sides of Equation (6) which is not known. However, since the magnitudes of the ram pressure and the internal energy density of the companion are similar, the stripping timescale may be long, consistent with the observation of detectable H i in the companion.³⁵

It is now interesting to ask how many Local Group dwarf galaxies would have been detected, if they were distributed around NGC 2903 similarly to the MW. The result is dependent on both their distribution and H i content. Seventeen Local Group dwarf galaxies listed in Mateo (1998) had sufficient data (distances and H i data) that we could apply our 3σ2δV detection criterion (Section 6.1) to them. The result is that we could have detected seven of them (41%) in terms of sensitivity limits. However, these seven galaxies lie at radii between 490 kpc and 1.6 Mpc, in comparison to the effective projected radius of R_eff = 110 kpc (for a circularized field) of our survey. Since our survey probes a volume that is only 0.7% of the volume extending to 1.6 Mpc, it is unlikely that any of these seven galaxies would have fallen within our field of view. Although Mateo does not list H i data for the Large and Small Magellanic Clouds (LMC and SMC, respectively) it is clear, however, that we would have detected these systems. Of the 20 new SDSS dwarf Local Group galaxies listed in Simon & Geha (2007), only one (the dwarf irregular, Leo T) has H i content (Ryan-Weber et  al. 2008), the remainder being dwarf spheroidals or falling within parameter space intermediate between dwarf spheroidals and globular clusters. Leo T would be marginally detectable in our survey but, at a distance of 420 kpc (Irwin et  al. 2007), would also likely lie outside our field of view. In spite of our large survey region, therefore, we are still only probing the inner region of a possible dwarf galaxy population, were it distributed like our own.

As for HVCs, the HVC population around the MW is not easily translated to NGC 2903 since HVC distances (and therefore H i masses) are not well known. Also, H i column densities which are known (typically 10¹⁹ cm⁻²; Stanimirović et  al. 2006; Putman et  al. 2002), will be diluted by large unknown beam filling factors at the distance of NGC 2903 (234'' = 10 kpc). We can say, however, that the MW's HVC Complex C, with a mass of 4.9 × 10⁶ M_☉ at a distance of 10 kpc from the Sun (Thom et  al. 2008), should have been detected, provided it were separated in velocity from the bulk of the H i in NGC 2903 itself. Given the inclination of NGC 2903 and the nature of HVCs, we expect this criterion to have been met. Thus, NGC 2903 lacks such an HVC complex. Indeed, given that our search velocity range was over 1000 km s⁻¹ (Table 4) and that the median velocity FWHM of MW HVCs (36 km s⁻¹; Putman et  al. 2002) corresponds to 28 velocity channels in our data, it is surprising that no clear HVC detections have been made. Either NGC 2903 lacks HVCs, possibly due the fact that NGC 2903 is isolated, or its HVCs are of very low mass.

No dark starless companions have been detected around NGC 2903. This result is consistent with the ALFALFA survey results which indicate that all extragalactic H i objects can be identified with an optical counterpart (Saintonge et  al. 2008). The discovery of one new H i rich dwarf companion now places the total number of companions within our surveyed field (R_eff = 110 kpc) at two: the H i companion, N2903-HI-1, likely associated with a dwarf galaxy, J093039.96+214324.7, with a dynamical mass ∼ 3 × 10⁸M_☉ (mean of values in Table 6) and UGC 5806, which is likely a dwarf spheroidal galaxy (Section 7.2.1). Using the SDSS data for the latter galaxy and the same transformations as described in Section 7.2.3, the stellar mass of UGC 5806 is M_⋆ = 4.7 × 10⁷ M_☉. Gilmore et  al. (2007) have shown that mass-to-(V-band) light ratios, M/L_V, for dwarf spheroidal galaxies in the Local Group range from ≈ 4 to 600, with more luminous galaxies having systematically lower values of M/L_V. The absolute magnitude of UGC 5086 is M_V = −13.7 which is closest to Fornax, amongst Local Group dwarfs. If UGC 5086 has similar properties, then 3 ⩽ M/L_V ⩽ 20, implying that 1.0 × 10⁸ ⩽ M/M_☉ ⩽ 5.2 × 10⁸. Adopting the mid-point of this range gives an estimate of ≈ 3 × 10⁸ M_☉ for the total mass of UGC 5086.

