---
abstract: 'The NRAO VLBA was used at 1.4 GHz to image the inner 550 mas (23 lt-years) of the nearest known Seyfert 1 nucleus, in the Sm galaxy NGC4395. One continuum source was detected, with flux density $530\pm130~\mu$Jy, diameter $d < 11$ mas (0.46 lt-year), and brightness temperature $T_{\rm b} > 2.0\times10^6$ K. The spectral power $P\/$ of the VLBA source is intermediate between those of SagittariusA and SagittariusA$^*$ in the Galactic Center. For the VLBA source in NGC4395, the constraints on $T_b$, $d\/$, and $P\/$ are consistent with an origin from a black hole but exclude an origin from a compact starburst or a supernova remnant like CassiopeiaA. Moreover, the spectral powers of NGC4395 at 1.4 and 4.9 GHz appear to be too low and too constant to allow analogy with SN1988Z, a suggested prototype for models of compact supernova remnants. The variable and warm X-ray absorber in NGC4395 has a free-free optical depth much larger than unity at 1.4 GHz and, therefore, cannot fully cover the VLBA source.'
author:
- 'J. M. Wrobel and C. D. Fassnacht'
- 'L. C. Ho'
title: The Inner Light Year of the Nearest Seyfert 1 Nucleus in NGC4395
---

MOTIVATION
==========

The Seyfert 1 nucleus of NGC4395 holds three unique distinctions. First, in the optical regime it is the least luminous Seyfert nucleus known, with an absolute blue magnitude of only $M_B=-9.8$ mag. On energetics grounds it is therefore essential to explore both black hole and stellar origins for the Seyfert activity [@fil89; @fil93; @lir99]. Second, it is the only Seyfert nucleus known to be hosted by an Sm galaxy. Proving that the Seyfert traits of NGC4395 are inconsistent with a stellar origin, but consistent with a black hole origin, would thus bolster claims of black hole ubiquity in the local universe [@mag98]. Finally, NGC4395 lies at a distance $D=2.6$ Mpc [@row85], making it the nearest known Seyfert 1 nucleus. The corresponding scale is 10 mas $=$ 0.42 lt-year, so the inner light year can, in theory, be probed directly with the NRAO Very Long Baseline Array (VLBA). Direct imaging of radio emission on light-year scales can be a powerful discriminant between black hole and stellar origins: a black hole could launch jets [@fal00], while stellar processes could result in a compact starburst [@con91], a supernova remnant resembling Galactic ones [@gre84], or a compact supernova remnant [@ter95].

Recent observations of NGC4395 with the NRAO Very Large Array (VLA) showed an unresolved source at a frequency $\nu=1.4$ GHz, with a flux density $S=1680\pm94~\mu$Jy; a diameter $d < 550$ mas (23 lt-years); and a spectral index $\alpha = -0.60\pm0.08$ ($S \propto
\nu^{\alpha}$) between 1.4 and 4.9 GHz, indicating synchrotron emission from a nonthermal plasma [@ho01a]. A VLA source of this strength is too weak to be imaged with the VLBA if just traditional self-calibration techniques are applied. However, the prospects for successfully imaging NGC4395 with the VLBA are good if phase-referencing techniques are employed [@wro00b]. Section 2 of this Letter reports the detection of one VLBA source at 1.4 GHz, using phase-referenced observations of a 550-mas region in NGC4395. Section 3 examines the implications of this VLBA detection, plus published radio photometry, for black hole and stellar models for the origin of the radio emission. These data are shown to be consistent with an origin from a black hole but inconsistent with an origin from a compact starburst, a supernova remnant like CassiopeiaA, or a compact supernova remnant resembling SN1988Z.

OBSERVATIONS, CALIBRATION, AND IMAGING
======================================

The VLBA [@nap94] was used to observe NGC4395 and calibrators on 2000 April 11 UT. Data were acquired in dual circular polarizations with 4-level sampling and at a center frequency 1.43840 GHz with bandwidth 32 MHz. Phase-referenced observations were made in the nodding style. A 3-minute observation of NGC4395 was preceded and followed by a 2-minute observation of the phase, rate, and delay calibrator J1220+3431 [@wil98] about $1.5\arcdeg$ from NGC4395. Sources J1215+3448 and J1310+3220 were also observed, respectively, to check the astrometric accuracy and to align the phases of the independent baseband channels [@ma98]. Observation and correlation assumed a coordinate equinox of 2000. The [*a priori*]{} position adopted for NGC4395 was very close to that in @ho01a.