How many dark-matter clumps of total mass M_tot≳ 3 × 10⁸ M_☉ are expected from ΛCDM predictions? Since NGC 2903 has a total mass that is similar to the MW (Section 3), we can use the Via Lactea simulations of subhalo clumps from Diemand et  al. (2007a) for comparison. The fraction of all halo mass that is present in substructure within a projected radius of 110 kpc (their Figure 7) is f = 0.02. With an adopted total halo mass of 1.77 × 10¹²M_☉, this yields a total mass in halo substructure of M_hs = 3.54 × 10¹⁰ M_☉ for the projected radius. Over the substructure mass range of 4.6 × 10⁶ ⩽M_sub/M_☉ ⩽ 1 × 10¹⁰, the number of clumps per unit mass range is given by dN/dM_sub = K/(M_sub)², where K is a constant. Since the slope does not change with radius, we can compute K from M_hs = ∫K/(M_sub)² M_sub dM_sub, finding K = 4.6 × 10⁹M_☉. Finally, we compute the expected number of clumps with masses greater than M_sub from N(>M_sub) = K/M_sub. For M_sub = 3 × 10⁸ M_☉, this yields 15 clumps in comparison to the two observed. If the dynamical mass of N2903-HI-1 is higher than 3 × 10⁸ M_☉ (for example, if it is rotating and nearly face-on), then the discrepancy reduces. However, its mass would have to be as high as ∼5 × 10⁹M_☉ for there to be agreement with the theoretical expectation. Such a high value would require N2903-HI-1 to represent a rotating system in a special geometry with an inclination less than 10° (Section 7.2.3). Again, although this possibility cannot be ruled out completely, it is quite unlikely. Thus, it would appear that there is a discrepancy between the expected number of companions and the number observed.

Since we have a detection threshold that is lower still than the value of the detected companion, we can also ask how many dark-matter clumps would we expect to see, were they H i rich. For example, our lowest H i detection threshold is 2 × 10⁵ M_☉ (Table 4). If M_tot/M_HI ≈ 100 for a dwarf galaxy in the field (equivalent to the N2903-HI-1 value), then we could have detected companions of total mass as low as 2 × 10⁷ M_☉ via their H i. Therefore, within our field of view, using the above relation, we should have detected 230 galaxies as opposed to that observed. This is strongly discordant with ΛCDM. The conclusion is that, if this many dark-matter clumps exist in the region, then they are clearly not H i rich, i.e. they contain no H i or they contain H i at a level lower than 1%.

Using the Arecibo telescope with the ALFA receiver, we have mapped NGC 2903 and its environment with very high sensitivity and coverage. Our lowest point source detection limit is 2 × 10⁵  M_☉ and almost 40 thousand square kpc of sky has been fully covered. The Arecibo ALFA beams have been carefully characterized as a function of azimuth, allowing us to clean each beam as a function of azimuth from the H i datacube. With a velocity coverage of 1035 km s⁻¹ and fine velocity resolution (2.6 km s⁻¹), our combination of observing parameters makes this survey unique and among the most sensitive and complete of a nearby galaxy. Although details of NGC 2903, itself, are left to future work, our results show that the H i envelope around NGC 2903 is much larger than previously known, extending to at least three times the optical galaxy diameter.

The fact that we have targeted an apparently isolated, noninteracting galaxy to a very low-sensitivity limit has clearly been an advantage in the search for H i companions. The discovery of only one isolated H i companion, N2903-HI-1, which appears to have a small optical counterpart, is a significant result. The optical companion is likely a dwarf galaxy with a stellar mass approximately equal to its H i mass with the H i in a broad envelope, approximately eight times larger, around it. The best estimate of its dynamical mass is 3 × 10⁸  M_☉. We have no convincing HVC detections.

In the field surveyed, there are now two known companion galaxies, our new discovery as well as what is likely a dwarf spheroidal galaxy, UGC 5086, the latter with a total mass likely comparable to N2903-HI-1. In this region, ΛCDM scenarios (specifically, the Via Lactea model) predict 15 companions for a MW-type galaxy, with masses greater than 3 × 10⁸ M_☉. Given our H i detection limits, however, if companions to NGC 2903 contained H i at the 1% level in comparison to their total masses, then we should have detected 230 of them. If these clumps are present as predicted, they do not contain appreciable H i. They may be starless dark clumps or very low luminosity dark-matter dominated dwarf spheroidals.

We are grateful to students K. Marble of Queen's University and I. Bah, A. Altaf, and J. Goldstein of Lafayette College for their assistance with the data reductions. Many thanks also to P. Perillat and the Arecibo staff for their knowledge and assistance. J.A.I. gratefully acknowledges a grant from the Natural Sciences and Engineering Research Council of Canada. G.L.H. gratefully acknowledges grants from the Lafayette College Academic Research Committee. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the NSF, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/.

Facilities: Arecibo.