Data editing and calibration were done using the 1999 December 31 release of the NRAO AIPS software and following the strategies outlined by @ulv00. After data deletion based on [*a priori*]{} flags, data were deleted on all baselines involving any antenna observing below an elevation of $20\arcdeg$. Such elevation-based editing minimized differential ionospheric conditions between the lines of sight to J1220+3431 and NGC4395. Despite this step, it was impossible to calibrate the phases on baselines to the VLBA antenna on Manua Kea in Hawaii, and those baselines were also deleted. This resulted in an observed baseline range of 240–5800 km and, for NGC4395, a total of 129 baseline-hours of integration. VLBA system temperatures and gains were used to set the amplitude scale to an accuracy of about 5%, after first correcting for sampler errors. No self-calibrations were performed on NGC4395.

The AIPS task IMAGR was used to form and deconvolve an image of the Stokes $I\/$ emission from NGC4395, with the visibility data being naturally weighted to optimize image sensitivity. This image, given in Figure 1, was restored with an elliptical-Gaussian beam with FWHM dimensions of 13.9 mas (0.59 lt-year) by 10.7 mas (0.45 lt-year) and elongation orientation of $-20.3\arcdeg$, and has an rms noise value of 51 microjanskys ($\mu$Jy) per beam area. The left panel of Figure 1 shows that only one VLBA source was detected above 4.5 times the rms noise, within a region centered on the unresolved VLA detection and spanning 550 mas (23 lt-years), which matches the upper limit to the diameter of the VLA detection at 1.4 GHz [@ho01a]. A quadratic fit to the peak of the VLBA detection yielded a position of $\alpha(J2000) = 12^{h} 25^{m} 48^{s}.874$ and $\delta(J2000) =
33^{\circ} 32' 48''.69$. This position carries an absolute 2-dimensional error of 55 mas, set by the position error of J1220+3431 and verified with a phase-referenced image of J1215+3448. Given the signal-to-noise ratio of the VLBA detection of NGC4395, its position can, in principal, be determined with an error of 1-2 mas [@bal75] and efforts are underway to reach that limiting accuracy.

The right panel of Figure 1 displays the inner 76 mas (3.2 lt-years) centered on the position of the VLBA detection. The VLBA source seems to be slightly resolved but this apparent resolution is likely to be artifical, due to the weakness of the source and/or residual errors in the phase calibration. As a gauge of the importance of the latter effect, before self-calibration an image of the strong source J1215+3448 had a peak intensity only 88% of the peak after phase self-calibration. For the VLBA detection of NGC4395, (1) image integration over $N=5.8$ beam areas gives a total flux density of $530\pm130~\mu$Jy, where the error is the quadratic sum of a 5% scale error and $\sqrt{N}$ times the rms noise; and (2) a conservative estimate to the diameter is $d < 11$ mas (0.46 lt-year). Traits (1) and (2) imply that the VLBA detection has a brightness temperature $T_{\rm b} > 2.0\times10^6$ K.

IMPLICATIONS
============

Figure 2 shows that the VLA detections of NGC4395 [@ho01a] match the spectral powers and spectral index of SagittariusA in the Galactic Center [@ped89]. The VLA size limit for NGC4395, conveyed by the dimension of the left panel of Figure 1, is also comparable to the diameter of 26 lt-years for SagittariusA [@ped89]. But the VLBA detection of NGC4395 at 1.4 GHz is less powerful (Figure 2) and more compact (Figure 1, left panel) than SagittariusA. This hints that the VLBA source in NGC4395 could be an extragalactic analog to SagittariusA$^*$ [@fal98], thought to mark the massive black hole at the dynamical center of the Galaxy [@rei99]. An accurate position is not available for the dynamical center of NGC4395 [@swa99]. The position of the VLBA detection does agree with positions for the optical continuum [@cot99], X-ray continuum [@ho01b], and nuclear H$\alpha$ emission [@van98], but these agreements carry combined errors of order 1000 mas (42 lt-years). However, a stringent mass limit is available for a putative black hole in NGC4395: @fil01 report detection of the infrared triplet lines in absorption from echelle spectra taken with the Keck I telescope, yielding estimates for the strength of the stellar contribution to the nuclear light ($M_B=-7.3$ mag) and the central LOS velocity dispersion ($\sigma \sim
30$ km s$^{-1}$), which, in combination with the star cluster size from [*HST*]{} images, limit any black hole mass to $M_{\rm BH}
\lesssim 80,000~M_{\sun}$.

An accreting black hole could launch jets. While @fal00 use a spectral analysis to build a case for a jet origin for SagittariusA$^*$, morphological evidence for an outflow would be far more compelling. In the case of NGC4395, the VLBA source is unresolved at a linear resolution of 0.46 lt-year or 170 lt-days, and only 0.32$\pm$0.26 times as strong as the VLA source spanning 23 lt-years or less. A few nearby galaxies have had their radio nuclei probed on similar scales, or even finer scales down to a resolution of 10 lt-days (eg, NGC3031, Bietenholz, Bartel, & Rupen 2000). While jets or jet-like structures are invariably observed, those galaxies exhibit spectral powers at 1.5 GHz that are an order of magnitude, or more, above the power of the source in NGC4395 at comparable VLA resolutions [@wro01]. Still, the VLA and VLBA sources in NGC4258 at 1.5 GHz [@cec00] are only about a factor of ten more powerful than their counterparts in NGC4395. At a linear resolution of 0.94 lt-year, the VLBA source in NGC4258 is resolved into two jets elongated over 5.9 lt-years. Similar jets, if present in NGC4395, could account for some of the VLA flux density missing from Figure 1: a deeper VLBA image of NGC4395 is required to search for such jets. The present VLBA detection could then correspond to the brightest region in the jets, an hypothesis that could be tested with VLBA imaging at higher linear resolution.

On energetics grounds it is reasonable also to explore a stellar origin for the Seyfert activity in NGC4395 [@fil93; @lir99]. In the radio regime, stellar processes could result in a compact starburst, a supernova remnant resembling Galactic ones, or a compact supernova remnant.

For a compact starburst, @con91 use an empirical scaling relation between thermal and nonthermal radio continuum, based on more extended star-forming galaxies with thermal electron temperature $T_{\rm e} = 10^4$ K, to derive a limiting brightness temperature of $T_{\rm b} \lesssim 10^5$ K at 1.4 GHz. The VLBA source in NGC4395, with brightness temperature $T_{\rm b} > 2.0\times10^6$ K, clearly exceeds this limit, excluding a compact starburst origin.

The Galactic supernova remnant CassiopeiaA, of age $t \lesssim
400$ years, has spectral powers and a spectral index [@baa77; @gre84; @fil93] very similar to those derived from the VLA detections of NGC4395 [@ho01a]. These photometric similarities are displayed in Figure 2. The upper limit $d = 23$ lt-years to the diameter of the VLA source in NGC4395 is also consistent with the diameter $d = 13$ lt-years for CassiopeiaA [@gre84]. But the VLBA source in NGC4395 is about a third as powerful (Figure 2) and considerably more compact ($d < 0.46$ lt-year) than CassiopeiaA. This VLBA size constraint strongly excludes further analogy with CassiopeiaA.

Could the VLBA source in NGC4395 be a compact supernova remnant (cSNR) whose evolution is governed by a supernova expanding into dense circumstellar material [@ter95]? Such an interpretation encounters some successes and some difficulties [@fil93; @lir99]. In the optical regime, published images of NGC4395 plus the POSS revealed the presence of a starlike nucleus, visible at similar brightness levels since 1956 May 8 UT and implying an age $t \gtrsim
36$ years in 1992. @lir99 find that, while this slow blue photometric evolution could just be a signpost of a cSNR of age $t
\sim 300$ years, the observed line properties roughly match those expected for a cSNR of age $t \sim 34$ years. The large-amplitude variability observed on time scales of days to months, in both the optical and X-ray regimes, remains a problem for the cSNR model.

In the radio regime, the record of photometric evolution of the VLA source, while sparse, supports approximate constancy over 1–2 decades at 1.4 and 4.9 GHz, since 1982 [@mor99; @ho01a]. Also, @hee64 used the NRAO 91-m telescope in 1963 to set an upper limit of 0.2 Jy for the peak flux density of NGC4395 at 1.4 GHz. Is this record consistent with a cSNR origin? Models for cSNRs do not, as yet, predict radio light curves, although @ter95 do remark on the relevance of SN1988Z, a radio supernova, to their models. Figure 3 shows the model light curves for SN1988Z at 1.4 and 4.9 GHz, based on three years of monitoring in the radio regime [@van93] but extrapolated to an age of 50 years. The powers of the VLA detections of NGC4395, measured on 1982 Feb 8 UT and 1990 Mar 3 UT by @mor99 with matched resolutions, and on 1999 Aug 29 and Oct 31 UT by @ho01a also with matched resolutions, are plotted at their minimum possible ages assuming a reference date of 1956 May 8 UT. (No point appears for the VLBA detection due to concern about the effects of source resolution between VLA and VLBA scales.) The power of the 91-m upper limit, obtained during 1963 between Feb 1 UT and Dec 31 UT, is also plotted at the minimum possible age of about 7.2 years relative to the same reference date. Figure 3 shows that NGC4395 is observed to be less powerful than SN1988Z, by factors ranging from at least 60 at an age of about 7 years to about 100–700 at an age of 43 years. This latter discrepancy could reflect the failure of the SN1988Z model after a few decades but model problems are unlikely through the first decade [@hym95]. The spectral powers of NGC4395 at 1.4 and 4.9 GHz thus appear to be too low since 1963 and too steady since 1982 to allow analogy with SN1988Z. The former trait is more constraining than the latter, as the degree of variability could be artificially suppressed if only a small fraction of the VLA emission arises from a cSNR.

In summary, for the VLBA source in NGC4395, the constraints on $T_b$, $d\/$, and $P\/$ are consistent with an origin from a black hole but exclude an origin from a compact starburst or a supernova remnant like CassiopeiaA. Also, the spectral powers of NGC4395 at 1.4 and 4.9 GHz seem both too low and too stable for analogy with SN1988Z, a suggested prototype for models of compact supernova remnants. Future studies of the VLBA source in NGC4395 should focus on measuring its spectral index, reducing the upper limit to its diameter or seeking evidence for jet-like structures, assessing its astrometric stability (cf. Wrobel 2000), and imaging the missing VLA flux density.

Like other Seyfert galaxies, NGC4395 exhibits copious evidence for thermal nuclear plasma. Estimates of the associated free-free optical depths $\tau_{\rm ff}$ at 1.4 GHz can constrain the relative geometries of the thermal and nonthermal plasmas. Equation 1 of @ulv99 reduces to $\tau_{\rm ff} = 0.038~T_{\rm e}^{-1.35}~E$ at 1.4 GHz for a thermal plamsa with electron temperature $T_{\rm e}$ in units of K and emission measure $E\/$ in units of cm$^{-6}$ pc. While $T_{\rm e}$ is either readily observed or plausibly estimated, neither condition generally applies to $E$. However, a notable exception is the thermal plasma responsible for the variable warm X-ray absorber in NGC4395 [@iwa00]. For this plasma, $T_{\rm
e} = 10^6$ K is plausibly adopted, the column density is measured, and the variability time scale sets a lower limit to the density. The latter two quantities imply $E \gtrsim 2 \times 10^{13}$ cm$^{-6}$ pc, which, in combination with $T_{\rm e} = 10^6$ K, leads to $\tau_{\rm
ff} \gtrsim 6000$. The variable warm X-ray absorber in NGC4395 has a free-free optical depth much larger than unity at 1.4 GHz and, therefore, cannot fully cover the VLBA source. Arguments such as these should be folded into discussions of unifying structures for the inner regions of quasars (eg, Elvis 2000).

The authors thank Dr. J. Ulvestad for discussions. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration. NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Figure 1

Figure 2

Figure 3
